Mason-Pfizer monkey virus (MPMV) constitutive transport element (CTE) functions in a position-dependent manner

Expression, purification, and functional characterization of soluble recombinant full-length simian immunodeficiency virus (SIV) Pr55Gag

The simian immunodeficiency virus (SIV) precursor polypeptide Pr55^Gag drives viral assembly and facilitates specific recognition and packaging of the SIV genomic RNA (gRNA) into viral particles. While several studies have tried to elucidate the role of SIV Pr55^Gag by expressing its different components independently, studies using full-length SIV Pr55^Gag have not been conducted, primarily due to the unavailability of purified and biologically active full-length SIV Pr55^Gag. We successfully expressed soluble, full-length SIV Pr55^Gag with His₆-tag in bacteria and purified it using affinity and gel filtration chromatography. In the process, we identified within Gag, a second in-frame start codon downstream of a putative Shine-Dalgarno-like sequence resulting in an additional truncated form of Gag. Synonymously mutating this sequence allowed expression of full-length Gag in its native form. The purified Gag assembled into virus-like particles (VLPs) in vitro in the presence of nucleic acids, revealing its biological functionality. In vivo experiments also confirmed formation of functional VLPs, and quantitative reverse transcriptase PCR demonstrated efficient packaging of SIV gRNA by these VLPs. The methodology we employed ensured the availability of >95% pure, biologically active, full-length SIV Pr55^Gag which should facilitate future studies to understand protein structure and RNA-protein interactions involved during SIV gRNA packaging.

Visualization of Retroviral Gag-Genomic RNA Cellular Interactions Leading to Genome Encapsidation and Viral Assembly: An Overview

Retroviruses must selectively recognize their unspliced RNA genome (gRNA) among abundant cellular and spliced viral RNAs to assemble into newly formed viral particles. Retroviral gRNA packaging is governed by Gag precursors that also orchestrate all the aspects of viral assembly. Retroviral life cycles, and especially the HIV-1 one, have been previously extensively analyzed by several methods, most of them based on molecular biology and biochemistry approaches. Despite these efforts, the spatio-temporal mechanisms leading to gRNA packaging and viral assembly are only partially understood. Nevertheless, in these last decades, progress in novel bioimaging microscopic approaches (as FFS, FRAP, TIRF, and wide-field microscopy) have allowed for the tracking of retroviral Gag and gRNA in living cells, thus providing important insights at high spatial and temporal resolution of the events regulating the late phases of the retroviral life cycle. Here, the implementation of these recent bioimaging tools based on highly performing strategies to label fluorescent macromolecules is described. This report also summarizes recent gains in the current understanding of the mechanisms employed by retroviral Gag polyproteins to regulate molecular mechanisms enabling gRNA packaging and the formation of retroviral particles, highlighting variations and similarities among the different retroviruses.

Identification of Pr78Gag Binding Sites on the Mason-Pfizer Monkey Virus Genomic RNA Packaging Determinants

How retroviral Gag proteins recognize the packaging signals (Psi) on their genomic RNA (gRNA) is a key question that we addressed here using Mason-Pfizer monkey virus (MPMV) as a model system by combining band-shift assays and footprinting experiments. Our data show that Pr78^Gag selects gRNA against spliced viral RNA by simultaneously binding to two single stranded loops on the MPMV Psi RNA: (1) a large purine loop (ssPurines), and (2) a loop which partially overlaps with a mostly base-paired purine repeat (bpPurines) and extends into a GU-rich binding motif. Importantly, this second Gag binding site is located immediately downstream of the major splice donor (mSD) and is thus absent from the spliced viral RNAs. Identifying elements crucial for MPMV gRNA packaging should help in understanding not only the mechanism of virion assembly by retroviruses, but also facilitate construction of safer retroviral vectors for human gene therapy.

Role of Purine-Rich Regions in Mason-Pfizer Monkey Virus (MPMV) Genomic RNA Packaging and Propagation

A distinguishing feature of the Mason-Pfizer monkey virus (MPMV) packaging signal RNA secondary structure is a single-stranded purine-rich sequence (ssPurines) in close vicinity to a palindromic stem loop (Pal SL) that functions as MPMV dimerization initiation site (DIS). However, unlike other retroviruses, MPMV contains a partially base-paired repeat sequence of ssPurines (bpPurines) in the adjacent region. Both purine-rich sequences have earlier been proposed to act as potentially redundant Gag binding sites to initiate the process of MPMV genomic RNA (gRNA) packaging. The objective of this study was to investigate the biological significance of ssPurines and bpPurines in MPMV gRNA packaging by systematic mutational and biochemical probing analyses. Deletion of either ssPurines or bpPurines individually had no significant effect on MPMV gRNA packaging, but it was severely compromised when both sequences were deleted simultaneously. Selective 2′ hydroxyl acylation analyzed by primer extension (SHAPE) analysis of the mutant RNAs revealed only mild effects on structure by deletion of either ssPurines or bpPurines, while the structure was dramatically affected by the two simultaneous deletions. This suggests that ssPurines and bpPurines play a redundant role in MPMV gRNA packaging, probably as Gag binding sites to facilitate gRNA capture and encapsidation. Interestingly, the deletion of bpPurines revealed an additional severe defect on RNA propagation that was independent of the presence or absence of ssPurines or the gRNA structure of the region. These findings further suggest that the bpPurines play an additional role in the early steps of MPMV replication cycle that is yet to be identified.

Stabilizing role of structural elements within the 5´ Untranslated Region (UTR) and gag sequences in Mason-Pfizer monkey virus (MPMV) genomic RNA packaging

The Mason-Pfizer monkey virus (MPMV) genomic RNA (gRNA) packaging signal is a highly-structured element with several stem-loops held together by two phylogenetically conserved long-range interactions (LRIs) between U5 and gag complementary sequences. These LRIs play a critical role in maintaining the structure of the 5´ end of the MPMV gRNA. Thus, one could hypothesize that the overall RNA secondary structure of this region is further architecturally held together by three other stem loops (SL3, Gag SL1, and Gag SL2) comprising of sequences from the distal parts of the 5´untranslated region (5' UTR) to ~ 120 nucleotides into gag, excluding gag sequences involved in forming the U5-Gag LRIs. To provide functional evidence for the biological significance of these stem loops during gRNA encapsidation, these structural motifs were mutated and their effects on MPMV RNA packaging and propagation were tested in a single round trans-complementation assay. The mutant RNA structures were further studied by high throughput SHAPE (hSHAPE) assay. Our results reveal that sequences involved in forming these three stem loops do not play crucial roles at an individual level during MPMV gRNA packaging or propagation. Further structure-function analysis indicates that the U5-Gag LRIs have a more important architectural role in stabilizing the higher order structure of the 5´ UTR than the three stem loops which have a more secondary and perhaps indirect role in stabilizing the overall RNA secondary structure of the region. Our work provides a better understanding of the molecular interactions that take place during MPMV gRNA packaging.

Expression, purification, and characterization of biologically active full-length Mason-Pfizer monkey virus (MPMV) Pr78Gag

MPMV precursor polypeptide Pr78^Gag orchestrates assembly and packaging of genomic RNA (gRNA) into virus particles. Therefore, we have expressed recombinant full-length Pr78^Gag either with or without His₆-tag in bacterial as well as eukaryotic cultures and purified the recombinant protein from soluble fractions of the bacterial cultures. The recombinant Pr78^Gag protein has the intrinsic ability to assemble in vitro to form virus like particles (VLPs). Consistent with this observation, the recombinant protein could form VLPs in both prokaryotes and eukaryotes. VLPs formed in eukaryotic cells by recombinant Pr78^Gag with or without His₆-tag can encapsidate MPMV transfer vector RNA, suggesting that the inclusion of the His₆-tag to the full-length Pr78^Gag did not interfere with its expression or biological function. This study demonstrates the expression and purification of a biologically active, recombinant Pr78^Gag, which should pave the way to study RNA-protein interactions involved in the MPMV gRNA packaging process.

Gammaretroviral pol sequences act in cis to direct polysome loading and NXF1/NXT-dependent protein production by gag-encoded RNA

Background

All retroviruses synthesize essential proteins via alternatively spliced mRNAs. Retrovirus genera, though, exploit different mechanisms to coordinate the synthesis of proteins from alternatively spliced mRNAs. The best studied of these retroviral, post-transcriptional effectors are the trans-acting Rev protein of lentiviruses and the cis-acting constitutive transport element (CTE) of the betaretrovirus Mason-Pfizer monkey virus (MPMV). How members of the gammaretrovirus genus translate protein from unspliced RNA has not been elucidated.

Results

The mechanism by which two gammaretroviruses, XMRV and MLV, synthesize the Gag polyprotein (Pr65^Gag) from full-length, unspliced mRNA was investigated here. The yield of Pr65^Gag from a gag–only expression plasmid was found to be at least 30-fold less than that from an otherwise isogenic gag-pol expression plasmid. A frameshift mutation disrupting the pol open reading frame within the gag-pol expression plasmid did not decrease Pr65^Gag production and 398 silent nucleotide changes engineered into gag rendered Pr65^Gag synthesis pol-independent. These results are consistent with pol-encoded RNA acting in cis to promote Pr65^Gag translation. Two independently-acting pol fragments were identified by screening 17 pol deletion mutations. To determine the mechanism by which pol promoted Pr65^Gag synthesis, gag RNA in total and cytoplasmic fractions was quantitated by northern blot and by RT-PCR. The pol sequences caused, maximally, three-fold increase in total or cytoplasmic gag mRNA. Instead, pol sequences increased gag mRNA association with polyribosomes ~100-fold, a magnitude sufficient to explain the increase in Pr65^Gag translation efficiency. The MPMV CTE, an NXF1-binding element, substituted for pol in promoting Pr65^Gag synthesis. A pol RNA stem-loop resembling the CTE promoted Pr65^Gag synthesis. Over-expression of NXF1 and NXT, host factors that bind to the MPMV CTE, synergized with pol to promote gammaretroviral gag RNA loading onto polysomes and to increase Pr65^Gag synthesis. Conversely, Gag polyprotein synthesis was decreased by NXF1 knockdown. Finally, overexpression of SRp20, a shuttling protein that binds to NXF1 and promotes NXF1 binding to RNA, also increased gag RNA loading onto polysomes and increased Pr65^Gag synthesis.

Conclusion

These experiments demonstrate that gammaretroviral pol sequences act in cis to recruit NXF1 and SRp20 to promote polysome loading of gag RNA and, thereby license the synthesis of Pr65^Gag from unspliced mRNA.

Effect of posttranscriptional regulatory elements on transgene expression and virus production in the context of retrovirus vectors

Ineffective transgene expression in a sufficient amount of target cells is still a limitation in retroviral vector mediated gene therapy. Thus, we systematically evaluated four genetic modulators, (i) the woodchuck posttranscriptional regulatory element (WPRE), (ii) the mouse RNA transport element (RTE), (iii) the constitutive transport element (CTE) of the simian retrovirus type 1 (SRV-1), and (iv) the 5' untranslated region of the human heat shock protein 70 (Hsp70 5'UTR), all of them involved in the posttranscriptional control of mRNA nucleo/cytoplasmatic transport, RNA stability, and translation efficiency, in an MLV-based retrovirus vector context. Insertion of the WPRE into the retrovirus vector resulted in enhancement of transgene expression (EGFP) both in transfected virus producing cells as well as in infected recipient cells irrespective of the location in the vector. The best effect was observed with two copies of the WPRE, 3' of the transgene and in the 3' untranslated region of the vector backbone. However, oligomerization of this element does not further increase transgene expression. Presence of the WPRE resulted also in an increase in virus production. Introduction of the CTE and/or RTE in the retroviral vector did not alter transgene expression and infectious particle production. Positive effects were observed only in vectors harboring the CTE and/or RTE in combination with the WPRE. The activity of the Hsp70 5'UTR as a translational enhancer was found to be negligible in the context of the retroviral vector. However, interference of the Hsp70 5'UTR strong secondary structure with the packaging sequence of the viral RNA was experimentally excluded as being the cause of this. These data suggest that only the WPRE is a suitable element for the improvement of transgene expression and oncoretroviral vector production.

Close proximity of the MPMV CTE to the polyadenylation sequences is important for efficient function in the subgenomic context

The constitutive transport element (CTE) of Mason-Pfizer monkey virus (MPMV) is a short cis-acting sequence element critical for virus gene expression. Analogous to the Rev/Rev Responsive Element (RRE) of primate lentiviruses, CTE allows the nucleocytoplasmic transport of unspliced viral mRNAs. In fact, CTE can functionally replace Rev/RRE in the genomic context and has been used successfully in the expression of viral and cellular genes from expression vectors as well. However, unlike RRE, CTE accomplishes this by interacting with cellular factors, making CTE function independent of co-expressed trans factors. Thus, CTE has proven to be a valuable tool in the expression of heterologous genes. Our previous studies have shown that close proximity of CTE to the polyadenylation sequences is important for CTE function in the genomic context. However, it is controversial whether CTE needs to be located spatially close to the polyadenylation sequences in the subgenomic context. Since CTE is being frequently used in expression vectors, we investigated the position dependency of CTE in the heterologous, subgenomic background using both genetic and structural analyses. Our results reveal that similar to the genomic situation, close proximity of CTE to the polyadenylation sequences is important for its function in the heterologous subgenomic context.

Feline Immunodeficiency Virus Vectors for Efficient Transduction of Primary Human Synoviocytes: Application to an Original Model of Rheumatoid Arthritis

The potential of gene therapeutics is hindered by the limitations of the delivery systems presently available. Recently, human immunodeficiency virus (HIV) vectors have been developed that allow the efficient and stable transduction of primary nondividing cells in vivo. Because of the safety concerns raised by HIV vectors, we developed a gene delivery system derived from the ungulate lentivirus feline immunodeficiency virus (FIV). We describe in the present study the optimization of the safety and efficiency of this system that proved to be as potent as HIV vectors to transduce nondividing cells, with titers over 10(8) transducing units per ml. We used this tool to transduce TNF-alpha into human primary synoviocytes, and showed a high efficiency of transduction. TNF-alpha-transduced synoviocytes injected into the knee joints of severe combined immunodeficient (SCID) mice induced cell proliferation, as well as cartilage and bone erosion as soon as day 7, creating a standardized, humanized animal model relevant for rheumatoid arthritis. FIV vectors appear to be promising tools for biologic research and gene therapy.

Mutational analysis of the predicted secondary RNA structure of the Mason-Pfizer monkey virus packaging signal

The 5' end of the Mason-Pfizer monkey virus (MPMV) genomic RNA has been predicted to fold into a complex stem/loop structure that is thought to play a role in specific RNA encapsidation. In this study, we used a set of mutations that either abrogated or recreated the first four stem loops predicted within the 5' untranslated region (5' UTR) for effects on RNA packaging. Test of these mutations in our biological assay revealed that only stem loop 1 (SL1) was important for the packaging potential of MPMV, while mutations in none of the other stem loops affected packaging significantly. Interestingly, it was the primary sequence of SL1 RNA and not its secondary structure that affected packaging since compensatory mutations that reformed SL1 were unable to restore the packaging efficiency of the retroviral vector. Additionally, our mutational analysis reveals that stem loop 4, predicted to be the major packaging determinant of MPMV, does not seem to have a significant role in packaging. Finally, results of the biological effects of the structural mutations are discussed in relation to their effects on the folding potential of the various stem loops.

Genetic therapy for HIV/AIDS

Despite the tremendous success of highly active antiretroviral treatment (HAART) introduced nearly 8 years ago for the treatment of human immunodeficiency virus (HIV), innovative therapies, including gene transfer approaches, are still required for nearly half of the general patient population. A number of potential gene therapeutic targets for HIV have been identified and include both viral and cellular genes essential for viral replication. The diverse methods used to inhibit viral replication comprise RNA-based strategies such as ribozymes, RNA decoys, antisense messenger RNAs and small interfering RNA (siRNA) molecules. Other potential anti-HIV genes include dominant negative viral proteins, intracellular antibodies, intrakines and suicide genes, all of which have had a modicum of success in vitro. Cellular targets include CD4+ T cells, macrophages and their progenitors. The greatest gene transfer efficiency has been achieved using retroviral or, more recently, lentiviral vectors. A limited number of Phase I clinical trials suggest that the general method is safe. It is proposed that a national network for HIV gene therapy (similar to the AIDS Clinical Trial Groups) may be the best way to determine which approaches should proceed clinically.

Sequences within the gag gene of feline immunodeficiency virus (FIV) are important for efficient RNA encapsidation

Feline immunodeficiency virus (FIV)-based retroviral vector systems are being developed for human gene therapy. Consequently, it has become important to know the precise sequence requirements for the packaging of FIV genome so that such sequences can be eliminated from transfer vectors post-transduction for improved safety. Recently, we have shown that sequences both within the 5'-untranslated leader region (UTR) and the 5'-end of gag are required for efficient packaging and transduction of FIV-based vectors. However, the extent of gag sequence important in the encapsidation process is not clear as well as their relative contribution to packaging. In this study, using a biologically relevant packaging system, we demonstrate that at the most 100 bp of gag sequences are sufficient for efficient RNA packaging in conjunction with the 5'-UTR and no other sequences within the next 600 bp of gag exist that affect packaging. In addition, we show that sequences within gag do not simply act as spatial elements to stabilize other structural determinants of packaging located within the 5'-UTR, but are important in themselves for the encapsidation process.

Sequences within both the 5' untranslated region and the gag gene are important for efficient encapsidation of Mason-Pfizer monkey virus RNA

It has previously been shown that the 5' untranslated leader region (UTR), including about 495 bp of the gag gene, is sufficient for the efficient encapsidation and propagation of Mason-Pfizer monkey virus (MPMV) based retroviral vectors. In addition, a deletion upstream of the major splice donor, SD, has been shown to adversely affect MPMV RNA packaging. However, the precise sequence requirement for the encapsidation of MPMV genomic RNA within the 5' UTR and gag remains largely unknown. In this study, we have used a systematic deletion analysis of the 5' UTR and gag gene to define the cis-acting sequences responsible for efficient MPMV RNA packaging. Using an in vivo packaging and transduction assay, our results reveal that the MPMV packaging signal is primarily found within the first 30 bp immediately downstream of the primer binding site. However, its function is dependent upon the presence of the last 23 bp of the 5' UTR and approximately the first 100 bp of the gag gene. Thus, sequences that affect MPMV RNA packaging seem to reside both upstream and downstream of the major splice donor with the downstream region responsible for the efficient functioning of the upstream primary packaging determinant.

Enhancement of antibody responses to an HIV-2 DNA envelope vaccine using an expression vector containing a constitutive transport element

Because immune responses to DNA vaccines in humans remains suboptimal, strategies need to be devised to facilitate expression of the vaccine in vivo. One method to improve response to a DNA vaccine is to construct plasmid vectors with leader sequences and post-transcriptional elements that facilitate export of transcribed RNA. In this study, we sought to determine if a mammalian expression vector (pND-14) containing a tissue plasminogen activator (TPA) leader sequence and a constitutive transport element (CTE) from simian retrovirus was superior to other mammalian expression vectors containing a post-transcriptional regulatory element (PRE) from hepatitis B virus (pCMV-link) or a minimal mammalian expression vector (pVAX1). Toward this objective, we evaluated protein expression of the HIV-2 envelope gene (gp140) in vitro and immune responses in immunized mice. We found that pVAX1 produced three- to fourfold lower levels of gp140 in vitro (5 ng/ml) in contrast to the pCMV-link and pND-14 vectors. When we immunized groups of mice intradermally with two of the HIV-2 gp140 DNA vaccine constructs, we found that pND-14 induced higher levels of envelope-specific systemic and mucosal antibodies than pCMV-link. We conclude that expression vectors for DNA vaccines should contain TPA and CTE sequences to facilitate immune responses.

Defective lentiviral vectors are efficiently trafficked by HIV-1 and inhibit its replication

Gene therapy against HIV infection should involve vector-mediated delivery of anti-HIV therapeutic genes into T-lymphocytes and macrophages or, alternatively, hematopoietic progenitors. Transduction of mature cells with defective vectors would have limited success because the vector would disappear with cell turnover. However, if a vector could be trafficked by wild-type HIV, initial transduction of a majority of the population would not be required, as the vector would be able to spread. We describe HIV-1-based lentiviral vectors that are efficiently packaged and trafficked by HIV-1, allowing a small number of cells initially transduced to spread the vector within a nontransduced cell population. We examined whether the presence or absence of the rev gene and the Rev-responsive element (RRE) would have a noticeable effect on the ability of lentiviral vectors to be trafficked and to inhibit HIV-1 replication. We found that replacement of rev/RRE with a constitutive transport element from Mason-Pfizer monkey virus had no apparent effect on trafficking and did not change the intrinsic inhibitory abilities of the vectors. We also constructed a rev/RRE-independent HIV-1-derived vector carrying a trans-dominant negative mutant of HIV-1 Rev, RevM10. This vector was less efficiently trafficked by HIV-1 and, despite the presence of an anti-HIV-1 gene, RevM10, was less efficient at inhibiting HIV-1 replication when introduced into a target T-cell population.

Primate and feline lentivirus vector RNA packaging and propagation by heterologous lentivirus virions

Development of safe and effective gene transfer systems is critical to the success of gene therapy protocols for human diseases. Currently, several primate lentivirus-based gene transfer systems, such as those based on human and simian immunodeficiency viruses (HIV/SIV), are being tested; however, their use in humans raises safety concerns, such as the generation of replication-competent viruses through recombination with related endogenous retroviruses or retrovirus-like elements. Due to the greater phylogenetic distance from primate lentiviruses, feline immunodeficiency virus (FIV) is becoming the lentivirus of choice for human gene transfer systems. However, the safety of FIV-based vector systems has not been tested experimentally. Since lentiviruses such as HIV-1 and SIV have been shown to cross-package their RNA genomes, we tested the ability of FIV RNA to get cross-packaged into primate lentivirus particles such as HIV-1 and SIV, as well as a nonlentiviral retrovirus such as Mason-Pfizer monkey virus (MPMV), and vice versa. Our results reveal that FIV RNA can be cross-packaged by primate lentivirus particles such as HIV-1 and SIV and vice versa; however, a nonlentivirus particle such as MPMV is unable to package FIV RNA. Interestingly, FIV particles can package MPMV RNA but cannot propagate the vector RNA further for other steps of the retrovirus life cycle. These findings reveal that diverse retroviruses are functionally more similar than originally thought and suggest that upon coinfection of the same host, cross- or copackaging may allow distinct retroviruses to generate chimeric variants with unknown pathogenic potential.

Enhancement of mucosal immune response against HIV-1 Gag by DNA immunization

In order to examine the feasibility of Gag-expression DNA as a potential candidate for HIV vaccine using a mouse model, we injected DNA into mice either intramuscularly or by using a gene gun. Both methods induced a low level of antibody production. However, after booster immunization with p24 protein emulsified with complete Freund's adjuvant via a footpad, we found that only the preceding intramuscular DNA immunization induced an anti-Gag Th1-type (IgG(2a)) antibody response, in addition to the enhancement of a Th2-type (IgG(1)) antibody response. Importantly, when mice were boosted intranasally with p24 and cholera toxin, intramuscular DNA injection was found to enhance both systemic and mucosal Gag-specific immune responses. These results indicate that intramuscular DNA immunization confers the inducibility of memory cells, which circulate around various mucosal tissues. Therefore, intramuscular DNA priming, followed by a mucosal booster immunization, could be considered as a regimen applicable to HIV vaccine.

Improved titers of HIV-based lentiviral vectors using the SRV-1 constitutive transport element

The development of lentiviral vectors that use Rev-independent mechanisms of nuclear export for their genomic RNA could facilitate the construction of novel anti-HIV vectors. We have improved the titers of Rev-independent lentiviral vectors having the SRV-1 CTE by mutating the major splice donor and acceptor sites present in the vector and by relocalization of the CTE sequences adjacent to the HIV-1 3'LTR. These two modifications have additive beneficial effects on vector titers and packaging efficiency. Packaging these CTE+ vectors expressing marker genes with a Rev-dependent HIV-1 helper vector yields higher titers than are obtained using a Rev-dependent lentiviral vector.

Multiple copies of the Mason-Pfizer monkey virus constitutive RNA transport element lead to enhanced HIV-1 Gag expression in a context-dependent manner

Retroviral gene expression requires nuclear export and translation of incompletely spliced RNA. In the case of human immunodeficiency virus (HIV), this is facilitated by the viral Rev protein binding to its cognate RNA response element (RRE), while other retroviruses contain constitutive transport elements (CTE) binding to cellular factors. These CTE can substitute for the HIV-1 Rev/RRE system, albeit with reduced efficiency. Here, we show that multimeric copies of the CTE restore HIV-1 protein expression to levels comparable to or higher than Rev/RRE in various cell lines from different species. We suggest that multimerization of export factors is important for CTE function, as reported for Rev. CTE function was not affected when the element was displaced from its natural position close to the poly(A) signal, while insertion of an intron into the 3'-untranslated region (3'-UTR) severely reduced CTE activity. In this case, cytoplasmic RNA degradation was observed, which may be mediated by nonsense-mediated RNA decay. In contrast, Rev-dependent gene expression was insensitive to an intron in the 3'-UTR. Finally, we show that the putative CTE-binding protein RNA helicase A is not specifically translocated into the cytoplasm upon overexpression of CTE-containing RNA.

Efficient transduction of nondividing cells by optimized feline immunodeficiency virus vectors

Second- and third-generation three-plasmid vector systems, termed FELIX, were constructed from feline immunodeficiency virus (FIV). To enhance vector production, the weak FIV long terminal repeat promoter was replaced with the human cytomegalovirus enhancer/promoter. To construct a minimal system in which Gag-Pol was the only viral protein present, the cytoplasmic transport element was used in place of the FIV Rev-RRE system to facilitate nuclear export of Gag-Pol and the transfer vector. Unconcentrated vector titers routinely exceeded 1 x 10(6) IU/mL for most constructs tested. Second- and optimized third-generation vectors were capable of efficiently infecting G1/S- and G2/M-arrested cells. FIV-based FELIX vectors transduced human dendritic cells, hepatocytes, and aortic smooth muscle with efficiencies similar to that of a control 3T3 cell line. All three of these primary cell types were transducible by both the second- and third-generation FELIX vectors, demonstrating that FIV Gag-Pol alone contains the determinants necessary for transduction of primary cells. In cross-packaging tests, we observed that HIV Gag-Pol does not substantially package FIV vectors; consequently, use of such vectors in human immunodeficiency virus-infected cells should not lead to efficient mobilization of the inserted gene. Thus, this FIV-based vector system offers high efficiency and stable delivery of genes to numerous nondividing and primary cell types, opening new avenues for biological inquiry into normal human cells.

Nucleocytoplasmic export of type D simian retrovirus genomic RNA: identification of important genetic subregions and interacting cellular proteins

The simian retrovirus (SRV) genome contains a constitutive transport element (CTE) within its 3' intergenic region (IR) that mediates the nuclear export of unspliced SRV RNA. The serogroup 2 SRV CTE is predicted to form a stable stem-loop structure containing two major internal loops exhibiting 180 degrees inverse symmetry, with loop face sequences A, A', B, and B' and additional minor internal and terminal loops. To begin the identification of potential CTE-interacting proteins and to assess structural requirements for protein interaction, we conducted RNA mobility shift assays using IR fragments that obliterated this region's known stable stem-loop structure. Using immunoblotting assays, we have determined that RNA helicase A, implicated in the nuclear export of unspliced SRV genomic RNA, does not appear to interact directly with either the complete serogroup 2 SRV 3' IR or the subregion RNAs and that formation of RNA-protein complexes is conferred by interaction with other novel proteins. UV crosslinking of RNA-protein complexes, coupled with RNase T1/A digestion, indicates that a novel protein of 120 kDa molecular weight interacts with the complete CTE or with individual subregion RNAs. Transfection analyses indicate that SRV recombinants containing A, A', B, or B' sequences forming the faces for two open loops undergo RNA export; only the complete sense CTE recombinant or a second recombinant containing two subregions in sense orientation that reconstitute the 3' two-thirds of the 3' IR, and contain only A' and B that form the faces for two terminal loops, are capable of SRV RNA export. These experiments indicate that secondary structural determinants of the 3' IR and multiple protein interactions may be important factors in the nuclear export of unspliced SRV RNA.

Evaluation of novel human immunodeficiency virus type 1 Gag DNA vaccines for protein expression in mammalian cells and induction of immune responses

Human immunodeficiency virus (HIV)-specific cytotoxic T lymphocytes (CTL) are an important parameter of host defenses that limit viral replication after infection. Induction of effective CTL against conserved viral proteins such as Gag may be essential to the development of a safe and effective HIV type 1 (HIV-1) vaccine. DNA vaccination represents a novel strategy for inducing potent CD8(+) CTL responses in vivo. However, expression of HIV-1 structural proteins by DNA vectors has been hampered by a stringent requirement for coexpression with other viral components, such as Rev and RRE. Furthermore, even with Rev and RRE present, the level of expression of HIV-1 Gag, Pol, or Env is very low in murine cells. These problems have limited our ability to address the key issue of how to generate effective CTL responses to Gag in a mouse model. To overcome this problem, we compared several novel DNA expression vectors for HIV-1 Gag protein expression in primate and mouse cells and for generating immune responses in mice after DNA vaccination. A DNA vector containing wild type HIV-1 gag coding sequences did not induce detectable Gag expression in any of the cells tested. Attempts to increase nuclear export of Gag expression RNA by adding the constitutive transport element yielded only a moderate increase in Gag expression in monkey-derived COS cells and an even lower increase in Gag expression in HeLa cells or several mouse cell lines. In contrast, silent-site mutations in the HIV-1 gag coding sequences significantly increased Gag expression levels in all cells tested. Furthermore, this construct induced both Gag-specific antibody and CTL responses in mice after DNA vaccination. Using this construct, we achieved stable expression of HIV-1 Gag in the mouse cell line p815, which can now be used as a target cell for measuring HIV-1 Gag-specific CTL responses in immunized mice. The DNA vectors described in this study should make it possible to systematically evaluate the approaches for maximizing the induction of CTL responses against HIV-1 Gag in mouse and other animal systems.

Influence of Rex and intronic sequences on expression of spliced mRNAs produced by human T cell leukemia virus type I

Expression of the incompletely spliced HTLV-I mRNAs relies on the viral posttranscriptional activator Rex, whose interaction with the Rex-responsive element (RXRE) overcomes effects of cis-acting repressive sequences (CRSs). Studies based on heterologous reporter plasmids identified an intronic CRS in the 5' LTR and a CRS that overlaps with the RXRE. The present study investigated the effects of these elements in the context of spliced viral mRNAs encoding p21Rex (mRNA 1-3), Tax/Rex (mRNA 1-2-3), and Tof (mRNA 1-2-B). All three mRNAs were inefficiently expressed when transcribed in their mature intronless form, with the p21Rex mRNA showing the weakest expression. In contrast, efficient expression of p21Rex was obtained from a plasmid containing the 5' LTR and 3' portion of the genome that encoded a spliceable RNA. The defective expression of the intronless mRNAs reflected the inhibitory activity of the RXRE and the lack of 5' intronic sequences. Insertion of an intronic 5' LTR segment located upstream of the 5' CRS overcame Rex dependence conferred by the RXRE. The activity of this segment was mapped to the major splice donor and sequences overlapping with, but functionally distinct from, a previously described transcriptional enhancer. The three mRNAs responded differently to Rex and to insertion of the constitutive transport element of simian retrovirus type 1. Taken together, these results suggest that expression of the spliced mRNAs is controlled by the relative influence of positive and negative sequences present on the primary transcript as well as the Rex-RXRE interaction.

Minimum requirements for efficient transduction of dividing and nondividing cells by feline immunodeficiency virus vectors

The development of gene delivery vectors based on feline immunodeficiency virus (FIV) is an attractive alternative to vectors based on primate sources for the delivery of genes into humans. To investigate the requirements for efficient transduction of dividing and nondividing cells by vector particles based on FIV, a series of packaging and vector constructs was generated for which viral gene expression was minimized and from which unnecessary cis-acting sequences were deleted. Pseudotyped vector particles produced in 293T cells were used to transduce various target cells, including contact-inhibited human skin fibroblasts and growth-arrested HT1080 cells. FIV vectors in which the U3 promoter was replaced with the cytomegalovirus promoter gave rise to over 50-fold-higher titers than FIV vectors containing the complete FIV 5' long terminal repeat (LTR). Comparison of the transduction efficiencies of vectors containing different portions of the FIV Gag coding region indicates that at least a functional part of the FIV packaging signal (Psi) is located within an area which includes the 5' LTR and the first 350 bp of gag. Transduction efficiencies of vectors prepared without FIV vif and orf2 accessory gene expression did not differ substantially from those of vectors prepared with accessory gene expression in either dividing or nondividing cells. The requirement for FIV rev-RRE was, however, demonstrated by the inefficient production of vector particles in the absence of rev expression. Together, these results demonstrate the efficient transduction of nondividing cells in vitro by a multiply attenuated FIV vector and contribute to an understanding of the minimum requirements for efficient vector production and infectivity. In addition, we describe the ability of an FIV vector to deliver genes in vivo into hamster muscle tissue.

The 5' RNA terminus of spleen necrosis virus contains a novel posttranscriptional control element that facilitates human immunodeficiency virus Rev/RRE-independent Gag production

Previous work has shown that spleen necrosis virus (SNV) long terminal repeats (LTRs) are associated with Rex/Rex-responsive element-independent expression of bovine leukemia virus RNA and supports the hypothesis that SNV RNA contains a cis-acting element that interacts with cellular Rex-like proteins. To test this hypothesis, the human immunodeficiency virus type 1 (HIV) Rev/RRE-dependent gag gene was used as a reporter to analyze various SNV sequences. Gag enzyme-linked immunosorbent assay and Western blot analyses reveal that HIV Gag production is enhanced at least 20, 000-fold by the 5' SNV LTR in COS, D17, and 293 cells. Furthermore, SNV RU5 in the sense but not the antisense orientation is sufficient to confer Rev/RRE-independent expression onto a cytomegalovirus-gag plasmid. In contrast, the SNV 3' LTR and 3' untranslated sequence between env and the LTR did not support Rev-independent gag expression. Quantitative RNase protection assays indicate that the SNV 5' RNA terminus enhances cytoplasmic accumulation and polysome association of HIV unspliced and spliced transcripts. However, comparison of the absolute amounts of polysomal RNA indicates that polysome association is not sufficient to account for the significant increase in Gag production by the SNV sequences. Our analysis reveals that the SNV 5' RNA terminus contains a unique cis-acting posttranscriptional control element that interacts with hypothetical cellular Rev-like proteins to facilitate HIV RNA transport and efficient translation.

The upstream, direct repeat sequence of Prague A Rous sarcoma virus is deficient in mediating efficient Gag assembly and particle release

Rous sarcoma virus (RSV) contains two approximately 135-nt imperfect direct repeats composed of smaller repeats, dr1 (approximately 100 nt) and dr2 (approximately 36 nt), that are between the env and src genes and downstream of src in the 3' untranslated region, respectively. It has previously been shown that a Prague A RSV mutant in which both dr1 sequences are deleted is defective at several points in the virus life cycle, including unspliced RNA and env mRNA stability, unspliced RNA transport, and virus particle assembly. A defect in unspliced RNA transport occurs because a cytoplasmic transport element is present within the dr1. We have suggested that the defect of particle production may arise from the failure of the unspliced RNA to be targeted to sites in the cytoplasm where its translation is favorable for Gag protein assembly. In this report, we have further investigated the function of the direct repeats by comparing virus mutants containing either a single upstream or downstream dr1 sequence. Both mutants were delayed in replication compared to the wild-type; the mutant with a single upstream dr1 (delta DDR) is significantly more defective than the mutant with a single downstream dr1 (delta UDR). While both mutants appear capable of efficiently transporting unspliced RNA to the cytoplasm, the delta DDR mutant with only the upstream dr1 is defective in its ability to support Gag assembly and particle release. The replication defect cannot be repaired by placing the upstream dr1 at the location of the downstream dr1 in the 3' untranslated region. A single point mutation in the upstream dr1 (U to C) restored replication and particle production to near normal levels. The results suggest that unspliced RNA transport and Gag assembly functions may be mediated by different elements within the dr1 and that the Prague A upstream dr1 is defective in the latter but not the former function.

The HIV-1 Rev protein

The nuclear export of intron-containing HIV-1 RNA is critically dependent on the activity of Rev, a virally encoded sequence-specific RNA-binding protein. Rev shuttles between the nucleus and the cytoplasm and harbors both a nuclear localization signal and a nuclear export signal. These essential peptide motifs have now been shown to function by accessing cellular signal-mediated pathways for nuclear import and nuclear export. HIV-1 Rev therefore represents an excellent system with which to study aspects of transport across the nuclear envelope.