Inhibition of Moloney murine leukemia virus-induced leukemia in transgenic mice expressing antisense RNA complementary to the retroviral packaging sequences

Protection of shrimp Penaeus monodon from WSSV infection using antisense constructs

White spot syndrome caused by white spot syndrome virus (WSSV) is one of the most threatening diseases of shrimp culture industry. Previous studies have successfully demonstrated the use of DNA- and RNA-based vaccines to protect WSSV infection in shrimp. In the present study, we have explored the protective efficacy of antisense constructs directed against WSSV proteins, VP24, and VP28, thymidylate synthase (TS), and ribonucleotide reductase-2 (RR2) under the control of endogenous shrimp histone-3 (H3) or penaedin (Pn) promoter. Several antisense constructs were generated by inserting VP24 (pH3-VP24, pPn-VP24), VP28 (pH3-VP28, pPn-VP28), TS (pH3-TS, pPn-TS), and RR2 (pH3-RR2) in antisense orientation. These constructs were tested for their protective potential in WSSV infected cell cultures, and their effect on reduction of the viral load was assessed. A robust reduction in WSSV copy number was observed upon transfection of antisense constructs in hemocyte cultures derived from Penaeus monodon and Scylla serrata. When tested in vivo, antisense constructs offered a strong protection in WSSV challenged P. monodon. Constructs expressing antisense VP24 and VP28 provided the best protection (up to 90 % survivability) with a corresponding decrease in the viral load. Our work demonstrates that shrimp treated with antisense constructs present an efficient control strategy for combating WSSV infection in shrimp aquaculture.

Therapeutic effect of a Gag-nuclease fusion protein against retroviral infection in vivo

Recently, remarkable progress has been made in developing effective combination drug therapies that can control but not cure retroviral replication. Even when effective, these drug regimens are toxic, they require demanding administration schedules, and resistant viruses can emerge. Thus the need for new gene-based therapies continues. In one such approach, capsid-targeted viral inactivation (CTVI), nucleases fused to viral coat proteins are expressed in infected cells and become incorporated during virion assembly. CTVI can eliminate infectious murine retrovirus titer in tissue culture. Here we describe transgenic mice expressing fusions of the Moloney murine leukemia virus (Mo-MuLV) Gag protein to staphylococcal nuclease. This work tests the protective effect and demonstrates in vivo proof-of-principle of CTVI in transgenic mice expressing endogenous proviral copies of Mo-MuLV. The antiviral protein-expressing mice are phenotypically normal, attesting to the lack of toxicity of the fusion protein. The Mo-MuLV infection was much less virulent in transgenic littermates than in nontransgenic littermates. Gag-nuclease expression reduced infectious titers in blood up to 10-fold, decreased splenomegaly and leukemic infiltration, and increased life spans up to 2.5-fold in transgenic relative to nontransgenic infected animals. These results suggest that gene therapies based on similar fusion proteins, designed to attack human immunodeficiency virus or other retroviruses, could provide substantial therapeutic benefits.

Expression of heterologous promoters in Lutzomyia longipalpis and Phlebotomus papatasi (Diptera: Psychodidae) cell lines

To establish a transient expression system for genes introduced into sand fly cell lines, we tested the expression of the luciferase reporter gene under control of different promoters. Towards this end, we lipofected cell lines obtained from New and Old World sand flies, LL-5 from Lutzomyia longipalpis Lutz & Neiva and PP-9 from Phlebotomus papatasi Scopoli, respectively. The relative levels of luciferase expression were studied under control of Drosophila melanogaster Meigen heat shock protein 70 (hsp70), human cytomegalovirus, simian virus 40 or Junonia coenia (Hübner) densovirus (P9) promoters. The Drosophila heat shock protein 70 promoter, originating from insect genes, functioned as a strong promoter in both cell lines. Promoters from the different virus genes also were capable of driving transgene expression in both cell lines.

Dose-response resistance to HIV-1/MuLV pseudotype virus ex vivo in a hairpin ribozyme transgenic mouse model

We have investigated the efficacy of a hairpin ribozyme targeting the 5' leader sequence of HIV-1 RNA in a transgenic model system. Primary spleen cells derived from transgenic or control mice were infected with HIV-1/MuLV pseudotype virus. A significantly reduced susceptibility to infection in ribozyme-expressing transgenic spleen cells (P = 0.01) was shown. Variation of transgene-expression levels between littermates revealed a dose response between ribozyme expression and viral resistance, with an estimated cut off value below 0.2 copies of hairpin ribozyme per cell. These findings open up possibilities for studies on ribozyme efficacy and anti-HIV-1 gene therapy.

Antisense transgenics in animals

Antisense transgenesis provides a methodology for ablating gene expression in targeted tissues through the use of tissue-specific or controllable promoters. The two major features to be considered in the design of a construct for injection are (1) the target sequence and (2) the promoter to be used. Information is provided to help the investigator make decisions in these regards. The standard methodology of making transgenics is not replicated but some hints as how best to use a transgenic facility are provided. An overview of methods for studying transgene expression and target gene suppression is given. In summary, antisense transgenesis may provide a more readily achievable method for tissue-specific ablation of a gene's function than controllable knockouts provide.

Examination of antisense RNA and oligodeoxynucleotides as potential inhibitors of avian leukosis virus replication in RP30 cells

Avian leukosis virus (ALV) is an economically important pathogen of chickens. Both antisense RNA and antisense oligodeoxynucleotides (ODN) have been used to diminish the replication and spread of other retroviruses. The use of antisense RNA and ODN to inhibit ALV replication has been examined in cultured RP30 cells. Using an expression system that constitutively transcribes antisense ALV RNA, one transfected cell clone showed a significant reduction in virus growth. However, this effect was not reproducibly observed in other transfected cell lines or in cells in which the antisense transcript was expressed from a regulatable promoter, even though a substantial amount of antisense transcript was generated. Antisense ODN complementary to several different target sites near the 5' end of the ALV genome were also tested for antiviral activity, by comparison of antisense ODN effects to those of randomized sequence controls. An antisense ODN complementary to the ALV primer binding site demonstrated a reproducible reduction in viral replication. However, when the corresponding region was specifically employed as a target for intracellular antisense RNA expression, there again was no significant inhibition of ALV. These results suggest that in vivo expression of antisense RNA is unlikely to be an effective way to generate transgenic poultry that are resistant to field strains of ALV.

Antiviral ribozymes. New jobs for ancient molecules

Catalytic RNAs are a genetic property not only of some particular viroids or viruses, but also are more common naturally among eukaryotes and even prokaryotes than earlier expected. However, the major interest in ribozymes results from their potential for development of "tailor-made" cDNA constructions designed to be transcribed into catalytic RNAs that will recognize by hybridization and destroy by specific cleavage their cellular or viral RNA targets. The efficiency of an antiviral ribozyme is determined by both the accessibility and sequence conservation of the target region, as well as the design of the ribozyme: its type, size, and composition of flanking sequences; expression rates; and cellular compartment localization. Until now the most frequently selected viral target is the human immunodeficiency virus, where an up to a 10(4)-fold inhibition in its progeny production has been achieved. Although the first generation ribozymes focused on improvements in basic design and expression rates, more recently the efficiency of antiviral catalytic activity has been increased by employing polyribozymes and/or multitarget ribozymes, as well as special constructions to enhance the cellular co-compartmentation of the ribozyme with its viral RNA target.

Antisense and ribozyme constructs in transgenic animals

Geneticists have long sought the ability to add or subtract individual genes from an organism's genome, or to be able to alter the level of expression of a gene in a targeted, developmentally and tissue-specific manner. The development of transgenic technology realized the possibilities of increasing the expression of a specific gene or the transfer of a new gene into an animal. Homologous recombination techniques allow the deletion or alteration of a gene in vivo. The production of transgenic animals incorporating a gene construct that expresses a complimentary antisense RNA to a targeted gene, or an antisense RNA incorporating a catalytic, ribozyme sequence, have been suggested as a potential mechanism for obtaining the developmentally and tissue-specific down-regulation of expression of a targeted gene in vivo. In this paper we review the current literature with respect to the application of antisense and ribozyme constructs in transgenic animals and conclude that such constructs can effectively downregulate the level of mRNA from a target gene, the amount of protein produced in the cell, and result in phenotypic consequences.

Transgenic cell lines for detection of animal viruses

Rapid diagnostic assays based on direct detection of viral antigen or nucleic acid are being used with increasing frequency in clinical virology laboratories. Virus culture, however, remains the only way to detect infectious virus and to analyze clinically relevant viral phenotypes, such as drug resistance. Growth of viruses in cell culture is labor intensive and time-consuming and requires the use of many different cell lines. Transgenic technology, together with increasing knowledge of the molecular pathways of virus replication, offers the possibility of using genetically modified cell lines to improve virus growth in cell culture and to facilitate detection of virus-infected cells. Genetically modifying cells so that they express a reporter gene only after infection with a specific virus can allow the detection of infectious virus by rapid and simple enzyme assays such as beta-galactosidase assays without the need for antibodies. Although transgenic cells have recently been successfully used for herpes simplex virus detection, much more work needs to be done to adapt this technology to other human viral pathogens such as cytomegalovirus and respiratory viruses. This review offers some strategies for applying this technology to a wide spectrum of animal viruses.

Transient inhibition of angiotensinogen production in transgenic mice bearing an antisense angiotensinogen gene

Angiotensinogen is the precursor of the biologically active hormone angiotensin II. Enzyme kinetic parameters suggest that concentrations of plasma angiotensinogen are rate limiting in the renin reaction. It is therefore assumed that a decrease in angiotensinogen synthesis in vivo would result in a decrease in angiotensin II plasma levels and then of blood pressure. To test this hypothesis, we generated a transgenic mouse line that carries an inducible antisense angiotensinogen gene. Transient inhibition of angiotensinogen synthesis could be demonstrated in these transgenic animals. However, the amounts of liver angiotensinogen message and plasma angiotensinogen concentrations were rapidly back to levels observed in control animals.

Ribozyme-mediated suppression of Moloney murine leukemia virus and human immunodeficiency virus type I replication in permissive cell lines

Several hammerhead ribozymes targeted to different sites within the retroviral packaging (psi) sequences of the Moloney murine leukemia virus (Mo-MLV) and the human immunodeficiency virus type 1 (HIV-1) were designed and shown to cleave target RNA in vitro at the chosen sites. The engineered ribozymes, as well as antisense sequence complementary to the Mo-MLV psi packaging region, were cloned into the 3' untranslated region of the neomycin-resistance gene (neo). This was coupled to the simian virus 40 early promoter within the pSV2neo vector. For the ribozymes against the Mo-MLV psi site, the constructs were transfected into Mo-MLV-infected and virus-producing mouse NIH 3T3 cells. With the exception of one of the single ribozymes (the one least effective in cutting target RNA in vitro), all of the constructs effectively (70-80%) suppressed retrovirus production. These results demonstrate a direct correlation between in vitro cleavage and in vivo ribozyme-mediated virus suppression. In addition, a ribozyme targeted to the HIV-1 psi packaging site was engineered into the same vector and transfected into the human T-cell line SupT1. The transfectants were cloned and then challenged with HIV-1. When compared to vector-transfected control cells, a significant reduction in HIV-1 production was observed as measured by p24 and syncytia formation assays. This study demonstrates a feasible approach to the suppression of retrovirus replication by targeting the psi packaging site with hammerhead ribozymes.

Reduced beta 2-microglobulin mRNA levels in transgenic mice expressing a designed hammerhead ribozyme

We have generated three artificial hammerhead ribozymes, denoted 'Rz-b', 'Rz-c' and 'Rz-d', with different specificities for exon II of the mouse beta-2-microglobulin (beta 2M) mRNA. In this study we tested for ribozyme mediated reduction of beta 2M mRNA in a cell line and in transgenic mice. Transfections of either of the Rz-b, Rz-c or Rz-d plasmids into a mouse cell-line (NIH/3T3) revealed reductions of beta 2M mRNA substrate in each case. Ribozyme expression in individual transfected clones was accompanied with an up to 80% reduction of beta 2M mRNA levels. Rz-c was selected for a transgenic study. Seven Rz-c transgenic founder animals were identified from which three ribozyme expressing families were established and analysed. Expression of the ribozyme transgene was tested for and detected in lung, kidney and spleen. Expression was accompanied with reduction of the beta 2M mRNA levels of heterozygous (Rz+/-) animals compared to non-transgenic litter mates. The effect was most pronounced in lung with more than 90% beta 2M mRNA reduction in individual mice. In summary, expression of our ribozymes in a cell free system, in a cell-line and in transgenic mice were all accompanied with reductions of beta 2M mRNA levels.

Antisense treatment of viral infection

In this chapter I have attempted to outline the rationale that underlies the antisense approach to treatment of virus infection, to catalog the effector molecules that are currently available, and to estimate the relative worth of each. In so doing I have tried to describe the criteria that might be employed in their design and the factors that may determine their efficacy in tissue culture and, perhaps, in vivo. Finally, I have described the few examples presently available that indicate that antisense approaches may one day be therapeutically useful in treatment of disease of viral or nonviral origin.

Ribozyme cleaves rex/tax mRNA and inhibits bovine leukemia virus expression

Bovine leukemia virus (BLV) encodes at least two regulatory proteins, Rex and Tax. Tax, the transactivating protein, stimulates the long terminal repeat to promote viral transcription and may be involved in tumorigenesis. Rex is involved in the transition from early expression of regulatory proteins to later expression of viral structural proteins. We have targeted ribozymes against the mRNA encoding Rex and Tax. The ribozymes consist of the hammer-head catalytic motif flanked by antisense sequences that hybridize with the complementary rex/tax mRNA. To evaluate cleavage in a cell-free system, we transcribed portions of rex/tax mRNA and incubated them with synthetic RNA ribozymes. A ribozyme was identified that cleaves > 80% of the target RNA. Synthetic DNA encoding this ribozyme was cloned into the expression vector pRc/RSV and transfected into BLV-infected bat lung cells. Intracellular cleavage of rex/tax mRNA was confirmed by reverse transcriptase PCR. In cells expressing the ribozyme, viral expression was markedly inhibited. Expression of the BLV core protein p24 was inhibited by 61%, and reverse transcriptase activity in supernatant was inhibited by 92%. Ribozyme inhibition of BLV expression suggests that cattle expressing these sequences may be able to control BLV replication.

Effect of downregulation of germline transcripts on immunoglobulin A isotype differentiation

In this study we determined the role of immunoglobulin (Ig) germline transcripts in the isotype switch differentiation of the cloned lymphoma B cell line CH12.LX. In initial studies, we showed that addition of transforming growth factor beta (TGF-beta) and interleukin 4 (IL-4), either alone or in combination, augment switching from membrane (m)IgM+ to mIgA+ cells, and that increased switching is preceded and paralleled by an increase in the steady-state level of alpha germline transcripts (alpha GLT). Interestingly, TGF-beta and IL-4 affect switching in different ways, as shown by the fact that IL-4 increases and TGF-beta decreases the number of dual-positive (mIgM+/mIgA+) cells; in addition, TGF-beta and IL-4 have different effects on the time course of induction of alpha GLT. In subsequent studies, we established that we could downregulate alpha GLT levels in CH12.LX B cells by transfecting an expression vector that can be induced to produce transcripts antisense to the I alpha exon. Using this approach we downregulated alpha GLT in CH12.LX B cells undergoing switching in the presence of TGF-beta and IL-4 and showed that such downregulation led to decreased switching, as evidenced by decreased appearance of dual-positive B cells as well as decreased IgA synthesis relative to IgM synthesis. This result was corroborated by the fact that incubation of CH12.LX cells with phosphorothio-oligo antisense DNA to I alpha sequence also led to a decrease in the number of dual-positive cells and in the IgA/IgM secretion ratio. In summary, IgA isotype differentiation in CH12.LX B cell, particularly the steps necessary for the elaboration of mIgM+/mIgA+ switch intermediate cells, is inhibited by downregulation of alpha GLT; it is therefore apparent that alpha GLT plays a key role in the initial stage of isotype switch differentiation.

Ribozymes as anti-HIV-1 therapeutic agents: principles, applications, and problems

An emerging strategy in the treatment of viral infections is the use of antisense DNA or RNA to pair with, and block expression of viral transcripts. RNA, in addition to being an informational molecule, can also possess enzymatic activity. Thus, by combining anti-sense and enzymatic functions into a single transcript, it is now possible to design catalytic RNAs, or ribozymes, which can specifically pair with virtually any viral RNA, and cleave the phosphodiester backbone at a specified location, thereby functionally inactivating the viral RNA. In carrying out this cleavage, the ribozyme is not itself altered, and is thus capable of recycling and cleaving other molecules, making it a true enzyme. There are several different catalytic motifs which possess enzymatic activity, and each one of these can be incorporated into an enzymatic antisense with site-specific cleavage capabilities. By focusing on one type of catalytic motif, the hammerhead, we describe the principles behind the development of ribozymes as transacting, site-specific ribonucleases, several applications of ribozymes in functional destruction of target RNAs, as well as several of the problems confronting their use. We also describe a liposome delivery system which facilitates intracellular inclusion of ribozymes, and may provide a means for therapeutic delivery of ribozymes to HIV-1 infected cells.