Fusion with an RNA binding domain to confer target RNA specificity to an RNase: design and engineering of Tat-RNase H that specifically recognizes and cleaves HIV-1 RNA in vitro

HIV Entry and Its Inhibition by Bifunctional Antiviral Proteins

HIV entry is a highly specific and time-sensitive process that can be divided into receptor binding, coreceptor binding, and membrane fusion. Bifunctional antiviral proteins (bAVPs) exploit the multi-step nature of the HIV entry process by binding to two different extracellular targets. They are generated by expressing a fusion protein containing two entry inhibitors with a flexible linker. The resulting fusion proteins exhibit exceptional neutralization potency and broad cross-clade inhibition. In this review, we summarize the HIV entry process and provide an overview of the design, antiviral potency, and methods of delivery of bAVPs. Additionally, we discuss the advantages and limitations of bAVPs for HIV prevention and treatment.

Control of HIV Infection In Vivo Using Gene Therapy with a Secreted Entry Inhibitor

HIV entry inhibitors are highly effective in controlling virus replication. We have developed a lentiviral vector that expresses a secreted entry inhibitor, soluble CD4 (sCD4), which binds to the HIV envelope glycoproteins and inactivates the virus. We have shown that sCD4 was secreted from gene-modified CD4⁺ T cells, as well as from human umbilical cord blood-derived CD34⁺ hematopoietic stem/progenitor cells (HSPCs), and protected unmodified HIV target cells from infection in vitro. To investigate the in vivo application of our approach, we injected gene-modified HSPCs into NOD/SCID/γc^null (NSG) mice. NSG hosts supported multi-lineage differentiation of human gene-modified HSPCs. Upon challenge with HIV, humanized mice capable of secreting sCD4 demonstrated a reduction of viral load over time compared to control humanized mice. In contrast to gene therapy approaches that render only gene-modified HIV target cells resistant to infection, our approach also showed protection of unmodified CD4⁺ T cells in the peripheral blood and tissues. Our findings provide support for the continuous delivery of secreted entry inhibitors via gene therapy as an alternative to oral administration of antiretroviral drugs or injection of antiretroviral proteins, including antibodies.

A novel gene therapy strategy using secreted multifunctional anti-HIV proteins to confer protection to gene-modified and unmodified target cells

Current human immunodeficiency virus type I (HIV) gene therapy strategies focus on rendering HIV target cells non-permissive to viral replication. However, gene-modified cells fail to accumulate in patients and the virus continues to replicate in the unmodified target cell population. We have designed lentiviral vectors encoding secreted anti-HIV proteins to protect both gene-modified and unmodified cells from infection. Soluble CD4 (sCD4), a secreted single chain variable fragment (sscFv(17b)) and a secreted fusion inhibitor (sFI(T45)) were used to target receptor binding, co-receptor binding and membrane fusion, respectively. Additionally, we designed bi- and tri-functional fusion proteins to exploit the multistep nature of HIV entry. Of the seven antiviral proteins tested, sCD4, sCD4-scFv(17b), sCD4-FI(T45) and sCD4-scFv(17b)-FI(T45) efficiently inhibited HIV entry. The neutralization potency of the bi-functional fusion proteins sCD4-scFv(17b) and sCD4-FI(T45) was superior to that of sCD4 and the Food and Drug Administration-approved fusion inhibitor T-20. In co-culture experiments, sCD4, sCD4-scFv(17b) and sCD4-FI(T45) secreted from gene-modified producer cells conferred substantial protection to unmodified peripheral blood mononuclear cells. In conclusion, continuous delivery of secreted anti-HIV proteins via gene therapy may be a promising strategy to overcome the limitations of the current treatment.

Current progress and challenges in HIV gene therapy

HIV-1 causes AIDS, a syndrome that affects millions of people globally. Existing HAART is efficient in slowing down disease progression but cannot eradicate the virus. Furthermore the severity of the side effects and the emergence of drug-resistant mutants call for better therapy. Gene therapy serves as an attractive alternative as it reconstitutes the immune system with HIV-resistant cells and could thereby provide a potential cure. The feasibility of this approach was first demonstrated with the 'Berlin patient', who was functionally cured from HIV/AIDS with undetectable HIV-1 viral load after transplantation of bone marrow harboring a naturally occurring CCR5 mutation that blocks viral entry. Here, we give an overview of the current status of HIV gene therapy and remaining challenges and obstacles.

Gene silencing by cell-penetrating, sequence-selective and nucleic-acid hydrolyzing antibodies

Targeting particular mRNAs for degradation is a fascinating approach to achieve gene silencing. Here we describe a new gene silencing tool exploiting a cell-penetrating, nucleic-acid hydrolyzing, single-domain antibody of the light-chain variable domain, 3D8 VL. We generated a synthetic library of 3D8 VL on the yeast surface by randomizing residues located in one of two β-sheets. Using 18-bp single-stranded nucleic acids as target substrates, including the human Her2/neu-targeting sequence, we selected 3D8 VL variants that had ∼100–1000-fold higher affinity and ∼2–5-fold greater selective hydrolyzing activity for target substrates than for off targets. 3D8 VL variants efficiently penetrated into living cells to be accumulated in the cytosol and selectively decreased the amount of target sequence-carrying mRNAs as well as the proteins encoded by these mRNAs with minimal effects on off-target genes. In particular, one 3D8 VL variant targeting the Her2 sequence showed more efficient downregulation of Her2 expression than a small-interfering RNA targeting the same Her2 sequence, resulting in apoptotic cell death of Her2-overexpressing breast cancer cells. Our results demonstrate that cell-penetrating 3D8 VL variants with sequence-selective, nucleic-acid-hydrolyzing activity can selectively degrade target mRNAs in the cytosol, providing a new gene silencing tool mediated by antibody.

Creating a ribonuclease T-tat that preferentially recognizes and hydrolyzes HIV-1 TAR RNA in vitro and in vivo

A ribonuclease, RNase T-tat, specifically designed to hydrolyze the TAR RNA of HIV-1 virus has been engineered. The protein was made by domain swapping the TAT peptide at the loop 3 position of ribonuclease T1. The RNase T-tat maintains a guanine-specific RNA hydrolytic activity, and characteristically displayed a specific affinity for the TAR RNA of HIV-1. In the in vitro and in vivo assays, the RNase T-tat preferentially inhibited the expression of TAR-bearing mRNA through cis-TAR targeting, suggesting that RNase T-tat may be potentially useful for the disruption of the initial stage of the transcription process of HIV-1 virus.

Targeted ribonuclease can inhibit replication of hepatitis B virus

AIM: To study the effect of a novel targeted ribonuclease (TN), the fusion protein of HBVc and human eosinophil-derived neurotoxin (hEDN), on the HBV replication in vitro.

METHODS: The gene encoding the targeted ribonuclease was cloned into pcDNA3.1 (-) to form recombinant eukaryotic expression vector p/TN. Control plasmids, including p/hEDN, p/HBVc, and p/TNmut in which a Lys113→Arg mutation was introduced by sequential PCR to eliminate the ribonuclease activity of hEDN, were also constructed. Liposome-mediated transfection of 2.2.15 cells by p/TN, p/TNmut, p/hEDN, p/HBVc, and pcDNA3.1 (-), or mock transfection was performed. After that, RT-PCR was used to verify the transgene expression. Morphology of the transfected cells was observed and MTT assay was performed to detect the cytotoxicity of transgene expression. Concentration of HBsAg in the supernatant of the transfected cells was measured using solid-phase radioimmunoassay.

RESULTS: Transgenes were successfully expressed in 2.2.15 cells. No obvious cytotoxic effect of transgene expression on 2.2.15 cells was found. The HBsAg concentration in the p/TN transfected cells was reduced by 58% compared with that of mock transfected cells. No such an effect was found in all other controls.

CONCLUSION: The targeted ribonuclease can inhibit HBV replication in vitro while it has no cytotoxicity on host cells. The targeted ribonuclease may be used as a novel antiviral agent for human HBV infection.

Targeted RNases: a feasibility study for use in HIV gene therapy

A targeted RNase would be ideal for gene therapy of several acquired and inherited disorders. Such an RNase may be engineered to contain a ribonucleolytic domain and a specific target RNA binding domain. To demonstrate the feasibility of this approach, an RNase targeted against human immunodeficiency virus (HIV) RNA--Tev-RNase T1--was designed and tested for its use in HIV-1 gene therapy. A human CD4+ T lymphoid (MT4) cell line and human peripheral blood lymphocytes (PBLs) were transduced with retroviral vectors lacking or expressing the tevT1 gene. Expression of enzymatically functional Tev-RNase T1 protein and its lack of toxicity was demonstrated in stable MT4 transductants. Compared with control cells lacking this protein, both transduced MT4 cells and PBLs expressing Tev-RNase T1 delayed HIV-1 replication. Tev-RNase T1 was shown to act after integration, since HIV-1 proviral DNA could be detected, but the amount of HIV-1 RNA produced in MT4 cells and PBLs was significantly decreased. This study demonstrates the feasibility of a targeted RNase strategy for therapeutic use.

From housekeeper to microsurgeon: the diagnostic and therapeutic potential of ribonucleases

The RNA population in cells is controlled post-transcriptionally by ribonucleases (RNases) of varying specificity. Angiogenin, neurotoxins, and plant allergens are among many proteins with RNase activity or significant homology to known RNases. RNase activity in serum and cell extracts is elevated in a variety of cancers and infectious diseases. RNases are regulated by specific activators and inhibitors, including interferons. Many of these regulatory molecules are useful lead compounds for the design of drugs to control tumor angiogenesis, allergic reactions, and viral replication. One RNase (Onconase) and several RNase activators are now in clinical trials for cancer treatment or inhibition of chronic virus infections. Several others, alone or conjugated with specific cell binding molecules, are being developed for their antifungal, antiviral, and antitumor cell activity.