Influence of chromosomal position and copy number of a white-directed ribozyme gene on the suppression of eye pigmentation in Drosophila melanogaster

Hammerhead ribozymes reduce central nervous system (CNS)-derived neuronal nitric oxide synthase messenger RNA in a human cell line

Catalytic RNA molecules (ribozymes) have been widely used specifically to suppress gene expression. Neuronal nitric oxide synthase (nNOS) is an important molecule involved in normal central nervous system function (e.g. vasodilation, neurotransmission.) and disease (e.g. oxidative stress). This report is an investigation of the hammerhead ribozyme function and potential in the central nervous system using nNOS as a model. Two antisense hammerhead ribozymes, nNOS-RZ1 and nNOS-RZ2, were designed and constructed against nNOS messenger RNA (mRNA). In vitro (cell-free) experiments demonstrated the ability of both ribozymes to cleave nNOS RNA targets. Ribozyme-mediated reduction of the endogenous nNOS mRNA in human TGW-I-nu neuroblastoma cells was demonstrated by plasmid- and adenovirus-mediated transfections. These results may form the basis for studying neuronal gene expression and for designing RNA-directed therapeutic strategies for neurological diseases that involve oxidative stress.

Episomal expression of a hammerhead ribozyme directed against plum pox virus

Two related antisense RNAs directed against plum pox virus (PPV) were expressed episomally in Nicotiana clevelandii by infection with recombinant potato virus X (PVX). One recombinant PVX expressed an ordinary PPV antisense RNA of about 400 nucleotides, while the other expressed a related antisense RNA that carried the catalytic domain of a hammerhead ribozyme. Inoculation with the latter recombinant PVX resulted in the accumulation of ribozyme RNA that was catalytically active when tested in vitro with a PPV substrate RNA. Plants that had been inoculated with recombinant PVX viruses, expressing either PPV-directed antisense or ribozyme sequences or GUS RNA as a control, were challenged with PPV by a sequential second inoculation. In plants that expressed PPV antisense sequences, the appearance of PPV disease symptoms was delayed for 3-5 days. Quantification of PPV 1 week after inoculation showed that the protective effect by the episomally expressed catalytic antisense RNA was stronger than that of the ordinary antisense RNA. However, eventually all plants tested accumulated comparable titers of PPV.

Essential role of mitochondrially encoded large rRNA for germ-line formation in Drosophila embryos

In Drosophila, pole cells, the progenitors of the germ line, are induced by the factors localized in the posterior pole region of oocytes and cleavage embryos, or germ plasm. Polar granules in germ plasm are electron-dense structures and have been proposed to contain factors essential for pole cell formation. Mitochondrially encoded large ribosomal RNA (mtlrRNA) has been identified as a component of polar granules. We previously have shown that mtlrRNA is able to rescue embryos that fail to form pole cells as a result of UV irradiation. However, there is a possibility that the function of mtlrRNA is limited to UV-irradiated embryos, and the question of whether mtlrRNA is required for the normal pathway leading to pole cell formation remains unanswered. In this study, we report that the reduction of mtlrRNA in germ plasm by injecting anti-mtlrRNA ribozymes into cleavage embryos leads to their inability to form pole cells. Other components of germ plasm, namely oskar mRNA, germ cell-less mRNA, and Vasa and Tudor proteins appear to be unaffected in these ribozyme-injected embryos. These results support an essential role for mtlrRNA in pole cell formation. We propose that mitochondrially encoded molecules participate in a key event in early cell-type specification.

Regulation of CAT protein by ribozyme and antisense mRNA in transgenic mice

Transgenic mouse lines were engineered to express stably antisense mRNA or antisense mRNA containing catalytic ribozyme (rbz) structures complementary to bacterial chloramphenicol acetyltransferase (CAT) gene transcripts. One transgenic line expressed antisense mRNA that specifically targeted full-length CAT coding sequences (ACAT). Another transgenic line expressed full-length antisense CAT mRNA which was modified by mutagensis to include four rbz cassettes (rbz-ACAT) in order to compare antisense versus antisense-rbz function in vivo. Preliminary data were also collected from a transgenic mouse line expressing antisense mRNA targeting 72% of the 5' region of CAT coding sequences (5' ACAT). All constructs contained similar control elements in their design. Promoter elements were derived from the bovine alpha s1-casein gene, while the small t intron and 3' control sequences were derived from SV40. The ability of these various constructs to down-regulate CAT protein levels was compared by analysis of CAT protein production in lactating double-hemizygous transgenic female mice. Every double-hemizygous mouse analysed expressed mRNA from the alpha s1-casein-CAT construct (Clarke et al., 1994) and equivalent levels of mRNA from one of the three antisense constructs. Transgenic mouse lines expressing both ACAT and CAT mRNA down-regulated CAT protein levels by 90% of that found in the CAT only transgenic population. Similarly, double-hemizygous transgenic lines expressing both rbz-ACAT and CAT mRNA regulated CAT protein levels by 87%. Preliminary data suggests that expression of mRNA from 5' ACAT/CAT double-hemizygote mice allowed approximately 67% down-regulation of normal CAT protein levels. We conclude that incorporation of multiple ribozymes within the full-length antisense CAT construct does not enhance the effectiveness of antisense mRNA in the down-regulation of CAT protein production in our system.

Colocalization of antisense RNAs and ribozymes with their target mRNAs

The use of complementary RNA sequences such as antisense RNAs and ribozymes to regulate the expression of specific genes in eukaryotic cells has been well-documented, particularly with their application to both human gene therapy and plant biotechnology. Despite the simplicity of this approach, this technique usually results in only partial suppression of gene expression and, in some instances, even fails to regulate the gene of interest. The variation observed with antisense RNA and ribozyme-mediated regulation is further complicated by the many factors with the potential to impact on the effectiveness of these RNAs. Recent advances in the understanding of the global architecture of the nucleus, chromatin structure, and RNA metabolism provide useful and necessary information for designing novel approaches to improving antisense RNA and ribozyme regulation. These studies predict that the position of genes within the nucleus is not random and that transcripts produced from these genes follow specific tracks in migrating to the cell cytoplasm. These observations have the potential to impact significantly on the ways in which RNA-mediated forms of gene regulation are applied. The purpose of this review is to discuss the concept of colocalizing antisense RNAs and ribozymes with their target mRNAs and to introduce a variety of approaches aimed at achieving this goal.

The structure, function and application of the hammerhead ribozyme

The hammerhead ribozyme is one of the smallest ribozymes known and catalyses the site-specific hydrolysis of a phosphodiester bond. This small ribozyme is of interest for two reasons. It offers a convenient system to study the structure/function relationship of a nucleotide sequence, and is a potential vehicle for the inhibition of gene expression. The first part of the review summarizes the sequence requirements of the hammerhead, its three-dimensional structure and the proposed mechanism, in addition to ribozyme specificity and turnover. The second part of the review focuses on the in vivo application of the ribozyme. The processes involved in designing ribozymes for efficient cleavage in vivo are described, together with possible delivery strategies.

A helix 1-extended hairpin ribozyme exhibits altered cleavage behavior in vitro

The catalytic domain of a hairpin ribozyme was incorporated at the 3'-end of a 254-base antisense RNA directed against the RNA of human immunodeficiency virus type 1 (HIV-1), generating a hairpin ribozyme with a largely extended helix 1. In parallel, a catalytic antisense RNA based on a hammerhead ribozyme was directed toward the same cleavage motif in the HIV-1 target. Both ribozymes were expected to create identical cleavage products. Cleavage analysis in vitro confirmed that the hammerhead ribozyme delivered the expected cleavage products. However, the helix 1-extended hairpin ribozyme catalyzed additional RNA cleavage at several unexpected sites, which were mapped. Some of the 3' cleavage products had other nucleotides than G at their 5'-terminus, indicating that the helix 1-extended hairpin ribozyme was able to cleave bonds other than NpG+1. Inspection of the sequence context of the different cleavage sites suggested that unconventional helices 2 in combination with an asymmetric loop A consisting of up to 32 unpaired nucleotides in the substrate strand were formed. A second variant of a helix 1-extended hairpin ribozyme that differed in two nucleotides gave consistent results.

Antisense approaches to the gene therapy of cancer--'Recnac'

This review has looked at the wide-ranging research initiatives in the field of antisense technology. It starts with the philosophy behind antisense DNA and the production of antisense RNA from genetic constructs and raises the various problems which are being addressed. These include uptake into cells, targeting the substrate sequence and cells, the stability of the antisense molecules and pharmokinetic considerations within animals. The review talks of the positive results attained in vitro and in vivo in animal and plant experiments but also addresses the problems many workers have faced in the field. It attempts to resolve these differences in terms of the need for further understanding of the mechanisms by which the positive results have been obtained. The novel use of catalytic ribozymes (RNA) in downregulating genes is also discussed in similar terms to antisense DNA and RNA. By taking a case study with a human leukaemia the review delves into the mysteries of how different results can be resolved by improving the design of ribozymes thereby increasing specificity and preventing aberrant reactions. It is concluded that despite a lack of understanding of how the biological effects have come about in vitro and in vivo the clinical and research developments should resolve the issue of antisense potential for rational drug development.