Low transposition frequency of the medaka fish Tol2 element may be due to extranuclear localization of its transposase

Functional analysis of the catalytic triad of the hAT-family transposase TcBuster

TcBuster is a hAT-family DNA transposon from the red flour beetle, Tribolium castaneum. The TcBuster transposase is of interest for genome engineering as it is highly active in insect and mammalian cells. To test the predicted catalytic triad of TcBuster, each residue of the catalytic triad of a haemagglutinin-tagged TcBuster transposase was individually mutated to a structurally conserved amino acid. Using a drug-resistant colony assay for transposon integration, we found that the D223N, D289N, and E589Q mutants of TcBuster transposase were inactive in human cells. We used a modified chromatin immunoprecipitation assay to determine that each mutant maintained binding to TcBuster transposon inverted repeat elements. Although the catalytic mutants retained their transposon binding properties, mutants displayed altered expression and localization in human cells. None of the catalytic mutants formed characteristic TcBuster transposase rodlet structures, and the D223N and D289N mutants were not able to be detected by immunofluorescence microscopy. Immunoblot analysis demonstrated that the E589Q mutant is less abundant than wild-type TcBuster transposase. Cells transfected with either TcBuster or TcBuster-E589Q transposase were imaged by structured illumination microscopy to quantify differences in the length of the transposase rodlets. The average length of the TcBuster transposase rodlets (N = 39) was 3.284 μm while the E589Q rodlets (N = 33) averaged 1.157 μm (p < 0.0001; t-test). The catalytic triad mutations decreased overall protein levels and disrupted transposase rodlet formation while nuclear localization and DNA binding to the inverted repeat elements were maintained. Our results may have broader implications for the overproduction inhibition phenomenon observed for DNA transposons.

Temporal self-regulation of transposition through host-independent transposase rodlet formation

Transposons are highly abundant in eukaryotic genomes, but their mobilization must be finely tuned to maintain host organism fitness and allow for transposon propagation. Forty percent of the human genome is comprised of transposable element sequences, and the most abundant cut-and-paste transposons are from the hAT superfamily. We found that the hAT transposase TcBuster from Tribolium castaneum formed filamentous structures, or rodlets, in human tissue culture cells, after gene transfer to adult mice, and ex vivo in cell-free conditions, indicating that host co-factors or cellular structures were not required for rodlet formation. Time-lapsed imaging of GFP-laced rodlets in human cells revealed that they formed quickly in a dynamic process involving fusion and fission. We delayed the availability of the transposon DNA and found that transposition declined after transposase concentrations became high enough for visible transposase rodlets to appear. In combination with earlier findings for maize Ac elements, these results give insight into transposase overproduction inhibition by demonstrating that the appearance of transposase protein structures and the end of active transposition are simultaneous, an effect with implications for genetic engineering and horizontal gene transfer.

Oxidative stress and regulation of Pink1 in zebrafish (Danio rerio)

Oxidative stress-mediated neuronal dysfunction is characteristic of several neurodegenerative disorders, including Parkinson's disease (PD). The enzyme tyrosine hydroxylase (TH) catalyzes the formation of L-DOPA, the rate-limiting step in the biosynthesis of dopamine. A lack of dopamine in the striatum is the most characteristic feature of PD, and the cause of the most dominant symptoms. Loss of function mutations in the PTEN-induced putative kinase (PINK1) gene cause autosomal recessive PD. This study explored the basic mechanisms underlying the involvement of pink1 in oxidative stress-mediated PD pathology using zebrafish as a tool. We generated a transgenic line, Tg(pink1:EGFP), and used it to study the effect of oxidative stress (exposure to H2O2) on pink1 expression. GFP expression was enhanced throughout the brain of zebrafish larvae subjected to oxidative stress. In addition to a widespread increase in pink1 mRNA expression, mild oxidative stress induced a clear decline in tyrosine hydroxylase 2 (th2), but not tyrosine hydroxylase 1 (th1) expression, in the brain of wild-type larvae. The drug L-Glutathione Reduced (LGR) has been associated with anti-oxidative and possible neuroprotective properties. Administration of LGR normalized the increased fluorescence intensity indicating pink1 transgene expression and endogenous pink1 mRNA expression in larvae subjected to oxidative stress by H2O2. In the pink1 morpholino oliogonucleotide-injected larvae, the reduction in the expression of th1 and th2 was partially rescued by LGR. The pink1 gene is a sensitive marker of oxidative stress in zebrafish, and LGR effectively normalizes the consequences of mild oxidative stress, suggesting that the neuroprotective effects of pink1 and LGR may be significant and useful in drug development.

Tol2: a versatile gene transfer vector in vertebrates

The medaka fish Tol2 element is an autonomous transposon that encodes a fully functional transposase. The transposase protein can catalyze transposition of a transposon construct that has 200 and 150 base pairs of DNA from the left and right ends of the Tol2 sequence, respectively. These sequences contain essential terminal inverted repeats and subterminal sequences. DNA inserts of fairly large sizes (as large as 11 kilobases) can be cloned between these sequences without reducing transpositional activity. The Tol2 transposon system has been shown to be active in all vertebrate cells tested thus far, including zebrafish, Xenopus, chicken, mouse, and human. In this review I describe and discuss how the Tol2 transposon is being applied to transgenic studies in these vertebrates, and possible future applications.

Fish transposons and their potential use in aquaculture

A large part of repetitive DNA of vertebrate genomes have been identified as transposon elements (TEs) or mobile sequences. Although TEs detected to date in most vertebrates are inactivated, active TEs have been found in fish and a salmonid TE has been successfully reactivated by molecular genetic manipulation from inactive genomic copies (Sleeping Beauty, SB). Progress in the understanding of the dynamics, control and evolution of fish TEs will allow the insertion of selected sequences into the fish genomes of germ cells to obtain transgenics or to identify genes important for growth and/or of somatic cells to improve DNA vaccination. Expectations are high for new possible applications to fish of this well developed technology for mammals. Here, we review the present state of knowledge of inactive and active fish TEs and briefly discuss how their possible future applications might be used to improve fish production in aquaculture.