Excision of the piggyBac transposable element in vitro is a precise event that is enhanced by the expression of its encoded transposase

Transposon Invasion of the Paramecium Germline Genome Countered by a Domesticated PiggyBac Transposase and the NHEJ Pathway

Sequences related to transposons constitute a large fraction of extant genomes, but insertions within coding sequences have generally not been tolerated during evolution. Thanks to their unique nuclear dimorphism and to their original mechanism of programmed DNA elimination from their somatic nucleus (macronucleus), ciliates are emerging model organisms for the study of the impact of transposable elements on genomes. The germline genome of the ciliate Paramecium, located in its micronucleus, contains thousands of short intervening sequences, the IESs, which interrupt 47% of genes. Recent data provided support to the hypothesis that an evolutionary link exists between Paramecium IESs and Tc1/mariner transposons. During development of the macronucleus, IESs are excised precisely thanks to the coordinated action of PiggyMac, a domesticated piggyBac transposase, and of the NHEJ double-strand break repair pathway. A PiggyMac homolog is also required for developmentally programmed DNA elimination in another ciliate, Tetrahymena. Here, we present an overview of the life cycle of these unicellular eukaryotes and of the developmentally programmed genome rearrangements that take place at each sexual cycle. We discuss how ancient domestication of a piggyBac transposase might have allowed Tc1/mariner elements to spread throughout the germline genome of Paramecium, without strong counterselection against insertion within genes.

Progress with schistosome transgenesis

Genome sequences for Schistosoma japonicum and Schistosoma mansoni are now available. The schistosome genome encodes ~13,000 protein encoding genes for which the function of only a minority is understood. There is a valuable role for transgenesis in functional genomic investigations of these new schistosome gene sequences. In gain-of-function approaches, transgenesis can lead to integration of transgenes into the schistosome genome which can facilitate insertional mutagenesis screens. By contrast, transgene driven, vector-based RNA interference (RNAi) offers powerful loss-of-function manipulations. Our laboratory has focused on development of tools to facilitate schistosome transgenesis. We have investigated the utility of retroviruses and transposons to transduce schistosomes. Vesicular stomatitis virus glycoprotein (VSVG) pseudotyped murine leukemia virus (MLV) can transduce developmental stages of S. mansoni including eggs. We have also observed that the piggyBac transposon is transpositionally active in schistosomes. Approaches with both VSVG-MLV and piggyBac have resulted in somatic transgenesis and have lead to integration of active reporter transgenes into schistosome chromosomes. These findings provided the first reports of integration of reporter transgenes into schistosome chromosomes. Experience with these systems is reviewed herewith, along with findings with transgene mediated RNAi and germ line transgenesis, in addition to pioneering and earlier reports of gene manipulation for schistosomes.

A method for stable transgenesis of radial glia lineage in rat neocortex by piggyBac mediated transposition

Methods that combine lineage tracing with cellular transgenesis are needed in order to determine mechanisms that specify neural cell types. Currently available methods include viral infection and Cre-mediated recombination. In utero electroporation (IUE) has been used in multiple species to deliver multiple transgenes simultaneously into neural progenitors. In standard IUE, most plasmids remain episomal, are lost during cell division, and so transgenes are not expressed in the complete neural lineage. Here we combine IUE with a binary piggyBac transposon system (PB-IUE), and show that unlike conventional IUE, a single embryonic transfection of neocortical radial glia with a piggyBac transposon system results in stable transgene expression in the neural lineage of radial glia: cortical neurons, astrocytes, oligodendrocytes, and olfactory bulb interneurons. We also developed a modular toolkit of donor and helper plasmids with different promoters that allows for shRNA, bicistronic expression, and trangenesis in subsets of progenitors. As a demonstration of the utility of the toolkit we show that transgenesis of epidermal growth factor receptor (EGFR) expands the number of astrocytes and oligodendrocyrtes generated from progenitors. The relative ease of implementation and experimental flexibility should make the piggyBac IUE method a valuable new tool for tracking and manipulating neural lineages.

Insect transgenesis: current applications and future prospects

The ability to manipulate the genomes of many insects has become a practical reality over the past 15 years. This has been led by the identification of several useful transposon vector systems that have allowed the identification and development of generalized, species-specific, and tissue-specific promoter systems for controlled expression of gene products upon introduction into insect genomes. Armed with these capabilities, researchers have made significant strides in both fundamental and applied transgenics in key model systems such as Bombyx mori, Tribolium casteneum, Aedes aegypti, and Anopheles stephensi. Limitations of transposon systems were identified, and alternative tools were developed, thus significantly increasing the potential for applied transgenics for control of both agricultural and medical insect pests. The next 10 years promise to be an exciting time of transitioning from the laboratory to the field, from basic research to applied control, during which the full potential of gene manipulation in insect systems will ultimately be realized.

Reprogramming of somatic cells

Reprogramming of adult somatic cells into pluripotent stem cells may provide an attractive source of stem cells for regenerative medicine. It has emerged as an invaluable method for generating patient-specific stem cells of any cell lineage without the use of embryonic stem cells. A revolutionary study in 2006 showed that it is possible to convert adult somatic cells directly into pluripotent stem cells by using a limited number of pluripotent transcription factors and is called as iPS cells. Currently, both genomic integrating viral and nonintegrating nonviral methods are used to generate iPS cells. However, the viral-based technology poses increased risk of safety, and more studies are now focused on nonviral-based technology to obtain autologous stem cells for clinical therapy. In this review, the pros and cons of the present iPS cell technology and the future direction for the successful translation of this technology into the clinic are discussed.

Hyperactive piggyBac gene transfer in human cells and in vivo

We characterized a recently developed hyperactive piggyBac (pB) transposase enzyme [containing seven mutations (7pB)] for gene transfer in human cells in vitro and to somatic cells in mice in vivo. Despite a protein level expression similar to that of native pB, 7pB significantly increased the gene transfer efficiency of a neomycin resistance cassette transposon in both HEK293 and HeLa cultured human cells. Native pB and SB100X, the most active transposase of the Sleeping Beauty transposon system, exhibited similar transposition efficiency in cultured human cell lines. When delivered to primary human T cells ex vivo, 7pB increased gene delivery two- to threefold compared with piggyBac and SB100X. The activity of hyperactive 7pB transposase was not affected by the addition of a 24-kDa N-terminal tag, whereas SB100X manifested a 50% reduction in transposition. Hyperactive 7pB was compared with native pB and SB100X in vivo in mice using hydrodynamic tail-vein injection of a limiting dose of transposase DNA combined with luciferase reporter transposons. We followed transgene expression for up to 6 months and observed approximately 10-fold greater long-term gene expression in mice injected with a codon-optimized version of 7pB compared with mice injected with native pB or SB100X. We conclude that hyperactive piggyBac elements can increase gene transfer in human cells and in vivo and should enable improved gene delivery using the piggyBac transposon system in a variety of cell and gene-therapy applications.

piggyBac transposon somatic mutagenesis with an activated reporter and tracker (PB-SMART) for genetic screens in mice

Somatic forward genetic screens have the power to interrogate thousands of genes in a single animal. Retroviral and transposon mutagenesis systems in mice have been designed and deployed in somatic tissues for surveying hematopoietic and solid tumor formation. In the context of cancer, the ability to visually mark mutant cells would present tremendous advantages for identifying tumor formation, monitoring tumor growth over time, and tracking tumor infiltrations and metastases into wild-type tissues. Furthermore, locating mutant clones is a prerequisite for screening and analyzing most other somatic phenotypes. For this purpose, we developed a system using the piggyBac (PB) transposon for somatic mutagenesis with an activated reporter and tracker, called PB-SMART. The PB-SMART mouse genetic screening system can simultaneously induce somatic mutations and mark mutated cells using bioluminescence or fluorescence. The marking of mutant cells enable analyses that are not possible with current somatic mutagenesis systems, such as tracking cell proliferation and tumor growth, detecting tumor cell infiltrations, and reporting tissue mutagenesis levels by a simple ex vivo visual readout. We demonstrate that PB-SMART is highly mutagenic, capable of tumor induction with low copy transposons, which facilitates the mapping and identification of causative insertions. We further integrated a conditional transposase with the PB-SMART system, permitting tissue-specific mutagenesis with a single cross to any available Cre line. Targeting the germline, the system could also be used to conduct F1 screens. With these features, PB-SMART provides an integrated platform for individual investigators to harness the power of somatic mutagenesis and phenotypic screens to decipher the genetic basis of mammalian biology and disease.

Genetic manipulation of schistosomes--progress with integration competent vectors

Draft genome sequences for Schistosoma japonicum and S. mansoni are now available. The schistosome genome encodes ∼13,000 protein-encoding genes for which the functions of few are well understood. Nonetheless, the new genes represent potential intervention targets, and molecular tools are being developed to determine their importance. Over the past 15 years, noteworthy progress has been achieved towards development of tools for gene manipulation and transgenesis of schistosomes. A brief history of genetic manipulation is presented, along with a review of the field with emphasis on reports of integration of transgenes into schistosome chromosomes.

Somatic genetics empowers the mouse for modeling and interrogating developmental and disease processes

With recent advances in genomic technologies, candidate human disease genes are being mapped at an accelerated pace. There is a clear need to move forward with genetic tools that can efficiently validate these mutations in vivo. Murine somatic mutagenesis is evolving to fulfill these needs with tools such as somatic transgenesis, humanized rodents, and forward genetics. By combining these resources one is not only able to model disease for in vivo verification, but also to screen for mutations and pathways integral to disease progression and therapeutic intervention. In this review, we briefly outline the current advances in somatic mutagenesis and discuss how these new tools, especially the piggyBac transposon system, can be applied to decipher human biology and disease.

Expression of hGM-CSF in silk glands of transgenic silkworms using gene targeting vector

The silk gland of the silkworm is a highly specialized organ that has the wonderful ability to synthesize and secrete silk protein. To express human granucyto-macrophage colony-stimulating factor (hGM-CSF) in the posterior silk glands of gene-targeted silkworms, a targeting vector pSK-FibL-L-A3GFP-PH-GMCSF-LPA-FibL-R was constructed, harboring a 1.2 kb portion of the left homogenous arm (FibL-L), a 0.5 kb portion of the right homogenous arm (FibL-R), fibroin H-chain-promoter-driven hGM-CSF and silkworm actin 3-promoter-driven gfp. The targeting vector was then introduced into the eggs of silkworm, and the transgenic silkworms were verified by PCR and DNA hybridization after being screened for the gfp gene. Western blotting analysis using an antibody against hGM-CSF demonstrated a specific band with a molecular weight of 22 kD in the silk glands of the G3 generation transgenic silkworms. The level of expression of hGM-CSF in the posterior silk glands of the G3 generation transgenic silkworms was approximately 2.70 ng/g of freeze-dried powdered posterior silk gland. These results showed that the heterologous gene could be introduced into the silkworm genome and expressed successfully. Further more, the exogenous genes existing in the G5 transgenic silkworm identified by PCR confirmed its integration stability. In addition, the silk glands containing expressed hGM-CSF performed the function of significantly increasing leukocyte count of CY-treated mice in a time-and-dose-dependent manner.

Size matters: versatile use of PiggyBac transposons as a genetic manipulation tool

Transposons have been promising elements for gene integration, and the Sleeping Beauty (SB) system has been the major one for many years, although there have been several other transposon systems available, for example, Tol2. However, recently another system known as PiggyBac (PB) has been introduced and developed for fulfilling the same purposes, for example, mutagenesis, transgenesis and gene therapy and in some cases with improved transposition efficiency and advantages over the Sleeping Beauty transposon system, although improved hyperactive transposase has highly increased the transposition efficacy for SB. The PB systems have been used in many different scientific research fields; therefore, the purpose of this review is to describe some of these versatile uses of the PiggyBac system to give readers an overview on the usage of PiggyBac system.

Advancements in reprogramming strategies for the generation of induced pluripotent stem cells

Direct reprogramming of somatic cells into induced pluripotent stem (iPS) cells has emerged as an invaluable method for generating patient-specific stem cells of any lineage without the use of embryonic materials. Following the first reported generation of iPS cells from murine fibroblasts using retroviral transduction of a defined set of transcription factors, various new strategies have been developed to improve and refine the reprogramming technology. Recent developments provide optimism that the generation of safe iPS cells without any genomic modification could be derived in the near future for the use in clinical settings. This review summarizes current and evolving strategies in the generation of iPS cells, including types of somatic cells for reprogramming, variations of reprogramming genes, reprogramming methods, and how the advancement iPS cells technology can lead to the future success of reproductive medicine.

Development of the piggyBac transposable system for Plasmodium berghei and its application for random mutagenesis in malaria parasites

BACKGROUND

The genome of a number of species of malaria parasites (Plasmodium spp.) has been sequenced in the hope of identifying new drug and vaccine targets. However, almost one-half of predicted Plasmodium genes are annotated as hypothetical and are difficult to analyse in bulk due to the inefficiency of current reverse genetic methodologies for Plasmodium. Recently, it has been shown that the transposase piggyBac integrates at random into the genome of the human malaria parasite P. falciparum offering the possibility to develop forward genetic screens to analyse Plasmodium gene function. This study reports the development and application of the piggyBac transposition system for the rodent malaria parasite P. berghei and the evaluation of its potential as a tool in forward genetic studies. P. berghei is the most frequently used malaria parasite model in gene function analysis since phenotype screens throughout the complete Plasmodium life cycle are possible both in vitro and in vivo.

RESULTS

We demonstrate that piggyBac based gene inactivation and promoter-trapping is both easier and more efficient in P. berghei than in the human malaria parasite, P. falciparum. Random piggyBac-mediated insertion into genes was achieved after parasites were transfected with the piggyBac donor plasmid either when transposase was expressed either from a helper plasmid or a stably integrated gene in the genome. Characterization of more than 120 insertion sites demonstrated that more than 70 most likely affect gene expression classifying their protein products as non-essential for asexual blood stage development. The non-essential nature of two of these genes was confirmed by targeted gene deletion one of which encodes P41, an ortholog of a human malaria vaccine candidate. Importantly for future development of whole genome phenotypic screens the remobilization of the piggyBac element in parasites that stably express transposase was demonstrated.

CONCLUSION

These data demonstrate that piggyBac behaved as an efficient and random transposon in P. berghei. Remobilization of piggyBac element shows that with further development the piggyBac system can be an effective tool to generate random genome-wide mutation parasite libraries, for use in large-scale phenotype screens in vitro and in vivo.

A piggyBac transposon-based mutagenesis system for the fission yeast Schizosaccharomyces pombe

The TTAA-specific transposon piggyBac (PB), originally isolated from the cabbage looper moth, Trichoplusia ni, has been utilized as an insertional mutagenesis tool in various eukaryotic organisms. Here, we show that PB transposes in the fission yeast Schizosaccharomyces pombe and leaves almost no footprints. We developed a PB-based mutagenesis system for S. pombe by constructing a strain with a selectable transposon excision marker and an integrated transposase gene. PB transposition in this strain has low chromosomal distribution bias as shown by deep sequencing-based insertion site mapping. Using this system, we obtained loss-of-function alleles of klp5 and klp6, and a gain-of-function allele of dam1 from a screen for mutants resistant to the microtubule-destabilizing drug thiabendazole. From another screen for cdc25-22 suppressors, we obtained multiple alleles of wee1 as expected. The success of these two screens demonstrated the usefulness of this PB-mediated mutagenesis tool for fission yeast.

The relationship between internal domain sequences of piggyBac and its transposition efficiency in BmN cells and Bombyx mori

The piggyBac transposon, which includes terminal inverted repeat sequences and internal domain (ID) sequences, is widely used as a tool for insect transformation. To optimize this system for transgenic research on Bombyx mori, we examined the effects of the amount of the transposase plasmid and its ID sequences on the expression of green fluorescent protein (GFP). Four kinds of transposon plasmids, pB[A3GFP]-1 with the full length of ID sequences, pB[A3GFP]-2 having only the 3' ID sequence, pB[A3GFP]-3 without ID sequences, and pB[A3GFP]-4 containing 333 bp of the 5' ID sequence, and 179 bp of the 3' ID sequence were constructed with GFP as the marker. After transfecting these four plasmids into BmN cells, we analyzed the transfecting efficiency by comparing the GFP positive to negative cell ratio. Our results indicated that plasmid pB-4 got the highest ratio on the 22nd day. Moreover, the GFP positive to negative cell ratio increased with higher amount of transposase plasmid without overproduction inhibition. Furthermore, we injected three piggyBac transposon plasmids, pB[A3GFP]-1, pB[3xP3GFP]-3, and pB[3xP3RFP]-4 harboring different markers into preblastoderm stage eggs of B. mori, and found that the transformation efficiency of pB[3xP3RFP]-4 was 3.8 folds higher than pB[A3GFP]-1, whereas pB[3xP3GFP]-3 failed to produce transformants. Our results suggested that pB-4 may be one of the best piggyBac transposon plasmids currently available for germline transformation in B. mori.

Characterization of a group of MITEs with unusual features from two coral genomes

Background

Miniature inverted-repeat transposable elements (MITEs), which are common in eukaryotic genomes, are small non-coding elements that transpose by utilizing transposases encoded by autonomous transposons. Recent genome-wide analyses and cross-mobilization assays have greatly improved our knowledge on MITE proliferation, however, specific mechanisms for the origin and evolution of MITEs are still unclear.

Principal Findings

A group of coral MITEs called CMITE were identified from two corals, Acropora millepora and Acropora palmata. CMITEs conform to many common characteristics of MITEs, but also present several unusual features. The most unusual feature of CMITEs is conservation of the internal region, which is more conserved between MITE families than the TIRs. The origin of this internal region remains unknown, although we found one CMITE family that seems to be derived from a piggyBac-like transposon in A. millepora. CMITEs can form tandem arrays, suggesting an unconventional way for MITEs to increase copy numbers. We also describe a case in which a novel transposable element was created by a CMITE insertion event.

Conclusions

To our knowledge, this is the first report of identification of MITEs from coral genomes. Proliferation of CMITEs seems to be related to the transposition machinery of piggyBac-like autonomous transposons. The highly conserved internal region of CMITEs suggests a potential role for this region in their successful transposition. However, the origin of these unusual features in CMITEs remains unclear, and thus represents an intriguing topic for future investigations.

A domesticated piggyBac transposase plays key roles in heterochromatin dynamics and DNA cleavage during programmed DNA deletion in Tetrahymena thermophila

This study suggests that a TPB2 piggyBac transposase has evolved to facilitate heterochromatin assembly and carry out the final DNA excision step of programmed DNA deletion in Tetrahymena thermophila. TPB2 appears to have gone through a domestication process to become a host gene and be maintained in the macronuclear genome.

Transposons comprise large fractions of eukaryotic genomes and provide genetic reservoirs for the evolution of new cellular functions. We identified TPB2, a homolog of the piggyBac transposase gene that is required for programmed DNA deletion in Tetrahymena. TPB2 was expressed exclusively during the time of DNA excision, and its encoded protein Tpb2p was localized in DNA elimination heterochromatin structures. Notably, silencing of TPB2 by RNAi disrupts the final assembly of these heterochromatin structures and prevents DNA deletion to occur. In vitro studies revealed that Tpb2p is an endonuclease that produces double-strand breaks with four-base 5′ protruding ends, similar to the ends generated during DNA deletion. These findings suggest that Tpb2p plays a key role in the assembly of specialized DNA elimination chromatin architectures and is likely responsible for the DNA cleavage step of programmed DNA deletion.

Expression of the hGM-CSF in the silk glands of germline of gene-targeted silkworm

To express human granulocyte-macrophage colony-stimulating factor (hGM-CSF) gene in the silk glands of transformation silkworm (Bombyx mori) based on gene-targeting, two fragments from fibroin heavy chain gene (fib-H) of silkworm were cloned and sequenced. One fragment contains the 1st exon and its downstream 1st intron's partial sequence; and the other fragment contains the 1st intron's partial sequence and the 2nd exon's partial sequence. Then the two fragments, as homologous arm, were inserted into pSK to generate a gene-targeted vector, pSK-HL-A3GFP-FLP-GM-CSF-FLPA-HR in which a gfp gene driven by A3 promoter and an hGM-CSF gene under the control of fibroin light chain (fib-L) promoter were included. The vector was transferred into the silkworm eggs using sperm-mediated gene transfer. After being screened for green fluorescent, the transformation silkworm was obtained, whose genome was verified by PCR and dot hybridization to confirm whether the target genes had been integrated into the silkworm genome. Furthermore, in the posterior silk glands of the G4 generation transformation silkworms, a specific band with the molecular weight of 22kDa could be detected by Western blotting with an antibody against hGM-CSF, and the expression level of the hGM-CSF estimated by ELISA was approximately 1.26ng per gram fresh posterior silk gland.

PiggyMac, a domesticated piggyBac transposase involved in programmed genome rearrangements in the ciliate Paramecium tetraurelia

Programmed genome rearrangements drive functional gene assembly in ciliates during the development of the somatic macronucleus. The elimination of germline sequences is directed by noncoding RNAs and is initiated by DNA double-strand breaks, but the enzymes responsible for DNA cleavage have not been identified. We show here that PiggyMac (Pgm), a domesticated piggyBac transposase, is required for these rearrangements in Paramecium tetraurelia. A GFP-Pgm fusion localizes in developing macronuclei, where rearrangements take place, and RNAi-mediated silencing of PGM abolishes DNA cleavage. This is the first in vivo evidence suggesting an essential endonucleolytic function of a domesticated piggyBac transposase.

An efficient and reversible transposable system for gene delivery and lineage-specific differentiation in human embryonic stem cells

Unraveling the therapeutic potential of human embryonic stem cells (hESC) requires tools to modify their genome. We have engineered the PiggyBac transposable element to create an efficient system for gene delivery in hESCs. This redesigned system, named "ePiggyBac," can deliver up to 18 Kb inserts, and transgene expression is observed in almost 90% of hES cells. ePiggyBac transposons can also carry insulators, inducible expression cassettes, and short hairpin RNAs for gain- and loss-of-function approaches. In hES cells, ePiggyBac's efficiency is superior to that of viral vectors and previously described transposons, including other PiggyBac-based systems. In addition, ePiggyBac transgenes can be removed from the hESC genome without leaving any mutation. We used this system to direct hESC differentiation toward a neuronal phenotype. We then removed the transposons to obtain transgene-free neuronal precursors and neurons. The ability to create fully reversible genetic modifications represents an important step toward clinical applications of hESCs.

Mutational analysis of highly conserved aspartate residues essential to the catalytic core of the piggyBac transposase

Background

The piggyBac mobile element is quickly gaining popularity as a tool for the transgenesis of many eukaryotic organisms. By studying the transposase which catalyzes the movement of piggyBac, we may be able to modify this vector system to make it a more effective transgenesis tool. In a previous publication, Sarkar A, Sim C, Hong YS, Hogan JR, Fraser MJ, Robertson HM, and Collins FH have proposed the presence of the widespread 'DDE/DDD' motif for piggyBac at amino acid positions D268, D346, and D447.

Results

This study utilizes directed mutagenesis and plasmid-based mobility assays to assess the importance of these residues as the catalytic core of the piggyBac transposase. We have functionally analyzed individual point-mutations with respect to charge and physical size in all three proposed residues of the 'DDD' motif as well as another nearby, highly conserved aspartate at D450. All of our mutations had a significant effect on excision frequency in S2 cell cultures. We have also aligned the piggyBac transposase to other close family members, both functional and non-functional, in an attempt to identify the most highly conserved regions and position a number of interesting features.

Conclusion

We found all the designated DDD aspartates reside in clusters of amino acids that conserved among piggyBac family transposase members. Our results indicate that all four aspartates are necessary, to one degree or another, for excision to occur in a cellular environment, but D450 seems to have a tolerance for a glutamate substitution. All mutants tested significantly decreased excision frequency in cell cultures when compared with the wild-type transposase.

Analysis of the piggyBac transposase reveals a functional nuclear targeting signal in the 94 c-terminal residues

Background

The piggyBac transposable element is a popular tool for germ-line transgenesis of eukaryotes. Despite this, little is known about the mechanism of transposition or the transposase (TPase) itself. A thorough understanding of just how piggyBac works may lead to more effective use of this important mobile element. A PSORTII analysis of the TPase amino acid sequence predicts a bipartite nuclear localization signal (NLS) near the c-terminus, just upstream of a putative ZnF (ZnF).

Results

We fused the piggyBac TPase upstream of and in-frame with the enhanced yellow fluorescent protein (EYFP) in the Drosophila melanogaster inducible metallothionein protein. Using Drosophila Schneider 2 (S2) cells and the deep red fluorescent nuclear stain Draq5, we were able to track the pattern of piggyBac localization with a scanning confocal microscope 48 hours after induction with copper sulphate.

Conclusion

Through n and c-terminal truncations, targeted internal deletions, and specific amino acid mutations of the piggyBac TPase open reading frame, we found that not only is the PSORTII-predicted NLS required for the TPase to enter the nucleus of S2 cells, but there are additional requirements for negatively charged amino acids a short length upstream of this region for nuclear localization.

Chromosomal transposition of PiggyBac in mouse embryonic stem cells

Transposon systems are widely used for generating mutations in various model organisms. PiggyBac (PB) has recently been shown to transpose efficiently in the mouse germ line and other mammalian cell lines. To facilitate PB's application in mammalian genetics, we characterized the properties of the PB transposon in mouse embryonic stem (ES) cells. We first measured the transposition efficiencies of PB transposon in mouse embryonic stem cells. We next constructed a PB/SB hybrid transposon to compare PB and Sleeping Beauty (SB) transposon systems and demonstrated that PB transposition was inhibited by DNA methylation. The excision and reintegration rates of a single PB from two independent genomic loci were measured and its ability to mutate genes with gene trap cassettes was tested. We examined PB's integration site distribution in the mouse genome and found that PB transposition exhibited local hopping. The comprehensive information from this study should facilitate further exploration of the potential of PB and SB DNA transposons in mammalian genetics.

PiggyBac transposon-mediated gene transfer in human cells

Transposons are mobile genetic elements that can be used to integrate transgenes into host cell genomes. The piggyBac transposon system has been used for transgenesis of insects and for germline mutagenesis in mice. We compared transposition activity of piggyBac with Sleeping Beauty (SB), a widely used transposon system for preclinical gene therapy studies. An engineered piggyBac transposon with minimal length 5' and 3' terminal repeats exhibited greater transposition activity in transfected cultured human cells than a well-characterized hyperactive SB system. PiggyBac excision was very precise as evidenced by the typical absence of "footprint" mutations at the site of transposon excision. We mapped 575 piggyBac integration sites in human cells to determine site selectivity of genomic integration. PiggyBac demonstrated non-random integration site selectivity that differed from that previously reported for SB, including a higher preference for integrations in regions surrounding transcriptional start sites and within long terminal repeat elements. Importantly, overproduction inhibition was not observed with piggyBac, a major limitation of the SB system. This permitted the generation of combination "helper-independent" piggyBac transposase-transposon vectors that exhibited a 2-fold increase of transposition activity in human cells as compared with cells transfected with separate transposon and transposase plasmids. We conclude that piggyBac is a transposon system with certain properties, including high efficiency and lack of overproduction inhibition that are advantageous in preclinical development of transposon-based gene therapy.

piggyBac transposon mediated transgenesis of the human blood fluke, Schistosoma mansoni

The transposon piggyBac from the genome of the cabbage looper moth Trichoplusia ni has been observed in the laboratory to jump into the genomes of key model and pathogenic eukaryote organisms including mosquitoes, planarians, human and other mammalian cells, and the malaria parasite Plasmodium falciparum. Introduction of exogenous transposons into schistosomes has not been reported but transposon-mediated transgenesis of schistosomes might supersede current methods for functional genomics of this important human pathogen. In the present study we examined whether the piggyBac transposon could deliver reporter transgenes into the genome of Schistosoma mansoni parasites. A piggyBac donor plasmid modified to encode firefly luciferase under control of schistosome gene promoters was introduced along with 7-methylguanosine capped RNAs encoding piggyBac transposase into cultured schistosomula by square wave electroporation. The activity of the helper transposase mRNA was confirmed by Southern hybridization analysis of genomic DNA from the transformed schistosomes, and hybridization signals indicated that the piggyBac transposon had integrated into numerous sites within the parasite chromosomes. piggyBac integrations were recovered by retrotransposon-anchored PCR, revealing characteristic piggyBac TTAA footprints in the vicinity of the endogenous schistosome retrotransposons Boudicca, SR1, and SR2. This is the first report of chromosomal integration of a transgene and somatic transgenesis of this important human pathogen, in this instance accomplished by mobilization of the piggyBac transposon.

Construction and characterization of new piggyBac vectors for constitutive or inducible expression of heterologous gene pairs and the identification of a previously unrecognized activator sequence in piggyBac

BACKGROUND

We constructed and characterized several new piggyBac vectors to provide transposition of constitutively- or inducibly-expressible heterologous gene pairs. The dual constitutive control element consists of back-to-back copies of a baculovirus immediate early (ie1) promoter separated by a baculovirus enhancer (hr5). The dual inducible control element consists of back-to-back copies of a minimal cytomegalovirus (CMVmin) promoter separated by a synthetic operator (TetO7), which drives transcription in the presence of a mutant transcriptional repressor plus tetracycline.

RESULTS

Characterization of these vectors revealed an unexpected position effect, in which heterologous genes adjacent to the 3'- terminal region ("rightward" genes) were consistently expressed at higher levels than those adjacent to the 5'-terminal region ("leftward" genes) of the piggyBac element. This position effect was observed with all six heterologous genes examined and with both transcriptional control elements. Further analysis demonstrated that this position effect resulted from stimulation of rightward gene expression by the internal domain sequence of the 3'-terminal region of piggyBac. Inserting a copy of this sequence into the 5'- terminal repeat region of our new piggyBac vectors in either orientation stimulated leftward gene expression. Representative piggyBac vectors designed for constitutive or inducible expression of heterologous gene pairs were shown to be functional as insect transformation vectors.

CONCLUSION

This study is significant because (a) it demonstrates the utility of a strategy for the construction of piggyBac vectors that can provide constitutive or inducible heterologous gene pair expression and (b) it reveals the presence of a previously unrecognized transcriptional activator in piggyBac, which is an important and increasingly utilized transposable element.

Interplasmid transposition demonstrates piggyBac mobility in vertebrate species

The piggyBac transposon is an extremely versatile helper-dependent vector for gene transfer and germ line transformation in a wide range of invertebrate species. Analyses of genome sequencing databases have identified piggyBac homologues among several sequenced animal genomes, including the human genome. In this report we demonstrate that this insect transposon is capable of transposition in primate cells and embryos of the zebrafish, Danio rerio. piggyBac mobility was demonstrated using an interplasmid transposition assay that has consistently predicted the germ line transformation capabilities of this mobile element in several other species. Both transfected COS-7 primate cells and injected zebrafish embryos supported the helper-dependent movement of tagged piggyBac element between plasmids in the characteristic cut-and-paste, TTAA target-site specific manner. These results validate piggyBac as a valuable tool for genetic analysis of vertebrates.

Functional genomics of Plasmodium falciparum through transposon-mediated mutagenesis

Plasmodium falciparum is the causative agent for the most lethal form of human malaria, killing millions annually. Genetic analyses of P. falciparum have been relatively limited due to the lack of robust techniques to manipulate this parasite. Development of transfection technologies and whole genome analyses have helped in understanding the complex biology of this parasite. Even with this wealth of information functional genomics approaches are still very limited in P. falciparum due to the cumbersome and inefficient methods of genetic manipulation. This review focuses on a recently developed, highly efficient method for transposon-based mutagenesis and transgene expression in P. falciparum that will allow functional genomics studies to be performed proficiently on this deadly malaria parasite. By using a piggyBac-based transposition system, multiple random integrations have been obtained into the genome of the parasite. This technique could hence be employed to set up several biological screens in this lethal protozoan parasite that may lead to identification of novel drug targets and vaccine candidates.

piggyBac internal sequences are necessary for efficient transformation of target genomes

A previously reported piggyBac minimal sequence cartridge, which is capable of efficient transposition in embryo interplasmid transposition assays, failed to produce transformants at a significant frequency in Drosophila melanogaster compared with full-length or less extensive internal deletion constructs. We have re-examined the importance of these internal domain (ID) sequences for germline transformation using a PCR strategy that effectively adds increasing lengths of ID sequences to each terminus. A series of these piggyBac ID synthetic deletion plasmids containing the 3xP3-ECFP marker gene are compared for germline transformation of D. melanogaster. Our analyses identify a minimal sequence configuration that is sufficient for movement of piggyBac vectored sequences from plasmids into the insect genome. Southern hybridizations confirm the presence of the piggyBac transposon sequences, and insertion site analyses confirm these integrations target TTAA sites. The results verify that ID sequences adjacent to the 5' and 3' terminal repeat domains are crucial for effective germline transformation with piggyBac even though they are not required for excision or interplasmid transposition. Using this information we reconstructed an inverted repeat cartridge, ITR1.1k, and a minimal piggyBac transposon vector, pXL-BacII-ECFP, each of which contains these identified ID sequences in addition to the terminal repeat configuration previously described as essential for mobility. We confirm in independent experiments that these new minimal constructs yield transformation frequencies similar to the control piggyBac vector. Sequencing analyses of our constructs verify the position and the source of a point mutation within the 3' internal repeat sequence of our vectors that has no apparent effect on transformation efficiency.

Promoter and piggyBac activities within embryos of the potato tuber moth, Phthorimaea operculella, Zeller (Lepidoptera: Gelechiidae)

Potato production in tropical and subtropical countries suffers from damage caused by the potato tuber moth (PTM), Phthorimaea operculella. The aim of this research was the development of the components required for a germline transformation system for the PTM. We tested three components that are critical to genetic transformation systems for insects: promoter activity, marker gene expression, and transposable element function. We compared the transcriptional activities of five different promoters, hsp70, hsp82, actin5C, polyubiquitin and immediate early 1 gene (ie1), within PTM embryos. The ie1 promoter, flanked by the hr5 enhancer element, showed a very high level of transcriptional activity compared to the other promoters. The fluorescence activity of EGFP was also determined and transient expression of EGFP was detected in 57% of injected embryos. The transpositional activity of the piggyBac transposable element was tested in an interplasmid transposition assay. The piggyBac element was shown to be mobile within the embryonic soma of the PTM with a transposition frequency of 4.2 x 10(-5) transpositions/donor plasmid. Incorporating a transactivator plasmid expressing the immediate early protein (IE1) from the Bombyx mori nuclear polyhedrosis virus enhanced the efficiency of piggyBac mobility.

Evolution of the Transposable Element Pokey in the Ribosomal DNA of Species in the Subgenus Daphnia (Crustacea: Cladocera)

Pokey is a member of the piggyBac (previously called the TTAA-specific) family of transposons and inserts into a conserved region of the large subunit ribosomal RNA gene. This location is a "hot spot" for insertional activity, as it is known to contain other arthropod transposable elements. However, Pokey is unique in that it is the first DNA transposon yet known to insert into this region. All other insertions are class I non-LTR retrotransposons. This study surveyed variation in Pokey elements through phylogenetic analysis of the 3' ends of Pokey elements from ribosomal DNA (rDNA) in species from the nominate subgenus of the genus Daphnia (Crustacea: Cladocera). The results suggest that Pokey has been stably, vertically inherited within rDNA over long periods of evolutionary time. No evidence was found to support horizontal transfer, which commonly occurs in other DNA transposons, such as P and mariner. Furthermore, Pokey has diverged into sublineages that have persisted across speciation events in some groups. In addition, a new highly divergent paralogous Pokey element was discovered in the rDNA of one species.

piggyBac-based insertional mutagenesis in the presence of stably integrated P elements in Drosophila

P element-mediated mutagenesis has been used to disrupt an estimated 25% of genes essential for Drosophila adult viability. Mutation of all genes in the fly genome, however, poses a problem, because P elements show significant hotspots of integration. In addition, advanced screening scenarios often require the use of P element-based tools like the generation of germ-line mosaics using FLP recombinase-mediated recombination or gene misexpression using the UAS/Gal4 system. These techniques are P element-based and can therefore not be combined with the use of P elements as mutagenic agents. To circumvent these limitations, we have developed an insertional mutagenesis system using non-P element transposons. An enhanced yellow fluorescent protein-marked piggyBac-based mutator element was mobilized by a piggyBac specific transposase source expressed from a Hermes-based jump-starter transposon marked with enhanced cyan fluorescent protein. In a pilot screen, we have generated 798 piggyBac insertions on FRT bearing third chromosomes of which 9% have sustained a putatively piggyBac-related lethal hit. The FRTs present on the target chromosome remained stably integrated during the screen and could subsequently be used to generate germ-line clones associated with maternal and zygotic phenotypes. PCR-based analysis of insertion loci shows that 57% of the insertions are in genes for which no P element insertions have been reported. Our data demonstrate the potential of this technique to facilitate the quest for saturation mutagenesis of the Drosophila genome. The system is Drosophila nonspecific and potentially applicable in a broad spectrum of nonmodel organisms.

piggyBac-based insertional mutagenesis and enhancer detection as a tool for functional insect genomics

Transposon mutagenesis provides a fundamental tool for functional genomics. Here we present a non-species-specific, combined enhancer detection and binary expression system based on the transposable element piggyBac: For the different components of this insertional mutagenesis system, we used widely applicable transposons and distinguishable broad-range transformation markers, which should enable this system to be operational in nonmodel arthropods. In a pilot screen in Drosophila melanogaster, piggyBac mutator elements on the X chromosome were mobilized in males by a Hermes-based jumpstarter element providing piggyBac transposase activity under control of the alpha1-tubulin promoter. As primary reporters in the piggyBac mutator elements, we employed the heterologous transactivators GAL4delta or tTA. To identify larval and adult enhancer detectors, strains carrying UASp-EYFP or TRE-EYFP as secondary reporter elements were used. Tissue-specific enhancer activities were readily observed in the GAL4delta/UASp-based systems, but only rarely in the tTA/TRE system. Novel autosomal insertions were recovered with an average jumping rate of 80%. Of these novel insertions, 3.8% showed homozygous lethality, which was reversible by piggyBac excision. Insertions were found in both coding and noncoding regions of characterized genes and also in noncharacterized and non-P-targeted CG-number genes. This indicates that piggyBac will greatly facilitate the intended saturation mutagenesis in Drosophila.

Stable transformation of a Mamestra brassicae (lepidoptera) cell line with the lepidopteran-derived transposon piggyBac

Cabbage moth cells were transfected with the vector pBac[3xP3-EGFPafm] and helper phsp-pBac. Seventeen percent of the transfected cells showed stable EGFP-expression. This indicates successful and stable transformation of M. brassicae cells with a piggyBac-derived vector. Genomic integration of Bac[3xP3-EGFPafm] in stably transformed cells was confirmed by Southern blots and inverse PCR. Since the integrations are stable, and transfection with pBac[3xP3-EGFPafm] alone did not yield in transformations, no cross-reacting transposase activity seems present in M. brassicae cells. Moreover, Southern blotting with a probe for piggyBac transposase indicated the absence of piggyBac-related elements in the genome of Mamestra brassicae. Due to the tissue specificity of the 3xP3-EGFP marker for eye and nervous tissues, it is intriguing that 3xP3-EGFP can successfully be used to identify stably transformed M. brassicae cells of cell line IZD-MB0503, which is hemocyte-derived. Sequence analysis of the insertion sites showed that piggyBac inverted repeats were adjacent to TTAA sequences on both termini in all the clones. The present results are particularly important as they suggest that piggyBac can be used for transgenesis of cabbage moth cells.

Germ line transformation of the yellow fever mosquito, Aedes aegypti, mediated by transpositional insertion of a piggyBac vector

Mosquito-vectored diseases such as yellow fever and dengue fever continue to have a substantial impact on human populations world-wide. Novel strategies for control of these mosquito vectored diseases can arise through the development of reliable systems for genetic manipulation of the insect vector. A piggyBac vector marked with the Drosophila melanogaster cinnabar (cn) gene was used to transform the white-eyed khw strain of Aedes aegypti. Microinjection of preblastoderm embryos resulted in four families of cinnabar transformed insects. An overall transformation frequency of 4%, with a range of 0% to as high as 13% for individual experiments, was achieved when using a heat-shock induced transposase providing helper plasmid. Southern hybridizations indicated multiple insertion events in three of four transgenic lines, while the presence of duplicated target TTAA sites at either ends of individual insertions confirmed characteristic piggyBac transposition events in these three transgenic lines. The transgenic phenotype has remained stable for more than twenty generations. The transformations effected using the piggyBac element establish the potential of this element as a germ-line transformation vector for Aedine mosquitoes.

Genetic transformation systems in insects

The past 5 years have witnessed the emergence of techniques that permit the stable genetic transformation of a number of non-drosophilid insect species. These transposable-element-based strategies, together with virus-based techniques that allow the expression of genes to be quickly examined in insects, provide insect scientists with a first generation of genetic tools that can begin to be harnessed to further increase our understanding of gene function and regulation in insects. We review and compare the characteristics of these gene transfer systems and conclude that, although significant progress has been made, these systems still do not meet the requirements of robust genetic tools. We also review risk assessment issues arising from the generation and probable release of genetically engineered insects.

Germ-line transformation of pink bollworm (Lepidoptera: gelechiidae) mediated by the piggyBac transposable element

The pink bollworm, Pectinophora gossypiella, is a world-wide pest of cultivated cotton. In certain growing regions populations are suppressed by a sterile release strategy. Efforts to improve the sterile insect technique as well as our understanding of lepidopteran biology could benefit greatly from a germ-line transformation system. We report transformation of pink bollworm with a piggyBac transposable element carrying the enhanced green flourescent protein (EGFP) marker gene. This vector-marker system resulted in recovery of transgenics at a rate of approximately 3.5%. Integration of the transforming construct that was typical of piggyBac was demonstrated by Southern analysis and sequence determination of transposon flanks. Expression of the EGFP marker was visualized by fluorescent microscopy and Western Blot analysis. Maintenance of transformed strains indicates that the transgene segregates in a Mendelian fashion and has been stable over fourteen generations to date.

The lepidopteran transposon vector, piggyBac, mediates germ-line transformation in the Mediterranean fruit fly

The piggyBac (IFP2) short inverted terminal repeat transposable element from the cabbage looper Trichoplusia ni was tested for gene transfer vector function as part of a bipartite vector-helper system in the Mediterranean fruit fly Ceratitis capitata. A piggyBac vector marked with the medfly white gene was tested with a normally regulated piggyBac transposase helper at two different concentrations in a white eye host strain. Both experiments yielded transformants at an approximate frequency of 3-5%, with a total of six lines isolated having pigmented eyes with various levels of coloration. G1 transformant siblings from each line shared at least one common integration, with several sublines having an additional second integration. For the first transformant line isolated, two integrations were determined to be stable for 15 generations. For five of the lines, a piggyBac-mediated transposition was verified by sequencing the insertion site junctions isolated by inverse PCR that identified a characteristic piggyBac TTAA target site duplication. The efficient and stable transformation of the medfly with a lepidopteran vector represents transposon function over a relatively large evolutionary distance and suggests that the piggyBac system will be functional in a broad range of insects.