Enhanced expression and stable transmission of transgenes flanked by inverted terminal repeats from adeno-associated virus in zebrafish

The Promising Role of a Zebrafish Model Employed in Neural Regeneration Following a Spinal Cord Injury

Spinal cord injury (SCI) is a devastating event that results in a wide range of physical impairments and disabilities. Despite the advances in our understanding of the biological response to injured tissue, no effective treatments are available for SCIs at present. Some studies have addressed this issue by exploring the potential of cell transplantation therapy. However, because of the abnormal microenvironment in injured tissue, the survival rate of transplanted cells is often low, thus limiting the efficacy of such treatments. Many studies have attempted to overcome these obstacles using a variety of cell types and animal models. Recent studies have shown the utility of zebrafish as a model of neural regeneration following SCIs, including the proliferation and migration of various cell types and the involvement of various progenitor cells. In this review, we discuss some of the current challenges in SCI research, including the accurate identification of cell types involved in neural regeneration, the adverse microenvironment created by SCIs, attenuated immune responses that inhibit nerve regeneration, and glial scar formation that prevents axonal regeneration. More in-depth studies are needed to fully understand the neural regeneration mechanisms, proteins, and signaling pathways involved in the complex interactions between the SCI microenvironment and transplanted cells in non-mammals, particularly in the zebrafish model, which could, in turn, lead to new therapeutic approaches to treat SCIs in humans and other mammals.

Cellular and animal models for facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is one of the most common forms of muscular dystrophy and presents with weakness of the facial, scapular and humeral muscles, which frequently progresses to the lower limbs and truncal areas, causing profound disability. Myopathy results from epigenetic de-repression of the D4Z4 microsatellite repeat array on chromosome 4, which allows misexpression of the developmentally regulated DUX4 gene. DUX4 is toxic when misexpressed in skeletal muscle and disrupts several cellular pathways, including myogenic differentiation and fusion, which likely underpins pathology. DUX4 and the D4Z4 array are strongly conserved only in primates, making FSHD modeling in non-primate animals difficult. Additionally, its cytotoxicity and unusual mosaic expression pattern further complicate the generation of in vitro and in vivo models of FSHD. However, the pressing need to develop systems to test therapeutic approaches has led to the creation of multiple engineered FSHD models. Owing to the complex genetic, epigenetic and molecular factors underlying FSHD, it is difficult to engineer a system that accurately recapitulates every aspect of the human disease. Nevertheless, the past several years have seen the development of many new disease models, each with their own associated strengths that emphasize different aspects of the disease. Here, we review the wide range of FSHD models, including several in vitro cellular models, and an array of transgenic and xenograft in vivo models, with particular attention to newly developed systems and how they are being used to deepen our understanding of FSHD pathology and to test the efficacy of drug candidates.

Injection of an SV40 transcriptional terminator causes embryonic lethality: a possible zebrafish model for screening nonhomologous end-joining inhibitors

Introduction

DNA repair by the nonhomologous end joining (NHEJ) pathway promotes tumor recurrence after chemotherapy and radiotherapy. Discovery of rapid and high-throughput techniques to screen for an effective NHEJ inhibitor drug is imperative for the suppression of NHEJ during tumor treatment. However, traditional screening methods are too cumbersome to meet the current need. Zebrafish is an ideal model for drug screening due to the specificity of its early embryonic development and similarity of tumor cell generation. By exploiting the high frequency of NHEJ in early embryonic development, we established a model that uses a transcriptional terminator signal fragment from the Simian virus 40 (SV40) to cause embryonic lethality. SV40 fragment-induced embryonic lethality was alleviated by 5,6-bis ((E)-benzylideneamino)-2-mercaptopyrimidin-4-ol or C18H14N4OS (SCR7), an NHEJ inhibitor.

Materials and methods

A 122 bp SV40 terminator fragment (10 ng/µL) was microinjected into zebrafish zygotes. SV40 fragment integration into the zebrafish embryonic genome was detected by Southern blot using a DNA probe for the SV40 terminator. Embryonic lethality rates were observed 24 and 48 h after microinjection. A nonhomologous recombinant inhibitor, SCR7 (5 µM), was used to alleviate embryonic lethality.

Results

Microinjection of zebrafish embryos with the SV40 terminator fragment (10 ng/µL) caused a progressive increase in mortality over time. Using Southern blots, we confirmed that SV40 terminator sequences were integrated into the zebrafish embryonic genome. This phenomenon was effectively alleviated by addition of SCR7.

Conclusion

Injection of an SV40 terminator into zebrafish embryos may cause embryonic lethality due to NHEJ during early zebrafish development. The high mortality of zebrafish embryos could be alleviated by using the NHEJ inhibitor, SCR7. The zebrafish model presented here is simpler and more convenient than traditional methods of screening for NHEJ inhibitors and can be utilized in large-scale drug screens for NHEJ inhibitors and for the development of novel anticancer drugs.

Imaging Myelination In Vivo Using Transparent Animal Models

Myelination by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system is essential for nervous system function and health. Despite its importance, we have a relatively poor understanding of the molecular and cellular mechanisms that regulate myelination in the living animal, particularly in the CNS. This is partly due to the fact that myelination commences around birth in mammals, by which time the CNS is complex and largely inaccessible, and thus very difficult to image live in its intact form. As a consequence, in recent years much effort has been invested in the use of smaller, simpler, transparent model organisms to investigate mechanisms of myelination in vivo. Although the majority of such studies have employed zebrafish, the Xenopus tadpole also represents an important complementary system with advantages for investigating myelin biology in vivo. Here we review how the natural features of zebrafish embryos and larvae and Xenopus tadpoles make them ideal systems for experimentally interrogating myelination by live imaging. We outline common transgenic technologies used to generate zebrafish and Xenopus that express fluorescent reporters, which can be used to image myelination. We also provide an extensive overview of the imaging modalities most commonly employed to date to image the nervous system in these transparent systems, and also emerging technologies that we anticipate will become widely used in studies of zebrafish and Xenopus myelination in the near future.

Zebrafish and Medaka: new model organisms for modern biomedical research

Although they are primitive vertebrates, zebrafish (Danio rerio) and medaka (Oryzias latipes) have surpassed other animals as the most used model organisms based on their many advantages. Studies on gene expression patterns, regulatory cis-elements identification, and gene functions can be facilitated by using zebrafish embryos via a number of techniques, including transgenesis, in vivo transient assay, overexpression by injection of mRNAs, knockdown by injection of morpholino oligonucleotides, knockout and gene editing by CRISPR/Cas9 system and mutagenesis. In addition, transgenic lines of model fish harboring a tissue-specific reporter have become a powerful tool for the study of biological sciences, since it is possible to visualize the dynamic expression of a specific gene in the transparent embryos. In particular, some transgenic fish lines and mutants display defective phenotypes similar to those of human diseases. Therefore, a wide variety of fish model not only sheds light on the molecular mechanisms underlying disease pathogenesis in vivo but also provides a living platform for high-throughput screening of drug candidates. Interestingly, transgenic model fish lines can also be applied as biosensors to detect environmental pollutants, and even as pet fish to display beautiful fluorescent colors. Therefore, transgenic model fish possess a broad spectrum of applications in modern biomedical research, as exampled in the following review.

Inducible and combinatorial gene manipulation in mouse brain

We have deployed recombinant adeno-associated viruses equipped with tetracycline-controlled genetic switches to manipulate gene expression in mouse brain. Here, we show a combinatorial genetic approach for inducible, cell type-specific gene expression and Cre/loxP mediated gene recombination in different brain regions. Our chemical-genetic approach will help to investigate 'when', 'where', and 'how' gene(s) control neuronal circuit dynamics, and organize, for example, sensory signal processing, learning and memory, and behavior.

Optimization of eGFP expression using a modified baculovirus expression system

The baculovirus gene expression system is an efficient and safe protein expression system, since baculoviruses cannot replicate in mammalian cells. In order to improve the transduction efficiency and increase the reporter gene expression levels delivered by baculoviruses, we tested in the baculovirus expression cassette the Woodchuck hepatitis virus response element (WPRE), and AAV-derived inverted terminal repeats (ITRs) and the truncated vesicular stomatitis virus G protein (VSV-GED). The results showed that WPRE and VSV-GED have synergistic effects and could enhance the expression efficiency of enhanced green fluorescence protein (eGFP), and that ITRs effectively extended the duration of eGFP expression. We also demonstrated that the efficiency of eGFP expression varied under the control of the CMV, CBA, EF1-α or WSSV ie1 promoters in different cell lines.

Expression of DUX4 in zebrafish development recapitulates facioscapulohumeral muscular dystrophy

Facioscapulohumeral muscular dystrophy (FSHD) is a common form of muscular dystrophy characterized by an asymmetric progressive weakness and wasting of the facial, shoulder and upper arm muscles, frequently accompanied by hearing loss and retinal vasculopathy. FSHD is an autosomal dominant disease linked to chromosome 4q35, but the causative gene remains controversial. DUX4 is a leading candidate gene as causative of FSHD. However, DUX4 expression is extremely low in FSHD muscle, and there is no DUX4 animal model that mirrors the pathology in human FSHD. Here, we show that the misexpression of very low levels of human DUX4 in zebrafish development recapitulates the phenotypes seen in human FSHD patients. Microinjection of small amounts of human full-length DUX4 (DUX4-fl) mRNA into fertilized zebrafish eggs caused asymmetric abnormalities such as less pigmentation of the eyes, altered morphology of ears, developmental abnormality of fin muscle, disorganization of facial musculature and/or degeneration of trunk muscle later in development. Moreover, DUX4-fl expression caused aberrant localization of myogenic cells marked with α-actin promoter-driven enhanced green fluorescent protein outside somite boundary, especially in head region. These abnormalities were rescued by coinjection of the short form of DUX4 (DUX4-s). Our results suggest that the misexpression of DUX4-fl, even at extremely low level, can recapitulate the phenotype observed in FSHD patients in a vertebrate model. These results strongly support the current hypothesis for a role of DUX4 in FSHD pathogenesis. We also propose that DUX4 expression during development is important for the pathogenesis of FSHD.

The viral TRAF protein (ORF111L) from infectious spleen and kidney necrosis virus interacts with TRADD and induces caspase 8-mediated apoptosis

Infectious spleen and kidney necrosis virus (ISKNV) is the type species of the Megalocytivirus genus of the Iridoviridae family. It causes a serious and potentially pandemic disease in wild and cultured fishes. ISKNV infection induces evident apoptosis in mandarin fish (Siniperca chuatsi) and zebrafish (Danio renio). However, the mechanism is still unknown. After a genome-wide bioinformatics analysis of ISKNV-encoded proteins, the ISKNV open reading frame 111L (ORF111L) shows a high similarity to the tumour necrosis factor receptor-associated factor (TRAF) encoded by fish, mice and mammals, which is essential for apoptotic signal transduction. Moreover, ORF111L was verified to directly interact with the zebrafish TNF receptor type 1 associated death domain protein (TRADD). A recombinant plasmid containing the DNA sequence of ORF111L was constructed and microinjected into zebrafish embryos at the 1–2 cell stage to investigate its biological function in vivo. ORF111L overexpression in the embryos resulted in increased apoptosis. ORF111L-induced apoptosis was clearly associated with significant caspase 8 upregulation and activation. The knockdown of zebrafish caspase 8 expression effectively blocked the apoptosis induced by ORF111L overexpression. Significantly, ORF111L overexpression resulted in much stronger effect on caspase 8 and caspase 3 upregulation compared to zebrafish TRAF2. This is the first report of a viral protein similar to TRAF that interacts with TRADD and induces caspase 8-mediated apoptosis, which may provide novel insights into the pathogenesis of ISKNV infection.

Isolation and transcriptome analysis of adult zebrafish cells enriched for skeletal muscle progenitors

INTRODUCTION

Over the past 10 years, the use of zebrafish for scientific research in the area of muscle development has increased dramatically. Although several protocols exist for the isolation of adult myoblast progenitors from larger fish, no standardized protocol exists for the isolation of myogenic progenitors from adult zebrafish muscle.

METHODS

Using a variant of a mammalian myoblast isolation protocol, zebrafish muscle progenitors have been isolated from the total dorsal myotome. These zebrafish myoblast progenitors can be cultured for several passages and then differentiated into multinucleated, mature myotubes.

RESULTS

Transcriptome analysis of these cells during myogenic differentiation revealed a strong downregulation of pluripotency genes, while, conversely, showing an upregulation of myogenic signaling and structural genes.

CONCLUSIONS

Together these studies provide a simple, yet detailed method for the isolation and culture of myogenic progenitors from adult zebrafish, while further promoting their therapeutic potential for the study of muscle disease and drug screening.

Non-homologous end joining plays a key role in transgene concatemer formation in transgenic zebrafish embryos

This study focused on concatemer formation and integration pattern of transgenes in zebrafish embryos. A reporter plasmid based on enhanced green fluorescent protein (eGFP) driven by Cytomegalovirus (CMV) promoter, pCMV-pax6in-eGFP, was constructed to reflect transgene behavior in the host environment. After removal of the insertion fragment by double digestion with various combinations of restriction enzymes, linearized pCMV-pax6in-eGFP vectors were generated with different combinations of 5′-protruding, 3′-protruding, and blunt ends that were microinjected into zebrafish embryos. Repair of double-strand breaks (DSBs) was monitored by GFP expression following religation of the reporter gene. One-hundred-and-ninety-seven DNA fragments were amplified from GFP-positive embryos and sequenced to analyze the repair characteristics of different DSB end combinations. DSBs involving blunt and asymmetric protruding ends were repaired efficiently by direct ligation of blunt ends, ligation after blunting and fill-in, or removed by cutting. Repair of DSBs with symmetric 3′-3′ protrusions was less efficient and utilized template-directed repair. The results suggest that non-homologous end joining (NHEJ) was the principal mechanism of exogenous gene concatemer formation and integration of transgenes into the genome of transgenic zebrafish.

Detection of calcium transients in embryonic stem cells and their differentiated progeny

A central issue in stem cell biology is the determination of function and activity of differentiated stem cells, features that define the true phenotype of mature cell types. Commonly, physiological mechanisms are used to determine the functionality of mature cell types, including those of the nervous system. Calcium imaging provides an indirect method of determining the physiological activities of a mature cell. Camgaroos are variants of yellow fluorescent protein that act as intracellular calcium sensors in transfected cells. We expressed one version of the camgaroos, Camgaroo-2, in mouse embryonic stem (ES) cells under the control of the CAG promoter system. Under the control of this promoter, Camgaroo-2 fluorescence was ubiquitously expressed in all cell types derived from the ES cells that were tested. In response to pharmacological stimulation, the fluorescence levels in transfected cells correlated with cellular depolarization and hyperpolarization. These changes were observed in both undifferentiated ES cells as well as ES cells that had been neurally induced, including putative neurons that were differentiated from transfected ES cells. The results presented here indicate that Camgaroo-2 may be used like traditional fluorescent proteins to track cells as well as to study the functionality of stem cells and their progeny.

Transcriptional targeting to brain cells: Engineering cell type-specific promoter containing cassettes for enhanced transgene expression

Transcriptional targeting using a mammalian cellular promoter to restrict transgene expression to target cells is often desirable for gene therapy. This strategy is, however, hindered by relatively weak activity of some cellular promoters, which may lead to low levels of gene expression, thus declining therapeutic efficacy. Here we outline the advances accomplished in the area of transcriptional targeting to brain cells, with a particular focus on engineering gene cassettes to augment cell type-specific expression. Among the effective approaches that improve gene expression while retaining promoter specificity are promoter engineering to change authentic sequences of a cellular promoter and the combined use of a native cellular promoter and other cis-acting elements. Success in achieving high level and sustained transgene expression only in the cell types of interest would be of importance in allowing gene therapy to have its impact on patient treatment.

Multiple regulatory elements mediating neuronal-specific expression of zebrafish sodium channel gene, scn8aa

Zebrafish scn8aa sodium channels mediate the majority of sodium conductance, which is essential for the embryonic locomotor activities. Here, we investigated the transcriptional regulation of scn8aa in developing zebrafish embryos by constructing a GFP reporter driven by a 15-kb fragment of scn8aa gene designed as scn8aa:GFP. GFP expression patterns of scn8aa:GFP temporally and spatially recapitulated the expression of endogenous scn8aa mRNA during zebrafish embryonic development, with one exception in the inner nuclear layer of the retina. Three novel elements, along with an evolutionarily conserved element shared with mouse SCN8A, modulated neuronal-specific expression of scn8aa. The deletion of each positive element reduced the expression levels in neurons without inducing ectopic GFP expression in non-neuronal cells. Our results demonstrate that these four regulatory elements function cooperatively to enhance scn8aa expression in the zebrafish nervous system.

Intrathecal injection of naked plasmid DNA provides long-term expression of secreted proteins

Therapeutic benefit has been reported to result from intrathecal (i.t.) injection of transgene vectors, including naked DNA. However, most studies using naked DNA have measured only the transgene expression of intracellular proteins. Here we demonstrate that i.t. injection of naked DNA can result in long-term expression of secreted proteins. Plasmids expressing either secreted alkaline phosphatase (SEAP) or human interleukin-10 (hIL-10) were injected into the i.t. space in rats, and transgene products were repeatedly measured in the cerebrospinal fluid (CSF). Both SEAP and hIL-10 were maximal at 1 and 2 days after the injection and still detectable at 4 months. The utilization of a plasmid having two features that are hypothesized to increase gene expression (matrix attachment regions (MARs) and lack of CpG dinucleotides) resulted in a significant increase in gene expression. Reinjection of SEAP or hIL-10 plasmids after 4 months significantly increased protein levels at 1 and 14 days after the reinjection. SEAP was uniformly distributed between the DNA delivery site (approximately vertebral level T13) and the lumbar puncture site (L5/L6 inter-vertebral space), was reduced at the cisterna magna, and was detectable, though at much lower levels, in serum. These data suggest that naked DNA has the potential to be used as a therapeutic tool for applications that require long-term release of transgenes into the CSF.

CONDITIONAL PRODUCTION OF A FUNCTIONAL FISH GROWTH HORMONE IN THE TRANSGENIC LINE OF NANNOCHLOROPSIS OCULATA (EUSTIGMATOPHYCEAE)(1)

Plasmid phr-YPGHc, containing the fish growth hormone (GH) cDNA driven by a heat shock protein 70A promoter and a RUBISCO SSU 2 promoter, was transferred into the protoplast of marine microalga Nannochloropsis oculata (Droop) D. J. Hibberd by electroporation. Four transgenic clones were obtained in which the transferred phr-YPGHc was integrated into the genome and existed stably at least until the 50th generation. When we treated these transgenic microalgae by heat shock, the heterologous fish GH was produced in the amount of 0.42 to 0.27 μg · mL(-1) from the 50 mL of medium. We incubated artemia with the wildtype and transgenic N. oculata for 6 h and then fed these microalgae-treated artemia to red-tilapia larvae. After feeding, the growth of larvae that were fed artemia incubated with transgenic microalgae was greater (i.e., statistically significant: P < 0.05) than that of larvae that were fed artemia incubated with nontransgenic microalgae: 316% versus 104% in weight gain, and 217% versus 146% in body length increase, respectively. Therefore, the N. oculata enables production of functional GH, and we propose that it might be an excellent bioreactor material.

Meganuclease and transposon mediated transgenesis in medaka

From among a plethora of various gene delivery methods, the researcher must choose the right one according to availability for a given species and the precise application the transgenic animal is intended for. Here we review the progress in meganuclease and Sleeping Beauty transposon mediated transgenesis over recent years with a focus on medaka and zebrafish. We present a side-by-side comparison of these two approaches based on their biologic properties and provide interesting perspectives for future experiments and applications, which are different for the two techniques because of their distinct modes of action.

Robust, persistent transgene expression in human embryonic stem cells is achieved with AAVS1-targeted integration

Silencing and variegated transgene expression are poorly understood problems that can interfere with gene function studies in human embryonic stem cells (hESCs). We show that transgene expression (enhanced green fluorescent protein [EGFP]) from random integration sites in hESCs is affected by variegation and silencing, with only half of hESCs expressing the transgene, which is gradually lost after withdrawal of selection and differentiation. We tested the hypothesis that a transgene integrated into the adeno-associated virus type 2 (AAV2) target region on chromosome 19, known as the AAVS1 locus, would maintain transgene expression in hESCs. When we used AAV2 technology to target the AAVS1 locus, 4.16% of hESC clones achieved AAVS1-targeted integration. Targeted clones expressed Oct-4, stage-specific embryonic antigen-3 (SSEA3), and Tra-1-60 and differentiated into all three primary germ layers. EGFP expression from the AAVS1 locus showed significantly reduced variegated expression when in selection, with 90% +/- 4% of cells expressing EGFP compared with 57% +/- 32% for randomly integrated controls, and reduced tendency to undergo silencing, with 86% +/- 7% hESCs expressing EGFP 25 days after withdrawal of selection compared with 39% +/- 31% for randomly integrated clones. In addition, quantitative polymerase chain reaction analysis of hESCs also indicated significantly higher levels of EGFP mRNA in AAVS1-targeted clones as compared with randomly integrated clones. Transgene expression from the AAVS1 locus was shown to be stable during hESC differentiation, with more than 90% of cells expressing EGFP after 15 days of differentiation, as compared with approximately 30% for randomly integrated clones. These results demonstrate the utility of transgene integration at the AAVS1 locus in hESCs and its potential clinical application.

Nodal signals mediate interactions between the extra-embryonic and embryonic tissues in zebrafish

In many vertebrates, extra-embryonic tissues are important signaling centers that induce and pattern the germ layers. In teleosts, the mechanism by which the extra-embryonic yolk syncytial layer (YSL) patterns the embryo is not understood. Although the Nodal-related protein Squint is expressed in the YSL, its role in this tissue is not known. We generated a series of stable transgenic lines with GFP under the control of squint genomic sequences. In all species, nodal-related genes induce their own expression through a positive feedback loop. We show that two tissue specific enhancers in the zebrafish squint gene mediate the response to Nodal signals. Expression in the blastomeres depends upon a conserved Nodal response element (NRE) in the squint first intron, while expression in the extra-embryonic enveloping layer (EVL) is mediated by an element upstream of the transcription start site. Targeted depletion experiments demonstrate that the zebrafish Nodal-related proteins Squint and Cyclops are required in the YSL for endoderm and head mesoderm formation. Thus, Nodal signals mediate interactions between embryonic and extra-embryonic tissues in zebrafish that maintain nodal-related gene expression in the margin. Our results demonstrate a high degree of functional conservation between the extra-embryonic tissues of mouse and zebrafish.

Hypocretin/orexin overexpression induces an insomnia-like phenotype in zebrafish

As many as 10% of humans suffer chronic sleep disturbances, yet the genetic mechanisms that regulate sleep remain essentially unknown. It is therefore crucial to develop simple and cost-effective vertebrate models to study the genetic regulation of sleep. The best characterized mammalian sleep/wake regulator is hypocretin/orexin (Hcrt), whose loss results in the sleep disorder narcolepsy and that has also been implicated in feeding behavior, energy homeostasis, thermoregulation, reward seeking, addiction, and maternal behavior. Here we report that the expression pattern and axonal projections of embryonic and larval zebrafish Hcrt neurons are strikingly similar to those in mammals. We show that zebrafish larvae exhibit robust locomotive sleep/wake behaviors as early as the fifth day of development and that Hcrt overexpression promotes and consolidates wakefulness and inhibits rest. Similar to humans with insomnia, Hcrt-overexpressing larvae are hyperaroused and have dramatically reduced abilities to initiate and maintain rest at night. Remarkably, Hcrt function is modulated by but does not require normal circadian oscillations in locomotor activity. Our zebrafish model of Hcrt overexpression indicates that the ancestral function of Hcrt is to promote locomotion and inhibit rest and will facilitate the discovery of neural circuits, genes, and drugs that regulate Hcrt function and sleep.

Myogenic regulatory factors Myf5 and Myod function distinctly during craniofacial myogenesis of zebrafish

The functions of Myf5 and Myod are well known in trunk myogenesis. However, the roles that Myf5 and Myod play during craniofacial myogenesis are far from well known. We observed that zebrafish myf5 was detected in the primordia of the obliques, lateral rectus, sternohyoideus, and pharyngeal mesoderm cores. In contrast, myod transcripts were expressed in all head muscle precursors at later stages. Knockdown of myf5 revealed that Myf5 was required for the development of the obliques, lateral rectus, sternohyoideus, and all pharyngeal muscles, whereas knockdown of myod proved that Myod was required for the development of superior rectus, medial rectus, inferior rectus, lateral rectus, and the ventral pharyngeal muscles. myod mRNA did not rescue the loss of the cranial muscle caused by injecting myf5-morpholino, or vice versa, suggesting that the functions of Myf5 and Myod were not redundant in head paraxial mesoderm, a finding different from their functions in trunk myogenesis. Myf5, but not Myod, was required for the forward migration of myf5-positive oblique precursors. All evidences reveal that Myf5 and Myod function independently during cranial myogenesis. On the basis of the expression patterns of myf5 and myod, we propose a model to present how Myf5 and Myod are involved in head myogenesis of zebrafish.

Recombinant baculovirus containing the diphtheria toxin A gene for malignant glioma therapy

Insect baculoviruses are capable of infecting mammalian glial cells in the central nervous system. We investigated in the current study the feasibility of using the viruses as toxin gene vectors to eliminate malignant glioma cells in the brain. We first confirmed that glioma cells were permissive to baculovirus infection, with variable transduction efficiencies at 100 viral particles per cell and ranging between 35% and 70% in seven human and rat glioma cell lines. We then developed a recombinant baculovirus vector accommodating the promoter of glial fibrillary acidic protein (GFAP) to minimize possible side effects caused by overexpression of a therapeutic gene in sensitive neurons. We placed the GFAP promoter into a baculovirus expression cassette, in which the enhancer of human cytomegalovirus immediate-early gene and the inverted terminal repeats of adeno-associated virus were employed to improve the relatively low transcriptional activity of the cellular promoter. This recombinant baculovirus significantly improved transduction in glioma cells, providing the efficiency in C6 rat glioma cells up to 96%. When used to produce the A-chain of diphtheria toxin intracellularly in a rat C6 glioma xenograft model, the baculovirus effectively suppressed tumor development. The new baculovirus vector circumvents some of the inherent problems associated with mammalian viral vectors and provides an additional option for cancer gene therapy.

A hybrid vector for ligand-directed tumor targeting and molecular imaging

Merging tumor targeting and molecular-genetic imaging into an integrated platform is limited by lack of strategies to enable systemic yet ligand-directed delivery and imaging of specific transgenes. Many eukaryotic viruses serve for transgene delivery but require elimination of native tropism for mammalian cells; in contrast, prokaryotic viruses can be adapted to bind to mammalian receptors but are otherwise poor vehicles. Here we introduce a system containing cis-elements from adeno-associated virus (AAV) and single-stranded bacteriophage. Our AAV/phage (AAVP) prototype targets an integrin. We show that AAVP provides superior tumor transduction over phage and that incorporation of inverted terminal repeats is associated with improved fate of the delivered transgene. Moreover, we show that the temporal dynamics and spatial heterogeneity of gene expression mediated by targeted AAVP can be monitored by positron emission tomography. This new class of targeted hybrid viral particles will enable a wide range of applications in biology and medicine.

Recombinant Avian Adeno-Associated Virus: Transgene ExpressionIn Vivoand Enhancement of ExpressionIn Vitro

Recombinant avian adeno-associated viruses coding for the LacZ gene were used to inoculate embryonating chicken eggs, to assess the usefulness of the system for the expression of a transgene in vivo. The results obtained indicate significantly higher levels of expression of the reporter gene at various time intervals in the embryos inoculated with the recombinant virus in comparison with the mock-inoculated controls. At the embryo level, significant differences were evident at 120 hr postinoculation; hatched chicks showed transgene expression up to 14 days of age. In a second experiment, different cell-line cultures were transfected with plasmids encoding for a reporter gene flanked by the avian adeno-associated virus inverted terminal repeats (ITR), either alone or in the presence of the major nonstructural proteins of the virus (Rep 78/68) to assess the ability of these proteins and DNA elements to enhance gene expression. Results indicate that the inclusion of the viral ITR alone or during coexpression of the Rep proteins significantly enhances the expression of the transgene in all cell lines tested, as evidenced by the detection of the beta-galacrosidase protein through chemiluminescence reactions and staining of transfected monolayers.

Astrocytic expression of transgene in the rat brain mediated by baculovirus vectors containing an astrocyte-specific promoter

Therapeutic gene expression in glial cells has been tested for the treatment of neurological diseases in animal models. Many of such studies used the promoter of the glial fibrillary acidic protein (GFAP) to restrict gene expression to astrocytes. We have investigated in the current study whether it is possible to improve the transcriptional activity of the cellular promoter, while maintaining its cell-type specificity. We constructed an expression cassette containing a hybrid cytomegalovirus (CMV) enhancer/GFAP promoter and placed it into baculovirus vectors, a type of viral vectors capable of transducing astrocytes. In another vector design, we used inverted terminal repeats (ITRs) from adeno-associated virus (AAV) to flank the expression cassette. The recombinant baculoviruses with the hybrid promoter improved gene expression levels over two orders of magnitude in glial cell lines and by 10-fold in the rat brain when compared to the baculoviruses with the GFAP promoter alone. The expression was further improved by ITR flanking, reaching levels higher than that mediated by the baculovirus vectors with the CMV immediate-early enhancer/promoter (CMV promoter). Using these recombinant baculoviruses, we observed extended in vivo transgene expression in the rat brain at 90 days postinjection, by which time the gene expression from baculovirus vectors with the GFAP or CMV promoter had already become undetectable. The astrocyte specificity of the GFAP promoter was preserved in the engineered expression cassette with the CMV enhancer and the AAV ITRs, as demonstrated by immunohistological analysis of brain samples and an axonal retrograde transport assay. Taken together, our findings suggest that these baculovirus vectors may serve as useful tools for astrocyte-specific gene expression in the brain.

Biomolecular imaging based on far-red fluorescent protein with a high two-photon excitation action cross section

Received October 14, 2005; revised January 7, 2006; accepted January 9, 2006; posted January 12, 2006 (Doc. ID 65391) The two-photon excitation action cross section of Hc-Red fluorescent proteins (Hc-RFPs) is measured and found to be of the same order as that of enhanced green fluorescent proteins. With a 618 nm emission wavelength in the far-red region and with an excitation wavelength around 1200 nm, Hc-RPF-based two-photon fluorescence microscopy (2PFM) can offer deep penetration capability inside live samples and is ideal for in vivo gene expression study and biomolecular imaging in live objects. In vivo 2PFM of the developing heart deep inside a transgenic zebrafish embryo tagged by Hc-RFP is also successfully demonstrated.

Adeno-associated virus inverted terminal repeats improve neuronal transgene expression mediated by baculoviral vectors in rat brain

Baculoviral vectors can transduce neurons in the CNS but mediate only transient expression of transgenes. We have developed a new baculoviral vector in which the inverted terminal repeats (ITRs) of adeno-associated virus are used to flank a luciferase reporter gene cassette harboring a neuron-specific promoter. When tested in rat brain, the new viral vector was able to provide transgene expression for at least 90 days. Immunohistological analysis demonstrated that ITR flanking did not affect the cellular preference of the neuronal promoter in the context of baculovirus. These findings establish an effective way to engineer baculoviral vectors in order to achieve sustained expression of a functional gene for gene therapy for neurodegenerative disorders and physiological studies of neurons.

Improved neuronal transgene expression from an AAV-2 vector with a hybrid CMV enhancer/PDGF-beta promoter

BACKGROUND

Adeno-associated virus type 2 (AAV-2) vectors are highly promising tools for gene therapy of neurological disorders. After accommodating a cellular promoter, AAV-2 vectors are able to drive sustained expression of transgene in the brain. This study aimed to develop AAV-2 vectors that also facilitate a high level of neuronal expression by enhancing the strength of a neuron-specific promoter, the human platelet-derived growth factor beta-chain (PDGF) promoter.

METHODS AND RESULTS

A hybrid promoter approach was adopted to fuse the enhancer of human cytomegalovirus immediately early (CMV) promoter to the PDGF promoter. In cultured cortex neurons, AAV-2 vectors containing the hybrid promoter augmented transgene expression up to 20-fold over that mediated by titer-matched AAV-2 vectors with the PDGF promoter alone and 4-fold over the CMV enhancer/promoter. Injection of AAV-2 vectors with the hybrid promoter into the rat striatum resulted in neuron-specific transgene expression, the level of which was about 10-fold higher than those provided by the two control AAV-2 expression cassettes at 4 weeks post-injection and maintained for at least 12 weeks. Gene expression in the substantia nigra through possible retrograde transport of the AAV-2 vectors injected into the striatum was not obvious. After direct injection of AAV-2 vectors into the substantia nigra, transgene expression driven by the hybrid promoter was observed specifically in dopaminergic neurons and its level was about 3 and 17 times higher than that provided by the PDGF promoter alone and the CMV enhancer/promoter, respectively.

CONCLUSIONS

Enhanced transgene capacity plus neuron-specificity of the AAV-2 vectors developed in this study might prove valuable for gene therapy of Parkinson's disease.

Conditional expression of a myocardium-specific transgene in zebrafish transgenic lines

To develop the first heart-specific tetracycline (Tet)-On system in zebrafish, we constructed plasmids in which the cardiac myosin light chain 2 promoter of zebrafish was used to drive the reverse Tet-controlled transactivator (rtTA) and the green fluorescent protein (GFP) reporter gene was preceded by an rtTA-responsive element. In the zebrafish fibroblast cell-line, rtTA-M2, one of rtTA's derivatives, demonstrated the highest increase in luciferase activity upon doxycycline (Dox) induction. We then generated two germ lines of transgenic zebrafish: line T03 was derived from microinjection of a plasmid containing rtTA-M2 and a plasmid containing a responsive reporter gene, whereas line T21 was derived from microinjection of a single dual plasmid. Results showed that line T21 was superior to line T03 in terms of greater GFP intensity after induction and with of minimal leakiness before induction. The photographic images of induced GFP in the heart of F2 larvae showed that the fluorescent level of GFP was dose-responsive. The level of GFP expressed in the F3 3 days postfertilization larvae that were treated with Dox for 1 hr decreased gradually after the withdrawal of the inducer; and the fluorescent signal disappeared after 5 days. The GFP induction and reduction were also tightly controlled by Dox in the F3 adult fish from line T21. This Tet-On system developed in zebrafish shows much promise for the study of the gene function in a specific tissue at the later developmental stage.

Proximal upstream region of zebrafish bone morphogenetic protein 4 promoter directs heart expression of green fluorescent protein

We examined the activity of the bone morphogenetic protein 4 (BMP4) promoter in zebrafish embryos via transient and stable transgenic expression analyses in order to obtain a better understanding of the regulation of BMP4 tissue-specific expression. Transient expression studies showed that the 9.0-kb BMP4 promoter/upstream region drove green fluorescent protein (GFP) expression mainly in the heart. Deletion analyses indicated the existence of multiple regulatory elements in the 7.5-kb BMP4 promoter/proximal upstream region. In addition, a coinjection experiment further demonstrated the 2.4-kb Bgl II-Hind III DNA region contains major positive regulatory elements. In addition, stable transgenic lines were established to further confirm the heart-specificity of this segment in BMP4 promoter. The results showed that GFP was mainly localized in the myocardium of developing ventricles of 48-hpf (hours postfertilization), 72-hpf, and 100-hpf transgenic F(1) embryos. Together, these results indicate that the 7.5-kb BMP4 promoter/proximal upstream region specifically contains regulatory elements for BMP4 expression in the heart, while regulatory elements for other endogenous BMP4-expressing tissues may reside in more distal regions and/or in introns.

Inverted terminal repeat sequences of adeno-associated virus enhance the antibody and CD8(+) responses to a HIV-1 p55Gag/LAMP DNA vaccine chimera

The immune responses to an HIV-1 p55Gag vaccine encoded as a DNA chimera with the lysosomal associated membrane protein-1 (LAMP) have been examined for the effect of the addition of the inverted terminal repeat (ITR) sequences of the adeno-associated virus (AAV) to the DNA plasmid construct, and of packaging the LAMP/gag gene as a recombinant AAV vector (rAAV). DNA plasmids encoding Gag and the LAMP/Gag protein chimera were constructed in two vectors, the pcDNA3.1 and a corresponding plasmid containing the ITR sequences (pITR) flanking the expression elements of the plasmid, and the pITR LAMP/gag DNA plasmid was encapsidated in the rAAV vector. Human 293 cells transfected in vitro with LAMP/gag plasmids either in pcDNA3.1 or pITR produced much Gag protein in cell extracts (1.6 and 2.2 ng of Gag/mg of protein, respectively). The immune responses of mice to immunization with these constructs were examined under three protocols: DNA prime/DNA boost, DNA prime/rAAV boost, and a single rAAV immunization. The results demonstrated that under DNA prime/DNA boost protocol, the "naked" DNA vaccines encoding the LAMP/gag chimera, either as pcDNA3.1 or pITR DNA plasmid constructs, elicited strong CD4(+) T cell responses. In contrast, significantly higher levels of CD8(+) and antibody responses were observed with the pITR-DNA constructs. Immunization with the rAAV vector under the DNA prime/rAAV boost protocol resulted in sustained T cell responses and a markedly increased antibody response, predominantly of the IgG(1) isotype resulting from the activation of the Th2 subset of CD4(+) T cells, that was sustained for at least 5 months after immunization.

Requirements for adeno-associated virus-derived non-viral vectors to achieve stable and site-specific integration of plasmid DNA in liver carcinoma cells

BACKGROUND AND AIMS

Adeno-associated virus (AAV) is the only known virus capable of site-specific genomic integration in human cells. Thus, AAV-based vectors may be an attractive option to achieve prolonged transgene expression in human cells. We therefore studied the minimal elements of gene therapy vectors necessary for stable integration and tested the effectiveness of this approach in hepatoma cells.

METHODS

Plasmids were constructed that contained a GFPneo fusion transgene with or without the AAV-inverted terminal repeats (ITRs). In addition, Rep protein was either encoded in CIS or supplied in TRANS by co-transfections. Stable clones were analyzed by Southern blotting for site-specific integration.

RESULTS

The ITRs alone conferred neither stable nor site-specific transgene integration. Expression of Rep protein in CIS or TRANS resulted in an increased frequency of integration regardless of the presence of ITRs. It was shown that in the absence of the ITRs, other Rep-binding site (RBS) like sequences such as the ColE1 sequence present in plasmid backbones can function as RBS. Site-specific integration was achieved in up to 26% of clones derived from hepatoma cells.

CONCLUSION

Both expression of Rep proteins and inclusion of a RBS are necessary for enhanced and stable integration of AAV-based non-viral vectors. A novel two-plasmid system capable of achieving stable and site-specific gene transfer in hepatoma cells is introduced.

Time-lapse microscopy of brain development

Zebrafish embryos represent an ideal vertebrate model organism for noninvasive intravital imaging because of their optical clarity, external embryogenesis, and fast development. Many different labeling techniques have been adopted from other model organisms or newly developed to address a wealth of different developmental questions directly inside the living organism. The parallel advancements in the field of optical imaging let us now observe dynamic processes at the cellular and subcellular resolution. Combined with the repertoire of available surgical and genetic manipulations, zebrafish embryos provide the powerful and almost unique possibility to observe the interplay of molecular signals with cellular, morphological, and behavioral changes directly within a living and developing vertebrate organism. A bright future for zebrafish is yet to come, let there be light.

Transposon-mediated enhancer trapping in medaka

We tested the Sleeping Beauty transposable element for its ability to efficiently insert transgenes into the genome of medaka (Oryzias latipes), an important model system for vertebrate development. We show that the SB transposon efficiently mediates integration of a reporter gene into the fish germ line. In pilot experiments, we established 174 transgenic lines with a transgenesis efficiency of 32%. Transgenes are stably transmitted to, and expressed in, subsequent generations. Interestingly, the transgenic lines show novel expression patterns with temporal and spatial specificity at a rate of 12% (21/174), likely due to both, enhancing and silencing position effects. Furthermore, promoter-dependent GFP expression in injected fish embryos is tightly correlated with germ line transmission, facilitating easy selection of founder fish. Thus, the SB transposon/transposase system provides a highly efficient tool for transgenesis in general and for the generation of novel reporter gene expression patterns in particular.

Transgenic zebrafish with fluorescent germ cell: a useful tool to visualize germ cell proliferation and juvenile hermaphroditism in vivo

Juvenile zebrafish are hermaphroditic; undifferentiated gonads first develop into ovary-like tissues, which then either become ovaries and produce oocytes (female) or degenerate and develop into testes (male). In order to fully capture the dynamic processes of germ cells' proliferation and juvenile hermaphroditism in zebrafish, we established transgenic lines TG(beta-actin:EGFP), harboring an enhanced green fluorescent protein (EGFP) gene driven by a medaka beta-actin promoter. In TG(beta-actin:EGFP), proliferating germ cells and female gonads strongly expressed EGFP, but fluorescence was only dimly detected in male gonads. Based on the fluorescent (+) or nonfluorescent (-) appearance of germ cells seen in living animals, three distinct groups were evident among TG(beta-actin:EGFP). Transgenics in ++ group (44%) were females, had fluorescent germ cells as juveniles, and female gonads continuously fluoresced throughout sexual maturation. Transgenics in +- (23%) and -- (33%) groups were males. Fluorescent germ cells were transiently detected in +- transgenics from 14 to 34 days postfertilization (dpf), but were not detected in -- transgenics throughout their life span. Histological analyses showed that 26-dpf-old transgenics in ++, +-, and -- groups all developed ovary-like tissues: Germ cells in -- group juveniles arrested at the gonocyte stage and accumulated low quantities of EGFP, while those in ++ group juveniles highly proliferated into diplotene to perinucleolar stages and accumulated high quantities of EGFP. In +- group juveniles, degenerating oocytes, gonocytes, and spermatogonia were coexistent in transiently fluorescent gonads. Therefore, the fluorescent appearance of gonads in this study was synchronous with the differentiation of ovary-like tissues. Thus, TG(beta-actin:EGFP) can be used to visualize germ cells' proliferation and juvenile hermaphroditism in living zebrafish for the first time.

Germ-line transmission of a myocardium-specific GFP transgene reveals critical regulatory elements in the cardiac myosin light chain 2 promoter of zebrafish

In response to the lack of a transgenic line of zebrafish labeled with heart-specific fluorescence in vivo to serve as a research model, we cloned a 1.6-kb polymerase chain reaction (PCR) -product containing the upstream sequence (-870 bp), exon 1 (39 bp), intron 1 (682 bp), and exon 2 (69 bp) of the zebrafish cardiac myosin light chain 2 gene, (cmlc2). A germ-line transmitted zebrafish possessing a green fluorescent heart was generated by injecting this PCR product fused with the green fluorescent protein (GFP) gene with ends consisting of inverted terminal repeats of an adeno-associated virus. Green fluorescence was intensively and specifically expressed in the myocardial cells located both around the heart chambers and the atrioventricular canal. Neither the epicardium nor the endocardium showed fluorescent signals. The GFP expression in the transgenic line faithfully recapitulated with the spatial and temporal expression of the endogenous cmlc2. Promoter analysis showed that the fragment consisting of nucleotides from -210 to 34 (-210/34) was sufficient to drive heart-specific expression, with a -210/-73 motif as a basal promoter and a -210/-174 motif as an element involved in suppressing ectopic (nonheart) expression. Interestingly, a germ-line of zebrafish whose GFP appeared ectopically in all muscle types (heart, skeletal, and smooth) was generated by injecting the fragment including a single nucleotide mutation from G to A at -119, evidence that A at -119 combined with neighboring nucleotides to create a consensus sequence for binding myocyte-specific enhancer factor-2.

Molecular structure and developmental expression of three muscle-type troponin T genes in zebrafish

Troponin T (Tnnt), a troponin component, interacts with tropomyosin and is crucial to the regulation of striated muscle contraction. To gain insight into the molecular evolution and developmental regulation of Tnnt gene (Tnnt) in lower vertebrates, zebrafish Tnnt1 (slow Tnnt), Tnnt2 (cardiac Tnnt), and Tnnt3b (fast Tnnt isoform b) were characterized. The polypeptides of zebrafish Tnnt1, Tnnt2, and Tnnt3b were conserved in the central tropomyosin- and C-terminal troponin I-binding domains. However, the N-terminal hypervariable regions were highly extended and rich in glutamic acid in polypeptides of Tnnt1 and Tnnt2, but not Tnnt3b. The Tnnt2 and Tnnt3b contain introns, whereas Tnnt1 is intron-free. During development, large to small, alternatively spliced variants were detected in Tnnt2, but not in Tnnt1 or Tnnt3. Whole-mount in situ hybridization showed zebrafish Tnnt1 and Tnnt2 are activated during early somitogenesis (10 hr postfertilization, hpf) and cardiogenesis (14 hpf), respectively, but Tnnt3b is not activated until middle somitogenesis (18 hpf). Tnnt2 and Tnnt3b expression was cardiac- and fast-muscle specific, but Tnnt1 was expressed in both slow and fast muscles. We propose that three, distinct, muscle-type Tnnt evolved after the divergence of fish and deuterostome invertebrates. In zebrafish, the developmental regulation of Tnnt during somitogenesis and cardiogenesis is more restricted and simpler than in tetrapods. These new findings may provide insight into the developmental regulation and molecular evolution of vertebrate Tnnt.

Windows into development: historic, current, and future perspectives on transgenic zebrafish

The recent explosion of transgenic zebrafish lines in the literature demonstrates the value of this model system for detailed in vivo analysis of gene regulation and morphogenetic movements. The optical clarity and rapid early development of zebrafish provides the ability to follow these events as they occur in live, developing embryos. This article will review the development of transgenic technology in zebrafish as well as the current and future uses of transgenic zebrafish to explore the dynamic environment of the developing vertebrate embryo.

Dynamics of transgene expression in human glioblastoma cells mediated by herpes simplex virus/adeno-associated virus amplicon vectors

One of the challenges in gene therapy is to ensure stable transgene expression at the site of disease with a high degree of accuracy and safety. In this paper, we examine both viral and cellular elements that may affect the level of transgene expression mediated by herpes simplex virus type 1 (HSV-1) adeno-associated virus (AAV) amplicon vectors. These elements include the AAV inverted terminal repeats (ITRs), the AAV Rep proteins, and the allelic status of 19q in human glioma cell lines. The latter is of particular interest because the AAV integration site (AAVS1) is located on the long arm of chromosome 19 and 30-40% of human glioblastoma tumors are reported to have loss of heterozygosity in this region of chromosome 19q. Fluorescence-activated cell-sorting analysis results indicate that inclusion of minimal or full-length AAV ITRs in HSV-1 amplicon vectors markedly increases the efficiency of transgene expression. On the other hand, insertion of the AAV rep gene decreases the level of transgene expression, apparently because of the cytotoxic effects of Rep proteins. Further, the levels of transgene expression appear to be independent of 19q allelic status or the number of endogenous AAVS1 sequences in the various glioma cell lines studied. Taken together, these data support employing AAV ITRs, in the context of HSV-1 amplicon vectors, to enhance short-term levels of transgene expression.