Spermatozoa, DNA binding and transgenic animals

Spontaneous uptake of exogenous DNA by goat spermatozoa and selection of donor bucks for sperm-mediated gene transfer

Sperm-mediated gene transfer (SMGT) has been long heralded as a faster and cheaper alternative to more commonly used methods of producing transgenic animals. In this study, the capra semen ejaculates were pooled together and then incubated in vitro with DIG-labeled DNA. The binding and internalizing rates were observed by the in situ hybridization methods. We also compared the standard sperm parameters and the efficiencies of interaction with exogenous DNA of 60 individuals to select donor bucks for SMGT. It was showed that labeled exogenous DNA was detected in different localizations in spermatozoa but genuine DNA uptake, in contrast to mere binding, seems to be limited to the postacrosomal region. The removal of seminal plasma increased significantly (P < 0.01) the capability in picking up exogenous DNA. Use of frozen-thawed semen (without cryoprotectant agents) and Triton X-100 treatment also increased significantly (P < 0.01) the DNA-binding capacity, but reduced the sperm viability. The binding rates (the proportion of labeled-DNA positive spermatozoa to all the spermatozoa counted) of 60 buck individuals were in the range of 3.08-73.39%, and the internalizing rates (the proportion of DNaseI-treated labeled-DNA positive spermatozoa to all the spermatozoa counted) were 4.83-70.00%. About 8.34% (5/60) bucks showed high binding, but low internalizing ability. Chi-square test showed that there was significant difference among the breeds (x(2) = 26.515, P < 0.01). Eight individual bucks that demonstrated high DNA uptake were selected for SMGT. It was demonstrated that the goat spermatozoa was capable of spontaneous uptake of exogenous DNA. Seminal fluid inhibits DNA uptake and that membrane disruption increases DNA binding but greatly diminishes uptake.

Transgene transmission in South American catfish (Rhamdia quelen) larvae by sperm-mediated gene transfer

The silver catfish (Rhamdia quelen) is an endemic American fish species. The sperm of each species has its own peculiarities and biological characteristics, which influence the success of mass DNA transfer methods. Our objective in this study was to evaluate different sperm-mediated gene transfer (SMGT) methods to obtain transgenic silver catfish. Different treatments for the incorporation of a foreign pEGFP plasmid group were used: (1) dehydrated/rehydrated (DR), (2) dehydrated/rehydrated/electroporated (DRE), (3) electroporated (E), (4) incubated with seminal plasma (INC); and (5) incubated in the absence of seminal plasma (INCSP). Sperm motility, time of activity duration (TAD), fertilization rate (FR), hatching rate (HR) and sperm morphology were also evaluated. The polymerase chain reaction (PCR) positivity rates for the presence of the transgene were: DRE 60%; DR 40%; E 25%; INC 5% and INCSP 25%. The rates of embryo EGFP expression were: DRE 63%; DR 44%; E 34%; INC 8% and INCSP 38%. The fertilization rate in the control and DRE treatments groups were higher than in the DR group, but the E,INC and INCSP treatment groups had the lowest rate. The hatching rates of the DRE, DR and control groups were higher than in the INCSP, INC and E treatment groups (P>0.05). There were no differences among the DRE and DR, E and DR, E and INCSP groups in expression and PCR positivity rates of enhanced green fluorescent protein (EGFP) in embryos. Scanning electron microscopy also did not show any change in sperm morphology among treatment groups. To the best of our knowledge, this is the first report on transgene transmission of exogenous DNA into silver catfish larvae through SMGT technology.

Animal transgenesis: an overview

Transgenic animals are extensively used to study in vivo gene function as well as to model human diseases. The technology for producing transgenic animals exists for a variety of vertebrate and invertebrate species. The mouse is the most utilized organism for research in neurodegenerative diseases. The most commonly used techniques for producing transgenic mice involves either the pronuclear injection of transgenes into fertilized oocytes or embryonic stem cell-mediated gene targeting. Embryonic stem cell technology has been most often used to produce null mutants (gene knockouts) but may also be used to introduce subtle genetic modifications down to the level of making single nucleotide changes in endogenous mouse genes. Methods are also available for inducing conditional gene knockouts as well as inducible control of transgene expression. Here, we review the main strategies for introducing genetic modifications into the mouse, as well as in other vertebrate and invertebrate species. We also review a number of recent methodologies for the production of transgenic animals including retrovirus-mediated gene transfer, RNAi-mediated gene knockdown and somatic cell mutagenesis combined with nuclear transfer, methods that may be more broadly applicable to species where both pronuclear injection and ES cell technology have proven less practical.

A unique method to produce transgenic embryos in ovine, porcine, feline, bovine and equine species

Transgenesis is an essential tool in many biotechnological applications. Intracytoplasmic sperm injection (ICSI)-mediated gene transfer is a powerful technique to obtain transgenic pups; however, most domestic animal embryos do not develop properly after ICSI. An additional step in the protocol, namely assistance by haploid chemical activation, permits the use of ICSI-mediated gene transfer to generate transgenic preimplantation embryos in a wide range of domestic species, including ovine, porcine, feline, equine and bovine. In the present study, spermatozoa from five species were coincubated with pCX-EGFP plasmid and injected into metaphase II oocytes. The chemical activation protocol consisted of ionomycin plus 6-dimethylaminopurine. We detected high proportions of fluorescent EGFP embryos for all five species (23–60%), but with a high frequency of mosaic expression (range 60–85%). To our knowledge, this is the first study to produce exogenous DNA expression in feline and equine embryos. Chemical activation reduces the lag phase of egfp expression in ovine embryos. Our results show that this unique method could be used to obtain ovine, porcine, feline, bovine and equine transgenic preimplantation embryos.

Is there a relationship between the chromatin status and DNA fragmentation of boar spermatozoa following freezing-thawing?

In this study a radioisotope method, which is based on the quantitative measurements of tritiated-labeled actinomycin D ((3)H-AMD) incorporation into the sperm nuclei ((3)H-AMD incorporation assay), was used to assess the chromatin status of frozen-thawed boar spermatozoa. This study also tested the hypothesis that frozen-thawed spermatozoa with altered chromatin were susceptible to DNA fragmentation measured with the neutral comet assay (NCA). Boar semen was diluted in lactose-hen egg yolk-glycerol extender (L-HEY) or lactose ostrich egg yolk lipoprotein fractions-glycerol extender (L-LPFo), packaged into aluminum tubes or plastic straws and frozen in a controlled programmable freezer. In Experiment 1, the chromatin status and DNA fragmentation were measured in fresh and frozen-thawed spermatozoa from the same ejaculates. There was a significant increase in sperm chromatin destabilization and DNA fragmentation in frozen-thawed semen as compared with fresh semen. The proportions of spermatozoa labeled with (3)H-AMD were concurrent with elevated levels of sperm DNA fragmentation in K-3 extender, without cryoprotective substances, compared with L-HEY or L-LPFo extender. Regression analysis revealed that the results of the (3)H-AMD incorporation assay and NCA for frozen-thawed spermatozoa were correlated. Boars differed significantly in terms of post-thaw sperm DNA damage. In Experiment 2, the susceptibility of sperm chromatin to decondensation was assessed using a low concentration of heparin. Treatment of frozen-thawed spermatozoa with heparin revealed enhanced (3)H-AMD binding, suggesting nuclear chromatin decondensation. The deterioration in post-thaw sperm viability, such as motility, mitochondrial function and plasma membrane integrity, was concurrent with increased chromatin instability and DNA fragmentation. This is the first report to show that freezing-thawing procedure facilitated destabilization in the chromatin structure of boar spermatozoa, resulting in an unstable DNA that was highly susceptible to fragmentation.

Transgenic farm animals: an update

The first transgenic livestock species were reported in 1985. Since then microinjection of foreign DNA into pronuclei of zygotes has been the method of choice. It is now being replaced by more efficient protocols based on somatic nuclear transfer that also permit targeted genetic modifications. Lentiviral vectors and small interfering ribonucleic acid (siRNA) technology are also becoming important tools for transgenesis. In 2006 the European Medicines Agency (EMEA) gave green light for the commercialistion of the first recombinant protein produced in the milk of transgenic animals. Recombinant antithrombin III will be launched as ATryn for prophylactic treatment of patients with congenital antithrombin deficiency. This important milestone will boost the research activities in farm animal transgenesis. Recent developments in transgenic techniques of farm animals are discussed in this review.

Application of transgenesis in livestock for agriculture and biomedicine

Microinjection of foreign DNA into pronuclei of a fertilized oocyte has predominantly been used for the generation of transgenic livestock. This technology works reliably, but is inefficient and results in random integration and variable expression patterns in the transgenic offspring. Nevertheless, remarkable achievements have been made with this technology. By targeting expression to the mammary gland, numerous heterologous recombinant human proteins have been produced in large amounts which could be purified from milk of transgenic goats, sheep, cattle and rabbit. Products such as human anti-thrombin III, alpha-anti-trypsin and tissue plasminogen activator are currently in advanced clinical trials and are expected to be on the market within the next few years. Transgenic pigs that express human complement regulating proteins have been tested in their ability to serve as donors in human organ transplantation (i.e. xenotransplantation). In vitro and in vivo data convincingly show that the hyperacute rejection response can be overcome in a clinically acceptable manner by successful employing this strategy. It is anticipated that transgenic pigs will be available as donors for functional xenografts within a few years. Similarly, pigs may serve as donors for a variety of xenogenic cells and tissues. The recent developments in nuclear transfer and its merger with the growing genomic data allow a targeted and regulatable transgenic production. Systems for efficient homologous recombination in somatic cells are being developed and the adaptation of sophisticated molecular tools, already explored in mice, for transgenic livestock production is underway. The availability of these technologies are essential to maintain "genetic security" and to ensure absence of unwanted side effects.

Effective generation of transgenic pigs and mice by linker based sperm-mediated gene transfer

BACKGROUND

Transgenic animals have become valuable tools for both research and applied purposes. The current method of gene transfer, microinjection, which is widely used in transgenic mouse production, has only had limited success in producing transgenic animals of larger or higher species. Here, we report a linker based sperm-mediated gene transfer method (LB-SMGT) that greatly improves the production efficiency of large transgenic animals.

RESULTS

The linker protein, a monoclonal antibody (mAb C), is reactive to a surface antigen on sperm of all tested species including pig, mouse, chicken, cow, goat, sheep, and human. mAb C is a basic protein that binds to DNA through ionic interaction allowing exogenous DNA to be linked specifically to sperm. After fertilization of the egg, the DNA is shown to be successfully integrated into the genome of viable pig and mouse offspring with germ-line transfer to the F1 generation at a highly efficient rate: 37.5% of pigs and 33% of mice. The integration is demonstrated again by FISH analysis and F2 transmission in pigs. Furthermore, expression of the transgene is demonstrated in 61% (35/57) of transgenic pigs (F0 generation).

CONCLUSIONS

Our data suggests that LB-SMGT could be used to generate transgenic animals efficiently in many different species.

Transgenic livestock: premises and promises

Microinjection of DNA constructs into pronuclei of zygotes has been the method of choice for the generation of transgenic livestock. However, this procedure is characterized by low efficiency (1-4% transgenic offspring), random integration and variable expression of the transgene as well as a considerable proportion of mosaicism. Furthermore, it is extremely time consuming and costly. As a consequence, commercial application has focused on the production of recombinant proteins in the mammary gland of transgenic animals and xenotransplantation, e.g. the use of porcine organs in human organ transplantation. In addition, transgenic pigs carrying a modified porcine growth hormone (hMt-pGH) construct show significant improvements in economically important traits without adverse side effects of a GH overproduction. Improvements of transgenic technology will likely come from the generation of appropriate cell lines suitable for transfection or even homologous recombination and their subsequent use in nuclear transfer. Additionally, in the mouse a number of sophisticated molecular tools have been developed that allow precise modifications of the genome. These include the application of artificial chromosomes from yeast (YAC) or bacteria (BAC) for position-independent and copy-number-dependent expression of a transgene, the Tet-system (tetracycline inducible) for a tight temporal control of transgene expression, as well as conditional mutagenesis by applying site-specific DNA recombinases (e.g. Cre, FLP). The successful adaptation of these molecular tools to livestock will enable the fulfillment of many of the promises originally thought to be achievable when transgenic livestock were first reported.

Electroporation of salmon sperm for gene transfer: efficiency, reliability, and fate of transgene

Uptake of exogenous DNA by electroporated salmon sperm for gene transfer is being investigated. Our studies show that electroporated salmon sperm cells were more efficient and more reliable than untreated sperm in picking up exogenous DNA and subsequently transferring the DNA into salmon embryos. Indirect evidence suggest that some of the exogenous DNA was internalized in the sperm nuclei. The taken up DNA retained its integrity as demonstrated by PCR. The foreign DNA was detected in 15-month-old fish, and had a mosaic pattern of distribution. Integration of the foreign DNA occurred infrequently, and the expression of the foreign genes was poor. The potential of sperm-mediated gene transfer as a routine protocol for mass gene transfer in salmon will be dependent on the improvement of integration and expression of the foreign gene.

Sperm-mediated transgenesis

The idea of using a sperm cell for introducing exogenous DNA into an oocyte at the time of fertilization would be brilliant if only we were sure that it can be done. Since 1989, contradictory reports have appeared in the literature and, at present, no consensus has been reached on the topic. Given the potential impact of this method for the generation of transgenic animals, for both mammalian and non-mammalian species, this review summarizes what has been achieved in this field. While some aspects, such as the binding of DNA molecules to spermatozoa, have now a solid experimental base, others, such as the generation of real transgenic individuals, are still based on disputed evidence. A critical analysis of the most relevant data will be presented in order to provide the tools for an objective evaluation of the efficiency of this method.