Gene transfer in the chicken by sperm-mediated methods

Genetically engineered birds; pre-CRISPR and CRISPR era

Generating biopharmaceuticals in genetically engineered bioreactors continues to reign supreme. Hence, genetically engineered birds have attracted considerable attention from the biopharmaceutical industry. Fairly recent genome engineering methods have made genome manipulation an easy and affordable task. In this review, we first provide a broad overview of the approaches and main impediments ahead of generating efficient and reliable genetically engineered birds, and various factors that affect the fate of a transgene. This section provides an essential background for the rest of the review, in which we discuss and compare different genome manipulation methods in the pre-CRISPR and CRISPR era in the field of avian genome engineering.

The effect of methyl-beta-cyclodextrin on DNA absorption and quality of posttransfected sperm

Sperm can be selected as a natural vector for the production of transgenic animals. Methyl-beta-cyclodextrin (MBCD) removes cholesterol from the phospholipid membrane of sperm and improves the efficiency of DNA uptake by sperm. In experiment 1, fresh sperm was treated with various concentrations of MBCD. The direct effects of MBCD on sperm parameters were monitored. In experiment 2, different concentrations of MBCD (0, 1, 2, and 4 mmol) were assessed for the transfection of genetically exogenous construction to rooster sperm. Washed semen was divided into 5 equal groups for the incubation and transfection with a pcDNA3.1+/hG-CSF vector (exogenous DNA) as follows; Treatment I-Control (washed semen without DNA); Treatment II-Control (washed semen with DNA); Treatment III-(washed semen incubated with DNA and 1 mmol MBCD); Treatment IV-(washed semen incubated with DNA and 2 mmol MBCD); and Treatment V-(washed semen incubated with DNA and 4 mmol MBCD). We demonstrated that rooster spermatozoa spontaneously can uptake exogenous DNA; this was assessed using exogenous DNA amplification (sperm genomic DNA used as a template for PCR reaction) after DNase I treatment. In addition, total motility (TM), progressive motility (PM), velocity parameters [curvilinear velocity (VCL), straight linear velocity (VSL), sperm track straightness (STR), linearity (LIN)], membrane integrity (MI), and membrane functionality were posttransfectionally evaluated. The concentrations of 1 and 2 mmol MBCD significantly (P < 0.05) improved the motion characteristics and membrane integrity of fresh sperm. The presence of hG-CSF in rooster sperm was detected by PCR and based on sperm analyses MBCD (1 mmol) improved the percentage of motility (98.9 ± 0.81), membrane functionality (64 ± 1.64), and MI (76.2 ± 1.65) after transfection when compared with the other groups (P < 0.05). For the production of transgenic chicken, hens were inseminated (AI) by transfected sperm treated with 1 and 0 mmol MBCD. A PCR analysis of the blood samples and dead embryo tissues of chicks did not reveal the transgene integration.

Transfection efficiency and cytotoxicity of polyethyleneimine-coated magnetic iron oxide nanoparticles in rooster sperm cells

Since the morphology of the rooster spermatozoa is different to other animal spermatozoa, the aim of the current study was to investigate the transfection efficiency and cytotoxicity of polyethyleneimine (PEI) coated magnetic iron oxide nanoparticles (MION) on these cells. Liposome/nucleic acid (NA) complexes and PEI-coated MION linked to the labeled oligonucleotides were used. Viability and percentage of exogenous nucleic acid uptake of spermatozoa were measured by flow cytometry analyses. The results showed a significant increase in exogenous nucleic acid uptake by rooster spermatozoa (P < 0.001) when treated with MION-NA complexes in comparison to liposome/NA. There were no significant differences between efficiency of lipid-based transfection agent and MION (P > 0.05) during short incubation period. MION with or without magnetic field, did not show significant cytotoxicity while the lipid-based transfection agent significantly decreased (P < 0.05) the viability of rooster spermatozoa. Results of this study showed that magnetofection and lipofection were both effective methods which increased exogenous nucleic acid uptake by rooster spermatozoa. However, the magnetofection method was more successful in maintaining the cell's survival than lipofection method.

Dimethyl sulphoxide and electrolyte-free medium improve exogenous DNA uptake in mouse sperm and subsequently gene expression in the embryo

SummaryOne of the methods to generate transgenic animals is called sperm-mediated gene transfer (SMGT). Mature sperm cells can take up exogenous DNA molecules intrinsically and transfer them into the oocyte during fertilization. This study assessed the effect of dimethyl sulfoxide (DMSO) and electrolyte-free medium (EFM) on DNA uptake (EGFP-N1plasmid) in mouse sperm. Sperms cells cultured in human tubular fluid (HTF) without any treatment were considered as the control group. Sperms cells that were incubated in EFM and HTF with DNA/DMSO at 4°C were classified into EFM and HTF groups. Sperm motility and viability were assessed following treatment. In vitro fertilization (IVF) with sperm in all groups was performed. Fertilization, embryo development and GFP-positive blastocyst rates were analyzed and compared. The result showed that sperm motility and viability in EFM were better than those in the HTF group. The rate of development to reach the blastocyst stage and GFP-positive blastocysts was significantly higher in the EFM group compared with the HTF group (P<0.05). Our data demonstrate that sperm stored in the EFM group can improve the efficiency of SMGT for the generation of GFP-positive blastocysts.

Primordial germ cell-mediated transgenesis and genome editing in birds

Transgenesis and genome editing in birds are based on a unique germline transmission system using primordial germ cells (PGCs), which is quite different from the mammalian transgenic and genome editing system. PGCs are progenitor cells of gametes that can deliver genetic information to the next generation. Since avian PGCs were first discovered in nineteenth century, there have been numerous efforts to reveal their origin, specification, and unique migration pattern, and to improve germline transmission efficiency. Recent advances in the isolation and in vitro culture of avian PGCs with genetic manipulation and genome editing tools enable the development of valuable avian models that were unavailable before. However, many challenges remain in the production of transgenic and genome-edited birds, including the precise control of germline transmission, introduction of exogenous genes, and genome editing in PGCs. Therefore, establishing reliable germline-competent PGCs and applying precise genome editing systems are critical current issues in the production of avian models. Here, we introduce a historical overview of avian PGCs and their application, including improved techniques and methodologies in the production of transgenic and genome-edited birds, and we discuss the future potential applications of transgenic and genome-edited birds to provide opportunities and benefits for humans.

Is passive transmission of non-viral vectors through artificial insemination of sperm-DNA mixtures sufficient for chicken transgenesis?

DNA uptake in the post-acrosomal region of the spermatozoa takes place exclusively in immotile spermatozoa that are naturally unable to fertilize eggs. The present study aimed to assess whether passive transmission of non-viral vectors to the surrounding areas of chicken embryos could be an alternate mechanism in chicken sperm-mediated gene transfer. First, the presence of nucleases in rooster seminal plasma was evaluated. Semen ejaculates from five roosters were centrifuged and the supernatant was incubated with pBL2 for 1 h. A robust nuclease cocktail was detected in the rooster semen. To overcome these nucleases, plasmid-TransIT combinations were incubated with semen for 1 h. Incubation of exogenous DNA in the lipoplex structure could considerably bypass the semen nuclease effect. Then, intravaginal insemination of 1 × 10(9) sperm mixed with lipoplexes (40 µg pBL2:40 µl TransIT) was carried out in 15 virgin hens. Neither the epithelial tissue from the inseminated female reproductive tracts nor the produced embryos following artificial insemination showed the transgene. To remove any bias in the transgene transmission possibility, the plasmid-TransIT admixture was directly injected in close vicinity of the embryos in newly laid eggs. Nonetheless, none of the produced fetuses or chicks carried the transgene. In conclusion, the results of the present study revealed a nuclease admixture in rooster seminal plasma, and passive/active transmission of the non-viral vector into close vicinity of the chicken embryo was inefficient for producing transgenic chicks.

The impact of exogenous DNA on the structure of sperm of olive flounder (Paralichthys olivaceus)

Sperm-mediated gene transfer (SMGT) is a promising transgenic technology that relies on the capability of sperm to internalize exogenous DNA. In marine fish, however, the interaction between sperm and exogenous DNA appears to be deficient. Here, we demonstrated significant DNase activity in the seminal plasma of the olive flounder. When incubated with naked-DNA, the spermatozoa lost their structural integrity, including the head, mitochondria and flagellum, in an incubation time-dependent manner. However, internalization of a liposome-DNA complex resulted in the structural integrity of the spermatozoa being maintained, even when using incubation times of up to 50min. We concluded that in the olive flounder, SMGT is possible by integrating liposome-DNA complexes, rather than naked-DNA alone, into the sperm. In brief, removal of the seminal plasma and packaging the exogenous DNA were necessary for successful SMGT in the olive flounder.

[Generation of goat H-FABP overexpression transgenic mice by testicular injection]

To explore the possibility of transgenic animals by testicular injection, the goat heart-type fatty acid binding protein (H-FABP) expression vector pEGFP-H-FABP was injected into the testis of 6 mice randomly by liposome mediated transfection. By detection of testis slice, sperm fluorescence and sperm DNA PCR, the exogenous gene was expressed in the parental mice. The exogenous gene was expressed at different levels in both the F1 generation mice gave birthed by treated male mice and normal female mice and the F2 generation mice generated by mating F1 could be detected that the exogenous gene expressed at different levels with the positive rates of 4% and 30.23%, respectively. The results suggested that testicu-lar injection, as an effective method to generate transgenic animal, could realize the stable integration of exogenous gene. The amelioration and maturity of testicular injection provides theoretical and practical significance in generation of trans-genic animals and even in the animal trait improvement and breeding.

Exogenous DNA internalisation by sperm cells is improved by combining lipofection and restriction enzyme mediated integration

1. Three types of exogenous DNA inserts, i.e. complete linearised pVIVO2-GFP/LacZ vector (9620 bp), the LacZ gene (5317 bp) and the GFP gene (2152 bp) were used to transfect chicken spermatozoa through simple incubation of sperm cells with insert. 2. PCR assay, Dot Blot hybridisation and Southern hybridisation showed the successful internalisation of exogenous DNA by chicken sperm cells. 3. Lipofection and Restriction Enzyme Mediated Integration (REMI) were used to improve the rate of internalisation of exogenous DNA by sperm cells. 4. Results from dot blot as well as Southern hybridisation were semi-quantified and improved exogenous DNA uptake by sperm cells through lipofection and REMI. Stronger signals were observed from hybridisation of LacZ as well as GFP specific probe with the DNA from lipofected exogenous DNA transfected sperm DNA in comparison with those transfected with nude exogenous DNA.

Sperm and testis mediated DNA transfer as a means of gene therapy

Assisted reproductive technologies (ART) have revolutionized the treatment of infertility. However, many types of infertility may still not be addressable by ART. With recent successes in identifying many of the genetic factors responsible for male infertility and the future prospect of whole individual human genome sequencing to identify disease causing genes, the possible use of gene therapy for treating infertility deserves serious consideration. Gene therapy in the sperm and testis offers both opportunities and obstacles. The opportunities stem from the fact that numerous different approaches have been developed for introducing transgenes into the sperm and testis, mainly because of the interest in using sperm mediated gene transfer and testis mediated gene transfer as ways to generate transgenic animals. The obstacles arise from the fact that it may be very difficult to carry out gene therapy of the testis and sperm without also affecting the germline. Here we consider new developments in both sperm and testis mediated gene transfer, including the use of viral vectors, as well as the technical and ethical challenges facing those who would seek to use these approaches for gene therapy as a way to treat male infertility.

In vivoGene Transfer into Testis and Sperm: Developments and Future Application

Despite significant advances in the treatment of infertility via assisted reproductive technology (ART), the underlying causes of idiopathic male infertility still remain unclear. Accumulating evidence suggests that disorders associated with testicular gene expression may play an important role in male infertility. To be able to fully study the molecular mechanisms underlying spermatogenesis and fertilization, it is necessary to manipulate gene expression in male germ cells. Since there is still no reliable method of recapitulating spermatogenesis culture, the development of alternative transgenic approaches is paramount in the study of gene function in testis and sperm. Established methods of creating transgenic animals rely heavily upon injection of DNA into the pronucleus or the injection of transfected embryonic stem cells into blastocysts to form chimeras. Despite the success of these two approaches for making transgenic and knockout animals, concerns remain over costs and the efficiency of transgene integration. Consequently, efforts are in hand to evaluate alternative methodologies. At present, there is much interest in developing approaches that utilize spermatozoa as vectors for gene transfer. These approaches, including testis mediated gene transfer (TMGT) and sperm mediated gene transfer (SMGT), have great potential as tools for infertility research and in the creation of transgenic animals. The aim of this short review is to briefly describe developments in this field and discuss how these gene transfer methods might be used effectively in future research and clinical arenas.

Progress in gene transfer by germ cells in mammals

Use of germ cells as vectors for transgenesis in mammals has been well developed and offers exciting prospects for experimental and applied biology, agricultural and medical sciences. Such approach is referred to as either male germ cell mediated gene transfer (MGCMGT) or female germ cell mediated gene transfer (FGCMGT) technique. Sperm-mediated gene transfer (SMGT), including its alternative method, testis-mediated gene transfer (TMGT), becomes an established and reliable method for transgenesis. They have been extensively used for producing transgenic animals. The newly developed approach of FGCMGT, ovary-mediated gene transfer (OMGT) is also a novel and useful tool for efficient transgenesis. This review highlights an overview of the recent progress in germ cell mediated gene transfer techniques, methods developed and mechanisms of nucleic acid uptake by germ cells.

Use of sperm plasmid DNA lipofection combined with REMI (restriction enzyme-mediated insertion) for production of transgenic chickens expressing eGFP (enhanced green fluorescent protein) or human follicle-stimulating hormone

Linearized p-eGFP (plasmid-enhanced green fluorescent protein) or p-hFSH (plasmid human FSH) sequences with the corresponding restriction enzyme were lipofected into sperm genomic DNA. Sperm transfected with p-eGFP were used for artificial insemination in hens, and in 17 out of 19 of the resultant chicks, the exogenous DNA was detected in their lymphocytes as determined by PCR and expressed in tissues as determined by (a) PCR, (b) specific emission of green fluorescence by the eGFP, and (c) Southern blot analysis. A complete homology was found between the Aequorea Victoria eGFP DNA and a 313-bp PCR product of extracted DNA from chick blood cells. Following insemination with sperm lipofected with p-hFSH, transgenic offspring were obtained for two generations as determined by detection of the transgene for human FSH (PCR) and expression of the gene (RT-PCR and quantitative real-time PCR) and the presence of the protein in blood (radioimmunoassay). Data demonstrate that lipofection of plasmid DNA with restriction enzyme is a highly efficient method for the production of transfected sperm to produce transgenic offspring by direct artificial insemination.

Development of Transgenic Chickens Expressing Human Parathormone Under the Control of a Ubiquitous Promoter by Using a Retrovirus Vector System

Transgenic chickens, ubiquitously expressing a human protein, could be a very useful model system for studying the role of human proteins in embryonic development as well as for efficiently producing pharmaceutical drugs as bioreactors. Human parathormone (hPTH) secreted from parathyroid glands plays a significant role in calcium homeostasis and is an important therapeutic agent for the treatment of osteoporosis in humans. Here, by using a robust replication-defective Moloney murine leukemia virus-based retrovirus vector encapsidated with vesicular stomatitis virus G glycoprotein, we generated transgenic chickens expressing hPTH under the control of a ubiquitous Rous sarcoma virus promoter. The recombinant retrovirus was injected into the subgerminal cavity of freshly laid eggs at the blastodermal stage. After 21 d of incubation, 42 chicks hatched from 473 retrovirus-injected eggs. All 42 living chicks were found to express the vector-encoded hPTH gene in diverse organs, as revealed by PCR and reverse transcription-PCR analysis by using primer pairs specific for hPTH. Four days after hatching, 6 chicks died and 14 chicks showed phenotypic deformities. At 18 wk of age, only 3 G(0) chickens survived. They also released the hPTH hormone in their blood and transmitted the hPTH gene to G(1) embryos. However, although the embryos were alive at d 18 of incubation, none hatched. An electrochemiluminescence immunoassay further showed that the hPTH expression level was markedly elevated in mammalian cells infected by the retrovirus vector. Thus, we demonstrated that transgenic chickens, expressing a human protein under the control of a ubiquitous promoter, not only could be an efficient bioreactor for the production of pharmaceutical drugs, but also could be useful for studies on the role of human proteins in embryonic development. To our knowledge, this is the first report on the production of a human protein (hPTH) in transgenic chickens under the control of a ubiquitous promoter by using a replication-defective Moloney murine leukemia virus-based retrovirus vector system.

Quantification of DNA binding, uptake, transmission and expression in bovine sperm mediated gene transfer by RT-PCR: Effect of transfection reagent and DNA architecture

In this study, we compared the transfection effectiveness of liposomes with the new transfection reagent FuGene 6 in bovine sperm mediated gene transfer (SMGT). Furthermore, we examined whether plasmid architecture affects overall efficiency by comparing two plasmids, one of them bearing an additional murine nontranscribed spacer (nts) insert (CMV-INF-tau-IRES-EGFP versus CMV-INF-tau-IRES-EGFP-nts). To accomplish that, we quantified plasmid binding and uptake to spermatozoon and transfer and expression of foreign DNA into embryos by real time PCR. More plasmids bound to spermatozoa when treated with FuGene 6 than with liposome treatment (p<0.05) reaching highest counts in plasmids bearing the nts sequence (p<0.05). Mean number of plasmids taken up was significantly (p<0.05) affected by transfection strategy (1-3 versus 15-81 versus 120-162) with plasmids bearing the nts sequence being 2-8 fold more effective (p<0.05). Culture of SMGT derived embryos up to day 9 did not result in any difference in terms of cleavage rate (64.2-84.2%) and development to blastocyst stage (18.8-26.3%) between different groups. Insert of the nts fragment significantly (p<0.05) affected mean number of transmitted plasmids to 4-cell stage embryos (44 versus 7) and relative INF-tau mRNA expression level in day 9 blastocysts (7-8 fold). However, only six blastocysts (3.6%) exhibited green fluorescence indicating low EGFP protein production. In conclusion, we were able to show effectiveness of sperm mediated gene transfer is significantly affected by choice of transfection reagent and by plasmid architecture.

Direct gene delivery to murine testis as a possible means of transfection of mature sperm and epithelial cells lining epididymal ducts

The use of a sperm cell to introduce exogenous DNA into an oocyte at the time of fertilization is of interest for the simple production of transgenic mice, and is now called 'sperm-mediated gene transfer (SMGT)'. In vivo transfection of sperm cells has been developed as an alternative method for SMGT and can be carried out by direct gene delivery into an interstitial space in a testis (now called 'testis-mediated gene transfer [TMGT]'), into the vas deferens, or into seminiferous tubules. This review summarizes what has been achieved in the field of in vivo gene transfer using sperm cells. (Reprod Med Biol 2006; 5: 1-7).

Transgenic animals: current and alternative strategies

Transgenic animal technology is one of the most fascinating technologies developed in the last two decades. It allows us to address questions in life sciences that no other methods have achieved. The impact on biomedical and biological research, as well as commercial interests are overwhelming. The questions accompanying this fast growing technology and its diversified applications attract the attention from a variety of entities. Still, one of the most fundamental problems remaining is the search for an efficient and reliable gene delivery system for creating transgenic animals. The traditional method of pronuclear microinjection has displayed great variability in success among species. While an acceptable efficiency in the production of transgenic mice has been attained, the relative low efficiency (<1%) in creating transgenic livestock has become one of the barriers for its application. In the past decades, improvements in producing transgenic livestock have made a slow progression, however, the recent advancement in cloning technology and the ability to create transgenic livestock in a highly efficient manner, have opened the gate to a new era in transgenic technology. Discoveries of new gene delivery systems have created an enthusiastic atmosphere that has made this technology so unique. This review focuses on gene delivery strategies as well as various approaches that may assist the advancement of transgenic efficiency in large animals.

Sperm-mediated gene transfer: applications and implications

Recent developments in studies of sperm-mediated gene transfer (SMGT) now provide solid ground for the notion that sperm cells can act as vectors for exogenous genetic sequences. A substantive body of evidence indicates that SMGT is potentially useable in animal transgenesis, but also suggests that the final fate of the exogenous sequences transferred by sperm is not always predictable. The analysis of SMGT-derived offspring has shown the existence of integrated foreign sequences in some cases, while in others stable modifications of the genome are difficult to detect. The appearance of SMGT-derived modified offspring on the one hand and, on the other hand, the rarity of actual modification of the genome, suggest inheritance as extrachromosomal structures. Several specific factors have been identified that mediate distinct steps in SMGT. Among those, a prominent role is played by an endogenous reverse transcriptase of retrotransposon origin. Mature spermatozoa are naturally protected against the intrusion of foreign nucleic acid molecules; however, particular environmental conditions, such as those occurring during human assisted reproduction, can abolish this protection. The possibility that sperm cells under these conditions carry genetic sequences affecting the integrity or identity of the host genome should be critically considered. These considerations further suggest the possibility that SMGT events may occasionally take place in nature, with profound implications for evolutionary processes.

Retrovirus-mediated gene transfer and expression of EGFP in chicken

Here, we successfully demonstrate expression of the EGFP (enhanced green fluorescence protein) gene in chickens using replication-defective MLV (murine leukemia virus)-based retrovirus vectors encapsidated with VSV-G (vesicular stomatitis virus G glycoprotein). The recombinant retrovirus was injected beneath the blastoderm of non-incubated chicken embryos (stage X). After 12 days incubation, all of the eight living embryos assayed were found to express this vector-encoded EGFP gene, which was under the control of the RSV (Rous Sarcoma Virus) promoter, in diverse organ tissues, including head, beak, neck, wing, hock, tail, toes, heart, amnion, and yolk sac. Surprisingly, despite the presumed cytotoxicity of EGFP, some embryos hatched and survived and these had prominent green fluorescent spots, both in internal organs and externally.

Development of transgenic chickens expressing enhanced green fluorescent protein

In this work we demonstrated the successful production of transgenic chickens expressing the enhanced green fluorescence protein (EGFP) gene. Replication-defective recombinant retroviruses produced from vesicular stomatitis virus G glycoprotein pseudotyped retrovirus vector system were injected beneath the blastoderm of non-incubated chicken embryos (stage X). From 129 injected eggs, 13 chicks hatched after 21 days of incubation. All hatched chicks were found to express vector-encoded EGFP gene, which was under the control of the Rous sarcoma virus promoter and boosted post-transcriptionally by woodchuck hepatitis virus post-transcriptional regulatory element sequence. Green fluorescent signals, indicative of the EGFP gene expression, were detected in various body parts, including head, limb, eye, toe, and several internal organs. Genomic incorporation of the transgene was also proven by Southern blot assay. Our results show the exceptional versatile effectiveness of the EGFP gene as a marker in the gene expression-related studies which therefore would be very helpful in establishing a useful transgenic chicken model system for studies on embryo development and for efficient production of transgenic chickens as bioreactors.

Association of Rabbit Sperm Cells with Exogenous DNA

The ability of rabbit spermatozoa to bind exogenous DNA during sperm-mediated gene transfer (SMGT) was tested in our study. Fresh collected semen, or fully capacitated sperm cells, was co-cultured with plasmid DNA labeled with tetramethyrodamine-6-dUTP. Fluorescent spermatozoa were counted before and after DNaseI treatment. Results showed that fluorescent-labeled plasmid DNA could be taken up by capacitated rabbit sperm cells. 66% spermatozoa carried exogenous DNA in the presence of lipofectin. Bovine serum albumin could block this process effectively. Associated DNA was mainly located in the posterior area of the sperm head. In order to verify whether exogenous DNA was carried into the embryo and expressed in the offspring, further SMGT experiments were carried out using the pHM-CR plasmid which contains LacZ and Neomycin genes. beta-galactosidase was expressed in different stages of embryo development and in the tissues of young rabbit as detected by using X-gal staining. Large portion of embryos survived under the selection pressure in G418 containing medium, after SMGT. Transgene integration was further verified by PCR analysis. These results confirmed the ability of rabbit sperm cells to carry transgene into the embryo during in vitro fertilization.

Expression of porcine growth hormone gene in transgenic rabbits as reported by green fluorescent protein

Sperm-mediated gene transfer was used to produce transgenic rabbits that expressed the porcine growth hormone gene under the control of a metallothionein promoter. The gene that encodes the selectable marker green fluorescent protein (GFP) was inserted downstream of the transgene. After lipofectin-mediated gene transfer into sperm cells and after subsequent in vitro fertilization using the transfected sperm cells, 32% of the cultured blastocysts exhibited bright green fluorescence when stimulated with blue light. Of the 74 adult rabbits and five fetal rabbits (age, gestational day 15), 2 fetuses and 29 rabbits were GFP-positive as indicated by PCR analysis. Southern blot analysis of their genomic DNA showed that 13 of 21 GFP-positive rabbits were transgenic. GFP expression was observed in different tissues of transgenic rabbits and the growth rate of four GFP-positive rabbits was greater than that of controls. PCR analysis showed that one of six F1 offspring was transgenic. These results suggest that lipofectin-mediated gene transfer into sperm cells can be used to efficiently produce transgenic rabbits.

Regulation of foreign DNA uptake by mouse spermatozoa

We have studied some features of DNA uptake in both mature and immature mammalian spermatozoa. Mature sperm collected from the cauda epididymis are able to incorporate foreign DNA in a buffer containing only salts and calcium. Immature spermatozoa, however, are unable to bind DNA. This seems to be caused by the lack of a functional receptor in the sperm membrane since once this membrane is disrupted by sonication, DNA can be detected in the postacrosome region of the sperm nucleus, matching the distribution of the mature spermatozoa. Comparison between the DNA binding proteins of mature and immature spermatozoa allowed us to identify two bands that could be part of the putative membrane receptor for the DNA. On the other hand, DNA uptake in mature sperm is prevented by the seminal plasma. We have identified two components of the seminal plasma, a calcium-dependent DNase present in the seminal vesicle fluid and several DNA binding proteins secreted by the ventral prostate, that could account for the inhibitory activity. Taken as a whole, our results indicate that DNA uptake by the mammalian spermatozoa is a very specific and highly regulated phenomenon.

Generation of genetically modified mice by spermatozoa transfection in vivo: preliminary results

Mouse vas deferens were injected with a plasmid DNA encoding the GFP (Green Fluorescent Protein). The night after injection males were mated with normal oestrus females, and the offspring were analyzed. From 53 newborns, 4 were found positive by PCR for the GFP gene. In these positive animals, some tissues showed expression for GFP as evidenced by a strong green cytoplasmic fluorescence. GFP expression was particularly patent in the liver (hepatocytes), kidney (renal corpuscle and tubules), abdominal wall, and lung. These preliminary results indicate the possibility to use this method as a simple alternative procedure to create transgenic animals, and it could be especially helpful in species in which the microinjection procedure is not feasible.

Sperm cell mediated transgenesis: a review

Transgenesis would be greatly streamlined if sperm cells could be used as transgene vectors. Attempts to 'force' sperm cells to take up transgenes may be more worthy of further study than 'autouptake' attempts. However, thorough proof of its effectiveness will be required if sperm cell-mediated gene transfer is to be accepted, given the evolutionary implications associated with the possibility of sperm cells being able to transfer exogenous DNA.

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.

Liposomes fuse with sperm cells and induce activation by delivery of impermeant agents

Sperm cell activation is a critical step in fertilization. To directly investigate the cell signaling events leading to sperm activation it is necessary to deliver membrane impermeant agents into the cytoplasm. In this study, the use of liposomes as possible agent-loading vectors was examined using (1) the octadecylrhodamine B (R18) and NBD phosphatidylethanolamine (NBD DHPE)/rhodamine phosphatidylethanolamine (rhod DHPE) fusion assays in bulk samples, (2) membrane transfer of fluorescence from liposome membranes labeled with R18 and rhodamine-tagged phosphatidylethanolamine (TRITC DHPE), and (3) lumenal transfer of impermeant calcium ions from liposomes to sperm cells, a process that stimulated sperm cell activation. Intermediate-sized unilamellar liposomes (98.17+/-15.34 nm) were prepared by the detergent-removal technique using sodium cholate as the detergent and a phosphatidylcholine/phosphatidylethanolamine/cholesterol (2:1:1 mole ratio) lipid composition. In the R18 fusion assays, self-quenching increased logarithmically with increasing concentrations of R18 in the liposome membranes; addition of unlabeled sperm to R18-labeled liposomes lead to a rapid release of self-quenching. In the NBD DHPE/rhod DHPE resonance energy transfer (RET) fusion assay, RET was rapidly reduced under similar conditions. In addition, individual sperm became fluorescent when TRITC DHPE-labeled liposomes were incubated with unlabeled sperm cells. Incubation of sperm cells with empty liposomes did not significantly affect sperm cell activation and did not alter cell morphology. However, incubation with Ca (10 mM)-loaded liposomes resulted in a time-dependent increase in sperm cell activation (7.5-fold over controls after 15 min). We conclude that liposomes can be used for direct loading of membrane-impermeant agents into sea squirt sperm cell cytoplasm, and that delivery occurs via fusion and content intermixing.

Foreign DNA introduced into the vas deferens is gained by mammalian spermatozoa

The uptake of exogenous DNA by mouse and rat spermatozoa was analyzed using in vitro and in vivo methods. Two DNA constructs were used, one containing the Growth hormone (GH) gene and the other the c-myc oncogene linked to the alphaA-crystallin promoter (CPV-1 plasmid). For the in vitro approach, washed epididymal spermatozoa were incubated for 2 hr in the presence of linearized DNA. For in vivo experiments, DNA was injected into the proximal region of the vas deferens, and spermatozoa were recovered 6 hr later. In situ hybridization employing fluorescent markers and electron microscopy were used to localize the exogenous genes in spermatozoa. The precise localization of the foreign DNA in spermatozoa was visualized by tridimensional reconstructions using a confocal laser microscopy. Uptake of exogenous DNA occurred in 60-70% of the spermatozoa after in vitro or in vivo treatments. A positive signal was detected in the sperm nucleus and was not affected by DNase treatments. Incorporation of exogenous DNA was also evaluated by slot blot and PCR techniques using the DNA isolated from the sperm nuclei and the corresponding labelled probes. Comparison of a nucleotide sequence between the DNA isolated from in vivo treated spermatozoa and CPV-1 plasmid showed a 98.6% identity. These results show the in vivo capacity of spermatozoa to incorporate exogenous DNA, the ability of this DNA to reach the nucleus, and also demonstrate that epididymal and vas deferens secretions do not block these capacities.

Development of transgenic Xenopus laevis with a high C-src gene expression

Xenopus laevis larvae with an elevated expression of c-src were generated by mating a transgenic X. laevis male frog carrying proviral Rous sarcoma virus (RSV) long terminal repeat (LTR) and most of the pol gene sequences in its sperm DNA and a normal X. laevis female frog. Offspring (15-20%) with a higher dosage of c-Src, detected in disorganized myotomal musculature and in cerebral and spinal neuronal cells by immunohistochemical analysis, developed abnormally, with edemas (in most cases), head deformities, and eye and axial system defects. In the remaining embryos, a small increase in c-src expression seemed to be compatible with normal embryogenesis. The dosage of c-Src correlated with the dosage of RSV LTR integrated in frog DNA as revealed by Southern and polymerase chain reaction (PCR) analyses. Authenticity of the integrated RSV LTR including enhancer sequence was proved by sequencing. Probing of total RNA from aberrant larvae demonstrated several times higher dosage of c-src mRNA in their tissues than in control tadpoles. We hypothesize that the integrated RSV regulatory sequences can stimulate the expression of c-src proto-oncogene of X. laevis above a threshold that interferes with the early developmental program of frog embryos.

Spermatozoa, DNA binding and transgenic animals

The idea that sperm cells could be used as an effective tool for introducing exogenous DNA into an oocyte at fertilization is generally regarded with scepticism. However, in recent years, several investigators have been working on different aspects of this intriguing research topic. In the present review, their results are summarised and discussed. Sections have been dedicated to the way DNA molecules bind to spermatozoa of different species, to the events regulating such binding, to the fate of the DNA within sperm cells, and to the attempts made to produce transgenic animals with this method. The data available on the interaction between DNA and spermatozoa begin to explain how this event takes place and how it is regulated. However, the stable integration of exogenous genes into the genome of adult animals mediated by sperm cells is a very rare event, although several reports describe forms of partial success. Available evidence suggests that changes to the DNA molecules, occurring mostly within the oocyte, represents the limiting step in the production of transgenic animals using spermatozoa as vectors of exogenous genes. At present there are not enough data to understand what happens to sperm-associated DNA upon its entrance into the oocyte at fertilization. Therefore, it has not yet been resolved whether sperm-mediated gene transfer is a possible way to manipulate the genome or if evolution has imposed some unsurpassible barriers to its use.

Sperm-mediated gene transfer in the silkworm Bombyx mori

Plasmid DNA containing the CAT reporter gene was injected into the testis of V instar silkworm larvae. The persistence, expression, and transmission of the injected DNA were monitored in the injected individuals till eclosion as well as in the progeny. The DNA injected into the testis persisted extrachromosomally during the entire period of metamorphosis and was also transferred into the egg via sperm during fertilization. Injected plasmids were rescued from the moths that emerged from the injected larvae and also from the eggs laid by the moths that copulated with injected males. Positive signals for CAT assay in the experimental samples suggested that the injected DNA was internalized in the testicular cells and sperm. The persistence, expression, and transmission of the DNA injected into the testes indicate that sperm-mediated gene transfer is possible in the silkworm, Bombyx mori.

Activation of endogenous nucleases in mature sperm cells upon interaction with exogenous DNA

Mature sperm cells, either of epididymal origin or ejaculated and depleted of seminal fluid, are spontaneously able to bind exogenous DNA molecules which are subsequently internalized into sperm nuclei. Southern blot analysis showed that the internalized DNA was specifically cleaved by sperm endonucleases and showed typical fragmentation patterns of localized hypersensitivity. Nucleases were activated in response to the internalization of exogenous DNA by sperm cells and their activity increased with the DNA concentration. Nuclease activation was efficient in epididymal sperm cells, while being drastically reduced in ejaculated washed spermatozoa. Nucleases were Ca++ dependent, and were, respectively, inhibited and activated by preincubating sperm cells with Aurintricarboxylic Acid (ATA) and Ca++ Ionophore A23187, which are known to, respectively, inhibit and activate apoptosis in somatic cells. Moreover, nuclease activation also caused a partial degradation of the sperm endogenous chromosomal DNA; cleaved DNA fragments were released from the sperm cells to the medium. Taken together, these results suggest that a metabolically active process similar to apoptosis is triggered in the nuclei of mature sperm cells upon interaction with exogenous DNA.

Integration of foreign DNA sequences into mouse sperm genome

Mouse epididymal sperm cells have the spontaneous ability to take up exogenous DNA. A proportion of the sperm-bound DNA is further internalized into sperm nuclei. In this work, we have followed up the fate of the foreign DNA upon internalization into nuclei. We have found that the internalized plasmid DNA becomes tightly associated with the nuclear scaffold, is extensively rearranged, and undergoes recombination with the sperm genomic DNA. Sequence analysis of two randomly selected clones independently recovered by plasmid rescue from pSV2CAT plasmid-challenged sperm cells shows that DNA fragments from the plasmid are integrated into the mouse sperm genome. The sites of integration are identical in both clones, suggesting that these events do not occur randomly, but take place at preferential sites. A topoisomerase II consensus sequence is found adjacent to one end of the integration site, suggesting a possible role of this enzyme in the process of nonhomologous recombination.

Association of rous sarcoma virus DNA with Xenopus laevis spermatozoa and its transfer to ova through fertilization

Mature Xenopus laevis spermatozoa are capable of binding plasmid pAPrC carrying the complete Rous sarcoma virus (RSV) DNA. Each sperm cell associates, on an average, with 70-160 molecules of the plasmid DNA in a DNase resistant form, if the spermatozoa were exposed to the DNA at a concentration of 1.0-1.4 micrograms/10(7) sperm cells. Fertilization with pAPrC-treated spermatozoa induced developmental malformations in 25-30% of embryos. Immunohistochemical analysis of tissue sections from defective animals revealed aberrations in myotomal structures, and increased expression of pp60src protein in myoblasts, neuronal tube, and epidermis. The presence of characteristic v-src and RSV-long terminal repeat (LTR) sequences in X. laevis DNA was detected by PCR analysis. Embryonic RNA hybridized with a src-specific and an RSV-LTR specific probes indicating expression of the viral DNA. Plasmid DNAs without the v-src gene (pATV9) or completely free of any RSV sequences (pBR322) did not induce any changes in embryonic development. Our results provide evidence that the pBR322-cloned DNA form of the RSV genome associates with frog sperm cells in a DNase-resistant manner suggesting internalization and may be subsequently carried into eggs during the process of artificial fertilization. Correlation between the defective morphogenesis of X. laevis and increased expression of the src gene as well as an interference of RSV DNA with the developmental programs of frog embryos are discussed.