Retrovirus-mediated modification of male germline stem cells in rats

Targeted Gene Editing in Porcine Germ Cells

As the genetic mutations driving human disease are identified, there is an increasing need for a biomedical model that can accurately represent the disease of interest and provide a platform for potential therapeutic testing. Pigs are a better model for human disease than rodents because of their genetic and physiological similarities to humans. However, current methods to generate porcine models are both technically challenging and expensive. Germline genetic modification through gene edited spermatogonia provides an effective alternative to how these models are developed. Here, we report an improved technique of gene editing in spermatogonia of pigs using CRISPR-Cas9 to generate different edits that reflect the genotypes of human diseases.

Targeted Gene Editing in Porcine Spermatogonia

To study the pathophysiology of human diseases, develop innovative treatments, and refine approaches for regenerative medicine require appropriate preclinical models. Pigs share physiologic and anatomic characteristics with humans and are genetically more similar to humans than are mice. Genetically modified pigs are essential where rodent models do not mimic the human disease phenotype. The male germline stem cell or spermatogonial stem cell (SSC) is unique; it is the only cell type in an adult male that divides and contributes genes to future generations, making it an ideal target for genetic modification. Here we report that CRISPR/Cas9 ribonucleoprotein (RNP)-mediated gene editing in porcine spermatogonia that include SSCs is significantly more efficient than previously reported editing with TALENs and allows precise gene editing by homology directed repair (HDR). We also established homology-mediated end joining (HMEJ) as a second approach to targeted gene editing to enable introduction of larger transgenes and/or humanizing parts of the pig genome for disease modeling or regenerative medicine. In summary, the approaches established in the current study result in efficient targeted genome editing in porcine germ cells for precise replication of human disease alleles.

The study and manipulation of spermatogonial stem cells using animal models

Spermatogonial stem cells (SSCs) are a rare group of cells in the testis that undergo self-renewal and complex sequences of differentiation to initiate and sustain spermatogenesis, to ensure the continuity of sperm production throughout adulthood. The difficulty of unequivocal identification of SSCs and complexity of replicating their differentiation properties in vitro have prompted the introduction of novel in vivo models such as germ cell transplantation (GCT), testis tissue xenografting (TTX), and testis cell aggregate implantation (TCAI). Owing to these unique animal models, our ability to study and manipulate SSCs has dramatically increased, which complements the availability of other advanced assisted reproductive technologies and various genome editing tools. These animal models can advance our knowledge of SSCs, testis tissue morphogenesis and development, germ-somatic cell interactions, and mechanisms that control spermatogenesis. Equally important, these animal models can have a wide range of experimental and potential clinical applications in fertility preservation of prepubertal cancer patients, and genetic conservation of endangered species. Moreover, these models allow experimentations that are otherwise difficult or impossible to be performed directly in the target species. Examples include proof-of-principle manipulation of germ cells for correction of genetic disorders or investigation of potential toxicants or new drugs on human testis formation or function. The primary focus of this review is to highlight the importance, methodology, current and potential future applications, as well as limitations of using these novel animal models in the study and manipulation of male germline stem cells.

Beyond the mouse monopoly: studying the male germ line in domestic animal models

Spermatogonial stem cells (SSCs) are the foundation of spermatogenesis and essential to maintain the continuous production of spermatozoa after the onset of puberty in the male. The study of the male germ line is important for understanding the process of spermatogenesis, unravelling mechanisms of stemness maintenance, cell differentiation, and cell-to-cell interactions. The transplantation of SSCs can contribute to the preservation of the genome of valuable individuals in assisted reproduction programs. In addition to the importance of SSCs for male fertility, their study has recently stimulated interest in the generation of genetically modified animals because manipulations of the male germ line at the SSC stage will be maintained in the long term and transmitted to the offspring. Studies performed mainly in the mouse model have laid the groundwork for facilitating advancements in the field of male germ line biology, but more progress is needed in nonrodent species in order to translate the technology to the agricultural and biomedical fields. The lack of reliable markers for isolating germ cells from testicular somatic cells and the lack of knowledge of the requirements for germ cell maintenance have precluded their long-term maintenance in domestic animals. Nevertheless, some progress has been made. In this review, we will focus on the state of the art in the isolation, characterization, culture, and manipulation of SSCs and the use of germ cell transplantation in domestic animals.

Experimental methods to preserve male fertility and treat male factor infertility

Infertility is a prevalent condition that has insidious impacts on the infertile individuals, their families, and society, which extend far beyond the inability to have a biological child. Lifestyle changes, fertility treatments, and assisted reproductive technology (ART) are available to help many infertile couples achieve their reproductive goals. All of these technologies require that the infertile individual is able to produce at least a small number of functional gametes (eggs or sperm). It is not possible for a person who does not produce gametes to have a biological child. This review focuses on the infertile man and describes several stem cell-based methods and gene therapy approaches that are in the research pipeline and may lead to new fertility treatment options for men with azoospermia.

Effects of extracellular matrices and lectin Dolichos biflorus agglutinin on cell adhesion and self-renewal of bovine gonocytes cultured in vitro

Surface molecules of primitive male germ cells, gonocytes, are essential components for regulating cell adhesion and maintaining self-renewal in mammalian species. In domestic animals, the stage-specific glycan epitope α-N-acetylgalactosamine (GalNAc) is recognised by the lectin Dolichos biflorus agglutinin (DBA) and is found on the surface of gonocytes and spermatogonia. Gonocytes from bovine testis formed mouse embryonic stem-like cell colonies on plates that had been coated with DBA or extracellular matrix (ECM) components, such as gelatin (GN), laminin (LN) and poly-L-lysine (PLL). The number of colonies on the DBA-coated plate was significantly higher than that on the GN-, LN- and PLL-coated plates. Pretreating gonocytes with DBA to neutralise the terminal GalNAc residues strongly suppressed colony formation. Furthermore, expression of a germ cell-specific gene and pluripotency-related transcription factors was increased considerably on the DBA-coated plates. These results suggest that the GalNAc residues on gonocytes can recognise precoated DBA on plates and the resulting GalNAc–DBA complexes support germ cell and stem cell potentials of gonocytes in vitro. These glycan complexes, through the GalNAc epitope, may provide a suitable microenvironment for the adhesion and cell proliferation of gonocytes in culture.

Lentiviral modification of enriched populations of bovine male gonocytes

Undifferentiated germ cells have the capacity to develop into sperm capable of fertilizing oocytes and contributing genetic material to subsequent generations. The most primitive prepubertal undifferentiated germ cells include gonocytes and undifferentiated spermatogonia, including spermatogonial stem cells (SSC). Gonocytes, present in the testis at birth, differentiate into SSC, which maintain spermatogenesis for the remainder of the male's life. Because of their capacity to contribute to lifelong spermatogenesis, undifferentiated germ cells are attractive targets for genetic modification to produce transgenic animals, including cattle. To maximize the efficiency of genetic modification of bovine gonocytes and SSC, effective enrichment techniques need to be developed. Selection of bovine gonocytes using differential plating was improved 8-fold (P < 0.001) when using a combination of extracellular matrix proteins, including laminin, fibronectin, collagen type IV, and gelatin, compared to using laminin and gelatin alone. Selected cells labeled with PKH26 formed colonies of donor-derived germ cells after transplantation into recipient mouse testes, indicating putative stem cell function. Significantly more colonies (P < 0.001) per 1 × 10(5) viable transplanted cells were formed from isolated nonadherent cells (203 ± 23.2) compared to adherent (20 ± 2.7) or Percoll (45.5 ± 4.5) selected cells. After selection, some gonocytes were transduced using a lentiviral vector containing the transgene for the enhanced green fluorescent protein. Transduction efficiency was 17%. Collectively, these data demonstrate effective methods for the selection and genetic modification of bovine undifferentiated germ cells.

Rat placentation: an experimental model for investigating the hemochorial maternal-fetal interface

The rat possesses hemochorial placentation with deep intrauterine trophoblast cell invasion and trophoblast-directed uterine spiral artery remodeling; features shared with human placentation. Recognition of these similarities spurred the establishment of in vitro and in vivo research methods using the rat as an animal model to address mechanistic questions regarding development of the hemochorial placenta. The purpose of this review is to provide the requisite background to help move the rat to the forefront in placentation research.

Generation of genetically modified animals using spermatogonial stem cells

Spermatogonial stem cells (SSCs) provide the foundation for spermatogenesis, and are unique tissue-specific stem cells because of their ability to transmit genetic information to offspring. Generation of knockout mice using mouse SSCs became feasible after the successful establishment of protocols for the transplantation and long-term culture of these cells, called germline stem (GS) cells. Furthermore, SSCs can acquire pluripotentiality similar to that of embryonic stem (ES) cells, in addition to their highly differentiated spermatogenic potential. These ES-like cells, called multipotent GS (mGS) cells, are capable of generating knockout mice in a manner similar to that of ES cells. The use of GS and mGS cells for animal transgenesis has added a new dimension to gene-targeting technology using ES cells and somatic cell nuclear transfer, which has limited application. Furthermore, for regenerative medicine purposes, the use of mGS will settle problems such as ethics issues and immunological rejection associated with ES cells, as well as risks of insertional mutagenesis associated with integrated genes into induced pluripotent stem cells.

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.

Factors determining the risk of inadvertent retroviral transduction of male germ cells after in utero gene transfer in sheep

The possibility of permanent genetic changes to the germline is central to the bioethics of in utero gene therapy (IUGT) because of the concern of inadvertent potentially deleterious alterations to the gene pool. Despite presumed protection of the male germline due to early germ cell (GC) compartmentalization, we reported that GCs within the developing ovine testes are transduced at low levels after retrovirus-mediated IUGT, thus underscoring the need for a thorough understanding of GC development in clinically predictive models to determine the optimal time to perform IUGT and avoid germline modification. In the present studies, we used the fetal sheep model to analyze GCs for phenotype, location, proliferation, and incidence of transduction after IUGT at various fetal ages to learn when during development the nascent germline is likely to be at greatest risk of retrovirus-mediated alteration. Our studies show that although GCs were transduced at all injection ages, the levels of transduction varied by nearly 700-fold as a function of the age at transfer. After remaining largely quiescent as they migrated to/settled within nascent sex cords, GCs began active cycling before cord closure was complete, suggesting this is likely the point at which they would be most susceptible to retroviral transduction.Furthermore, we observed that compartmentalization of GCs continued into early postnatal life, suggesting the male germline may be vulnerable to low-level inadvertent retroviral vector modification throughout fetal life, but that this risk can be minimized by performing IUGT later in gestation.

Worldwide trends in the use of animals in research: the contribution of genetically-modified animal models

The Three Rs--Reduction, Replacement and Refinement--which were first proposed in 1959 by Russell and Burch, have become widely accepted principles in the governance of humane animal research. However, there is substantial variation in the ways in which different countries document the numbers and types of research animals used, making it difficult to determine how effectively the Three Rs are being implemented. Here, we provide the first data illustrating worldwide trends in animal use for research purposes. To document global trends in animal use, we sampled 2691 articles from 24 countries, published between 1983 and 2007, in four scientific journals. We show that the percentage of articles reporting animal use has risen in the past 15 years. The rising popularity of genetic modification methods has contributed to this trend: reported genetically-modified animal use has more than doubled since 1997. We also show that mice are the most commonly-used species for genetic modification, and that, even in 2007, relatively inefficient random integration methods were still widely used to achieve genetic modification. These results illustrate shortcomings in the effort to implement the Three Rs in animal research.

Spermatogonial culture medium: an effective and efficient nutrient mixture for culturing rat spermatogonial stem cells

An economical and simplified procedure to derive and propagate fully functional lines of undifferentiated rat spermatogonia in vitro is presented. The procedure is based on the formulation of a new spermatogonial culture medium termed SG medium. The SG medium is composed of a 1:1 mixture of Dulbecco modified Eagle medium:Ham F12 nutrient, 20 ng/ml of GDNF, 25 ng/ml of FGF2, 100 microM 2-mercaptoethanol, 6 mM l-glutamine, and a 1x concentration of B27 Supplement Minus Vitamin A solution. Using SG medium, six individual spermatogonial lines were derived from the testes of six separate Sprague-Dawley rats. After proliferating over a 120-day period in SG medium, stem cells within the spermatogonial cultures effectively regenerated spermatogenesis in testes of busulfan-treated recipient rats, which transmitted the donor cell haplotype to more than 75% of progeny by natural breeding. Subculturing in SG medium did not require protease treatment and was achieved by passaging the loosely bound spermatogonial cultures at 1:3 dilutions onto fresh monolayers of irradiated DR4 mouse fibroblasts every 12 days. Spermatogonial lines derived and propagated using SG medium were characterized as homogeneous populations of ZBTB16(+) DAZL(+) cells endowed with spermatogonial stem cell potential.

Male germ cell transplantation

Transplantation of male germ line stem cells from a donor animal to the testes of an infertile recipient was first described in 1994. Donor germ cells colonize the recipient's testis and produce donor-derived sperm, such that the recipient male can distribute the genetic material of the germ cell donor. Germ cell transplantation represents a functional reconstitution assay for male germ line stem cells and as such has vastly increased our ability to study the biology of stem cells in the testis and define phenotypes of infertility. First developed in rodents, the technique has now been used in a number of animal species, including domestic mammals, chicken and fish. There are three major applications for this technology in animals: first, to study fundamental aspects of male germ line stem cell biology and male fertility; second, to preserve the reproductive potential of genetically valuable individuals by male germ cell transplantation within or between species; third, to produce transgenic sperm by genetic manipulation of isolated germ line stem cells and subsequent transplantation. Transgenesis through the male germ line has tremendous potential in species in which embryonic stem cells are not available and somatic cell nuclear transfer has limited success. Therefore, transplantation of male germ cells is a uniquely valuable approach for the study, preservation and manipulation of male fertility in animals.

Transduction and transplantation of spermatogonia into the testis of ram lambs through the extra-testicular rete

Spermatogonial transplantation will provide a new way to study spermatogenesis in domestic animals, disseminate male genetics and produce transgenic animals, if efficiency can be improved. We evaluated a 'surgical' method for transplanting donor cells into testes of ram lambs, where the head of the epididymis is reflected, and a catheter introduced into the extra-testicular rete testis. We also tested transduction of ram spermatogonia with a lentiviral (LV) vector as a means to identify permanent colonization, and introduce genes into donor cells. Eight ram lambs, 11- to 13-week olds, were the recipients: in five, spermatogonia were injected into one testis, and the contralateral testis was an un-manipulated control: in two, spermatogonia were injected into one testis and the contralateral was sham-injected: in one, both testes were injected. Six lambs received spermatogonia labelled with a cell-tracking dye and these were collected 1 or 2 weeks after transplantation; three lambs received spermatogonia transduced with a LV vector driving the expression of enhanced Green Fluorescence Protein and these were collected after 2 months. Donor cells were detected by immunohistochemistry in tubules of seven of nine recipient testes. Approximately 22% of tubule cross-sections contained donor cells immediately after transplantation, and 0.2% contained virally transduced cells 2 months after transplantation. The onset of spermatogenesis was delayed, and there were lesions in both injected and sham-injected testes. Despite the effects of the surgery, elongated spermatids were present in one recipient testis 2 months after surgery. The results suggest that, after modifying the surgical and transduction techniques, this approach will be a means to produce good colonization by donor spermatogonia in sheep testes.

Expression of NANOG, but not POU5F1, points to the stem cell potential of primitive germ cells in neonatal pig testis

Gonocytes are primitive germ cells that are present in the neonatal testis and are committed to male germline development. Gonocytes differentiate to spermatogonia, which establish and maintain spermatogenesis in the postnatal testis. However, it is unknown whether large animal species have pluripotency-specific proteins in the testis. Nanog and Pou5f1 (Oct3/4) have been identified as transcription factors essential for maintaining pluripotency of embryonic stem cells in mice. Here, we show that NANOG protein was expressed in the germ cells of neonatal pig testes, but was progressively lost with age. NANOG was expressed in most of the lectin Dolichos biflorus agglutinin- and ZBTB16-positive gonocytes, which are known gonocyte-specific markers in pigs. NANOG was also expressed in Sertoli and interstitial cells of neonatal testes. Interestingly, POU5F1 expression was not detected at either the transcript or the protein level in neonatal pig testis. In the prepubertal testis, NANOG and POU5F1 proteins were primarily detected in differentiated germ cells, such as spermatocytes and spermatids, and rarely in undifferentiated spermatogonia. By using a testis transplantation assay, we found that germ cells from 2- to 4-day-old pigs could colonize and proliferate in the testes of the recipient mice, suggesting that primitive germ cells from neonatal pig testes have stem cell potential.

Genes involved in post-transcriptional regulation are overrepresented in stem/progenitor spermatogonia of cryptorchid mouse testes

Gene expression and consequent biological activity of adult tissue stem cells are regulated by signals emanating from the local microenvironment (niche). To gain insights into the molecular regulation of spermatogonial stem cells (SSCs), gene expression was characterized from SSCs isolated from their cognate niches of cryptorchid (stem cell-enriched), wild-type, and busulfan-treated (stem cell-depleted) mouse testes. Quantitative assessment of stem cell activity in each testis model was determined using an in vivo functional assay and correlated with gene expression using Affymetrix MGU74Av2 microarrays and the ChipStat algorithm optimized to detect gene expression from rare cells in complex tissues. We identified 389 stem/progenitor spermatogonia candidate genes, which exhibited significant overlap with genes expressed by embryonic, hematopoietic, and neural stem cells; enriched spermatogonia; and cultured SSCs identified in previous studies. Candidate cell surface markers identified by the microarray may facilitate the isolation and enrichment of stem and/or progenitor spermatogonia. Flow cytometric analyses confirmed the expression of chemokine receptor 2 (Ccr2) and Cd14 on a subpopulation cryptorchid testis cells (alpha6-integrin+, side scatter(lo)) enriched for SSCs. These cell surface molecules may mark progenitor spermatogonia but not SSCs because Ccr2+ and Cd14+ fractions failed to produce spermatogenesis upon transplantation to recipient testes. Functional annotation of candidate genes and subsequent immunohistochemistry revealed that proteins involved in post-transcriptional regulation are overrepresented in cryptorchid testes that are enriched for SSCs. Comparative analyses indicated that this is a recurrent biological theme among stem cells.

Germ cell transplantation for the propagation of companion animals, non-domestic and endangered species

The transplantation of spermatogonial stem cells between males results in a recipient animal producing spermatozoa carrying a donor's haplotype. First pioneered in rodents, this technique has now been used in several animal species. Importantly, germ cell transplantation was successful between unrelated, immuno-competent large animals, whereas efficient donor-derived spermatogenesis in rodents requires syngeneic or immuno-compromised recipients. Transplantation requires four steps: recipient preparation, donor cell isolation, transplantation and identifying donor-derived spermatozoa. There are two main applications for this technology. First, genetic manipulation of isolated germ line stem cells and subsequent transplantation will result in production of transgenic spermatozoa. Transgenesis through the male germ line has tremendous potential in species in which embryonic stem cells are not available and somatic cell nuclear transfer and reprogramming pose several problems. Second, spermatogonial stem cell transplantation within or between species offers a means of preserving the reproductive potential of genetically valuable individuals. This might have significance in the captive propagation of non-domestic animals of high conservation value. Transplantation of germ cells is a uniquely valuable approach for the study, preservation and manipulation of male fertility in mammalian species.

Adeno‐associated virus (AAV)‐mediated transduction of male germ line stem cells results in transgene transmission after germ cell transplantation

We explored whether exposure of mammalian germ line stem cells to adeno-associated virus (AAV), a gene therapy vector, would lead to stable transduction and transgene transmission. Mouse germ cells harvested from experimentally induced cryptorchid donor testes were exposed in vitro to AAV vectors carrying a GFP transgene and transplanted to germ cell-depleted syngeneic recipient testes, resulting in colonization of the recipient testes by transgenic donor cells. Mating of recipient males to wild-type females yielded 10% transgenic offspring. To broaden the approach to nonrodent species, AAV-transduced germ cells from goats were transplanted to recipient males in which endogenous germ cells had been depleted by fractionated testicular irradiation. Transgenic germ cells colonized recipient testes and produced transgenic sperm. When semen was used for in vitro fertilization (IVF), 10% of embryos were transgenic. Here, we report for the first time that AAV-mediated transduction of mammalian germ cells leads to transmission of the transgene through the male germ line. Equally important, this is also the first report of transgenesis via germ cell transplantation in a nonrodent species, a promising approach to generate transgenic large animal models for biomedical research.

Identification, Isolation, and In Vitro Culture of Porcine Gonocytes1

Gonocytes are primitive germ cells that reside in the seminiferous tubules of neonatal testes and give rise to spermatogonia, thereby initiating spermatogenesis. Due to a lack of specific markers, the isolation and culture of these cells has proven to be difficult in the pig. In the present study, we show that a lectin, Dolichos biflorus agglutinin (DBA), which has specific affinity for primordial germ cells (PCGs) in the genital ridge, binds specifically to gonocytes in neonatal pig testes. The specific affinity of DBA for germ cells was progressively lost with age. This suggests that DBA binds strongly to primitive germ cells, such as gonocytes, weakly to primitive spermatogonia, and not at all to spermatogonia. The presence of alkaline phosphatase (AP) activity in the germ cells of neonatal pig testis confirmed the existence of primitive germ cells. Gonocytes from neonatal pig testis were purified, and a cell population that consisted of approximately 70% gonocytes was obtained, as indicated by the DBA binding assay. Purified gonocytes were cultured in DMEM/F12 supplemented with 10% FBS in the absence of any specific growth factors for 7 days. The cells remained viable and proliferated actively in culture. Initially, the gonocytes grew as focal colonies that transformed to three-dimensional colonies by 7 days of culture. Cultured germ cells expressed SSEA-1, a marker for embryonic stem (ES) cells, and were negative for the expression of somatic cell markers. These results should help to establish a male germ cell line that could be used for studying spermatogenesis in vitro and for genetic modification of pigs.

Transplantation of germ line stem cells for the study and manipulation of spermatogenesis

Transplantation of male germ line stem cells from a fertile donor to the testis of an infertile recipient restores donor-derived spermatogenesis in the recipient testis and the resulting sperm pass the donor genotype to the offspring of the recipient. Germ cell transplantation has been an invaluable tool to elucidate the biology of male germ line stem cells and their niche in the testis, develop systems to isolate and culture spermatogonial stem cells, examine defects in spermatogenesis, correct male infertility and introduce genetic changes into the male germ line. Although most widely studied in rodents, germ cell transplantation has been applied to larger mammals, including primates. Recently, ectopic grafting of testis tissue from diverse donor species, including primates, into a mouse host has opened an additional possibility to study spermatogenesis and to produce fertile sperm from immature donors. Testis xenografts are ideally suitable to study toxicants or drugs with the potential to enhance or suppress male fertility without the necessity of performing experiments in the target species. Therefore, transplantation of germ cells or xenografting of testis tissue represent powerful approaches for the study, preservation, and manipulation of male fertility.

Clonal Origin of Germ Cell Colonies after Spermatogonial Transplantation in Mice1

Spermatogenesis originates from a small number of spermatogonial stem cells that can reinitiate spermatogenesis and produce germ cell colonies following transplantation into infertile recipient testes. Although several previous studies have suggested a single-cell origin of germ cell colonies, only indirect evidence has been presented. In this investigation, we tested the clonal origin hypothesis using a retrovirus, which could specifically mark an individual spermatogonial stem cell. Spermatogonial stem cells were infected in vitro with an enhanced green fluorescence protein-expressing retrovirus and subsequently transplanted into infertile recipient mice. Live haploid germ cells were recovered from individual colonies and were microinjected into eggs to create offspring. In total, 45 offspring were produced from five colonies, and 23 (51%) of the offspring were transgenic. Southern blot analysis indicated that the transgenic offspring from the single colony carried a common integration site, and the integration site was different among the transgenic offspring from different colonies. These results provide evidence that germ cell colonies develop from single spermatogonial stem cells.

Male germ cell transplantation in livestock

Male germ cell transplantation is a powerful approach to study the control of spermatogenesis with the ultimate goal to enhance or suppress male fertility. In livestock animals, applications can be expanded to provide an alternative method of transgenesis and an alternative means of artificial insemination (AI). The transplantation technique uses testis stem cells, harvested from the donor animal. These donor stem cells are injected into seminiferous tubules, migrate from the lumen to relocate to the basement membrane and, amazingly, they can retain the capability to produce donor sperm in their new host. Adaptation of the mouse technique for livestock is progressing, with gradual gains in efficiency. Germ cell transfer in goats has produced offspring, but not yet in cattle and pigs. In goats and pigs, the applications of germ cell transplantation are mainly in facilitating transgenic animal production. In cattle, successful male germ cell transfer could create an alternative to AI in areas where it is impractical. Large-scale culture of testis stem cells would enhance the use of elite bulls by providing a renewable source of stem cells for transfer. Although still in a developmental state, germ cell transplantation is an emerging technology with the potential to create new opportunities in livestock production.

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).

Spermatogonial stem cells: questions, models and perspectives

This review looks into the phylogeny of spermatogonial stem cells and describes their basic biological features. We are focusing on species-specific differences of spermatogonial stem cell physiology. We propose revised models for the clonal expansion of spermatogonia and for the potential existence of true stem cells and progenitors in primates but not in rodents. We create a new model for the species-specific arrangements of spermatogenic stages which may depend on the variable clonal expansion patterns. We also provide a brief overview of germ cell transplantation as a powerful tool for basic research and its potential use in a clinical setting.

Efficient and simple production of transgenic mice and rabbits using the new DMSO-sperm mediated exogenous DNA transfer method

A high efficient and simple transgenic technology on mice and rabbits to transfect spermatozoa with exogenous DNA/DMSO complex to obtain transgenic offspring, which is namely called DMSO-sperm mediated gene transfer (SMGT). Mouse sperm could be either directly transfected via injection into testis or cultured in vitro with the plasmed DNA containing the enhanced green fluorescent protein (EGFP) that could be expressed in the embryos and offspring. Then, 36 living transgenic rabbits were produced using the same technology, and the transgenic ratio of 56.3% was detected using PCR and Southern blot. As the controls, the transgenic ratios of 39.6% and 47.8% have also been tested using the liposomes mediated technology of Tfx-50 Reagent or Lipefectamin-2000, respectively. The results show that the female transgenic rabbits, as the mammary gland bioreactor models, could express the human tissue plasminogen activator mutant (htPAm) in their mammary cells when they are adult.

Spermatogonial stem cells

The biological activities of spermatogonial stem cells (SSCs) are the foundation for spermatogenesis and thus sustained male fertility. Therefore, understanding the mechanisms governing their ability to both self-renew and differentiate is essential. Moreover, because SSCs are the only adult stem cell to contribute genetic information to the next generation, they are an excellent target for genetic modification. In this chapter, we discuss two important approaches to investigate SSCs and their cognate niche microenvironment in the mouse, the SSC transplantation assay and the long-term serum-free SSC culture method. These techniques can be used to enhance our understanding of SSC biology as well as to produce genetically modified animals.

Germ cell transplantation and testis tissue xenografting in domestic animals

Transplantation of germ cells from fertile donor mice to the testes of infertile recipient mice results in donor-derived spermatogenesis and transmission of the donor's genetic material to the offspring of recipient animals. Germ cell transplantation provides a bioassay to study the biology of male germ line stem cells, develop systems to isolate and culture spermatogonial stem cells, examine defects in spermatogenesis and treat male infertility. Although most widely studied in rodents, germ cell transplantation has been applied to larger mammals. In domestic animals including pigs, goats and cattle, as well as in primates, germ cells can be transplanted to a recipient testis by ultrasonographic-guided cannulation of the rete testis. Germ cell transplantation was successful between unrelated, immuno-competent pigs and goats, whereas transplantation in rodents requires syngeneic or immuno-compromised recipients. Genetic manipulation of isolated germ line stem cells and subsequent transplantation will result in the production of transgenic sperm. Transgenesis through the male germ line has tremendous potential in domestic animal species where embryonic stem cell technology is not available and current options to generate transgenic animals are inefficient. As an alternative to transplantation of isolated germ cells to a recipient testis, ectopic grafting of testis tissue from diverse mammalian donor species, including horses and primates, into a mouse host represents a novel possibility to study spermatogenesis, to investigate the effects of drugs with the potential to enhance or suppress male fertility, and to produce fertile sperm from immature donors. Therefore, transplantation of germ cells or xenografting of testis tissue are uniquely valuable approaches for the study, preservation and manipulation of male fertility in domestic animals.

Long-Term Culture of Mouse Male Germline Stem Cells Under Serum-or Feeder-Free Conditions1

Spermatogonial stem cells are the only stem cells in the body that transmit genetic information to the next generation. These cells can be cultured for extended periods in the presence of serum and feeder cells. However, little is known about factors that regulate self-renewal division of spermatogonial stem cells. In this investigation we examined the possibility of establishing culture systems for spermatogonial stem cells that lack serum or a feeder cell layer. Spermatogonial stem cells could expand in serum-free conditions on mouse embryonic fibroblasts (MEFs), or were successfully cultivated without feeder cells on a laminin-coated plate. However, they could not expand when both serum and feeder cells were absent. Although the cells cultured on laminin differed phenotypically from those on feeder cells, they grew exponentially for at least 6 mo, and produced normal, fertile progeny following transplantation into infertile mouse testis. This culture system will provide a new opportunity for understanding the regulatory mechanism that governs spermatogonial stem cells.

The spermatogonial stem cell: from basic knowledge to transgenic technology

Differentiation of germ cells in the testis originates from a constantly renewed small pool of stem cells. They give rise to the first differentiated spermatogenic cells (spermatogonia). These committed cells will then follow a strictly defined succession of steps, starting with six synchronized mitotic cycles before reaching the first meiotic stages. Following a first identification of the spermatogonial stem cells on morphological and cytological criteria, a functional assay was devised, based on their ability to repopulate the testis of a sterile recipient. Purification and characterization of the stem fraction is in progress. Fundamental knowledge of the biology of the germ line and preclinical studies in several important fields will benefit of these advances, while gene transfer prior to reimplantation opens a new approach in transgenic technology.

Spermatogenesis and Germ Cell Transgene Expression in Xenografted Bovine Testicular Tissue1

The present study was conducted to evaluate the development of spermatogenesis and utility of using electroporation to stably transfect germ cells with the beta-galactosidase gene in neonatal bovine testicular tissue ectopically xenografted onto the backs of recipient nude mice. Bull testicular tissue from 4-wk donor calves, which contains a germ cell population consisting solely of gonocytes or undifferentiated spermatogonia, was grafted onto the backs of castrated adult recipient nude mice. Testicular grafts significantly increased in weight throughout the grafting period and the timing of germ cell differentiation in grafted tissue was consistent with postnatal testis development in vivo relative to the bull. Seminiferous tubule diameter also significantly increased with advancing time after grafting. At 1 wk after grafting, gonocytes in the seminiferous cords completed migration to the basement membrane and differentiated germ cell types could be observed 24 wk after grafting. The presence of elongating spermatids at 24 wk confirmed that germ cell differentiation occurred in the bovine tissue. Leydig cells in the grafted bovine tissue were also capable of producing testosterone in the castrated recipient mice from 4 wk to 24 wk after grafting at concentrations that were similar to levels in intact, nongrafted control mice. The testicular tissue that had been electroporated with a beta-galactosidase expression vector showed tubule-specific transgene expression 24 wk after grafting. Histological analysis showed that transgene expression was present in both Sertoli and differentiated germ cells but not in interstitial cells. The system reported here has the potential to be used for generation of transgenic bovine spermatozoa.

Transgenic mice produced by retroviral transduction of male germ line stem cells in vivo

Spermatogonial stem cells are the only stem cells in the postnatal body that can transmit parental genetic information to the offspring, making them an attractive target cell population for animal transgenesis. Although transgenic mice and rats were recently produced by retrovirus transduction of these cells in vitro, with transplantation of the transduced cells into infertile recipients, the difficulty of restoring fertility and preparing recipients using spermatogonial transplantation limits practical application of the technique. In this article, we describe a novel approach for producing transgenic animals by transducing spermatogonial stem cells in vivo using a retrovirus vector. Microinjection of retrovirus into immature seminiferous tubules resulted in the direct transduction of spermatogonial stem cells in situ, and the animals produced transgenic offspring after mating with females. Transgenic mice were produced in C57BL/6, BALB/C, A, and C3H backgrounds, with an average efficiency of 2.8%. The transgene was transmitted stably and expressed in the next generation. The technique overcomes the drawback of the in vitro-transduction approach, and will be useful as a novel method for producing transgenic animals as well as providing a means for analyzing the self-renewal and differentiation processes of spermatogonial stem cells in vivo.

Defining the spermatogonial stem cell

Through the use of donor cells from transgenic rats expressing GFP exclusively in the germline, we have defined culture conditions where male germ cells lose (on STO cells) or maintain (on MSC-1 cells) stem cell activity. A cadre of germ cell transcripts strikingly decrease in relative abundance as a function of testis age or culture time on STO cells, but only a subset of these transcripts (approximately 248) remain elevated when cultured on MSC-1 cells. If specific gene expression regulates stem cell activity, some or all of these transcripts are candidates as such regulators. We establish a spermatogonial stem cell index (SSCI) that reliably predicts relative stem cell activity in rat or mouse testis cell cultures, and through the use of an antibody to a robust signal (Egr3) within the index find intense signals in single or paired cells. As germ cells form longer interconnected chains (incomplete cytokinesis), the Egr3 signal disappears coincident with a loss of stem cell activity. Thus, molecular markers specific for spermatogonial stem cells establish a reliable and rapid means by which to define these cells in culture and alleviate the need for laborious testicular transfers in initial cell culture studies.

Gene Therapy: The Potential Applicability of Gene Transfer Technology to the Human Germline

The theoretical possibility of applying gene transfer methodologies to the human germline is explored. Transgenic methods for genetically manipulating embryos may in principle be applied to humans. In particular, microinjection of retroviral vector appears to hold the greatest promise, with transgenic primates already obtained from this approach. Sperm-mediated gene transfer offers potentially the easiest route to the human germline, however the requisite methodology is presently underdeveloped. Nuclear transfer (cloning) offers an alternative approach to germline genetic modification, however there are major health concerns associated with current nuclear transfer methods. It is concluded that human germline gene therapy remains for all practical purposes a future possibility that must await significant and important advances in gene transfer technology.

Endocrine aspects of cancer gene therapy

The field of cancer gene therapy is in continuous expansion, and technology is quickly moving ahead as far as gene targeting and regulation of gene expression are concerned. This review focuses on the endocrine aspects of gene therapy, including the possibility to exploit hormone and hormone receptor functions for regulating therapeutic gene expression, the use of endocrine-specific genes as new therapeutic tools, the effects of viral vector delivery and transgene expression on the endocrine system, and the endocrine response to viral vector delivery. Present ethical concerns of gene therapy and the risk of germ cell transduction are also discussed, along with potential lines of innovation to improve cell and gene targeting.

Stem cell and niche development in the postnatal rat testis

Adult tissue stem cells self-renew and differentiate in a way that exactly meets the biological demand of the dependent tissue. We evaluated spermatogonial stem cell (SSC) activity in the developing rat testis and the quality and accessibility of the stem cell niche in wild type, and two busulfan-treated models of rat pup recipient testes using an SSC transplantation technique as a functional assay. While our results revealed a 69-fold increase in stem cell activity during rat testis development from neonate to adult, only moderate changes in SSC concentration were observed, and stem cells from neonate, pup, and adult donor testes produce spermatogenic colonies of similar size. Analysis of the stem cell niche in recipient rat testes demonstrated that pup testes support high levels of donor stem cell engraftment when endogenous germ cells are removed or compromised by busulfan treatment. Fertility was established when rat pup donor testis cells were transplanted into fetal- or pup-busulfan-treated recipient rat pup testes, and the donor genotype was transmitted to subsequent generations. These results provide insight into stem cell/niche interactions in the rat testis and demonstrate that techniques originally developed in mice can be extended to other species for regenerative medicine and germline modification.

Fertility and germline transmission of donor haplotype following germ cell transplantation in immunocompetent goats

Transplantation of spermatogonial stem cells into syngeneic or immunosuppressed recipient mice or rats can result in donor-derived spermatogenesis and fertility. Recently, this approach has been employed to introduce a transgene into the male germline. Germ-cell transplantation in species other than laboratory rodents, if successful, holds great promise as an alternative to the inefficient methods currently available to generate transgenic farm animals that can produce therapeutic proteins in their milk or provide organs for transplantation to humans. To explore whether germ-cell transplantation could result in donor-derived spermatogenesis and fertility in immunocompetent recipient goats, testis cells were transplanted from transgenic donor goats carrying a human alpha-1 antitrypsin expression construct to the testes of sexually immature wild-type recipient goats. After puberty, sperm carrying the donor-derived transgene were detected in the ejaculates of two out of five recipients. Mating of one recipient resulted in 15 offspring, one of which was transgenic for the donor-derived transgene. This is the first report of donor cell-derived sperm production and transmission of the donor haplotype to the next generation after germ-cell transplantation in a nonrodent species. Furthermore, these results indicate that successful germ-cell transplantation is feasible between immunocompetent, unrelated animals. In the future, transplantation of genetically modified germ cells may provide a more efficient alternative for production of transgenic domestic animals.

Restoration of fertility by germ cell transplantation requires effective recipient preparation

Spermatogonial transplantation provides access to the mammalian germline and has been used in experimental animal models to study stem cell/niche biology and germline development, to restore fertility, and to produce transgenic models. The potential to manipulate and/or transplant the germline has numerous practical applications that transcend species boundaries. To make the transplantation technology more broadly accessible, it is necessary to develop practical recipient preparation protocols. In the current study, mouse recipients for spermatogonial transplantation were prepared by treating pregnant females with the chemotherapeutic agent busulfan at different times during gestation. Donor germ cells were introduced into the testes of male progeny between 5 and 12 days postpartum. Analysis of recipient animals revealed that busulfan treatment of pregnant females on 12.5 days postcoitum was the most effective; male progeny transplanted with donor germ cells became fertile and passed the donor genotype to 25% of progeny. This approach was effective because 1) the cytoablative treatment reduced (but did not abolish) endogenous spermatogenesis, creating space for colonization by donor stem cells, 2) residual endogenous germ cells contributed to a healthy testicular environment that supported robust donor and recipient spermatogenesis, and 3) fetal busulfan-treated males could be transplanted as pups, which have been established as better recipients than adults. Laboratory mice provide a valuable experimental model for developing the technology that now can be applied and evaluated in other species.

Male germ line stem cells: from cell biology to cell therapy

Research using stem cells has several applications in basic biology and clinical medicine. Recent advances in the establishment of male germ line stem cells provided researchers with the ability to identify, isolate, maintain, expand and differentiate the spermatogonia, the primitive male germ cells, as cell lines under in vitro conditions. The ability to culture and manipulate stem cell lines from male germ cells has gradually facilitated research into spermatogenesis and male infertility, to an extent beyond that facilitated by the use of somatic stem cells. After the introduction of exogenous genes, the spermatogonial cells can be transplanted into the seminiferous tubules of recipients, where the transplanted cells can contribute to the offspring. The present review concentrates on the origin, life cycle and establishment of stem cell lines from male germ cells, as well as the current status of transplantation techniques and the application of spermatogonial stem cell lines.