Functional assessment of self-renewal activity of male germline stem cells following cytotoxic damage and serial transplantation

Spermatogonial stem cell self-renewal and development

Spermatogenesis originates from spermatogonial stem cells (SSCs). Development of the spermatogonial transplantation technique in 1994 provided the first functional assay to characterize SSCs. In 2000, glial cell line-derived neurotrophic factor was identified as a SSC self-renewal factor. This discovery not only provided a clue to understand SSC self-renewing mechanisms but also made it possible to derive germline stem (GS) cell cultures in 2003. In vitro culture of GS cells demonstrated their potential pluripotency and their utility in germline modification. However, in vivo SSC analyses have challenged the traditional concept of SSC self-renewal and have revealed their relationship with the microenvironment. An improved understanding of SSC self-renewal through functional assays promises to uncover fundamental principles of stem cell biology and will enable us to use these cells for applications in animal transgenesis and medicine.

Cell-cycle-dependent colonization of mouse spermatogonial stem cells after transplantation into seminiferous tubules

Spermatogonial stem cells (SSCs) migrate to the niche upon introduction into the seminiferous tubules of the testis of infertile animals. However, only 5–10% of the transplanted cells colonize recipient testes. In this study, we analyzed the impact of cell cycle on spermatogonial transplantation. We used fluorescent ubiquitination-based cell cycle indicator transgenic mice to examine the influence of cell cycle on SSC activity of mouse germline stem (GS) cells, a population of cultured spermatogonia enriched for SSCs. GS cells in the G1 phase are more efficient than those in the S/G2-M phase in colonizing the seminiferous tubules of adult mice. Cells in the G1 phase not only showed higher expression levels of GFRA1, a component of the GDNF self-renewal factor receptor, but also adhered more efficiently to laminin-coated plates. Furthermore, this cell cycle-dependency was not observed when cells were transplanted into immature pup recipients, which do not have the blood-testis barrier (BTB) between Sertoli cells, suggesting that cells in the G1 phase may passage through the BTB more readily than cells in the S/G2-M phase. Thus cell cycle status is an important factor in regulating SSC migration to the niche.

Effect of genistein administration on the recovery of spermatogenesis in the busulfan-treated rat testis

Objective

Impairment of spermatogenesis has been identified as an inevitable side effect of cancer treatment. Although estrogen treatment stimulates spermatogenic recovery from the impaired spermatogenesis by suppressing the intra-testicular testosterone (ITT) level, side effects of estrogen are still major impediments to its clinical application in humans. Soybeans are rich in genistein, which is a phytoestrogen that binds to estrogen receptors and has an estrogenic effect. We investigated the effects of genistein administration on ITT levels, testis weight, and recovery of spermatogenesis in rats treated with a chemotherapeutic agent, busulfan.

Methods

Busulfan was administered intraperitoneally to rats, and then a GnRH agonist was injected subcutaneously into the back, or genistein was administered orally.

Results

The weight of the testes was significantly reduced by the treatment with busulfan. The testis weight was partially restored after busulfan treatment by additional treatment with either the GnRH agonist or genistein. Busulfan also induced atrophy of a high percentage of the seminiferous tubules, but this percentage was decreased by additional treatment with either the GnRH agonist or genistein. Treatment with genistein was effective at suppressing and maintaining ITT levels comparable to that in the GnRH agonist group.

Conclusion

Genistein effectively suppressed ITT levels and stimulated the recovery of spermatogenesis in rats treated with a chemotherapeutic drug. This suggests that genistein may be a substitute for estrogens, for helping humans to recover fertility after cancer therapy without the risk of side effects.

Oncofertility and the male cancer patient

OPINION STATEMENT

Oncofertility as a discipline plays an important, adjunctive role in the treatment of male patients with cancer. Despite recommendations by the American Society of Clinical Oncology, many clinicians managing malignancies in males fail to consistently incorporate fertility preservation as a routine aspect of health care. Providers involved in the treatment of oncologic patients should have an awareness of the impact of their prescribed treatments on reproductive potential, just as they would be knowledgeable of the potential deleterious effects of cancer therapies on vital organs such as the kidneys, lungs, and liver. Providers should then have a discussion with their patients regarding these potential adverse therapeutic effects or consult a fertility preservation specialist to discuss these matters and fertility preservation options with the patient. Cryopreservation of sperm remains an excellent option for male fertility preservation as it is readily available and results in storage of viable gametes for future use in the event of post treatment infertility. With the use of assisted reproductive techniques (ART), cryopreserved sperm may ultimately result in successful paternity, even in the setting of very low numbers of stored sperm. While sperm cryopreservation is usually an option for adolescent and adult males, fertility preservation in pre-pubertal males presents a more challenging problem. To date, no clinically proven methods are available to preserve fertility in these males. However, some centers do offer experimental protocols under the oversight of an IRB, such as testicular tissue cryopreservation in these males. The hope is that one day science will provide a mechanism for immature germ cells from the testicular tissue of these patients to be used in vivo or in vitro to facilitate reproduction. In closing, studies have shown that the patient's regard for his provider is enhanced when the issue fertility preservation is raised. While oncologic care is often fraught with time constraints and acute medical concerns, fertility preservation care in the male can typically be administered quickly and without disruption of the overall plan of care.

Effect of liver growth factor on both testicular regeneration and recovery of spermatogenesis in busulfan-treated mice

BACKGROUND

Some adult stem cells persist in adult tissue; however, we do not know how to stimulate stem cells in adults to heal injuries. Liver growth factor (LGF) is a biliprotein with hepatic mitogen activity. Its concentration increases markedly in the presence of any type of liver injury, and it shows in vivo therapeutic biological activity at extrahepatic sites.

METHODS

We have analyzed the effect of LGF on the replenishment of germinal cells in the testes of mice injected with busulfan, a common cancer drug that also specifically affects germ line stem cells and spermatogonia. We determined the testicular and epididymal weight, spermatozoal concentration in the epididymis and sperm motility, and performed a histological analysis.

RESULTS

Intraperitoneal administration of LGF was able to partially restore spermatogenesis, as well as sperm production and motility, in mice sterilized with busulfan. LGF treatment in busulfan-treated animals that have suffered a disruption of spermatogenesis can accelerate the reactivation of this process in most of the tubules, as shown in the histological analysis.

CONCLUSIONS

Our results suggest a potential use of LGF in the mobilization of testicular stem cells and in the restoration of spermatogenesis after busulfan-induced damage to the testicular germinal epithelium.

Spermatogonial stem cells in higher primates: are there differences from those in rodents?

Spermatogonial stem cells (SSCs) maintain spermatogenesis throughout the reproductive life of mammals. While A(single) spermatogonia comprise the rodent SSC pool, the identity of the stem cell pool in the primate spermatogenic lineage is not well established. The prevailing model is that primate spermatogenesis arises from A(dark) and A(pale) spermatogonia, which are considered to represent reserve and active stem cells respectively. However, there is limited information about how the A(dark) and A(pale) descriptions of nuclear morphology correlate with the clonal (A(single), A(paired), and A(aligned)), molecular (e.g. GFRalpha1 (GFRA1) and PLZF), and functional (SSC transplantation) descriptions of rodent SSCs. Thus, there is a need to investigate primate SSCs using criteria, tools, and approaches that have been used to investigate rodent SSCs over the past two decades. SSCs have potential clinical application for treating some cases of male infertility, providing impetus for characterizing and learning to manipulate these adult tissue stem cells in primates (nonhuman and human). This review recounts the development of a xenotransplant assay for functional identification of primate SSCs and progress dissecting the molecular and clonal characteristics of the primate spermatogenic lineage. These observations highlight the similarities and potential differences between rodents and primates regarding the SSC pool and the kinetics of spermatogonial self-renewal and clonal expansion. With new tools and reagents for studying primate spermatogonia, the field is poised to develop and test new hypotheses about the biology and regenerative capacity of primate SSCs.

Genetic influences in mouse spermatogonial stem cell self-renewal

Spermatogonial stem cells (SSCs) are slowly dividing cells that undergo self-renewal division to support spermatogenesis. Although the effects of genetic background in stem cell self-renewal have been well studied in hematopoietic stem cells, little is known about its effect on stem cells in other self-renewing tissues, including SSCs. To examine whether genetic factors are involved in regulation of SSC self-renewal, we first studied spermatogenesis in different inbred mouse strains (C57BL/6, DBA/2, AKR, BALB/C and C3H) after chemical damage caused by busulfan. Spermatogenesis in the DBA/2 and AKR strains was relatively resistant to busulfan treatment, whereas spermatogenesis was diminished in C57BL/6 mice and nearly ablated in C3H and BALB/C mice. Serial germ cell transplantation experiments provided functional evidence that SSCs with the DBA/2 background expanded more rapidly than those with the B6 background. Finally, we also employed the Germline Stem (GS) cell culture technique to examine the self-renewal activity in vitro. Although genetic manipulation of GS cells has been limited to those from the DBA/2 background, we produced transgenic offspring of the C3H background by electroporation of GS cells with a plasmid vector. Our results underscore the importance of genetic factors in SSC self-renewal. Furthermore, application of genetic modification techniques to GS cells with non-DBA/2 backgrounds extends the potential of a SSC-based approach in male germline modification.

Phenotypic plasticity of mouse spermatogonial stem cells

Background

Spermatogonial stem cells (SSCs) continuously undergo self-renewal division to support spermatogenesis. SSCs are thought to have a fixed phenotype, and development of a germ cell transplantation technique facilitated their characterization and prospective isolation in a deterministic manner; however, our in vitro SSC culture experiments indicated heterogeneity of cultured cells and suggested that they might not follow deterministic fate commitment in vitro.

Methodology and Principal Findings

In this study, we report phenotypic plasticity of SSCs. Although c-kit tyrosine kinase receptor (Kit) is not expressed in SSCs in vivo, it was upregulated when SSCs were cultured on laminin in vitro. Both Kit⁻ and Kit⁺ cells in culture showed comparable levels of SSC activity after germ cell transplantation. Unlike differentiating spermatogonia that depend on Kit for survival and proliferation, Kit expressed on SSCs did not play any role in SSC self-renewal. Moreover, Kit expression on SSCs changed dynamically once proliferation began after germ cell transplantation in vivo.

Conclusions/Significance

These results indicate that SSCs can change their phenotype according to their microenvironment and stochastically express Kit. Our results also suggest that activated and non-activated SSCs show distinct phenotypes.

Testicular stem cells for fertility preservation: preclinical studies on male germ cell transplantation and testicular grafting

Spermatogonial stem cells open novel strategies for preservation of testicular tissue and fertility preservation in boys and men exposed to gonadotoxic therapies. This review provides an update on the physiology of spermatogonial stem cells in rodent and primate testes. Species-specific differences must be considered when new technologies on testicular stem cells are considered. Germ cell transplantation is presented as one novel and promising strategy. Whereas this technique has become an important research tool in rodents, a clinical application must still be regarded as experimental and many aspects of the procedure need to be optimized prior to a safe and efficient clinical application in men. Testicular grafting opens another exciting strategy for fertility preservation. Autologous and xenologous transfer of immature tissue revealed a high regenerative potential of immature testicular tissue. Grafting was applied in rodents and primates and resulted in the generation of sperm. Further research is needed before an application in humans can be considered safe and efficient. Despite the current limitations in regard to the generation of sperm from cryopreserved male germline cells and tissues, protocols for cryopreservation of testicular tissue are available and reveal a promising outcome. Since future improvements of germ cell transplantation and grafting approaches can be assumed, bioptic retrieval and cryopreservation of testicular tissue fragments should be performed in oncological patients at high risk of fertility loss since this is their only option to maintain their fertility potential.

CABS1 is a novel calcium-binding protein specifically expressed in elongate spermatids of mice

Single intraperitoneal injection of busulfan at 20 mg/kg body weight to mature male mice induced the deletion of the spermatogenic cells, followed by the restoration of the spermatogenesis by the surviving undifferentiated spermatogonia. The changes of the protein contents in testis during these processes were analyzed by two-dimensional gel electrophoresis in order to identify the proteins expressed at the specific stages of spermatogenesis. An acidic protein that disappeared and recovered in the same time course as spermatids after the busulfan treatment was identified as CABS1 by mass spectrometry. It was found that CABS1 was specifically expressed in the elongate spermatids at steps 13 to 16 in stages I to VIII of the seminiferous epithelium cycle of the mouse, and then it localized to the principal piece of flagellum of the mature sperm in the cauda epididymis. We have found for the first time that CABS1 is a calcium-binding protein that binds calcium during the maturation in the epididymis.

Cryopreservation in liquid nitrogen of gonocytes from neonatal porcine testes stored at 4°C

Aim:  Gonocytes are primitive germ cells in neonatal male testes. Germ cells from the neonatal testes of mice have a self-renewal activity and have pluripotential characteristics in established stem-cell lines. Therefore, these germ cells are reliable sources for the preservation of genetic resources of domestic animals and endangered species. The aim of the present study was to examine the cryopreservation of porcine gonocytes in liquid nitrogen (LN₂) from neonatal testes that were freshly collected or stored at 4°C for 24 h. Methods:  Gonocytes were isolated as lectin Dolichos biflorus agglutinin (DBA) positive cells from porcine testes 2-5 days after birth. The effects of the cryoprotectants used in the cryopreservation of the gonocytes, which were isolated from testes stored at 4°C in various solutions for 24 h, were examined on the results of cell viability after cryopreservation and cell proliferation in culture. Testis tissues from stored testes were transplanted into immunodeficient mice to evaluate the ability of the gonocytes to differentiate 5 weeks after transplantation. Results:  The portion of the gonocytes that was isolated from stored testes at 4°C was approximately 70%. The viability of the gonocytes from stored testes was significantly higher in HEPES-supplemented Dulbecco's Modified Eagle's Medium/F12 (DMEM/F12) and HEPES-supplemented phosphate-buffered saline than from stock solutions without HEPES. The addition of 10% dimethylsulfoxide (DMSO) and 0.07 mmol/L sucrose to cryopreservation solutions supported high viability of gonocytes after freezing and thawing. The cryopreserved gonocytes formed colonies with DBA activity in DMEM/F12 supplemented with 10% fetal bovine serum 3 days after culture and continued to proliferate for at least 12 days in culture. The germ cells in the testis tissues that were xenografted into immunodefficient mice differentiated into primitive spermatogonia. Conclusion:  Gonocytes in the testis stored at 4°C for at least 24 h, isolated and cryopreserved can survive. The cryopreserved gonocytes differentiated in immunodefficient mice and proliferated along with the formation of colonies in vitro. (Reprod Med Biol 2008; 7: 153-160).

Changes in the expression of P-cadherin in the normal, cryptorchid and busulphan-treated rat testis

Adhesion between Sertoli cells and germ cells is important for spermatogenesis. Cadherins are Ca(2+)-dependent transmembrane proteins that mediate cell-cell adhesion. The aim of this study was to compare the expression of P-cadherin in unilaterally cryptorchid and busulphan-treated rat testes using immunohistochemistry. The pattern of expression of P-cadherin in the seminiferous epithelium changed with the stage of the seminiferous epithelium. The membranes of round spermatids and membranes and cytoplasm of spermatocytes were strongly positive. Our experiments revealed that busulphan treatment (2 doses - 10 mg/kg of body weight - 21 days apart) and cryptorchism led to destructive changes in the structure of seminiferous tubules, together with the decrease in P-cadherin expression. The expression of P-cadherin disappeared in the spermatids segregated from the epithelium while segregated spermatocytes remained still positive for P-cadherin during the 3- to 11-day cryptorchid period. In busulphan-treated animals, the expression of P-cadherin was dependent on the presence or absence of the spermatocytes and spermatids in the tubules. Strong positivity for P-cadherin was observed in the spermatocytes that re-appeared in the regenerating seminiferous epithelium. We suggest that P-cadherin participates in the architecture of adherens junctions in testis, plays an important role in maintaining normal spermatogenesis and that cryptorchism and busulphan treatment lead to adherens junction disintegration.

Male Germ Line Stem Cells Have an Altered Potential to Proliferate and Differentiate During Postnatal Development in Mice1

Spermatogonial stem cells (SSCs) continuously support spermatogenesis after puberty. However, accumulating evidence suggests that SSCs differ functionally during postnatal development. For example, mutant mice exist in which SSCs support spermatogenesis in the first wave after birth but cease to do so thereafter, resulting in infertility in adults. Studies using a retroviral vector have shown that the vector transduces pup SSCs more efficiently than adult SSCs, which suggests that pup SSCs divide more frequently. Thus, it is hypothesized that the SSCs in pup and adult testes have different characteristics. As an approach to testing this hypothesis in the present study, we investigated the proliferation kinetics of pup SSCs (6-9 days old) and their self-renewal/differentiation patterns for the first 2 mo after transplantation, and compared them to those of adult SSCs. Using serial transplantation, we found that the number of pup SSCs declined over the first week after transplantation. Thereafter, it increased ~4-fold by 1 mo and ~9-fold by 2 mo after transplantation, which indicates that pup SSCs continuously proliferate from 1 wk to 2 mo after transplantation. Compared to the proliferation of SSCs derived from adult intact testes, that of pup SSCs was lower at 1 mo but similar at 2 mo, indicating the delayed proliferation of pup SSCs. However, the pup SSCs regenerated spermatogenic colonies at 1 mo that were similar in length to those of SSCs from adult intact testes. Therefore, these results suggest that some functional differences exist in SSCs during postnatal development, and that these differences may affect the abilities of SSCs to self-renew and differentiate.

Functional identification of the actual and potential stem cell compartments in mouse spermatogenesis

To clarify the mechanisms that support the continuity of actively cycling tissues of long-lived organisms, we investigated the composition of a mouse spermatogenic stem cell system by pulse-chase of the undifferentiated spermatogonia, the population responsible for stem cell functions, in combination with transplantation and regeneration assays after pulse-labeling. We demonstrate that in addition to "actual stem cells," which are indeed self-renewing, a second population ("potential stem cells") also exists, which is capable of self-renewing but do not self-renew in the normal situation. Potential stem cells rapidly turn over in normal testes, suggesting that they belong to the transit-amplifying, rather than the dormant, population. During the long natural course, actual stem cells are occasionally lost and compensated for by progeny of their neighbors. In this process, potential stem cells are postulated to shift their modes from transit amplification to self-renewal, thus playing an essential role to ensure spermatogenesis integrity.

A revised model for spermatogonial expansion in man: lessons from non-human primates

We have recently described a revised scheme for spermatogonial expansion in non-human primates. We proposed that Apale-spermatogonia act as self-renewing progenitors and premeiotic germ cells are organized and divide as small clones. Here, we are revisiting the model described for man and propose a modified scheme for spermatogonial expansion. Our revised model shows high similarity to the scheme proposed for non-human primates and is in accordance with all previous and present data.

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.

Testicular xenografts: a novel approach to study cytotoxic damage in juvenile primate testis

The underlying primary damage to the testis caused by chemotherapeutic regimens during childhood is largely unknown. Xenografting of monkey testes was successfully applied in maturation of juvenile testis to the point of complete spermatogenesis. This allows us to manipulate developing primate testis without direct treatment of patients. This new model is validated establishing the effects of cytotoxic treatment in the immature primate testis. Male castrated nude mice received eight s.c. grafts of juvenile monkey testicular tissue and, 28 weeks later, were injected with busulfan (38 mg/kg, i.p.) or vehicle. Graft numbers, size, and histology were examined. Grafts showed pubertal induction of spermatogenesis to the level of pachytene spermatocytes at point of busulfan treatment and further progressed to the level of round spermatids in control samples at 4 weeks. Busulfan treatment caused a statistically significant decrease in the number of seminiferous tubules containing germ cells. Type B spermatogonia and more advanced stages of spermatogenesis were depleted. A statistically significant decrease to pretreatment level was observed in the number of type A pale and centrally located spermatogonia. Busulfan did not affect type A dark spermatogonia. Occasionally, elongating spermatids were detected in busulfan-treated grafts. Observations show that busulfan selectively destroys differentiating spermatogonia whereas some of the spermatocytes present at the moment of cytotoxic insult are able to continue differentiation. Data indicate that xenografting of testicular monkey tissue is a valid approach to detect the busulfan-induced germ cell damage and serves as a powerful experimental tool to study cytotoxic effects in developing primate testis.

The Niche for Spermatogonial Stem Cells in the Mammalian Testis

The theory of the "stem cell niche" was originally proposed for the hematopoietic system, and the existence of the niche as an actual entity was proved in the Drosophila germ cell system. Historically, mammalian spermatogenesis has been studied extensively as a prime example of a stem cell system, and studies have established a stem-progenitor hierarchical order of spermatogonia. In the niche on the basal lamina of seminiferous tubules, spermatogonial stem cells (SSCs) are secluded from the outside world and divide constantly to self-renew and differentiate. During the last 10 years, the development and exploitation of the germ cell transplantation method has expanded our understanding of the nature of SSCs and their niches. The ability to maintain and expand SSCs in vitro, which recently became possible, has further reinforced this research area as a mecca of stem cell biology. Nonetheless, the mammalian germ stem cell and its niche remain to be defined more strictly and precisely. We are still on a journey in search of the real stem cell and its true niche.

GDNF family receptor alpha1 phenotype of spermatogonial stem cells in immature mouse testes

Spermatogonial stem cells (SSCs) are essential for spermatogenesis, and these adult tissue stem cells balance self-renewal and differentiation to meet the biological demand of the testis. The developmental dynamics of SSCs are controlled, in part, by factors in the stem cell niche, which is located on the basement membrane of seminiferous tubules situated among Sertoli cells. Sertoli cells produce glial cell line-derived neurotrophic factor (GDNF), and disruption of GDNF expression results in spermatogenic defects and infertility. The GDNF signals through a receptor complex that includes GDNF family receptor alpha1 (GFRA1), which is thought to be expressed by SSCs. However, expression of GFRA1 on SSCs has not been confirmed by in vivo functional assay, which is the only method that allows definitive identification of SSCs. Therefore, we fractionated mouse pup testis cells based on GFRA1 expression using magnetic activated cell sorting. The sorted and depleted fractions of GFRA1 were characterized for germ cell markers by immunocytochemistry and for stem cell activity by germ cell transplantation. The GFRA1-positive cell fraction coeluted with other markers of SSCs, including ITGA6 and CD9, and was significantly depleted of KIT-positive cells. The transplantation results confirmed that a subpopulation of SSCs expresses GFRA1, but also that the stem cell pool is heterogeneous with respect to the level of GFRA1 expression. Interestingly, POU5F1-positive cells were enriched nearly 15-fold in the GFRA1-selected fraction, possibly suggesting heterogeneity of developmental potential within the stem cell pool.

Isolation of male germ-line stem cells; influence of GDNF

The identification and physical isolation of testis stem cells, a subset of type A spermatogonia, is critical to our understanding of their growth regulation during the first steps of spermatogenesis. These stem cells remain poorly characterized because of the paucity of specific molecular markers that permit us to distinguish them from other germ cells. Thus, the molecular mechanisms driving the first steps of spermatogenesis are still unknown. We show in the present study that GFR alpha-1, the receptor for GDNF (glial cell line-derived neurotrophic factor), is strongly expressed by a subset of type A spermatogonia in the basal part of the seminiferous epithelium. Using this characteristic, we devised a method to specifically isolate these GFR alpha-1-positive cells from immature mouse testes. The isolated cells express Ret, a tyrosine kinase transmembrane receptor that mediates the intracellular response to GDNF via GFR alpha-1. After stimulation with rGDNF, the isolated cells proliferated in culture and underwent the first steps of germ cell differentiation. Microarray analysis revealed that GDNF induces the differential expression of a total of 1124 genes. Among the genes upregulated by GDNF were many genes involved in early mammalian development, differentiation, and the cell cycle. This report describes the first isolation of a pure population of GFR alpha-1-positive cells in the testis and identifies signaling pathways that may play a crucial role in maintaining germ-line stem cell proliferation and/or renewal.

Murine male germ cell apoptosis induced by busulfan treatment correlates with loss of c-kit-expression in a Fas/FasL- and p53-independent manner

Male germ cell apoptosis has been extensively explored in rodents. In contrast, very little is known about the susceptibility of developing germ cells to apoptosis in response to busulfan treatment. Spontaneous apoptosis of germ cells is rarely observed in the adult mouse testis, but under the experimental conditions described here, busulfan-treated mice exhibited a marked increase in apoptosis and a decrease in testis weight. TdT-mediated dUTP-X nicked end labeling analysis indicates that at one week following busulfan treatment, apoptosis was confined mainly to spermatogonia, with lesser effects on spermatocytes. The percentage of apoptosis-positive tubules and the apoptotic cell index increased in a time-dependent manner. An immediate effect was observed in spermatogonia within one week of treatment, and in the following week, secondary effects were observed in spermatocytes. RT-PCR analysis showed that expression of the spermatogonia-specific markers c-kit and Stra 8 was reduced but that Gli I gene expression remained constant, which is indicative of primary apoptosis of differentiating type A spermatogonia. Three and four weeks after busulfan treatment, RAD51 and FasL expression decreased to nearly undetectable levels, indicating that meiotic spermatocytes and post-meiotic cells, respectively, were lost. The period of germ cell depletion did not coincide with increased p53 or Fas/FasL expression in the busulfan-treated testis, although p110Rb phosphorylation and PCNA expression were inhibited. These data suggest that increased depletion of male germ cells in the busulfan-treated mouse is mediated by loss of c-kit/SCF signaling but not by p53- or Fas/FasL-dependent mechanisms. Spermatogonial stem cells may be protected from cell death by modulating cell cycle signaling such that E2F-dependent protein expression, which is critical for G1 phase progression, is inhibited.

Regulation of Mouse Spermatogonial Stem Cell Self-Renewing Division by the Pituitary Gland1

Spermatogenesis originates in spermatogonial stem cells, which have the unique mode of replication. It is considered that a single stem cell can produce two stem cells (self-renewing division), one stem and one differentiating (asymmetric division), or two differentiating cells (differentiating division). However, little is known regarding how each type of division is regulated. In this investigation, we focused on the analysis of self- renewing division and examined the effect of the pituitary gland using two models of stem cell self-renewing division. In the first experiment using newborn mice, the administration of GnRH- analogue, which represses the release of gonadotropin, reduced the number of stem cells during postnatal testicular development, suggesting that the pituitary gland enhances stem cell self- renewing division. In the second experiment, however, the number of stem cells increased dramatically in hypophysectomized adult recipients after spermatogonial transplantation. Thus, the pituitary gland affects the self-renewing division of stem cells, but these contradictory results suggest that its role may be different depending on the stage of the testicular development.

Identification and enrichment of spermatogonial stem cells displaying side-population phenotype in immature mouse testis

In mammals, spermatogenesis is maintained by spermatogonial stem cells (SSC). In their niche, SSC divide to self-maintain and to produce a transit-amplifying population that eventually enters the meiotic cycle to give rise to spermatozoa. The low number of SSC and the lack of specific markers hinder their isolation and enrichment. Stem cells in several adult tissues can be identified by using their verapamil-sensitive Hoechst dye-effluxing properties, which define the characteristic "side population" (SP). Here we show, by multicolor flow cytometric analysis, that immature mouse testis contains a "side-population" (T-SP), which is Sca-1pos, Ep-CAMpos, EE2 pos, alpha6-integrin pos, and alpha(v)-integrin neg. A 13-fold enrichment in SSC activity was observed when sorted T-SP cells from ROSA 26 mice were transplanted in busulfan-treated mouse testis. Whereas an incomplete range of spermatogenic stages was encountered two months after transplantation of unsorted testicular cells, the transplantation of T-SP cells generated all associations of mouse germ cells representing the full range of spermatogenic stages. These data suggest that Hoechst staining and cell sorting might provide a novel approach to SSC enrichment in mammals.