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

The pluripotency factor LIN28 marks undifferentiated spermatogonia in mouse

BACKGROUND

Life-long production of spermatozoa depends on spermatogonial stem cells. Spermatogonial stem cells exist among the most primitive population of germ cells - undifferentiated spermatogonia. Transplantation experiments have demonstrated the functional heterogeneity of undifferentiated spermatogonia. Although the undifferentiated spermatogonia can be topographically divided into As (single), Apr (paired), and Aal (aligned) spermatogonia, subdivision of this primitive cell population using cytological markers would greatly facilitate characterization of their functions.

RESULTS

In the present study, we show that LIN28, a pluripotency factor, is specifically expressed in undifferentiated spermatogonia (As, Apr, and Aal) in mouse. Ngn3 also specifically labels undifferentiated spermatogonia. We used Ngn3-GFP knockin mice, in which GFP expression is under the control of all Ngn3 transcription regulatory elements. Remarkably, Ngn3-GFP is only expressed in approximately 40% of LIN28-positive As (single) cells. The percentage of Ngn3-GFP-positive clusters increases dramatically with the chain length of interconnected spermatogonia.

CONCLUSION

Our study demonstrates that LIN28 specifically marks undifferentiated spermatogonia in mice. These data, together with previous studies, suggest that the LIN28-expressing undifferentiated spermatogonia exist as two subpopulations: Ngn3-GFP-negative (high stem cell potential) and Ngn3-GFP-positive (high differentiation commitment). Furthermore, Ngn3-GFP-negative cells are found in chains of Ngn3-GFP-positive spermatogonia, suggesting that cells in the Aal spermatogonia could revert to a more primitive state.

Isolation and characterization of pluripotent human spermatogonial stem cell-derived cells

Several reports have documented the derivation of pluripotent cells (multipotent germline stem cells) from spermatogonial stem cells obtained from the adult mouse testis. These spermatogonia-derived stem cells express embryonic stem cell markers and differentiate to the three primary germ layers, as well as the germline. Data indicate that derivation may involve reprogramming of endogenous spermatogonia in culture. Here, we report the derivation of human multipotent germline stem cells (hMGSCs) from a testis biopsy. The cells express distinct markers of pluripotency, form embryoid bodies that contain derivatives of all three germ layers, maintain a normal XY karyotype, are hypomethylated at the H19 locus, and express high levels of telomerase. Teratoma assays indicate the presence of human cells 8 weeks post-transplantation but limited teratoma formation. Thus, these data suggest the potential to derive pluripotent cells from human testis biopsies but indicate a need for novel strategies to optimize hMGSC culture conditions and reprogramming.

Wnt signaling promotes proliferation and stemness regulation of spermatogonial stem/progenitor cells

Spermatogonial stem cells (SSCs) self-renew throughout life to produce progenitor cells that are able to differentiate into spermatozoa. However, the mechanisms underlying the cell fate determination between self-renewal and differentiation have not yet been delineated. Culture conditions and growth factors essential for self-renewal and proliferation of mouse SSCs have been investigated, but no information is available related to growth factors that affect fate determination of human spermatogonia. Wnts form a large family of secreted glycoproteins, the members of which are involved in cell proliferation, differentiation, organogenesis, and cell migration. Here, we show that Wnts and their receptors Fzs are expressed in mouse spermatogonia and in the C18-4 SSC line. We demonstrate that WNT3A induces cell proliferation, morphological changes, and cell migration in C18-4 cells. Furthermore, we show that beta-catenin is activated during testis development in 21-day-old mice. In addition, our study demonstrates that WNT3A sustained adult human embryonic stem (ES)-like cells derived from human germ cells in an undifferentiated stage, expressing essential human ES cell transcription factors. These results demonstrate for the first time that Wnt/beta-catenin pathways, especially WNT3A, may play an important role in the regulation of mouse and human spermatogonia.

A GRFa2/Prop1/stem (GPS) cell niche in the pituitary

BACKGROUND

The adult endocrine pituitary is known to host several hormone-producing cells regulating major physiological processes during life. Some candidates to progenitor/stem cells have been proposed. However, not much is known about pituitary cell renewal throughout life and its homeostatic regulation during specific physiological changes, such as puberty or pregnancy, or in pathological conditions such as tumor development.

PRINCIPAL FINDINGS

We have identified in rodents and humans a niche of non-endocrine cells characterized by the expression of GFRa2, a Ret co-receptor for Neurturin. These cells also express b-Catenin and E-cadherin in an oriented manner suggesting a planar polarity organization for the niche. In addition, cells in the niche uniquely express the pituitary-specific transcription factor Prop1, as well as known progenitor/stem markers such as Sox2, Sox9 and Oct4. Half of these GPS (GFRa2/Prop1/Stem) cells express S-100 whereas surrounding elongated cells in contact with GPS cells express Vimentin. GFRa2+-cells form non-endocrine spheroids in culture. These spheroids can be differentiated to hormone-producing cells or neurons outlining the neuroectoderm potential of these progenitors. In vivo, GPSs cells display slow proliferation after birth, retain BrdU label and show long telomeres in its nuclei, indicating progenitor/stem cell properties in vivo.

SIGNIFICANCE

Our results suggest the presence in the adult pituitary of a specific niche of cells characterized by the expression of GFRa2, the pituitary-specific protein Prop1 and stem cell markers. These GPS cells are able to produce different hormone-producing and neuron-like cells and they may therefore contribute to postnatal pituitary homeostasis. Indeed, the relative abundance of GPS numbers is altered in Cdk4-deficient mice, a model of hypopituitarism induced by the lack of this cyclin-dependent kinase. Thus, GPS cells may display functional relevance in the physiological expansion of the pituitary gland throughout life as well as protection from pituitary disease.

Signaling pathways in spermatogonial stem cells and their disruption by toxicants

Spermatogenesis is a complex biological process that is particularly sensitive to environmental insults such as chemicals and physical stressors. Exposure to specific chemicals has been shown to inhibit fertility through a negative impact on germ cell proliferation and differentiation that can lower sperm count. In addition, toxicants might produce DNA damages that could have negative consequences on the development of the offspring. This review describes spermatogonial stem cell development in the testis, signaling pathways that are crucial for self-renewal, and possible target molecules for environmental toxicants such as phthalate esters and nanoparticles.

Achieving high survival rate following cryopreservation after isolation of prepubertal mouse spermatogonial cells

PURPOSE

Isolating spermatogonia cells with high purity and viability and achieving better survival rate following cryopreservation

METHODS

Isolating the cells by Magnetic Activating Cell Sorting (MACS) method using anti CD49f (alpha6 integrin) antibody and Dynabeads and freezing in DMSO-based freezing mediums containing three different FBS concentrations of 50%, 60% and 70%.

RESULTS

The mean (+/-SD) purity of the isolated cells was 92.52+/-3.57 (range 92.43-98.25). The cells frozen in group I, II and III had mean 39.60+/-1.48 (range 37.98-41.62), 89.05+/-3.83 (range 80.83-90.33) and 90.52+/-1.71 (range 89.07-92.52) viability, respectively.

CONCLUSION

Higher viable cell counts and purity can be attained by the use of alpha6 integrin and magnetic beads. After the thawing of spermatogonial cells, optimum viability was achieved in freezing media containing 60% FBS.

Spermatogonial stem cells: unlimited potential

Recent reports have demonstrated that adult cells can be reprogrammed to pluripotency, but mostly with genes delivered using retroviruses. Some of the genes are cancer causing; thus, these adult-derived embryonic stem (ES)-like cells cannot be used for therapy to cure human diseases. Remarkably, it has also been demonstrated recently by several groups that, in mice, spermatogonial stem cells (SSCs) can be reprogrammed to ES-like cells without the necessity of exogenously added genes. SSCs constitute one of the most important stem cell systems in the body, not only because they produce spermatozoa that transmit genetic information from generation to generation, but also because of the recent studies showing their remarkable plasticity. Very little is known about SSCs in humans, except for the earlier work of Clermont and colleagues who demonstrated that there are Adark and Apale spermatogonia, with the Adark referred to as the reserve stem cells and the Apale being the renewing stem cells. We now demonstrate that G protein-coupled receptor 125 (GPR125) may be a marker for human SSCs. Putative human SSCs can also be reprogrammed to pluripotency. We were able to achieve this result without the addition of genes, suggesting that human SSCs have considerable potential for cell-based, autologous organ regeneration therapy for various diseases.

The molecular signature of spermatogonial stem/progenitor cells in the 6-day-old mouse testis

To characterize the molecular phenotype of spermatogonial stem cells (SSCs), we examined genes that are differentially expressed in the stem/progenitor spermatogonia compared to nonstem spermatogonia. We isolated type A spermatogonia (stem and nonstem type A) from 6-day-old mice using sedimentation velocity at unit gravity and further selected the stem/progenitor cell subpopulation by magnetic activated cell sorting with an antibody to GDNF-receptor-alpha-1 (GFRA1). It has been previously shown that GFRA1 is expressed in SSCs and is required for their stemness. The purity of the isolated cells was approximately 95% to 99% as indicated by immunocytochemistry using anti-GFRA1. Comparison of GFRA1-positive and GFRA1-negative spermatogonia by microarray analysis revealed 99 known genes and 12 uncharacterized transcripts that are overexpressed in the former cell population with a >2-fold change. Interestingly, the highest level of overexpression was observed for Csf1r, encoding the receptor for macrophage colony-stimulating factor (M-CSF, official symbol CSF1), which has a well-established role in the regulation of myeloid progenitor cells. Analysis of our microarray data with a bioinformatics software program (Ingenuity Systems) revealed the potential role of various signaling pathways in stem/progenitor spermatogonia and suggested a common pathway for GFRA1 and CSF1R that may lead to their proliferation. Further investigation to test this hypothesis has shown that CSF1 promotes cell proliferation in primary cultures of the isolated type A spermatogonia and in the spermatogonial-derived stem cell line C18-4. Semiquantitative RT-PCR and immunohistochemistry confirmed the previously mentioned microarray data. Collectively, this study provides novel molecular signatures for stem/progenitor spermatogonia and demonstrates a role for CSF1/CSF1R signaling in regulating their proliferation.

Regulation of spermatogonial stem cell self-renewal in mammals

Mammalian spermatogenesis is a classic adult stem cell-dependent process, supported by self-renewal and differentiation of spermatogonial stem cells (SSCs). Studying SSCs provides a model to better understand adult stem cell biology, and deciphering the mechanisms that control SSC functions may lead to treatment of male infertility and an understanding of the etiology of testicular germ cell tumor formation. Self-renewal of rodent SSCs is greatly influenced by the niche factor glial cell line-derived neurotrophic factor (GDNF). In mouse SSCs, GDNF activation upregulates expression of the transcription factor-encoding genes bcl6b, etv5, and lhx1, which influence SSC self-renewal. Additionally, the non-GDNF-stimulated transcription factors Plzf and Taf4b have been implicated in regulating SSC functions. Together, these molecules are part of a robust gene network controlling SSC fate decisions that may parallel the regulatory networks in other adult stem cell populations.

Spermatogonial stem cell self-renewal requires OCT4, a factor downregulated during retinoic acid-induced differentiation

The long-term production of billions of spermatozoa relies on the regulated proliferation and differentiation of spermatogonial stem cells (SSCs). To date only a few factors are known to function in SSCs to provide this regulation. Octamer-4 (OCT4) plays a critical role in pluripotency and cell survival of embryonic stem cells and primordial germ cells; however, it is not known whether it plays a similar function in SSCs. Here, we show that OCT4 is required for SSC maintenance in culture and for colonization activity following cell transplantation, using lentiviral-mediated short hairpin RNA expression to knock down OCT4 in an in vitro model for SSCs ("germline stem" [GS] cells). Expression of promyelocytic leukemia zinc-finger (PLZF), a factor known to be required for SSC self-renewal, was not affected by OCT4 knockdown, suggesting that OCT4 does not function upstream of PLZF. In addition to developing a method to test specific gene function in GS cells, we demonstrate that retinoic acid (RA) triggers GS cells to shift to a differentiated, premeiotic state lacking OCT4 and PLZF expression and colonization activity. Our data support a model in which OCT4 and PLZF maintain SSCs in an undifferentiated state and RA triggers spermatogonial differentiation through the direct or indirect downregulation of OCT4 and PLZF. The current study has important implications for the future use of GS cells as an in vitro model for spermatogonial stem cell biology or as a source of embryonic stem-like cells. Disclosure of potential conflicts of interest is found at the end of this article.

Regulation of the spermatogonial stem cell niche

Spermatogonial stem cells (SSCs) reside within specialized microenvironments called 'niches', which are essential for their maintenance and self-renewal. In the mammalian testis, the main components of the niche include the Sertoli cell, the growth factors that this nursing cell produces, the basement membrane, and stimuli from the vascular network between the seminiferous tubules. This review focuses on signalling pathways maintaining SSCs self-renewal and differentiation and describes potential mechanisms of regulation of the spermatogonial stem cell niche.

Gdnf signaling pathways within the mammalian spermatogonial stem cell niche

Mammalian spermatogenesis is a complex process in which male germ-line stem cells develop to ultimately form spermatozoa. Spermatogonial stem cells, or SSCs, are found in the basal compartment of the seminiferous epithelium. They self-renew to maintain the pool of stem cells throughout life, or they differentiate to generate a large number of germ cells. A balance between SSC self-renewal and differentiation in the adult testis is therefore essential to maintain normal spermatogenesis and fertility. Maintenance and self-renewal are tightly regulated by extrinsic signals from the surrounding microenvironment, called the spermatogonial stem cell niche. By physically supporting the SSCs and providing them with growth factors, the Sertoli cell is the main component of the niche. In addition, adhesion molecules that connect the SSCs to the basement membrane and cellular components of the interstitium between the seminiferous tubules are important regulators of the niche function. This review mainly focuses on glial cell line-derived neurotrophic factor (Gdnf), which is produced by Sertoli cells to maintain SSCs self-renewal, and the downstream signaling pathways induced by this crucial growth factor. Interactions between Gdnf and other signaling pathways that maintain self-renewal, as well as the role of novel SSC- and Sertoli cell-specific transcription factors, are also discussed.

Deriving multipotent stem cells from mouse spermatogonial stem cells: a new tool for developmental and clinical research

In recent years, embryonic stem (ES) cell-like cells have been obtained from cultured mouse spermatogonial stem cells (SSCs). These advances have shown that SSCs can transition from being the stem cell-producing cells of spermatogenesis to being multipotent cells that can differentiate into derivatives of all three germ layers. As such, they offer new possibilities for studying the mechanisms that regulate stem cell differentiation. The extension of these findings to human SSCs offers a route to obtaining personalized ES-like or differentiated cells for use in regenerative medicine. Here, we compare the different approaches used to derive ES-like cells from SSCs and discuss their importance to clinical and developmental research.

Expression of the pluripotency marker UTF1 is restricted to a subpopulation of early A spermatogonia in rat testis

The population of early A spermatogonia includes stem cells that possess spermatogonial stem cell properties. Recent reports suggest that these cells have the ability to regain pluripotent properties. Here, we show that expression of the pluripotency marker undifferentiated embryonic cell transcription factor 1 (UTF1) is restricted to distinct germ cells within the testis. In embryonic and neonatal testes, all gonocytes were found to strongly express UTF1. During further testicular development, expression of UTF1 was restricted to a subset of A spermatogonia and with the increase in age the number of cells expressing UTF1 decreased even more. Ultimately, in the adult rat testis, only a small subset of the A spermatogonia expressed UTF1. Remarkably, even in testes of vitamin A-deficient rats, in which the early A spermatogonia (A(s), A(pr), and A(al)) are the only type of spermatogonia, only a subset of the spermatogonia expressed UTF1. In the adult rat testis, expression of UTF1 is restricted to a subpopulation of the ZBTB16 (PLZF)-positive early A spermatogonia. Furthermore, the observed distribution pattern of UTF1-expressing cells over the different stages of the cycle of the seminiferous epithelium suggests that the expression of UTF1 is restricted to those A(s), A(pr), and short chains of A(al) spermatogonia that are in the undifferentiated state and therefore maintain the ability to differentiate into A1 spermatogonia in the next round of the epithelial cycle or possibly even in other directions when they are taken out of their testicular niche.

Study of the potential spermatogonial stem cell compartment in dogfish testis, Scyliorhinus canicula L

In the lesser-spotted dogfish (Scyliorhinus canicula), spermatogenesis takes place within spermatocysts made up of Sertoli cells associated with stage-synchronized germ cells. As shown in testicular cross sections, cysts radiate in maturational order from the germinative area, where they are formed, to the opposite margin of the testis, where spermiation occurs. In the germinative zone, which is located in a specific area between the tunica albuginea of the testis and the dorsal testicular vessel, individual large spermatogonia are surrounded by elongated somatic cells. The aim of this study has been to define whether these spermatogonia share characteristics with spermatogonial stem cells described in vertebrate and non-vertebrate species. We have studied their ultrastructure and their mitotic activity by 5'-bromo-2'-deoxyuridine (BrdU) incorporation and proliferating cell nuclear antigen (PCNA) immunodetection. Additionally, immunodetection of c-Kit receptor, a marker of differentiating spermatogonia in rodents, and of alpha- and beta-spectrins, as constituents of the spectrosome and the fusome, has been performed. Ultrastructurally, nuclei of stage I spermatogonia present the same mottled aspect in dogfish as undifferentiated spermatogonia nuclei in rodents. Moreover, intercellular bridges are not observed in dogfish spermatogonia, although they are present in stage II spermatogonia. BrdU and PCNA immunodetection underlines their low mitotic activity. The presence of a spectrosome-like structure, a cytological marker of the germline stem cells in Drosophila, has been observed. Our results constitute the first step in the study of spermatogonial stem cells and their niche in the dogfish.

Effects of ETV5 (ets variant gene 5) on testis and body growth, time course of spermatogonial stem cell loss, and fertility in mice

The transcription factor ets variant gene 5 (ETV5; also known as ERM) is essential for self-renewal of spermatogonial stem cells (SSCs). Mice with targeted disruption of Etv5 (Etv5(-/-)) undergo the first wave of spermatogenesis, but all SSCs are lost during this time, causing a Sertoli cell-only phenotype. This study examined body and testis growth and the time course of SSC loss in Etv5(-/-) mice to understand how loss of ETV5 impacts testicular and somatic development. Body weights were reduced in postnatal Etv5(-/-) males, indicating a role of ETV5 in growth. Testis weights and histology in Etv5(-/-) and wild-type (WT) males were similar at Postnatal Day 4, but testis weights were reduced at d8 and subsequently, indicating that ETV5 impacts postnatal testis growth. SSC density (SSCs per mum(2) of seminiferous tubule), estimated using an antibody against GFRA1, was similar in 4d WT and Etv5(-/-) mice. By 8 and 12d, GFRA1-positive cell density in Etv5(-/-) mice was decreased 17% and 32%, respectively, vs. WT. By 28d, GFRA1-positive cell density in Etv5(-/-) was reduced 95%, and GFRA1-positive cells were absent in 36d Etv5(-/-) males. In contrast to WT, 35- to 56-day-old Etv5(-/-) mice were infertile as assessed by natural breeding, artificial insemination, and in vitro fertilization, although motile sperm were present in epididymides of Etv5(-/-) mice during this time. In summary, initial testis development is normal in Etv5(-/-) mice despite decreased body weight, but SSC loss begins between 4 and 8d of age, indicating that ETV5 has effects beginning in the early neonatal period. Etv5(-/-) mice are infertile even when sperm is produced, indicating that ETV5 loss has other effects besides lack of SSC self-renewal that impair fertility.

Gdnf upregulates c-Fos transcription via the Ras/Erk1/2 pathway to promote mouse spermatogonial stem cell proliferation

Glial cell line-derived neurotrophic factor (GDNF) plays a crucial role in regulating the proliferation of spermatogonial stem cells (SSC). The signaling pathways mediating the function of GDNF in SSC remain unclear. This study was designed to determine whether GDNF signals via the Ras/ERK1/2 pathway in the C18-4 cells, a mouse SSC line. The identity of this cell line was confirmed by the expression of various markers for germ cells, proliferating spermatogonia, and SSC, including GCNA1, Vasa, Dazl, PCNA, Oct-4, GFRalpha1, Ret, and Plzf. Western blot analysis revealed that GDNF activated Ret tyrosine phosphorylation. All 3 isoforms of Shc were phosphorylated upon GDNF stimulation, and GDNF induced the binding of the phosphorylated Ret to Shc and Grb2 as indicated by immunoprecipitation and Western blotting. The active Ras was induced by GDNF, which further activated ERK1/2 phosphorylation. GDNF stimulated the phosphorylation of CREB-1, ATF-1, and CREM-1, and c-fos transcription. Notably, the increase in ERK1/2 phosphorylation, c-fos transcription, bromodeoxyuridine incorporation, and metaphase counts induced by GDNF, was completely blocked by pretreatment with PD98059, a specific inhibitor for MEK1, the upstream regulator of ERK1/2. GDNF stimulation eventually upregulated cyclin A and CDK2 expression. Together, these data suggest that GDNF induces CREB/ATF-1 family member phosphorylation and c-fos transcription via the Ras/ERK1/2 pathway to promote the proliferation of SSC. Unveiling GDNF signaling cascades in SSC has important implications in providing attractive targets for male contraception as well as for the regulation of stem cell renewal vs. differentiation.

GDNF maintains mouse spermatogonial stem cells in vivo and in vitro

Spermatogonial stem cells (SSCs) produce sperm throughout the post-pubertal life of a male. Transgenic loss- and gain-of-function mouse models have shown that their self-renewal and differentiation are controlled in vivo by glial-cell-line-derived neurotrophic factor (GDNF) in a dose-dependent manner. After this in vivo observation, the culture conditions for mouse SSCs were rapidly developed. GDNF together with other growth factors, hormones, and vitamins maintain proliferation and self-renewal of SSCs for years in vitro. Both serum-supplemented and serum-free culture methods have been described. The cells are cultivated either on feeder layer or laminin-coated dishes. First reports from random and targeted mutagenesis of SSCs have been published. Some cells in the spermatogonial stem cell culture transform to embryonic stem cell-like cells and form teratomas in nude mice. In general, the spermatogonial stem cells maintain their germline identity in long-term culture. The mechanism for transformation to embryonic stem cell-like cells is not known, but the data suggest that germline and embryonic stem cells are closely related. We describe in detail the culture system of SSCs developed by Dr. Takashi Shinohara in 2003.

Gfra1 silencing in mouse spermatogonial stem cells results in their differentiation via the inactivation of RET tyrosine kinase

Spermatogenesis is the process by which spermatogonial stem cells divide and differentiate into sperm. The role of growth factor receptors in regulating self-renewal and differentiation of spermatogonial stem cells remains largely unclear. This study was designed to examine Gfra1 receptor expression in immature and adult mouse testes and determine the effects of Gfra1 knockdown on the proliferation and differentiation of type A spermatogonia. We demonstrated that GFRA1 was expressed in a subpopulation of spermatogonia in immature and adult mice. Neither Gfra1 mRNA nor GFRA1 protein was detected in pachytene spermatocytes and round spermatids. GFRA1 and POU5F1 (also known as OCT4), a marker for spermatogonial stem cells, were co-expressed in a subpopulation of type A spermatogonia from 6-day-old mice. In addition, the spermatogonia expressing GFRA1 exhibited a potential for proliferation and the ability to form colonies in culture, which is a characteristic of stem cells. RNA interference assays showed that Gfra1 small interfering RNAs (siRNAs) knocked down the expression of Gfra1 mRNA and GFRA1 protein in type A spermatogonia. Notably, the reduction of Gfra1 expression by Gfra1 siRNAs induced a phenotypic differentiation, as evidenced by the elevated expression of KIT, as well as the decreased expression of POU5F1 and proliferating cell nuclear antigen (PCNA). Furthermore, Gfra1 silencing resulted in a decrease in RET phosphorylation. Taken together, these data indicate that Gfra1 is expressed dominantly in mouse spermatogonial stem cells and that Gfra1 knockdown leads to their differentiation via the inactivation of RET tyrosine kinase, suggesting an essential role for Gfra1 in spermatogonial stem cell regulation.

Glial cell line-derived neurotrophic factor: an intraovarian factor that enhances oocyte developmental competence in vitro

The success of early embryonic development depends on oocyte nuclear and cytoplasmic maturation. We have investigated whether glial cell line-derived neurotrophic factor (GDNF) affects the in vitro maturation (IVM) of porcine oocytes and their subsequent ability to sustain preimplantation embryo development. GDNF and both its coreceptors, GDNF family receptor alpha-1 (GFR alpha-1) and the rearranged during transformation (RET) receptor, were expressed in oocytes and their surrounding cumulus cells derived from small and large follicles. When included in IVM medium, GDNF significantly enhanced cumulus cell expansion of both small and large cumulus-oocyte complexes and increased the percentage of small follicle-derived oocytes maturing to the metaphase II stage, although nuclear maturation of large oocytes was not significantly affected. Examination of cyclin B1 protein expression as a measure of cytoplasmic maturation revealed that in the presence of GDNF, cyclin B1 levels were significantly increased in large follicle-derived oocytes, as well as in oocytes from small follicles to a level comparable to the untreated large group. After activation, a significantly higher percentage of both small and large oocytes that were matured in the presence of GDNF developed to the blastocyst stage compared with untreated controls. Indeed, GDNF enhanced the blastocyst rate of small oocytes to levels comparable to those obtained for large oocytes matured without GDNF. The effect of GDNF was specific; this was evident because its enhancement of nuclear maturation and embryo developmental potential was blocked by an antibody against GFR alpha-1. Our study provides the first functional evidence that GDNF affects oocyte maturation and preimplantation embryo developmental competence in a follicular stage-dependent manner. This finding may provide insights for improving the formulation of IVM culture systems, especially for oocytes from small follicles.

Spermatogenesis in immature mammals

Mammalian spermatogenesis has been studied extensively as a prime theme of male reproductive biology, especially for germ cell production, fertilization and development. Investigation of spermatogenesis has provided us with the opportunity to both study the male germ line stem cells and generate the transgenic animals. Spermatogenesis is conducted in the seminiferous tubules, which end in the rete testis. The organization of spermatogenesis means that the spermatogonia are uniformly distributed around the seminiferous tubules. The pubertal establishment and mature maintenance of spermatogenesis requires precursor cells. In bull testes at 4 weeks postnatal, gonocyte migration occurs and differentiated spermatogonia are recognized after 8 weeks. Within the period of 4-8 weeks of age, spermatogonial stem cell conversion and niche formation must occur. Spermatogonial stem cells are the only cells that can undergo self-renewal in spermatogenesis. Spermatogonial stem cell transplantation can potentially contribute to studies of gene expression during spermatogenesis and provide genetic progress in domestic animals. Bull spermatogonial stem cells have been demonstrated to be capable of colonizing recipient mouse seminiferous tubules. (Reprod Med Biol 2007; 6: 139-149).

In vitro culture of testicular germ cells: regulatory factors and limitations

Spermatogenesis is regulated mainly by endocrine factors and also by testicular paracrine/autocrine growth factors. These factors are produced by Sertoli cells, germ cells, peritubular cells and interstitial cells, mainly Leydig cells and macrophages. The interactions and the ratio between Sertoli and germ cells in the seminiferous tubules ensure successful spermatogenesis. In order to culture spermatogonial stem cells (SSCs) in vitro, researchers tried to overcome some of the obstacles -- such as the low number of stem cells in the testis, absence of specific markers to identify SSCs -- in addition to difficulties in keeping the SSCs alive in culture. Recently, some growth factors important for the proliferation and differentiation of SSCs were identified, such as glial cell line derived neurotrophic factor (GDNF), stem cell factor (SCF) and leukemia inhibitory factor (LIF); also, markers for SSCs at different stages were reported. Therefore, some groups succeeded in culturing SSCs (under limitations), or more differentiated cells and even were able to produce in vitro germ cells from embryonic stem cells. Thus, success in culturing SSCs is dependent on understanding the molecular mechanisms behind self-renewal and differentiation. Culture of SSCs should be a good tool for discovering new therapeutic avenue for some infertile men or for patients undergoing chemotherapy/radiotherapy (pre-puberty or post-puberty).

Role of Src family kinases and N-Myc in spermatogonial stem cell proliferation

Spermatogonial stem cells are required for the initiation of spermatogenesis and the continuous production of sperm. In addition, they can acquire pluripotency and differentiate into derivatives of the three embryonic germ layers when cultured in the appropriate conditions. Therefore, understanding the signaling pathways that lead to self-renewal or differentiation of these cells is of paramount importance for the treatment of infertility, the development of male contraceptives, the treatment of testicular cancers, and ultimately for tissue regeneration. In this report, we studied some of the signaling pathways triggered by glial cell line-derived neurotrophic factor (GDNF), a component of the spermatogonial stem cell niche produced by the somatic Sertoli cells. As model systems, we used primary cultures of mouse spermatogonial stem cells, a mouse spermatogonial stem cell line and freshly isolated testicular tubules. We report here that GDNF promotes spermatogonial stem cell proliferation through activation of members of the Src kinase family, and that these kinases exert their action through a PI3K/Akt-dependent pathway to up-regulate N-myc expression. Thus, to proliferate, spermatogonial stem cells activate mechanisms that are similar to the processes observed in brain stem cells and lung progenitors.

Unraveling the molecular targets pertinent to junction restructuring events during spermatogenesis using the Adjudin-induced germ cell depletion model

During spermatogenesis, extensive restructuring takes place at the Sertoli-Sertoli and Sertoli-germ cell interface, which is regulated via intriguing interactions among cytokines, proteases, protease inhibitors, kinases, phosphatases, and transcription factors. This in turn determines the steady-state levels of integral membrane proteins at the cell junctions. We sought to further expand these observations using the Adjudin model. Adjudin is a potential male contraceptive that targets Sertoli-germ cell adhesion, causing exfoliation of spermatids and spermatocytes, but not spermatogonia, from the seminiferous epithelium. This model thus provides the means to identify crucial regulatory molecules and signaling pathways pertinent to junction restructuring events during spermatogenesis. In this study, genome-wide expression profiling of rat testes after treatment with Adjudin at the time of extensive junction restructuring was performed. Differentially regulated genes, such as cytokines, proteases, protease inhibitors, cell junction-associated proteins, and transcription factors pertinent to junction restructuring were identified. These data were consistent with earlier findings; however, much new information was obtained which has been deposited at the Gene Expression Omnibus data repository website: http://www.ncbi.nih.gov/geo/ with Accession number: GSE5131. The primary signaling events pertinent to junction restructuring in the testis induced by Adjudin were also delineated using bioinformatics. These findings were also consistent with recently published reports. The identified molecular signatures or targets pertinent to junction dynamics in the testis as reported herein, many of which have not been investigated, thus offer a framework upon which the regulation of junction restructuring events at the Sertoli-Sertoli and Sertoli-germ cell interface pertinent to spermatogenesis can be further studied.

Secretory mechanisms and Ca2+ signaling in gametes: similarities to regulated neuroendocrine secretion in somatic cells and involvement in emerging pathologies

Recent studies demonstrate that regulated secretion in probably all mammalian cells, from gonadotropes to gametes, utilizes similar signaling systems, intracellular Ca(2+) regulation, Ca(2+)-dependent proteins, cytoskeletal participation, and SNARE-mediated fusion. Thus, highly specialized cells, like sperm and eggs, should no longer be considered to have evolved a cell-type specific secretory mechanism. In gametes, Ca(2+)-dependent proteins and enzymes transduce elevations of intracellular Ca(2+) into secretory events, i.e., exocytosis of the acrosome in sperm and cortical granules in the egg. Just as secretory deficiencies have clinical consequences in endocrine and exocrine cells, failure of secretion of cortical granules or the acrosome can result in failure of normal fertilization or fertilization followed by abnormal development. With the advent of human in vitro fertilization, such gamete pathologies have been recently identified and have led to new clinical procedures to achieve normal fertilization and pregnancies. A better understanding of the common Ca(2+)-dependent secretory pathways in both gametes and somatic cells should be beneficial to investigating mis-regulation in either cell type.

Microarray technology offers a novel tool for the diagnosis and identification of therapeutic targets for male infertility

Male infertility is now a major reproductive health problem because of an increasing number of environmental pollutants and chemicals, which eventually result in gene mutations. Genetic alterations caused by environmental factors account for a significant percentage of male infertility. Microarray technology is a powerful tool capable of measuring simultaneously the expression of thousands of genes expressed in a single sample. Eventually, advances in genetic technology will allow for the diagnosis of patients with male infertility due to congenital reasons or environmental factors. Since its introduction in 1994, microarray technology has made significant advances in the identification and characterization of novel or known genes possibly correlated with male infertility in mice, as well as in humans. This provides a rational basis for the application of microarray to establishing molecular signatures for the diagnosis and gene therapy targets of male infertility. In this review, the differential gene expression patterns characterized by microarray in germ and somatic cells at different steps of development or in response to stimuli, as well as a number of novel or known genes identified to be associated with male infertility in mice and humans, are addressed. Moreover, issues pertaining to measurement reproducibility are highlighted for the application of microarray data to male infertility.

Protein gene product 9.5 is a spermatogonia-specific marker in the pig testis: application to enrichment and culture of porcine spermatogonia

Identification and isolation of spermatogonial stem cells (SSCs) are a prerequisite for culture, genetic manipulation, and/or transplantation research. In this study, we established that expression of PGP 9.5 is a spermatogonia-specific marker in porcine testes. The expression pattern of PGP 9.5 in spermatogonia was compared to cell type-specific protein (GATA-4 or PLZF) expression in seminiferous tubules at different ages, and expression levels of PGP 9.5, Vasa, and Oct-4 were compared in different cell fractions. Enrichment of spermatogonia from 2-week-old (2wo) and 10-week-old (10wo) boars by adhesion to laminin, differential plating, or velocity sedimentation followed by differential plating was assessed by identification of spermatogonia using expression of PGP 9.5 as a marker. Compared to the initial samples, spermatogonia were enriched twofold in laminin-selected cells (P < 0.05), and fivefold either in cells remaining in suspension (fraction I) or in cells slightly attached to the culture dish (fraction II) (P < 0.05) after differential plating. Cells in fraction II appeared to be superior for future experiments due to higher viability (>90%) than in fraction I ( approximately 50%). Velocity sedimentation plus differential plating achieved cell populations containing up to 70% spermatogonia with good viability (>80%). Enriched spermatogonia from 2wo and 10wo testes could be maintained in a simple culture medium without additional growth factors for at least 2 weeks and continued to express PGP 9.5. These data provide the basis for future studies aimed at refining conditions of germ cell culture and manipulation prior to germ cell transplantation in pigs.

Expression of the adaptor protein m-Numb in mouse male germ cells

Numb is an adaptor protein that is asymmetrically inherited at mitosis and controls the fate of sibling cells in different species. The role of m-Numb (mammalian Numb) as an important cell fate-determining factor has extensively been described mostly in neural tissues, particularly in progenitor cells, in the mouse. Biochemical and genetic analyses have shown that Numb acts as an inhibitor of the Notch signaling pathway, an evolutionarily conserved pathway involved in the control of cell proliferation, differentiation, and apoptosis. In the present study, we sought to determine m-Numb distribution in germ cells in the postnatal mouse testis. We show that all four m-Numb isoforms are widely expressed during postnatal testis development. By reverse transcriptase-PCR and western blot analyses, we further identify p71 as the predominantly expressed isoform in germ cells. Moreover, we demonstrate through co-immunoprecipitation studies that m-Numb physically associates with Ap2a1, a component of the endocytotic clathrin-coated vesicles. Finally, we employed confocal immunofluorescence microscopy of whole mount seminiferous tubules and isolated germ cells to gain more insight into the subcellular localization of m-Numb. These morphological analyses confirmed m-Numb and Ap2a1 co-localization. However, we did not observe asymmetric localization of m-Numb neither in mitotic spermatogonial stem cells nor in more differentiated spermatogonial cells, suggesting that spermatogonial stem cell fate in the mouse does not rely on asymmetric partitioning of m-Numb.

c-Flip(L) is expressed in undifferentiated mouse male germ cells

Apoptosis represents a fundamental process during fetal/post-natal testis development. Therefore pro- and anti-apoptotic proteins are essential to regulate testis physiology. c-Flip(L) is a known inhibitor of caspase 8/10 activity; in this study its perinatal expression in mouse male germ cells was investigated. In testis sections and seminiferous tubule whole mount c-Flip(L) was found to be expressed in undifferentiated spermatogonia and to co-localize with germ stem cells markers. In vivo investigations in the vitamin-A deficient mouse, lacking differentiated germ cells, confirmed c-Flip(L) expression in undifferentiated spermatogonia. Further analyses showed Fas expression but no significant caspase 8/10 activity when c-Flip(L) was highly expressed. Altogether these data suggest that c-Flip may control the survival rate of undifferentiated spermatogonia.

Identification, molecular cloning, and cellular distribution of the rat homolog of minichromosome maintenance protein 7 (MCM7) in the rat testis

As part of a program to decipher the rat testicular proteome, we studied spermatogonia and identified numerous proteins including the human homolog of the Minichromosome Maintenance Protein 7 (MCM7). MCM7 has been implicated in DNA replication in various species, but had not been detected in the testis. Here we describe the cellular distribution of MCM7 transcripts and protein, and their testicular ontogenetic expression. The full-length coding region of the rat MCM7 was also characterized. Northern blot analyses showed that MCM7 transcripts are more abundant in the testis than other organs and confirmed the presence of the 2.4 kb MCM7 transcript at all ages studied. Interestingly, two additional transcripts of 3.2 and 1.6 kb were found from 26 days post partum onwards, when spermatocytes and spermatids accumulate within the tubules. This was confirmed in isolated cell types: the three MCM7 transcripts were observed in meiotic and post-meiotic germ cells. The 3.2 kb isoform has an extended 5' untranslated region (UTR) and the 1.6 kb transcript is the result of alternative splicing of five exons. Western blot and immunohistochemistry experiments evidenced abundant MCM7 in proliferating gonocytes and Sertoli cells in the fetal testis. In the adult testis, an intense signal was observed in spermatogonia and primary spermatocytes. We conclude that the Mcm7 is one example of genes that are differently transcribed and translated in somatic and spermatogenetic cells in mammals. Further work is required to determine the roles of MCM7 in spermatogonia and germ lineage.

Trophic factors counteract elevated FGF-2-induced inhibition of adult neurogenesis

The dentate gyrus of adult mammalian brain contains neural progenitor cells with self-renewal and multi-lineage potential. The lineage and maturation of the neural progenitors are determined by the composition and levels of the trophic factors in their microenvironment. In Alzheimer disease (AD) brain, especially the hippocampus, the level of basic fibroblast growth factor (FGF-2) is markedly elevated. Here we show that elevated FGF-2 enhances the division and nestin levels of cultured adult rat hippocampal progenitors but impairs neuronal lineage determination and maturation of these cells in culture. The trophic factors ciliary neurotrophic factor (CNTF), glial-derived neurotrophic factor (GDNF), and insulin-like growth factors-1 and -2 (IGF-1, IGF-2) as well as an Alzheimer peptidergic drug, Cerebrolysin((R)) (CL), in which we found these neurotrophic activities, counteract the effect of FGF-2 in inducing neuronal lineage (early neurogenesis). Whereas CNTF is the most active of the neurotrophic factors studied in promoting neurogenesis, CL, probably because of a combined effect of these factors, induces similar changes but without inhibiting cell proliferation. These findings suggest that CNTF, GDNF, IGF-1, and IGF-2 are promising therapeutic targets for AD and other diseases in which neurogenesis is probably inhibited.

Mechanistic insights into the regulation of the spermatogonial stem cell niche

Potential therapeutic use of stem cells in the treatment of human diseases depends on our ability to control the balance of their differentiation and self-renewal in vitro and in vivo. The stem cell "niche," or specialized microenvironment, is now recognized as one of the major contributors to this regulation in many species. Our recent study, which was reported in Nature, was the first to demonstrate that expression of a vertebrate animal transcription factor is essential for the maintenance of a stem cell niche. In that letter, targeted disruption of ERM (Ets-related molecule), which was localized only in the somatic support cell of the testis, the Sertoli cell, resulted in failure of self-renewal by spermatogonial stem cells, following the first wave of spermatogenesis. One of the more important conclusions drawn was the realization that regulation of the stem cell niche during the perinatal period, a phase characterized by rapid mitosis of both spermatogonial stem cells and Sertoli cells, differed from that in the adult. It appears that the ERM-regulated pathways are coincident with the termination of Sertoli cell proliferation and commencement of the cycle of spermatogenesis, which is sustained by the same cell that regulates the stem cell niche. Several likely targets for ERM regulation are discussed, as well as their potential implications for increasing our understanding of spermatogonial stem cell activity and the uniqueness of the Sertoli cell's immune privilege and possible utility for the protection of transplanted adult stem cells.

Identifying genes important for spermatogonial stem cell self-renewal and survival

Spermatogonial stem cells (SSCs) are the foundation for spermatogenesis and, thus, preservation of a species. Because of stem cell rarity, studying their self-renewal is greatly facilitated by in vitro culture of enriched biologically active cell populations. A recently developed culture method identified glial cell line-derived neurotrophic factor (GDNF) as the essential growth factor that supports in vitro self-renewal of SSCs and results in an increase in their number. This system is a good model to study mechanisms of stem cell self-renewal because of the well defined culture conditions, enriched cell population, and available transplantation assay. By withdrawing and replacing GDNF in culture medium, we identified regulated expression of many genes by using microarray analysis. The expression levels of six of these genes were dramatically decreased by GDNF withdrawal and increased by GDNF replacement. To demonstrate the biological significance of the identified GDNF-regulated genes, we examined the importance of the most responsive of the six, bcl6b, a transcriptional repressor. By using siRNA to reduce transcript levels, Bcl6b was shown to be crucial for SSC maintenance in vitro. Moreover, evaluation of Bcl6b-null male testes revealed degeneration and/or absence of active spermatogenesis in 24 +/- 7% of seminiferous tubules. These data suggest that Bcl6b is an important molecule in SSC self-renewal and validate the biological relevance of the GDNF-regulated genes identified through microarray analysis. In addition, comparison of data generated in this study to other stem cell types suggests that self-renewal in SSCs is regulated by distinctly different molecular mechanisms.

Novel regulators revealed by profiling Drosophila testis stem cells within their niche

Stem cells are defined by the fact that they both self-renew, producing additional stem cells, and generate lineal descendants that differentiate into distinct functional cell types. In Drosophila, a small germline stem cell population is influenced by a complex microenvironment, the stem cell niche, which itself includes a somatic stem cell population. While stem cells are unique, their immediate descendants retain considerable stem cell character as they mitotically amplify prior to differentiation and can be induced to de-differentiate into stem cells. Despite their importance, very few genes are known that are expressed in the stem cells or their early amplifying daughters. We present here whole-genome microarray expression analysis of testes specifically enriched for stem cells, their amplifying daughters, and their niche. These studies have identified a number of loci with highly specific stem cell expression and provide candidate downstream targets of Jak/Stat self-renewal signaling. Furthermore, functional analysis for two genes predicted to be enriched has enabled us to define novel regulators of the germline lineage. The gene list generated in this study thus provides a potent resource for the investigation of stem cell identity and regulation from functional as well as evolutionary perspectives.

Spermatogonial stem cells: characteristics and experimental possibilities

The continuation of the spermatogenic process throughout life relies on a proper regulation of self-renewal and differentiation of the spermatogonial stem cells. These are single cells situated on the basal membrane of the seminiferous epithelium. Only 0.03% of all germ cells are spermatogonial stem cells. They are the only cell type that can repopulate and restore fertility to congenitally infertile recipient mice following transplantation. Although numerous expression markers have been helpful in isolating and enriching spermatogonial stem cells, such as expression of THY-1 and GFRalpha-1 and absence of c-kit, no specific marker for this cell type has yet been identified. Much effort has been put into developing a protocol for the maintenance of spermatogonial cells in vitro. Recently, coculture systems of testicular cells on various feeder cells have made it possible to culture spermatogonial stem cells for a long period of time, as was demonstrated by the transplantation assay. Even expansion of testicular cells, including the spermatogonial stem cells, has been achieved. In these culture systems, hormones and growth factors are investigated for their role in the process of proliferation of spermatogonial stem cells. At the moment the best culture system known still consists of a mixture of testicular cells with about 1.33% spermatogonial stem cells. Recently pure SV40 large T immortalized spermatogonial stem cell lines have been established. These c-kit-negative cell lines did not show any differentiation in vitro or in vivo. A telomerase immortalized c-kit-positive spermatogonial cell line has been established that was able to differentiate in vitro. Spermatocytes and even spermatids were formed. However, spermatogonial stem cell activity by means of the transplantation assay was not tested for this cell line. Both the primary long-term cultures and immortalized cell lines have represented a major step forward in investigating the regulation of spermatogonial self-renewal and differentiation, and will be useful for identifying specific molecular markers.

Glial cell-line derived neurotrophic factor-mediated RET signaling regulates spermatogonial stem cell fate

Normal spermatogenesis is essential for reproduction and depends on proper spermatogonial stem cell (SSC) function. Genes and signaling pathways that regulate SSC function have not been well defined. We report that glial cell-line-derived neurotrophic factor (GDNF) signaling through the RET tyrosine kinase/GFRA1 receptor complex is required for spermatogonial self-renewal in mice. GFRA1 and RET expression was identified in a subset of gonocytes at birth, was restricted to SSCs during normal spermatogenesis, and RET expressing cells were abundant in a cryptorchid model of SSC self-renewal. We used the whole-testis transplantation technique to overcome the limitation of neonatal lethality of Gdnf-, Gfra1-, and Ret-deficient mice and found that each of these genes is required for postnatal spermatogenesis and not for embryological testes development. Each mutant testis shows severe SSC depletion by Postnatal Day 7 during the first wave of spermatogenesis. These defects were due to lack of SSC proliferation and an inability of SSCs to maintain an undifferentiated state. Our results demonstrate that GDNF-mediated RET signaling is critical for the fate of undifferentiated spermatogonia and that abnormalities in this pathway may contribute to male infertility and testicular germ cell tumors.

Capillary morphogenesis gene (CMG)-1 is among the genes differentially expressed in mouse male germ line stem cells and embryonic stem cells

We recently established a technique to expand male germ line stem (GS) cells in long-term culture without losing their spermatogenic capacity. To gain insight into the genetic program of these cells, we compared the mRNA expression profile of GS cells with that of embryonic stem (ES) cells using DNA microarrays. We found 79 genes that were upregulated in GS cells compared to ES cells, including synaptonemal complex protein-1, deleted in azoospermia-like, ubiquitin-conjugating enzyme E2B, and ubiquitin carboxy-terminal hydrolase L1, all of which are functionally important for spermatogenesis. In addition, we identified a cDNA encoding the mouse ortholog of capillary morphogenesis gene (CMG)-1. CMG-1 transcripts were predominantly produced in spermatogonia and spermatocytes in mouse testis. When CMG-1 expression was attenuated in a mouse spermatocyte-derived cell line, GC-2spd(ts), by a target-specific short interfering RNA, the morphology of the cells was changed and the expression of cyclin D2 was abrogated. A reporter assay using a genomic region upstream of the mouse cyclin D2 gene revealed that this downmodulation occurs at the transcriptional level. We detected FLAG-tagged CMG-1 protein in the nuclei of transfected COS7 cells, suggesting that CMG-1 may play a unique role in the transcriptional regulation of the cyclin D2 gene. The upregulated GS genes identified in this study will provide useful information for the future investigation of spermatogonial stem cells and the early phase of male germ cell differentiation.

β-nerve growth factor participates in an auto/paracrine pathway of regulation of the meiotic differentiation of rat spermatocytes

NGF appears to be involved in spermatogenesis. However, mice lacking NGF or TrkA genes do not survive more than a few days whereas p75(NTR) knockout mice are viable and fertile. Therefore, we addressed the effect of betaNGF on spermatogenesis by using the systems of rat germ cell culture we established previously. betaNGF did not modify the number of Sertoli cells, pachytene spermatocytes, secondary spermatocytes nor the half-life of round spermatids, but increased the number of secondary meiotic metaphases and decreased the number of round spermatids formed in vitro. These effects of betaNGF were reversible and maximal at about 4 x 10(-11) M. Conversely, K252a, a Trk-specific kinase inhibitor, enhanced the number of round spermatids above that of control cultures. The presence of betaNGF and its receptors TrkA and p75(NTR) was investigated in testis sections, in Sertoli cell and germ cell fractions, and in germ cell and Sertoli cell co-cultures. betaNGF was detected only in germ cells from pachytene spermatocytes of stages VII up to spermatids of stages IX-X. TrkA and p75(NTR) were detected in Sertoli cells and in these germ cells. Taken together, these results indicate that betaNGF should participate in an auto/paracrine pathway of regulation of the second meiotic division of rat spermatocytes in vivo.

In vitro cytotoxicity of nanoparticles in mammalian germline stem cells

Gametogenesis is a complex biological process that is particularly sensitive to environmental insults such as chemicals. Many chemicals have a negative impact on the germline, either by directly affecting the germ cells, or indirectly through their action on the somatic nursing cells. Ultimately, these effects can inhibit fertility, and they may have negative consequences for the development of the offspring. Recently, nanomaterials such as nanotubes, nanowires, fullerene derivatives (buckyballs), and quantum dots have received enormous national attention in the creation of new types of analytical tools for biotechnology and the life sciences. Despite the wide application of nanomaterials, there is a serious lack of information concerning their impact on human health and the environment. Thus, there are limited studies available on toxicity of nanoparticles for risk assessment of nanomaterials. The purpose of this study was to assess the suitability of a mouse spermatogonial stem cell line as a model to assess nanotoxicity in the male germline in vitro. The effects of different types of nanoparticles on these cells were evaluated by light microscopy, and by cell proliferation and standard cytotoxicity assays. Our results demonstrate a concentration-dependent toxicity for all types of particles tested, whereas the corresponding soluble salts had no significant effect. Silver nanoparticles were the most toxic while molybdenum trioxide (MoO(3)) nanoparticles were the least toxic. Our results suggest that this cell line provides a valuable model with which to assess the cytotoxicity of nanoparticles in the germ line in vitro.

Role of glial cell line-derived neurotrophic factor in germ-line stem cell fate

The overall goal of this study is to unravel the role(s) played by glial cell line-derived neurotrophic factor (GDNF) in the fate of spermatogonial stem cells. There is great interest in the biology of spermatogonial stem cells, or A(single) spermatogonia, because of their importance in the treatment of infertility, the development of contraceptives, and the understanding of the etiology of testicular cancer, particularly seminoma. In the mouse, spermatogonial stem cells express GFRalpha-1, the receptor for GDNF, and respond to this growth factor in vivo and in vitro. GDNF is produced by the adjacent Sertoli cells, which are part of the germ-line stem cell niche in vertebrates. We specifically isolated GFRalpha-1-positive spermatogonia using an immunomagnetic bead technique. We then stimulated the cells with 100 ng/mL of rGDNF for 10 hours; unstimulated cells served as negative controls. Microarray analysis, immunocytochemistry, and Western blotting revealed that Numb, a regulator of the Notch pathway, is upregulated by GDNF in spermatogonial stem cells. There are indications that in rats, mice, and humans, the Notch pathway promotes spermatogonial differentiation. We observed that an increase in Numb expression is concomitant with Notch degradation in these cells. Thus, through Numb, GDNF might inhibit differentiation and allows the maintenance of the stem cell pool in the mouse seminiferous epithelium.

Anchorage-independent growth of mouse male germline stem cells in vitro

Spermatogenesis originates from a small number of spermatogonial stem cells that reside on the basement membrane and undergo self-renewal division to support spermatogenesis throughout the life of adult animals. Although the recent development of a technique to culture spermatogonial stem cells allowed reproduction of self-renewal division in vitro, much remains unknown about how spermatogonial stem cells are regulated. In this study, we found that spermatogonial stem cells could be cultured in an anchorage-independent manner, which is characteristic of stem cells from other types of self-renewing tissues. Although the cultured cells grew slowly (doubling time, approximately 4.7 days), they expressed markers of spermatogonia, and grew exponentially for at least 5 months to achieve 1.5 x 10(10) -fold expansion. The cultured cells underwent spermatogenesis following transplantation into the seminiferous tubules of infertile animals and fertile offspring were obtained by microinsemination of germ cells that had developed within the testes of recipients of the cultured cells. These results indicate that spermatogonial stem cells can undergo anchorage-independent, self-renewal division, and suggest that stem cells have the common property to survive and proliferate in the absence of exogenous substrata.

Self renewal, expansion, and transfection of rat spermatogonial stem cells in culture

The use of a transgenic line of rats that express enhanced GFP (EGFP) exclusively in the germ line has allowed a separation of feeder layers and contaminating testis somatic cells from germ cells and the identification of a set of spermatogonial stem cell marker transcripts. With these molecular markers as a guide, we have now devised culture conditions where rat spermatogonial stem cells renew and proliferate in culture with a doubling time between 3 and 4 days. The marker transcripts increase in relative abundance as a function of time in culture, and the stem cells retain competency to colonize and develop into spermatids after transplantation to the testes of recipient rats. The cells also remain euploid after at least 12 passages. Cell lines could be isolated and cryopreserved and, upon subsequent thawing, continue to self renew. Transfection of the spermatogonial stem cells with a plasmid containing the neomycin phosphotransferase (neo) selectable marker resulted in selection of G418-resistant cell lines that effectively colonize recipient testes, suggesting that gene targeting is now feasible in the rat.