Reestablishment of spermatogenesis after more than 20 years of cryopreservation of rat spermatogonial stem cells reveals an important impact in differentiation capacity.

Treatment of cancer in children is increasingly successful but leaves many prepubertal boys suffering from infertility or subfertility later in life. A current strategy to preserve fertility in these boys is to cryopreserve a testicular biopsy prior to treatment with the expectation of future technologies allowing for the reintroduction of stem cells and restoration of spermatogenesis. Spermatogonial stem cells (SSCs) form the basis of male reproduction, differentiating into all germ cell types, including mature spermatozoa and can regenerate spermatogenesis following transplantation into an infertile testis. Here, we demonstrate that rat SSCs frozen for more than 20 years can be transplanted into recipient mice and produce all differentiating germ cell types. However, compared with freshly isolated cells or those frozen for a short period of time, long-frozen cells do not colonize efficiently and showed reduced production of spermatids. Single-cell RNA sequencing revealed similar profiles of gene expression changes between short- and long-frozen cells as compared with fresh immediately after thawing. Conversely, following transplantation, long-frozen samples showed enhanced stem cell signaling in the undifferentiated spermatogonia compartment, consistent with self-renewal and a lack of differentiation. In addition, long-frozen samples showed fewer round spermatids with detectable protamine expression, suggesting a partial block of spermatogenesis after meiosis resulting in a lack of elongating spermatids. These findings strongly suggest that prolonged cryopreservation can impact the success of transplantation to produce spermatogenesis, which may not be revealed by analysis of the cells immediately after thawing. Our analysis uncovered persistent effects of long-term freezing not found in other cryopreservation studies that lacked functional regeneration of the tissue and this phenomenon must be accounted for any future therapeutic application.

FGF9 promotes mouse spermatogonial stem cell proliferation mediated by p38 MAPK signalling.

OBJECTIVES: Fibroblast growth factor 9 (FGF9) is expressed by somatic cells in the seminiferous tubules, yet little information exists about its role in regulating spermatogonial stem cells (SSCs). MATERIALS AND METHODS: Fgf9 overexpression lentivirus was injected into mouse testes, and PLZF immunostaining was performed to investigate the effect of FGF9 on spermatogonia in vivo. Effect of FGF9 on SSCs was detected by transplanting cultured germ cells into tubules of testes. RNA-seq of bulk RNA and single cell was performed to explore FGF9 working mechanisms. SB203580 was used to disrupt p38 MAPK pathway. p38 MAPK protein expression was detected by Western blot and qPCR was performed to determine different gene expression. Small interfering RNA (siRNA) was used to knock down Etv5 gene expression in germ cells. RESULTS: Overexpression of Fgf9 in vivo resulted in arrested spermatogenesis and accumulation of undifferentiated spermatogonia. Exposure of germ cell cultures to FGF9 resulted in larger numbers of SSCs over time. Inhibition of p38 MAPK phosphorylation negated the SSC growth advantage provided by FGF9. Etv5 and Bcl6b gene expressions were enhanced by FGF9 treatment. Gene knockdown of Etv5 disrupted the growth effect of FGF9 in cultured SSCs along with downstream expression of Bcl6b. CONCLUSIONS: Taken together, these data indicate that FGF9 is an important regulator of SSC proliferation, operating through p38 MAPK phosphorylation and upregulating Etv5 and Bcl6b in turn.

Chemokine (C-X-C) Ligand 12 Facilitates Trafficking of Donor Spermatogonial Stem Cells.

The chemokine (C-X-C) receptor type 4 (CXCR4) is an early marker of primordial germ cells (PGCs) essential for their migration and colonization of the gonads. In spermatogonial stem cells (SSCs), the expression of CXCR4 is promoted by the self-renewal factor, glial cell line-derived neurotrophic factor (GDNF). Here, we demonstrate an important role of CXCR4 during donor mouse SSCs reoccupation of the endogenous niche in recipient testis. Silencing of CXCR4 expression in mouse SSCs dramatically reduced the number of donor stem cell-derived colonies, whereas colony morphology and spermatogenesis were comparable to controls. Inhibition of CXCR4 signaling using a small molecule inhibitor (AMD3100) during the critical window of homing also significantly lowered the efficiency of donor-derived SSCs to establish spermatogenic colonies in recipient mice; however, the self-renewal of SSCs was not affected by exposure to AMD3100. Rather, in vitro migration assays demonstrate the influence of CXCR4-CXCL12 signaling in promoting germ cell migration. Together, these studies suggest that CXCR4-CXCL12 signaling functions to promote homing of SSCs towards the stem cell niche and plays a critical role in reestablishing spermatogenesis.

Fertile offspring derived from mouse spermatogonial stem cells cryopreserved for more than 14 years.

BACKGROUND: Approximately 80% of childhood cancers can now be cured but a side effect of treatment results in about one-third of the surviving boys being infertile or severely subfertile when they reach reproductive age. Currently, more than 1 in 5000 men of reproductive age who are childhood cancer survivors suffer from this serious quality of life problem. It is possible to obtain a testicular biopsy before treatment to preserve the spermatogonial stem cells (SSCs) of the male by cryopreservation, but the results of long-term storage of SSCs on their subsequent functional ability to generate normal offspring has not been examined in any mammalian species. Moreover, it will be necessary to increase the number of these cryopreserved SSCs to remove any contaminating malignant cells and assure regeneration of spermatogenesis. METHODS AND RESULTS: In this report, we demonstrate that long-term cryopreservation (>14 years) of testis cells from mouse, rat, rabbit and baboon safeguards SSC viability, and that these cells can colonize the seminiferous tubules of recipient testes. Moreover, mouse and rat SSCs can be cultured and re-establish complete spermatogenesis, and fertile mouse progeny without apparent genetic or epigenetic errors were generated by the sperm produced. CONCLUSIONS: These findings provide a platform for fertility preservation in prepubertal boys undergoing gonadotoxic treatments.

Spermatogonial stem cell self-renewal requires ETV5-mediated downstream activation of Brachyury in mice.

Insight regarding mechanisms controlling gene expression in the spermatogonial stem cell (SSC) will improve our understanding of the processes regulating spermatogenesis and aid in treating problems associated with male infertility. In the present study, we explored the global gene expression profiles of the glial cell line-derived neurotrophic factor (GDNF)-regulated transcription factors Ets (E-twenty-six) variant gene 5 (Etv5); B-cell chronic lymphocytic leukemia (CLL)/lymphoma 6, member B (Bcl6b); and POU domain, class-3 transcription factor 1 (Pou3f1). We reasoned that these three factors may function as a core set of transcription factors, regulating genes responsible for maintaining the SSC population. Using transient siRNA oligonucleotides to individually target Etv5, Bcl6b, and Pou3f1 within mouse SSC cultures, we examined changes to the global gene expression profiles associated with these transcription factors. Only modest overlaps in the target genes regulated by the three factors were noted, but ETV5 was found to be a critical downstream regulator of GDNF signaling that mediated the expression of several known SSC self-renewal related genes, including Bcl6b and LIM homeobox 1 (Lhx1). Notably, ETV5 was identified as a regulator of Brachyury (T) and CXC chemokine receptor, type 4 (Cxcr4), and we showed that ETV5 binding to the Brachyury (T) gene promoter region is associated with an active state of transcription. Moreover, in vivo transplantation of SSCs following silencing of Brachyury (T) significantly reduced the number of donor cell-derived colonies formed within recipient mouse testes. These results suggest Brachyury is of biological importance and functions as part of GDNF/ETV5 signaling to promote self-renewal of mouse SSCs cultured in vitro.

MicroRNA-21 regulates the self-renewal of mouse spermatogonial stem cells.

MicroRNAs (miRs) play a key role in the control of gene expression in a wide array of tissue systems, where their functions include the regulation of self-renewal, cellular differentiation, proliferation, and apoptosis. However, the functional importance of individual miRs in controlling spermatogonial stem cell (SSC) homeostasis has not been investigated. Using high-throughput sequencing, we profiled the expression of miRs in the Thy1(+) testis cell population, which is highly enriched for SSCs, and the Thy1(-) cell population, composed primarily of testis somatic cells. In addition, we profiled the global expression of miRs in cultured germ cells, also enriched for SSCs. Our results demonstrate that miR-21, along with miR-34c, -182, -183, and -146a, are preferentially expressed in the Thy1(+) SSC-enriched population, compared with Thy1(-) somatic cells. Importantly, we demonstrate that transient inhibition of miR-21 in SSC-enriched germ cell cultures increased the number of germ cells undergoing apoptosis and significantly reduced the number of donor-derived colonies of spermatogenesis formed from transplanted treated cells in recipient mouse testes, indicating that miR-21 is important in maintaining the SSC population. Moreover, we show that in SSC-enriched germ cell cultures, miR-21 is regulated by the transcription factor ETV5, known to be critical for SSC self-renewal.

Glial cell line-derived neurotrophic factor and endothelial cells promote self-renewal of rabbit germ cells with spermatogonial stem cell properties.

Previous studies suggest that exogenous factors crucial for spermatogonial stem cell (SSC) self-renewal are conserved among several mammalian species. Since glial cell line-derived neurotrophic factor (GDNF) and fibroblast growth factor 2 (FGF2) are critical for rodent SSC self-renewal, we hypothesized that they might promote self-renewal of nonrodent SSCs. Therefore, we cultured testicular germ cells from prepubertal rabbits in the presence of GDNF and FGF2 and found they proliferated indefinitely as cellular clumps that displayed characteristics previously identified for rodent SSCs. The rabbit germ cells could not be maintained on mouse embryonic fibroblast (STO) feeders that support rodent SSC self-renewal in vitro but were rather supported on mouse yolk sac-derived endothelial cell (C166) feeder layers. Proliferation of rabbit germ cells was dependent on GDNF. Of critical importance was that clump-forming rabbit germ cells colonized seminiferous tubules of immunodeficient mice, proliferated for at least 6 mo, while retaining an SSC phenotype in the testes of recipient mice, indicating that they were rabbit SSCs. This study demonstrates that GDNF is a mitogenic factor promoting self-renewal that is conserved between rodent and rabbit SSCs; with an evolutionary separation of ∼ 60 million years. These findings provide a foundation to study the mechanisms governing SSC self-renewal in nonrodent species.

In vivo and in vitro aging is detrimental to mouse spermatogonial stem cell function.

The development of techniques to maintain the spermatogonial stem cell (SSC) in vivo and in vitro for extended periods essentially allows for the indefinite continuation of an individual germline. Recent evidence indicates that the aging of male reproductive function is due to failure of the SSC niche. SSCs are routinely cultured for 6 mo, and no apparent effect of culture over this period has been observed. To determine the effects of SSC aging, we utilized an in vitro culture system, followed by quantitative transplantation experiments. After culture for 6 mo, SSCs that had been aged in vivo for 1500 days had a slower proliferation rate than SSCs that were aged in vivo to 8 or 300 days. Examination of methylation patterns revealed no apparent difference in DNA methylation between SSCs that were aged 8, 300, or 1500 days before culture. Long-term culture periods resulted in a loss of stem cell potential without an obvious change in the visual appearance of the culture. DNA microarray analysis of in vivo- and in vitro-aged SSCs identified the differential expression of several genes important for SSC function, including B-cell CLL/lymphoma 6, member B (Bcl6b), Lim homeobox protein 1 (Lhx1), and thymus cell antigen 1, theta (Thy1). Collectively, these data indicate that, although both in vitro and in vivo aging are detrimental to SSC function, in vitro aging results in greater loss of function, potentially due to a decrease in core SSC self-renewal gene expression and an increase in germ cell differentiation gene expression.

Regulation of mouse spermatogonial stem cell differentiation by STAT3 signaling.

Homeostasis of many tissues is maintained by self-renewal and differentiation of stem cells. Spermatogenesis is one such system relying on the activity of spermatogonial stem cells (SSCs). Several key regulators of SSC self-renewal have been identified, yet knowledge of molecules that control SSC differentiation is undefined. In this study, we found that transient impairment of STAT3 signaling enhances SSC self-renewal in vitro without affecting general spermatogonial proliferation, indicating an alteration in the balance of SSC fate decisions that inhibited differentiation. Confirming this observation, short hairpin RNA-mediated stable reduction of STAT3 expression in cultured SSCs abolished their ability to differentiate beyond the undifferentiated spermatogonial stage following transplantation into recipient testes. Collectively, these results demonstrate that STAT3 promotes the differentiation of SSCs. In contrast, STAT3 plays a central role in maintaining self-renewal of mouse embryonic stem cells, and STAT signaling is essential for self-renewal of male germline stem cells in Drosophila.

The POU domain transcription factor POU3F1 is an important intrinsic regulator of GDNF-induced survival and self-renewal of mouse spermatogonial stem cells.

Continual spermatogenesis relies on a pool of spermatogonial stem cells (SSCs) that possess the capacity for self-renewal and differentiation. Maintenance of this pool depends on survival of SSCs throughout the lifetime of a male. Response to extrinsic stimulation from glial cell line-derived neurotrophic factor (GDNF), mediated by the PIK3/AKT signaling cascade, is a key pathway of SSC survival. In this study, we found that expression of the POU domain transcription factor POU3F1 in cultured SSCs is up-regulated via this mechanism. Reduction of Pou3f1 gene expression by short interfering RNA (siRNA) treatment induced apoptosis in cultured germ cell populations, and transplantation analyses revealed impaired SSC maintenance in vitro. POU3F1 expression was localized to spermatogonia in cross-sections of prepubertal and adult testes, implying a similar role in vivo. Through comparative analyses, we found that expression of POU5F1, another POU transcription factor implicated as essential for SSC self-renewal, is not regulated by GDNF in cultured SSCs. Transplantation analyses following siRNA treatment showed that POU5F1 expression is not essential for SSC maintenance in vitro. Additionally, expression of NODAL, a putative autocrine regulator of POU5F1 expression in mouse germ cells, could not be detected in SSCs isolated from testes or cultured SSCs. Collectively, these results indicate that POU3F1, but not POU5F1, is an intrinsic regulator of GDNF-induced survival and self-renewal of mouse SSCs.

Prepubertal human spermatogonia and mouse gonocytes share conserved gene expression of germline stem cell regulatory molecules.

In the human testis, beginning at approximately 2 months of age, gonocytes are replaced by adult dark (Ad) and pale (Ap) spermatogonia that make up the spermatogonial stem cell (SSC) pool. In mice, the SSC pool arises from gonocytes approximately 6 days after birth. During puberty in both species, complete spermatogenesis is established by cells that differentiate from SSCs. Essentially pure populations of prepubertal human spermatogonia and mouse gonocytes were selected from testis biopsies and validated by confirming the presence of specific marker proteins in cells. Stem cell potential of germ cells was demonstrated by transplantation to mouse testes, following which the cells migrated to the basement membrane of the seminiferous tubule and were maintained similar to SSCs. Differential gene expression profiles generated between germ cells and testis somatic cells demonstrated that expression of genes previously identified as SSC and spermatogonial-specific markers (e.g., zinc-finger and BTB-domain containing 16, ZBTB16) was greatly elevated in both human spermatogonia and mouse gonocytes compared to somatic cells. Several genes were expressed at significantly higher levels in germ cells of both species. Most importantly, genes known to be essential for mouse SSC self-renewal (e.g., Ret proto-oncogene, Ret; GDNF-family receptor alpha1, Gfr alpha1; and B-cell CLL/lymphoma 6, member B, Bcl6b) were more highly expressed in both prepubertal human spermatogonia and mouse gonocytes than in somatic cells. The results indicate remarkable conservation of gene expression, notably for self-renewal genes, in these prepubertal germline cells between two species that diverged phylogenetically approximately 75 million years ago.

Identification of glial cell line-derived neurotrophic factor-regulated genes important for spermatogonial stem cell self-renewal in the rat.

Spermatogonial stem cells (SSCs) provide the foundation for spermatogenesis throughout the life of a male. Because SSCs of many species can colonize the mouse testis, and glial cell line-derived neurotrophic factor (GDNF) is responsible for stimulating SSC self-renewal in rodents, we reasoned that molecular mechanisms of SSC self-renewal are similar across species. GDNF-regulated genes have been identified in mouse SSCs; however, downstream targets of GDNF are unknown in other species. The objective of this work was to identify GDNF-regulated genes in rat SSCs and to define the biological significance of these genes for rat SSC self-renewal. We conducted microarray analysis on cultured rat germ cells enriched for SSCs in the presence and absence of GDNF. Many GDNF-regulated genes were identified, most notably, Bcl6b and Etv5, which are important for mouse SSC self-renewal. Bcl6b was the most highly regulated gene in both the rat and mouse. Additionally, we identified three novel GDNF-regulated genes in rat SSCs: Bhlhe40, Hoxc4, and Tec. Small interfering RNA treatment for Bcl6b, Etv5, Bhlhe40, Hoxc4, and Tec resulted in a decrease in SSC number, as determined by transplantation, without a change in total cell number within the culture. These data indicate that, like in the mouse SSC, Bcl6b and Etv5 are important for rat SSC self-renewal, suggesting that these genes may be important for SSCs in all mammals. Furthermore, identification of three novel GDNF-regulated genes in the rat SSC extends our knowledge of SSC activity and broadens the foundation for understanding this process in higher species, including humans.

Spermatogonial stem cells derived from infertile Wv/Wv mice self-renew in vitro and generate progeny following transplantation.

Loss-of-function mutation of the Kit gene causes a severe defect in spermatogenesis that results in infertility due to the inability of its cognate ligand, KIT ligand (KITL), to stimulate spermatogonial proliferation and differentiation. Although self-renewal of mouse spermatogonial stem cells (SSCs) depends on glial cell line-derived neurotrophic factor (GDNF), there is no unequivocal evidence that SSCs with a KIT deficiency can self-renew in vivo or in vitro. In the testis of W(v)/W(v) mice, in which the KIT tyrosine kinase activity is impaired, spermatogonia with SSC phenotype were identified. When W(v)/W(v) spermatogonia were cultured in an SSC culture system supplemented with GDNF in a 10% O(2) atmosphere, they formed clumps and proliferated continuously. An atmosphere of 10% O(2) was better than 21% O(2) to support SSC self-renewal. When W(v)/W(v) clump-forming germ cells were transplanted into testes of infertile wild-type busulfan-treated mice, they colonized the seminiferous tubules but did not differentiate. However, when transplanted into the testes of infertile W/W(v) pups, they restored spermatogenesis and produced spermatozoa, and progeny were generated using microinsemination. These results clearly show that SSCs exist in W(v)/W(v) testes and that they proliferate in vitro similar to wild-type SSCs, indicating that a functional KIT protein is not required for SSC self-renewal. Furthermore, the results indicate that a defect of KIT/KITL signaling of W(v)/W(v) SSCs does not prevent spermatogonial differentiation and spermatogenesis in some recipient strains.

Colony stimulating factor 1 is an extrinsic stimulator of mouse spermatogonial stem cell self-renewal.

Self-renewal and differentiation of spermatogonial stem cells (SSCs) provide the foundation for testis homeostasis, yet mechanisms that control their functions in mammals are poorly defined. We used microarray transcript profiling to identify specific genes whose expressions are augmented in the SSC-enriched Thy1(+) germ cell fraction of mouse pup testes. Comparisons of gene expression in the Thy1(+) germ cell fraction with the Thy1-depleted testis cell population identified 202 genes that are expressed 10-fold or higher in Thy1(+) cells. This database provided a mining tool to investigate specific characteristics of SSCs and identify novel mechanisms that potentially influence their functions. These analyses revealed that colony stimulating factor 1 receptor (Csf1r) gene expression is enriched in Thy1(+) germ cells. Addition of recombinant colony stimulating factor 1 (Csf1), the specific ligand for Csf1r, to culture media significantly enhanced the self-renewal of SSCs in heterogeneous Thy1(+) spermatogonial cultures over a 63-day period without affecting total germ cell expansion. In vivo, expression of Csf1 in both pre-pubertal and adult testes was localized to clusters of Leydig cells and select peritubular myoid cells. Collectively, these results identify Csf1 as an extrinsic stimulator of SSC self-renewal and implicate Leydig and myoid cells as contributors of the testicular stem cell niche in mammals.

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

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

Glial cell line-derived neurotrophic factor regulation of genes essential for self-renewal of mouse spermatogonial stem cells is dependent on Src family kinase signaling.

Self-renewal and differentiation by spermatogonial stem cells (SSCs) is the foundation for continual spermatogenesis. SSC self-renewal is dependent on glial cell line-derived neurotrophic factor (GDNF); however, intracellular mechanisms stimulated by GDNF in SSCs are unknown. To investigate these mechanisms we utilized a culture system that maintains a mouse undifferentiated germ cell population enriched for self-renewing SSCs. In these cultures mRNA for the transcription factors Bcl6b, Erm, and Lhx1 are up-regulated by GDNF and decreased in its absence. The expression of all three molecules was further identified in undifferentiated spermatogonia in vivo. Using small interfering RNA to reduce expression and transplantation to quantify stem cell numbers, Bcl6b, Erm, and Lhx1 were shown to be important for SSC maintenance in vitro. Next, GDNF was shown to activate both Akt and Src family kinase (SFK) signaling in SSCs, and culture of SSCs with inhibitors to Akt or SFKs followed by transplantation analysis showed significant impairment of SSC maintenance in vitro. Apoptosis analysis revealed a significant increase in the percentage of apoptotic cells when Akt, but not SFK, signaling was impaired, indicating that multiple signaling pathways are responsible for SSC self-renewal and survival. Biochemical and gene expression experiments revealed that GDNF up-regulated expression of Bcl6b, Erm, and Lhx1 transcripts is dependent on SFK signaling. Overall, these data demonstrate that GDNF up-regulation of Bcl6b, Erm, and Lhx1 expression through SFK signaling is a key component of the intracellular mechanism for SSC self-renewal.

Efficient generation of transgenic rats through the male germline using lentiviral transduction and transplantation of spermatogonial stem cells.

Spermatozoa produced from spermatogonial stem cells (SSCs) are the vehicle by which genes of a male are passed to the next generation. A single SSC has the ability to self-renew and produce thousands of spermatozoa; therefore, it is an ideal target for genetic modification to efficiently generate transgenic animals in mammalian species. Rats are an important model organism for biological research; however, gene function studies have been difficult because of a limited ability to generate transgenic animals. Transgenic rat production through SSCs offers a means to overcome this obstacle. Because SSCs divide slowly both in vivo and in vitro, lentiviral vectors may be an ideal method for introducing stable genetic modification. Using a lentiviral vector, an enhanced green fluorescent protein (eGFP) transgene was introduced into the genome of cultured rat SSCs, which were microinjected into testes of immunodeficient mice to assess transduction efficiency. Approximately 40% of rat SSCs exposed to the lentiviral vector overnight carried the eGFP transgene and generated colonies of spermatogenesis. When transduced SSCs were transplanted into recipient rat testes, in which endogenous germ cells had been decreased but not eliminated by busulfan treatment, approximately 6% of offspring were transgenic. The transgene was stably integrated into the donor SSC genome and transmitted to and expressed by progeny in subsequent generations. Thus, lentiviral transduction of SSCs followed by transplantation is an effective means for generating transgenic rats through the male germline, and this approach may be applicable to other species in which existing methods are inadequate or not applicable.

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.

Effects of aging and niche microenvironment on spermatogonial stem cell self-renewal.

Aging is evident in most tissues and organ systems, but the mechanisms of aging are difficult to identify and poorly understood. Here, we test the hypothesis that aging results in uncorrected defects in stem cell and/or niche function, which lead to system failure. We used the spermatogonial stem cell (SSC) transplantation assay to determine the effect of aging on testis stem cell/niche function in mice. Between 12 and 24 months of age, male mice experienced a declining level of fertility associated with decreased testis weight, level of spermatogenesis, and total stem cell content. However, when stem cells were consecutively passaged at 3-month intervals to testes of young males, these stem cells continued to produce spermatogenesis for more than 3 years. Thus, SSC self-renewal continues long past the normal life span of the animal when the stem cell is continually maintained in a young niche/microenvironment. Moreover, these data suggest that infertility in old males results from deterioration of the SSC niche and failure to support an appropriate balance between stem cell self-renewal and differentiation.

Conservation of spermatogonial stem cell self-renewal signaling between mouse and rat.

Self-renewal of spermatogonial stem cells (SSCs) is the foundation for maintenance of spermatogenesis throughout life in males and for continuation of a species. The molecular mechanism underlying stem cell self-renewal is a fundamental question in stem cell biology. Recently, we identified growth factors necessary for self-renewal of mouse SSCs and established a serum-free culture system for their proliferation in vitro. To determine whether the stimulatory signals for SSC replication are conserved among different species, we extended the culture system to rat SSCs. Initially, a method to assess in vitro expansion of SSCs was developed by using flow cytometric analysis, and, subsequently, we found that a combination of glial cell line-derived neurotrophic factor, soluble glial cell line-derived neurotrophic factor-family receptor alpha-1 and basic fibroblast growth factor supports proliferation of rat SSCs. When cultured with the three factors, stem cells proliferated continuously for >7 months, and transplantation of the cultured SSCs to recipient rats generated donor stem cell-derived progeny, demonstrating that the cultured stem cells are normal. The growth factor requirement for replication of rat SSCs is identical to that of mouse; therefore, the signaling factors for SSC self-renewal are conserved in these two species. Because SSCs from many mammals, including human, can replicate in mouse seminiferous tubules after transplantation, the growth factors required for SSC self-renewal may be conserved among many different species. Furthermore, development of a long-term culture system for rat SSCs has established a foundation for germ-line modification of the rat by gene targeting technology.