Improved serum- and feeder-free culture of mouse germline stem cells

Trivial role for NSMCE2 during in vitro proliferation and differentiation of male germline stem cells

Spermatogenesis, starting with spermatogonial differentiation, is characterized by ongoing and dramatic alterations in composition and function of chromatin. Failure to maintain proper chromatin dynamics during spermatogenesis may lead to mutations, chromosomal aberrations or aneuploidies. When transmitted to the offspring, these can cause infertility or congenital malformations. The structural maintenance of chromosomes (SMC) 5/6 protein complex has recently been described to function in chromatin modeling and genomic integrity maintenance during spermatogonial differentiation and meiosis. Among the subunits of the SMC5/6 complex, non-SMC element 2 (NSMCE2) is an important small ubiquitin-related modifier (SUMO) ligase. NSMCE2 has been reported to be essential for mouse development, prevention of cancer and aging in adult mice and topological stress relief in human somatic cells. By using in vitro cultured primary mouse spermatogonial stem cells (SSCs), referred to as male germline stem (GS) cells, we investigated the function of NSMCE2 during spermatogonial proliferation and differentiation. We first optimized a protocol to generate genetically modified GS cell lines using CRISPR-Cas9 and generated an Nsmce2−/− GS cell line. Using this Nsmce2−/− GS cell line, we found that NSMCE2 was dispensable for proliferation, differentiation and topological stress relief in mouse GS cells. Moreover, RNA sequencing analysis demonstrated that the transcriptome was only minimally affected by the absence of NSMCE2. Only differential expression of Sgsm1 appeared highly significant, but with SGSM1 protein levels being unaffected without NSMCE2. Hence, despite the essential roles of NSMCE2 in somatic cells, chromatin integrity maintenance seems differentially regulated in the germline.

The nature and dynamics of spermatogonial stem cells

Spermatogonial stem cells (SSCs) are crucial for maintaining spermatogenesis throughout life, and understanding how these cells function has important implications for understanding male infertility. Recently, various populations of cells harbouring stem cell-like properties have been identified in rodent seminiferous tubules, but deciphering how these cells might fuel spermatogenesis has been difficult, and various models to explain SSC dynamics have been put forward. This Review provides an overview of the organization and timing of spermatogenesis and then discusses these models in light of recent studies of SSC markers, heterogeneity and cell division dynamics, highlighting the evidence for and against each model.

Nonrandom Germline Transmission of Mouse Spermatogonial Stem Cells

Genes are thought to be transmitted to offspring by random fertilization of a small number of oocytes with numerous spermatozoa. Here we analyzed the dynamics of male germline transmission by genetic marking and transplantation of spermatogonial stem cells (SSCs). We found that offspring deriving from a small number of specific SSCs appear within a limited time. Interestingly, the same SSC clones reappear later with an average functional lifespan of ∼124.4 days. Cyclic offspring production from SSCs was not caused by changes in SSC self-renewal activity because lineage-tracing analyses suggested that all SSCs actively proliferated. Selection appears to occur during the differentiating spermatogonia stage, when extensive apoptosis was observed. The pattern of germline transmission could be predicted using a mathematical model in which SSCs repeat cycles of transient spermatogenic burst and refractory periods. Thus, spermatogenesis is a regulated process whereby specific SSC clones are repeatedly recruited for fertilization with long-term cycles.

Long-Term Propagation of Porcine Undifferentiated Spermatogonia

Spermatogonial stem cells (SSCs) provide the foundation for spermatogenesis and fertility throughout the adult life of a male. Genetic manipulations of SSCs combined with germ cell transplantation present a novel approach for gene therapy and production of genetically modified animals. However, the rarity of SSCs within mammalian testes remains an impediment to related applications, making in vitro expansion of SSCs a prerequisite. Nevertheless, long-term culture systems of SSCs from large animals have not been established yet. In this study, we developed an optimized in vitro culture condition for porcine undifferentiated spermatogonia. The germ cells were isolated and enriched from 7-day-old porcine testes by an optimized differential planting. We tested different feeder layers and found that neonatal autologous Sertoli cells acted better than the SIM mouse embryo-derived thioguanine- and ouabain-resistant (STO) cell line and adult Sertoli cells. The effects of several growth factors were also investigated. Using neonatal Sertoli cells as feeder and Dulbecco's modified eagle medium: nutrient mixture F-12 (DMEM/F12) culture medium supplemented with 10% KSR and four cytokines, the undifferentiated spermatogonia can proliferate in vitro for at least 2 months without loss of stemness. The expression of SSC markers indicated that the cultured cells maintained SSC expression profiles. Moreover, xenotransplantation and in vitro induction showed that the long-term cultured cells preserved the capacity to colonize in vivo and differentiate in vitro, respectively, demonstrating the presence of SSCs in the cultured cells. In conclusion, the conditions described in this study can support the normal proliferation of porcine undifferentiated spermatogonia with stemness and normal karyotype for at least 2 months. This culture system will serve as a basic refinement in the future studies and facilitate studies on SSC biology and genetic manipulation of male germ cells.

Glycolysis-Optimized Conditions Enhance Maintenance of Regenerative Integrity in Mouse Spermatogonial Stem Cells during Long-Term Culture

The application of spermatogonial stem cell (SSC) transplantation for regenerating male fertility requires amplification of SSC number in vitro during which the integrity to re-establish spermatogenesis must be preserved. Conventional conditions supporting proliferation of SSCs from mouse pups have been the basis for developing methodology with adult human cells but are unrefined. We found that the integrity to regenerate spermatogenesis after transplantation declines with advancing time in primary cultures of pup SSCs and that the efficacy of deriving cultures from adult SSCs is limited with conventional conditions. To address these deficiencies, we optimized the culture environment to favor glycolysis as the primary bioenergetics process. In these conditions, regenerative integrity of pup and adult SSCs was significantly improved and the efficiency of establishing primary cultures was 100%. Collectively, these findings suggest that SSCs are primed for conditions favoring glycolytic activity, and matching culture environments to their bioenergetics is critical for maintaining functional integrity.

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Regenerative integrity of SSCs declines over time in conventional culture

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Glycolysis-optimized (GO) culture improves regenerative integrity of SSCs

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GO conditions enhance the long-term culture of SSCs from adult mice

Generation of Mouse Spermatogonial Stem-Cell-Colonies in A Non-Adherent Culture

OBJECTIVE

The properties of self-renewal and division in spermatogonial stem cells (SSCs) support spermatogenesis. There is a number of reported methods for in vitro SSC culture systems. The development of a culture system that effectively supports isolation and selfrenewal of germline stem cells (GSCs) is of tremendous benefit for clinical trials, experimental research, and as potential treatment for male infertility. The current study aims to consider the cultivation and behavior of GSCs in a non-adherent culture system.

MATERIALS AND METHODS

In this experimental study, we cultured testicular cells from neonatal mice in agarose coated plates in the presence of Dulbecco's modified Eagle's medium (DMEM) medium (CTRL group), 10% fetal bovine serum (FBS)+DMEM (10% group), and growth factor (G group) that contained 2% FBS, glial cell-derived neurotrophic factor (GDNF), epidermal growth factor (EGF), and fibroblast growth factor (FGF). Mouse spermatogonial stem-like colonies were isolated approximately 3 weeks after digestion of the testis tissue. After passages 2-3, the identity of the mouse spermatogonial stem-like cells was confirmed by immunocytochemistry, reverse transcription-polymerase chain reaction (RT-PCR), and flow cytometry against the germ cell markers α6, β1, c-Kit, Thy-1, c-Ret, Plzf, and Oct4. The statistical significance between mean values in different groups was determined by one-way analysis of variance (ANOVA).

RESULTS

We observed spermatogonial stem-like colonies in the G and 10% groups, but not the CTRL group. Immunocytochemistry, flow cytometry, and RT-PCR confirmed expressions of germ cell markers in these cells. In the spermatogonial stem-like cells, we observed a significant expression (P<0.05) of germ cell markers in the G and 10% groups versus the testis cells (T). Their proliferative and apoptotic activities were examined by Ki67 and PI/annexin V-FITC. Alkaline phosphatase assay showed that mouse spermato- gonial stem-like colonies were partially positive.

CONCLUSION

A non-adherent culture system could provide a favorable method for in vitro short-term culture of spermatogonial stem-like cell colonies.

Effective production of recipient male pigs for spermatogonial stem cell transplantation by intratesticular injection with busulfan

Germ cell transplantation has facilitated spermatogonial stem cell (SSC) and spermatogenesis research and shown great potential in the seed-breeding of domestic livestock. However, little progress has been made in large animals, primarily reflecting the difficulties in preparing sterile recipients. Here, we developed a novel protocol to prepare recipient pigs through the direct injection of busulfan into the cavum vaginale of the scrotums of Landrace-Large bi-crossbreeding male pigs and Seghers male pigs, two economically-important types of pigs, to eliminate endogenous spermatogonia. No severe diseases or weight loss was observed in either pig type after the injection with busulfan. Histologic analysis showed an advanced and dose-dependent germ cell loss, with complete germ cell loss observed in the highest dose group, 3.0 mg/kg in the Landrace-Large bi-crossbreeding pigs and 2.0 mg/kg in the Seghers pigs. A smaller seminiferous tubule diameter, a vacuolized seminiferous epithelium and the overproliferation interstitial cells, frequently observed in mouse germ cell deficiency models, were present in the most of the high-dose busulfan-treated groups. Molecular markers detected in Seghers pigs further confirmed the depletion of endogenous germ cells, providing an accessible niche for exogenous SSCs. This study provides a basis to prepare the transplantation recipients of SSCs in pigs.

BMP4 Cooperates with Retinoic Acid to Induce the Expression of Differentiation Markers in Cultured Mouse Spermatogonia

Spermatogenesis is sustained by the proliferation and differentiation of spermatogonial stem cells (SSCs). However, the molecules controlling these processes remain largely unknown. Here, we developed a simplified high concentration serum-containing system for the culture of mouse SSCs. Analysis of SSCs markers and transplantation results revealed that the cultured spermatogonia retained stem cell characteristics after long-term in vitro propagation. Using this culture system, the expression and function of bone morphogenetic protein 4 (BMP4) were explored. Immunostaining showed that BMP4 was predominantly expressed in germ cells and that its level increased as spermatogenesis progresses. BMP4 receptors BMPR1A and BMPRII were present in spermatogonia, spermatocytes, and round spermatids. Moreover, despite the mRNAs of these two genes being present in mouse Sertoli cells, only BMPRII was detected by using Western blotting assays. While exogenous BMP4 by itself did not induce the expression of Stra8 and c-Kit, two marker genes of differentiating spermatogonia, a significant cooperative effect of BMP4 and retinoic acid (RA) was observed. Moreover, pretreatment of cultured spermatogonia with the BMP4 antagonist Noggin could inhibit RA-induced expression of these two marker genes. In conclusion, BMP4 may exert autocrine effects and act cooperatively with RA to induce the differentiation of spermatogonia in vivo.

Enrichment of Mouse Spermatogonial Stem Cells by the Stem Cell Dye CDy1

Spermatogonial stem cells (SSCs) comprise a small population of germ cells with self-renewal potential. Previous studies have shown that SSCs share several common features with stem cells in other self-renewing tissues, including surface markers and proliferative machinery. However, studies of SSCs are severely handicapped by the small number of SSCs and the lack of SSC-specific markers. In the present study, we examined the utility of CDy1 and Rh123, both of which are used for the collection of stem cells in several self-renewing tissues. CDy1 stained germline stem (GS) cells, cultured spermatogonia enriched for SSC activity, after in vitro incubation without exerting toxic effects. Unlike previously reported stem cell-specific dyes, CDy1 was also useful for enrichment of SSCs in both GS cell culture and mature adult testes. Spermatogonial transplantation showed that ∼1 in 66.7 cells exhibited SSC activity after CDH1-based magnetic cell selection and CDy1 staining. In contrast, although Rh123 was previously used successfully to collect SSCs from cryptorchid testes, it was not possible to recover SSCs from both GS cell cultures and wild-type testes. Thus, CDy1 staining will provide a useful strategy for the enrichment of SSCs and may be used in conjunction with other reagents for the enrichment of SSCs.

Differentiation of Fetal Male Germline and Gonadal Progenitor Cells Is Disrupted in Organ Cultures Containing Knockout Serum Replacement

Germ cells occupy a unique place in development through their requirement to maintain underlying totipotency while producing highly differentiated gametes. This is reflected in the expression of key regulators of pluripotency in the fetal germline and the ability of these cells to form pluripotent stem cells and germ cell tumors. Culture of whole fetal testes, including germ cells, provides a key model for studying gonad and germline development, but it is critical that such models mimic physiological development as closely as possible. We aimed to determine the effects of differing culture conditions, including serum-free and serum-containing conditions, on fetal germ cell and testis development. We tested a commonly used model that employs knockout serum replacement (KSR) to provide more defined culture conditions than media containing fetal bovine serum (FBS). In FBS conditions, cell cycle parameters in germ and Sertoli cells closely resembled normal development. In contrast, KSR significantly inhibited male germ cell entry into mitotic arrest, a key milestone in male germline development. Moreover, KSR disrupted molecular control of cell cycle and inhibited the transcription of a range of male germ cell differentiation markers. In the somatic compartment, KSR stimulated proliferation and inhibited differentiation in Sertoli cells. These data demonstrate that KSR substantially alters germ and somatic cell differentiation in fetal testis culture and should not be used to replicate normal gonadal development. In contrast, basal media with or without serum support germ and supporting cell differentiation and cell cycle dynamics that are in line with in vivo characteristics.

Spermatogonial stem cells: Current biotechnological advances in reproduction and regenerative medicine

Spermatogonial stem cells (SSCs) are the germ stem cells of the seminiferous epithelium in the testis. Through the process of spermatogenesis, they produce sperm while concomitantly keeping their cellular pool constant through self-renewal. SSC biology offers important applications for animal reproduction and overcoming human disease through regenerative therapies. To this end, several techniques involving SSCs have been developed and will be covered in this article. SSCs convey genetic information to the next generation, a property that can be exploited for gene targeting. Additionally, SSCs can be induced to become embryonic stem cell-like pluripotent cells in vitro. Updates on SSC transplantation techniques with related applications, such as fertility restoration and preservation of endangered species, are also covered on this article. SSC suspensions can be transplanted to the testis of an animal and this has given the basis for SSC functional assays. This procedure has proven technically demanding in large animals and men. In parallel, testis tissue xenografting, another transplantation technique, was developed and resulted in sperm production in testis explants grafted into ectopical locations in foreign species. Since SSC culture holds a pivotal role in SSC biotechnologies, current advances are overviewed. Finally, spermatogenesis in vitro, already demonstrated in mice, offers great promises to cope with reproductive issues in the farm animal industry and human clinical applications.