Culture Conditions and Single Growth Factors Affect Fate Determination of Mouse Spermatogonial Stem Cells1

Hormonally and chemically defined expansion conditions for organoids of biliary tree Stem Cells

Wholly defined ex vivo expansion conditions for biliary tree stem cell (BTSC) organoids were established, consisting of a defined proliferative medium (DPM) used in combination with soft hyaluronan hydrogels. The DPM consisted of commercially available Kubota's Medium (KM), to which a set of small molecules, particular paracrine signals, and heparan sulfate (HS) were added. The small molecules used were DNA methyltransferase inhibitor (RG108), TGF- β Type I receptor inhibitor (A83-01), adenylate cyclase activator (Forskolin), and L-type Ca2+ channel agonist (Bay K8644). A key paracrine signal proved to be R-spondin 1 (RSPO1), a secreted protein that activates Wnts. Soluble hyaluronans, 0.05 % sodium hyaluronate, were used with DPM to expand monolayer cultures. Expansion of organoids was achieved by using DPM in combination with embedding organoids in Matrigel that was replaced with a defined thiol-hyaluronan triggered with PEGDA to form a hydrogel with a rheology [G*] of less than 100 Pa. The combination is called the BTSC-Expansion-Glycogel-System (BEX-gel system) for expanding BTSCs as a monolayer or as organoids. The BTSC organoids were expanded more than 3000-fold ex vivo in the BEX-gel system within 70 days while maintaining phenotypic traits indicative of stem/progenitors. Stem-cell-patch grafting of expanded BTSC organoids was performed on the livers of Fah-/- mice with tyrosinemia and resulted in the rescue of the mice and restoration of their normal liver functions. The BEX-gel system for BTSC organoid expansion provides a strategy to generate sufficient numbers of organoids for the therapeutic treatments of liver diseases.

Identification and Manipulation of Spermatogonial Stem Cells with the Aim of Inducing Spermatogenesis in Vitro

Assisted reproduction techniques for infertile men with non-obstructive azoospermia require a sufficient number of functional germ cells produced in vitro. Understanding the mechanisms that allow the resumption of spermatogenesis outside the testicular environment is crucial for fertility preservation in these patients. A review of the literature was conducted using databases such as PubMed, Scopus and Web of Science, with keywords including "spermatogonial stem cell," "germ cells," "male factor infertility," and "enrichment and propagation of SSCs in vitro." Currently, two models-"in vivo" and "in vitro"-have been developed for producing haploid germ cells. The "in vivo" models include spermatogonial stem cell transplantation and testicular xenograft techniques. In contrast, the "in vitro" models consist of conventional culture systems, organ culture, and three-dimensional culture systems, all designed to induce spermatogenesis in vitro. These culture systems enable the simulation of the seminiferous epithelium in vitro, which facilitates better regulation of cell-signaling pathways that control the self-renewal and differentiation of SSCs. This review provides up-to-date information on the organization of SSCs, focusing on the identification, proliferation, and differentiation of spermatogonia in vitro.

Spermatogonial stem cell technologies: applications from human medicine to wildlife conservation†

Spermatogonial stem cell (SSC) technologies that are currently under clinical development to reverse human infertility hold the potential to be adapted and applied for the conservation of endangered and vulnerable wildlife species. The biobanking of testis tissue containing SSCs from wildlife species, aligned with that occurring in pediatric human patients, could facilitate strategies to improve the genetic diversity and fitness of endangered populations. Approaches to utilize these SSCs could include spermatogonial transplantation or testis tissue grafting into a donor animal of the same or a closely related species, or in vitro spermatogenesis paired with assisted reproduction approaches. The primary roadblock to progress in this field is a lack of fundamental knowledge of SSC biology in non-model species. Herein, we review the current understanding of molecular mechanisms controlling SSC function in laboratory rodents and humans, and given our particular interest in the conservation of Australian marsupials, use a subset of these species as a case-study to demonstrate gaps-in-knowledge that are common to wildlife. Additionally, we review progress in the development and application of SSC technologies in fertility clinics and consider the translation potential of these techniques for species conservation pipelines.

Spermatogenic Stem Cells: Core Biology, Defining Features, and Utilities

The actions of spermatogenic stem cells (SSCs) provide the foundation for continual spermatogenesis and regeneration of the cognate lineage following cytotoxic insult or transplantation. Several decades of research with rodent models have yielded knowledge about the core biology, morphological features, and molecular profiles of mammalian SSCs. Translation of these discoveries to utilities for human fertility preservation, improving animal agriculture, and wildlife conservation are actively being pursued. Here, we provide overviews of these aspects covering both historical and current states of understanding.

The effect of epididymosomes on the development of frozen-thawed mouse spermatogonial stem cells after culture in a decellularized testicular scaffold and transplantation into azoospermic mice

PURPOSE

This study examined SSC proliferation on an epididymosome-enriched decellularized testicular matrix (DTM) hydrogel and spermatogenesis induction in azoospermic mice.

METHODS

Epididymosomes were extracted and characterized using SEM and western blotting. After cryopreservation, thawed SSCs were cultured in a hydrogel-based three-dimensional (3D) culture containing 10 ng/mL GDNF or 20 µg/mL epididymosomes. SSCs were assessed using the MTT assay, flow cytometry, and qRT-PCR after two weeks of culture. The isolated SSCs were microinjected into the efferent ducts of busulfan-treated mice. DiI-labeled SSCs were followed, and cell homing was assessed after two weeks. After 8 weeks, the testes were evaluated using morphometric studies and immunohistochemistry.

RESULTS

The expression of PLZF, TGF-β, and miR-10b did not increase statistically significantly in the 3D + GDNF and 3D + epididymosome groups compared to the 3D group. Among the groups, the GDNF-treated group exhibited the highest expression of miR-21 (*P < 0.05). Caspase-3 expression was lower in the epididymosome-treated group than in the other groups (***P < 0.001). Compared to the 3D and negative control groups, the 3D + epididymosomes and 3D + GDNF groups showed an increase in spermatogenic cells. Immunohistochemical results confirmed the growth and differentiation of spermatogonial cells into spermatids in the treatment groups.

CONCLUSION

The DTM hydrogel containing 20 µg/mL epididymosomes or 10 ng/mL GDNF is a novel and safe culture system that can support SSC proliferation in vitro to obtain adequate SSCs for transplantation success. It could be a novel therapeutic agent that could recover deregulated SSCs in azoospermic patients.

Postnatal development of mouse spermatogonial stem cells as determined by immunophenotype, regenerative capacity, and long-term culture-initiating ability: a model for practical applications

Spermatogonial stem cells (SSCs) are the foundation of life-long spermatogenesis. While SSC research has advanced greatly over the past two decades, characterization of SSCs during postnatal development has not been well documented. Using the mouse as a model, in this study, we defined the immunophenotypic profiles of testis cells during the course of postnatal development using multi-parameter flow cytometry with up to five cell-surface antigens. We found that the profiles progress over time in a manner specific to developmental stages. We then isolated multiple cell fractions at different developmental stages using fluorescent-activated cell sorting (FACS) and identified specific cell populations with prominent capacities to regenerate spermatogenesis upon transplantation and to initiate long-term SSC culture. The data indicated that the cell fraction with the highest level of regeneration capacity exhibited the most prominent potential to initiate SSC culture, regardless of age. Interestingly, refinement of cell fractionation using GFRA1 and KIT did not lead to further enrichment of regenerative and culture-initiating stem cells, suggesting that when a high degree of SSC enrichment is achieved, standard markers of SSC self-renewal or commitment may lose their effectiveness to distinguish cells at the stem cell state from committed progenitors. This study provides a significant information resource for future studies and practical applications of mammalian SSCs.

Mouse Tspyl5 promotes spermatogonia proliferation through enhancing Pcna-mediated DNA replication

CONTEXT

The human TSPY1 (testis-specific protein, Y-linked 1) gene is critical for spermatogenesis and male fertility. However, there have been difficulties with studying the mechanism underlying its function, partly due to the presence of the Tspy1 pseudogene in mice.

AIMS

TSPYL5 (TSPY-like 5), an autosomal homologous gene of TSPY1 showing a similar expression pattern in both human and mouse testes, is also speculated to play a role in male spermatogenesis. It is beneficial to understand the role of TSPY1 in spermatogenesis by investigating Tspyl5 functions.

METHODS

Tspyl5 -knockout mice were generated to investigate the effect of TSPYL5 knockout on spermatogenesis.

KEY RESULTS

Tspyl5 deficiency caused a decline in fertility and decreased the numbers of spermatogonia and spermatozoa in aged male mice. Trancriptomic detection of spermatogonia derived from aged Tspyl5 -knockout mice revealed that the Pcna -mediated DNA replication pathway was downregulated. Furthermore, Tspyl5 was proven to facilitate spermatogonia proliferation and upregulate Pcna expression by promoting the ubiquitination-degradation of the TRP53 protein.

CONCLUSIONS

Our findings suggest that Tspyl5 is a positive regulator for the maintenance of the spermatogonia pool by enhancing Pcna -mediated DNA replication.

IMPLICATIONS

This observation provides an important clue for further investigation of the spermatogenesis-related function of TSPY1 .

SPRY4-dependent ERK negative feedback demarcates functional adult stem cells in the male mouse germline†

Niche-derived growth factors support self-renewal of mouse spermatogonial stem and progenitor cells through ERK MAPK signaling and other pathways. At the same time, dysregulated growth factor-dependent signaling has been associated with loss of stem cell activity and aberrant differentiation. We hypothesized that growth factor signaling through the ERK MAPK pathway in spermatogonial stem cells is tightly regulated within a narrow range through distinct intracellular negative feedback regulators. Evaluation of candidate extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK)-responsive genes known to dampen downstream signaling revealed robust induction of specific negative feedback regulators, including Spry4, in cultured mouse spermatogonial stem cells in response to glial cell line-derived neurotrophic factor or fibroblast growth factor 2. Undifferentiated spermatogonia in vivo exhibited high levels of Spry4 mRNA. Quantitative single-cell analysis of ERK MAPK signaling in spermatogonial stem cell cultures revealed both dynamic signaling patterns in response to growth factors and disruption of such effects when Spry4 was ablated, due to dysregulation of ERK MAPK downstream of RAS. Whereas negative feedback regulator expression decreased during differentiation, loss of Spry4 shifted cell fate toward early differentiation with concomitant loss of stem cell activity. Finally, a mouse Spry4 reporter line revealed that the adult spermatogonial stem cell population in vivo is demarcated by strong Spry4 promoter activity. Collectively, our data suggest that negative feedback-dependent regulation of ERK MAPK is critical for preservation of spermatogonial stem cell fate within the mammalian testis.

Advances of three-dimensional (3D) culture systems for in vitro spermatogenesis

The loss of germ cells and spermatogenic failure in non-obstructive azoospermia are believed to be the main causes of male infertility. Laboratory studies have used in vitro testicular models and different 3-dimensional (3D) culture systems for preservation, proliferation and differentiation of spermatogonial stem cells (SSCs) in recent decades. The establishment of testis-like structures would facilitate the study of drug and toxicity screening, pathological mechanisms and in vitro differentiation of SSCs which resulted in possible treatment of male infertility. The different culture systems using cellular aggregation with self-assembling capability, the use of different natural and synthetic biomaterials and various methods for scaffold fabrication provided a suitable 3D niche for testicular cells development. Recently, 3D culture models have noticeably used in research for their architectural and functional similarities to native microenvironment. In this review article, we briefly investigated the recent 3D culture systems that provided a suitable platform for male fertility preservation through organ culture of testis fragments, proliferation and differentiation of SSCs.

Comparing the adult and pre-pubertal testis: Metabolic transitions and the change in the spermatogonial stem cell metabolic microenvironment

BACKGROUND

Survivors of childhood cancer often suffer from infertility. While sperm cryopreservation is not feasible before puberty, the patient's own spermatogonial stem cells could serve as a germ cell reservoir, enabling these patients to father their own children in adulthood through the isolation, in vitro expansion, and subsequent transplantation of spermatogonial stem cells. However, this approach requires large numbers of stem cells, and methods for successfully propagating spermatogonial stem cells in the laboratory are yet to be established for higher mammals and humans. The improvement of spermatogonial stem cell culture requires deeper understanding of their metabolic requirements and the mechanisms that regulate metabolic homeostasis.

AIM

This review gives a summary on our knowledge of spermatogonial stem cell metabolism during maintenance and differentiation and highlights the potential influence of Sertoli cell and stem cell niche maturation on spermatogonial stem cell metabolic requirements during development.

RESULTS AND CONCLUSIONS

Fetal human spermatogonial stem cell precursors, or gonocytes, migrate into the seminiferous cords and supposedly mature to adult stem cells within the first year of human development. However, the spermatogonial stem cell niche does not fully differentiate until puberty, when Sertoli cells dramatically rearrange the architecture and microenvironment within the seminiferous epithelium. Consequently, pre-pubertal and adult spermatogonial stem cells experience two distinct niche environments potentially affecting spermatogonial stem cell metabolism and maturation. Indeed, the metabolic requirements of mouse primordial germ cells and pig gonocytes are distinct from their adult counterparts, and novel single-cell RNA sequencing analysis of human and porcine spermatogonial stem cells during development confirms this metabolic transition. Knowledge of the metabolic requirements and their changes and regulation during spermatogonial stem cell maturation is necessary to implement laboratory-based techniques and enable clinical use of spermatogonial stem cells. Based on the advancement in our understanding of germline metabolism circuits and maturation events of niche cells within the testis, we propose a new definition of spermatogonial stem cell maturation and its amendment in the light of metabolic change.

Identification of quiescent FOXC2+ spermatogonial stem cells in adult mammals

In adult mammals, spermatogenesis embodies the complex developmental process from spermatogonial stem cells (SSCs) to spermatozoa. At the top of this developmental hierarchy lie a series of SSC subpopulations. Their individual identities as well as the relationships with each other, however, remain largely elusive. Using single-cell analysis and lineage tracing, we discovered both in mice and humans the quiescent adult SSC subpopulation marked specifically by forkhead box protein C2 (FOXC2). All spermatogenic progenies can be derived from FOXC2+ SSCs and the ablation of FOXC2+ SSCs led to the depletion of the undifferentiated spermatogonia pool. During germline regeneration, FOXC2+ SSCs were activated and able to completely restore the process. Germ cell-specific Foxc2 knockout resulted in an accelerated exhaustion of SSCs and eventually led to male infertility. Furthermore, FOXC2 prompts the expressions of negative regulators of cell cycle thereby ensures the SSCs reside in quiescence. Thus, this work proposes that the quiescent FOXC2+ SSCs are essential for maintaining the homeostasis and regeneration of spermatogenesis in adult mammals.

Propagation of porcine spermatogonial stem cells in serum-free culture conditions using knockout serum replacement

In vitro culture and expansion of spermatogonial stem cells (SSCs) is an essential prerequisite to enhancing livestock productivity through SSC transplantation. Most of the culture media have been observed to be supplemented with serum. However, the use of serum in culture media may exert detrimental effects on SSC maintenance in vitro. An attempt was made to culture SSCs by replacing serum with 5% 'Knockout Serum Replacement (KSR)' in Doom pig (Sus domesticus), one of the valued indigenous germplasm of North-East India. Testes from 7 to 15 days old piglets were used for isolation, enrichment and in vitro culture of putative SSCs using serum-based and serum-free culture media. The cells were characterized for SSC-specific pluripotent markers expression by immunofluorescence staining and quantitative real-time PCR. The diameter and number of SSC colonies were recorded on days 9, 20 and 30 of culture. Similar morphologies of the SSC colonies were observed in both serum-based and serum-free culture conditions. Colony diameter and colony number were non-significantly higher in serum-free than serum-based media. The cells from both the culture conditions showed high alkaline phosphatase activity. The expression of SSC-specific pluripotent markers was observed in immunofluorescence and quantitative real-time PCR study. The present study revealed that SSCs from porcine species could be maintained in vitro for up to 30 days in serum-free culture using 5% KSR, which is believed to be a promising protein source for improving livestock production, and health care along with their conservation.

Defining Gene Function in Spermatogonial Stem Cells Through Conditional Knockout Approaches

Mammalian male fertility is maintained throughout life by a population of self-renewing mitotic germ cells known as spermatogonial stem cells (SSCs). Much of our current understanding regarding the molecular mechanisms underlying SSC activity is derived from studies using conditional knockout mouse models. Here, we provide a guide for the selection and use of mouse strains to develop conditional knockout models for the study of SSCs, as well as their precursors and differentiation-committed progeny. We describe Cre recombinase-expressing strains, breeding strategies to generate experimental groups, and treatment regimens for inducible knockout models and provide advice for verifying and improving conditional knockout efficiency. This resource can be beneficial to those aiming to develop conditional knockout models for the study of SSC development and postnatal function.

Perspectives: Methods for Evaluating Primate Spermatogonial Stem Cells

Mammalian spermatogenesis is a complex, highly productive process generating millions of sperm per day. Spermatogonial stem cells (SSCs) are at the foundation of spermatogenesis and can either self-renew, producing more SSCs, or differentiate to initiate spermatogenesis and produce sperm. The biological potential of SSCs to produce and maintain spermatogenesis makes them a promising tool for the treatment of male infertility. However, translating knowledge from rodents to higher primates (monkeys and humans) is challenged by different vocabularies that are used to describe stem cells and spermatogenic lineage development in those species. Furthermore, while rodent SSCs are defined by their biological potential to produce and maintain spermatogenesis in a transplant assay, there is no equivalent routine and accessible bioassay to test monkey and human SSCs or replicate their functions in vitro. This chapter describes progress characterizing, isolating, culturing, and transplanting SSCs in higher primates.

Primary Cultures of Spermatogonia and Testis Cells

Spermatogonial stem cells (SSCs) maintain adult spermatogenesis in mammals by undergoing self-renewal and differentiation into spermatozoa. In order to study the biology of SSCs as related to spermatogenesis, an in vitro, long-term expansion system of SSCs constitutes an ideal tool. In this chapter, we describe a robust culture system for mouse and rat SSCs in vitro. In the presence of GDNF, GFRα1, and bFGF, SSCs maintained on STO feeder layers with serum-free medium continuously proliferate for over 6 months. Complete spermatogenesis in infertile recipient mice can be attained following transplantation of the cultured mouse and rat SSCs. Using the in vitro SSC culture systems, elucidation of stem cell biology can be advanced that significantly advances our understanding of spermatogenesis and male fertility.

Modeling mammalian spermatogonial differentiation and meiotic initiation in vitro

In mammalian testes, premeiotic spermatogonia respond to retinoic acid by completing an essential lengthy differentiation program before initiating meiosis. The molecular and cellular changes directing these developmental processes remain largely undefined. This wide gap in knowledge is due to two unresolved technical challenges: (1) lack of robust and reliable in vitro models to study differentiation and meiotic initiation; and (2) lack of methods to isolate large and pure populations of male germ cells at each stage of differentiation and at meiotic initiation. Here, we report a facile in vitro differentiation and meiotic initiation system that can be readily manipulated, including the use of chemical agents that cannot be safely administered to live animals. In addition, we present a transgenic mouse model enabling fluorescence-activated cell sorting-based isolation of millions of spermatogonia at specific developmental stages as well as meiotic spermatocytes.

Culture of Kenyan Goat (Capra hircus) Undifferentiated Spermatogonia in Feeder-Free Conditions

The undifferentiated spermatogonial population in mammalian testes contains a spermatogonial stem cell (SSC) population that can regenerate continual spermatogenesis following transplantation. This capacity has the potential to be exploited as a surrogate sires breeding tool to achieve widespread dissemination of desirable genetics in livestock production. Because SSCs are relatively rare in testicular tissue, the ability to expand a population in vitro would be advantageous to provide large numbers for transplantation into surrogate recipient males. Here, we evaluated conditions that would support long-term in-vitro maintenance of undifferentiated spermatogonia from a goat breed that is endemic to Kenyan livestock production. Single-cell suspensions enriched for undifferentiated spermatogonia from pre-pubertal bucks were seeded on laminin-coated tissue culture plates and maintained in a commercial media based on serum-free composition. The serum-free media was conditioned on goat fetal fibroblasts and supplemented with a growth factor cocktail that included glial cell line-derived neurotrophic factor (GDNF), leukemia inhibitory factor (LIF), stromal cell-derived factor (SDF), and fibroblast growth factor (FGF) before use. Over 45 days, the primary cultures developed a cluster morphology indicative of in-vitro grown undifferentiated spermatogonia from other species and expressed the germ cell marker VASA, as well as the previously defined spermatogonial marker such as promyelocytic leukemia zinc finger (PLZF). Taken together, these findings provide a methodology for isolating the SSC containing undifferentiated spermatogonial population from goat testes and long-term maintenance in defined culture conditions.

A novel high throughput screen to identify candidate molecular networks that regulate spermatogenic stem cell functions†

Spermatogenic regeneration is key for male fertility and relies on activities of an undifferentiated spermatogonial population. Here, a high-throughput approach with primary cultures of mouse spermatogonia was devised to rapidly predict alterations in functional capacity. Combining the platform with a large-scale RNAi screen of transcription factors, we generated a repository of new information from which pathway analysis was able to predict candidate molecular networks regulating regenerative functions. Extending from this database, the SRCAP-CREBBP/EP300 (Snf2-related CREBBP activator protein-CREB binding protein/E1A binding protein P300) complex was found to mediate differential levels of histone acetylation between stem cell and progenitor spermatogonia to influence expression of key self-renewal genes including the previously undescribed testis-specific transcription factor ZSCAN2 (zinc finger and SCAN domain containing 2). Single cell RNA sequencing analysis revealed that ZSCAN2 deficiency alters key cellular processes in undifferentiated spermatogonia such as translation, chromatin modification, and ubiquitination. In Zscan2 knockout mice, while spermatogenesis was moderately impacted during steady state, regeneration after cytotoxic insult was significantly impaired. Altogether, these findings have validated the utility of our high-throughput screening approach and have generated a transcription factor database that can be utilized for uncovering novel mechanisms governing spermatogonial functions.

Male fertility preservation and restoration strategies for patients undergoing gonadotoxic therapies†

Medical treatments for cancers or other conditions can lead to permanent infertility. Infertility is an insidious disease that impacts not only the ability to have a biological child but also the emotional well-being of the infertile individuals, relationships, finances, and overall health. Therefore, all patients should be educated about the effects of their medical treatments on future fertility and about fertility preservation options. The standard fertility preservation option for adolescent and adult men is sperm cryopreservation. Sperms can be frozen and stored for a long period, thawed at a later date, and used to achieve pregnancy with existing assisted reproductive technologies. However, sperm cryopreservation is not applicable for prepubertal patients who do not yet produce sperm. The only fertility preservation option available to prepubertal boys is testicular tissue cryopreservation. Next-generation technologies are being developed to mature those testicular cells or tissues to produce fertilization-competent sperms. When sperm and testicular tissues are not available for fertility preservation, inducing pluripotent stem cells derived from somatic cells, such as blood or skin, may provide an alternative path to produce sperms through a process call in vitro gametogenesis. This review describes standard and experimental options to preserve male fertility as well as the experimental options to produce functional spermatids or sperms from immature cryopreserved testicular tissues or somatic cells.

A scRNA-seq Approach to Identifying Changes in Spermatogonial Stem Cell Gene Expression Following in vitro Culture

Spermatogonial stem cell (SSC) function is essential for male fertility, and these cells hold potential therapeutic value spanning from human infertility treatments to wildlife conservation. As in vitro culture is likely to be an integral component of many therapeutic pipelines, we have elected to explore changes in gene expression occurring in undifferentiated spermatogonia in culture that may be intertwined with the temporal reduction in regenerative capacity that they experience. Single cell RNA-sequencing analysis was conducted, comparing undifferentiated spermatogonia retrieved from the adult mouse testis with those that had been subjected to 10 weeks of in vitro culture. Although the majority of SSC signature genes were conserved between the two populations, a suite of differentially expressed genes were also identified. Gene ontology analysis revealed upregulated expression of genes involved in oxidative phosphorylation in cultured spermatogonia, along with downregulation of integral processes such as DNA repair and ubiquitin-mediated proteolysis. Indeed, our follow-up analyses have provided the first depiction of a significant accumulation of ubiquitinated proteins in cultured spermatogonia, when compared to those residing in the testis. The data produced in this manuscript will provide a valuable platform for future studies looking to improve SSC culture approaches and assess their safety for utilisation in therapeutic pipelines.

Progress in germline stem cell transplantation in mammals and the potential usage

Germline stem cells (GSCs) are germ cells with the capacities of self-renewal and differentiation into functional gametes, and are able to migrate to their niche and reconstitute the fertility of recipients after transplantation. Therefore, GSCs transplantation is a promising technique for fertility recovery in the clinic, protection of rare animals and livestock breeding. Though this novel technique faces tremendous challenges, numerous achievements have been made after several decades' endeavor. This review summarizes the current knowledge of GSCs transplantation and its utilization in mammals, and discusses the application prospect in reproductive medicine and animal science.

Culture supplementation of bFGF, GDNF, and LIF alters in vitro proliferation, colony formation, and pluripotency of neonatal porcine germ cells

Gonocytes in the neonatal testis have male germline stem cell properties and as such have important potential applications in fertility preservation and regenerative medicine. Such applications require further studies aimed at increasing gonocyte numbers and evaluating their pluripotency in vitro. The objective of the present study was to test the effects of basic fibroblast growth factor (bFGF), glial cell line-derived neurotrophic factor (GDNF), and leukemia inhibitory factor (LIF) on in vitro propagation, colony formation, and expression of pluripotency markers of neonatal porcine gonocytes. Testis cells from 1-week-old piglets were cultured in basic media (DMEM + 15% FBS), supplemented with various concentrations of bFGF, GDNF, and LIF, either individually or in combinations, in a stepwise experimental design. Gonocytes and/or their colonies were evaluated every 7 days and the gonocyte- (DBA) and pluripotency-specific markers (POU5F1, SSEA-1, E-cadherin, and NANOG) assessed on day 28. Greatest gonocyte numbers and largest colonies were found in media supplemented with 10 ng/mL bFGF and 10 ng/mL bFGF + 100 ng/mL GDNF + 1500 U/mL LIF, respectively. The resultant gonocytes and colonies expressed both germ cell- and pluripotency-specific markers. These results shed light on the growth hormone requirements of porcine gonocytes for in vitro proliferation and colony formation.

Primary culture of germ cells that portray stem cell characteristics and recipient preparation for autologous transplantation in the rhesus monkey

Fertility preservation for prepubertal cancer patients prior to oncologic treatment is an emerging issue, and non‐human primates are considered to constitute suitable models due to the limited availability of human testicular tissues. However, the feasibility of spermatogonial stem cell (SSC) propagation in vitro and autologous testicular germ cell transplantation in vivo requires further exploration in monkeys. Herein, we characterized germ cells in macaque testes at 6 months (M), 18 M and 60 M of age, and effectively isolated the spermatogenic cells (including the spermatogonia) from macaque testes with high purity (over 80%) using combined approaches of STA‐PUT separation, Percoll gradients and differential plating. We also generated recipient monkey testes with ablated endogenous spermatogenesis using the alkylating agent busulfan in six macaques, and successfully mimicked autologous cell transplantation in the testes under ultrasonographic guidance. The use of trypan blue led to successful intratubular injection in 4 of 4 testes. Although SSCs in culture showed no significant propagation, we were able to maintain monkey testicular germ cells with stem cell characteristics for up to 3 weeks. Collectively, these data provided meaningful information for future fertility preservation and SSC studies on both non‐human primates and humans.

Reproductive stage- and season-dependent culture characteristics of enriched caprine male germline stem cells

The present study aims to evaluate season- and reproductive-stage dependent variation in culture characteristics and expression of pluripotency and adhesion markers in caprine-male germline stem cells (cmGSCs). For this, testes from pre-pubertal (4-6 months) and adult (~ 2 years) bucks during non-breeding (July-August; n = 4 each) and breeding (October-November; n = 4 each) seasons were used to isolated testicular cells by two-step enzymatic digestion. After cmGSCs enrichment by multiple methods (differential platting, Percoll density gradient centrifugation, and MACS), cell viability of CD90⁺ cells was assessed before co-cultured onto the Sertoli cell feeder layer up to 3^rd-passage (P-3). The culture characteristics of cmGSCs were compared during primary culture (P-0) and P-3 with different assays [BrdU-assay (proliferation), MTT-assay (senescence), and Cluster-forming activity-assay] and transcript expression analyses by qRT-PCR. Moreover, the co-localization of UCHL-1, CD90, and DBA was examined by a double-immunofluorescence method. In adult bucks, significantly (p < 0.05) higher cell numbers with the ability to proliferate faster and form a greater number of cell clusters, besides up-regulation of pluripotency and adhesion markers expression were observed during the breeding season than the non-breeding season. In contrast, such season-dependent variation was lacking in pre-pubertal bucks. The expression of transcripts during non-breeding seasons was significantly (p < 0.05) higher in pre-pubertal cmGSCs than in adult cells (UCHL-1 = 2.38-folds; CD-90 = 6.66-folds; PLZF = 20.87-folds; ID-4 = 4.75-folds; E-cadherin = 3.89-folds and β1-integrin = 5.70-folds). Overall, the reproductive stage and season affect the population, culture characteristics, and expression of pluripotency and adhesion specific markers in buck testis. These results provide an insight to develop an efficient system for successful cell culture processes targeting cmGSCs.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10616-021-00515-x.

Unique metabolic phenotype and its transition during maturation of juvenile male germ cells

Human male reproductive development has a prolonged prepubertal period characterized by juvenile quiescence of germ cells with immature spermatogonial stem cell (SSC) precursors (gonocytes) present in the testis for an extended period of time. The metabolism of gonocytes is not defined. We demonstrate with mitochondrial ultrastructure studies via TEM and IHC and metabolic flux studies with UHPLC-MS that a distinct metabolic transition occurs during the maturation to SSCs. The mitochondrial ultrastructure of prepubertal human spermatogonia is shared with prepubertal pig spermatogonia. The metabolism of early prepubertal porcine spermatogonia (gonocytes) is characterized by the reliance on OXPHOS fuelled by oxidative decarboxylation of pyruvate. Interestingly, at the same time, a high amount of the consumed pyruvate is also reduced and excreted as lactate. With maturation, prepubertal spermatogonia show a metabolic shift with decreased OXHPOS and upregulation of the anaerobic metabolism-associated uncoupling protein 2 (UCP2). This shift is accompanied with stem cell specific promyelocytic leukemia zinc finger protein (PLZF) protein expression and glial cell-derived neurotropic factor (GDNF) pathway activation. Our results demonstrate that gonocytes differently from mature spermatogonia exhibit unique metabolic demands that must be attained to enable their maintenance and growth in vitro.

WTAP Function in Sertoli Cells Is Essential for Sustaining the Spermatogonial Stem Cell Niche

Sertoli cells are the major component of the spermatogonial stem cell (SSC) niche; however, regulatory mechanisms in Sertoli cells that dictate SSC fate decisions remain largely unknown. Here we revealed features of the N⁶-methyladenosine (m⁶A) mRNA modification in Sertoli cells and demonstrated the functions of WTAP, the key subunit of the m⁶A methyltransferase complex in spermatogenesis. m⁶A-sequencing analysis identified 21,909 m⁶A sites from 15,365 putative m⁶A-enriched transcripts within 6,122 genes, including many Sertoli cell-specific genes. Conditional deletion of Wtap in Sertoli cells resulted in sterility and the progressive loss of the SSC population. RNA sequencing and ribosome nascent-chain complex-bound mRNA sequencing analyses suggested that alternative splicing events of transcripts encoding SSC niche factors were sharply altered and translation of these transcripts were severely dysregulated by Wtap deletion. Collectively, this study uncovers a novel regulatory mechanism of the SSC niche and provide insights into molecular interactions between stem cells and their cognate niches in mammals.

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WTAP is highly expressed in Sertoli cell and is essential in spermatogenesis

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Wtap knockout in Sertoli cell causes defective spermatogonial stem cell maintenance

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WTAP regulates transcription and translation of m⁶A-enriched genes in Sertoli cell

Roles of Stra8 and Tcerg1l in retinoic acid induced spermatogonial differentiation in mouse†

Retinoic acid (RA) induces spermatogonial differentiation, but the mechanism by which it operates remains largely unknown. We developed a germ cell culture assay system to study genes involved in spermatogonial differentiation triggered by RA. Stimulated by RA 8 (Stra8), a RA-inducible gene, is indispensable for meiosis initiation, and its deletion results in a complete block of spermatogenesis at the pre-leptotene/zygotene stage. To interrogate the role of Stra8 in RA mediated differentiation of spermatogonia, we derived germ cell cultures from the neonatal testis of both wild type and Stra8 knock-out mice. We provide the first evidence that Stra8 plays a crucial role in modulating the responsiveness of undifferentiated spermatogonia to RA and facilitates transition to a differentiated state. Stra8-mediated differentiation is achieved through the downregulation of a large portfolio of genes and pathways, most notably including genes involved in the spermatogonial stem cell self-renewal process. We also report here for the first time the role of transcription elongation regulator-1 like (Tcerg1l) as a downstream effector of RA-induced spermatogonial differentiation.

Changing our view of the Schistosoma granuloma to an ecological standpoint

Schistosomiasis, a neglected parasitic tropical disease that has plagued humans for centuries, remains a major public health burden. A primary challenge to understanding schistosomiasis is deciphering the most remarkable pathological feature of this disease, the granuloma - a highly dynamic and self-organized structure formed by both host and parasite components. Granulomas are considered a remarkable example of how parasites evolved with their hosts to establish complex and intimate associations. However, much remains unclear regarding life within the granuloma, and strategies to restrain its development are still lacking. Here we explore current information on the hepatic Schistosoma mansoni granuloma in the light of Ecology and propose that this intricate structure acts as a real ecosystem. The schistosomal granuloma is formed by cells (biotic component), protein scaffolds, fibres, and chemical compounds (abiotic components) with inputs/outputs of energy and matter, as complex as in classical ecosystems. We review the distinct cell populations ('species') within the granuloma and examine how they integrate with each other and interact with their microenvironment to form a multifaceted cell community in different space-time frames. The colonization of the hepatic tissue to form granulomas is explained from the point of view of an ecological succession whereby a community is able to modify its physical environment, creating conditions and resources for ecosystem construction. Remarkably, the granuloma represents a dynamic evolutionary system that undergoes progressive changes in the 'species' that compose its community over time. In line with ecological concepts, we examine the granuloma not only as a place where a community of cells is settled (spatial niche or habitat) but also as a site in which the functional activities of these combined populations occur in an orchestrated way in response to microenvironmental gradients such as cytokines and egg antigens. Finally, we assert how the levels of organization of cellular components in a granuloma as conventionally defined by Cell Biology can fit perfectly into a hierarchical structure of biological systems as defined by Ecology. By rethinking the granuloma as an integrating and evolving ecosystem, we draw attention to the inner workings of this structure that are central to the understanding of schistosomiasis and could guide its future treatment.

Long-Term Ex Vivo Expansion of Murine Spermatogonial Stem Cells in a Simple Serum-Free Medium

Spermatogonial stem cells (SSCs) possess both self-renewal and differentiation abilities to sustain lifelong production of enormous numbers of spermatozoa in males. SSCs hold a unique position among tissue-specific stem cells in adults because of their ability to transmit the genetic information to subsequent generations. Ex vivo expansion of SSCs in conjunction with their transplantation is highly invaluable to study SSCs and develop new reproductive technologies for therapeutic applications. In this chapter, we describe a culture system involving a simple serum-free medium for mouse SSCs. Elimination of the serum from the culture is important to enhance the effects of exogenous factors, which are rather masked by the serum, and to avert the serum-induced inflammatory responses of testicular mesenchymal cells, which cause adverse effects on SSC proliferation. Consequently, using this culture system has proven for the first time that glial cell line-derived neurotrophic factor (GDNF) was found to be the key factor to drive the self-renewing proliferation of SSCs, and fibroblast growth factor 2 enhanced the GDNF-dependent proliferation of SSCs. Besides determining these two key cytokines, the simplicity of the system enabled individual modification of its components to develop long-term cultures of rat and rabbit SSCs. The basics of these culture systems will enable development of the culture conditions for human and other mammalian SSCs in the near future.

Metabolic Requirements for Spermatogonial Stem Cell Establishment and Maintenance In Vivo and In Vitro

The spermatogonial stem cell (SSC) is a unique adult stem cell that requires tight physiological regulation during development and adulthood. As the foundation of spermatogenesis, SSCs are a potential tool for the treatment of infertility. Understanding the factors that are necessary for lifelong maintenance of a SSC pool in vivo is essential for successful in vitro expansion and safe downstream clinical usage. This review focused on the current knowledge of prepubertal testicular development and germ cell metabolism in different species, and implications for translational medicine. The significance of metabolism for cell biology, stem cell integrity, and fate decisions is discussed in general and in the context of SSC in vivo maintenance, differentiation, and in vitro expansion.

Application of platelet-rich plasma (PRP) improves self-renewal of human spermatogonial stem cells in two-dimensional and three-dimensional culture systems

Spermatogonial stem cells (SSCs) are very sensitive to chemotherapy and radiotherapy, so male infertility is a great challenge for prepubertal cancer survivors. Cryoconservation of testicular cells before cancer treatment can preserve SSCs from treatment side effects. Different two-dimensional (2D) and three-dimensional (3D) culture systems of SSCs have been used in many species as a useful technique to in vitro spermatogenesis. We evaluated the proliferation of SSCs in 2D and 3D culture systems of platelet-rich plasma (PRP). testicular cells of four brain-dead patients cultivated in 2D pre-culture system, characterization of SSCs performed by RT-PCR, flow cytometry, immunocytochemistry and their functionality assessed by xenotransplantation to azoospermia mice. PRP prepared and dosimetry carried out to determine the optimized dose of PRP. After preparation of PRP scaffold, cytotoxic and histological evaluation performed and SSCs cultivated into three groups: control, 2D culture by optimized dose of PRP and PRP scaffold. The diameter and number of colonies measured and relative expression of GFRa1 and c-KIT evaluated by real-time PCR. Results indicated the expression of PLZF, VASA, OCT4, GFRa1 and vimentin in colonies after 2D pre-culture, xenotransplantation demonstrated proliferated SSCs have proper functionality to homing in mouse testes. The relative expression of c-KIT showed a significant increase as compared to the control group (*: p < 0.05) in PRP- 2D group, expression of GFRa1 and c-KIT in PRP scaffold group revealed a significant increase as compared to other groups (***: p < 0.001). The number and diameter of colonies in the PRP-2D group showed a considerable increase (p < 0.01) as compared to the control group. In PRP- scaffold group, a significant increase (p < 0.01) was seen only in the number of colonies related to the control group. Our results suggested that PRP scaffold can reconstruct a suitable structure to the in vitro proliferation of SSCs.

Leptin promotes proliferation of neonatal mouse stem/progenitor spermatogonia

PURPOSE

To keep and increase spermatogonial stem cell number (SSC) is the only available option for pediatric cancer survivors to maintain fertility. Leptin is secreted by the epididymal white adipose tissue and has receptors on stem/progenitor spermatogonia. The purpose of this study is to demonstrate dose- and time-dependent proliferative effect of leptin on stem/progenitor spermatogonia cultures from prepubertal mice testes.

METHODS

CD90.2 (+) stem/progenitor spermatogonia were isolated from the C57BL/6 mouse testis on postnatal day 6 and placed in culture. The proliferative effect of leptin supplementation was assessed by colony formation (diameter and number), WST proliferation assays, and xCELLigence real-time cell analysis (RTCA) on days 3, 5, and 7 of culture. Expressions of p-ERK1/2, p-STAT3, total STAT3, and p-SHP2 levels were determined by western blot analysis.

RESULTS

Leptin supplementation of 100 ng/ml increased the diameter (p = 0.001) and number (p = 0.01) of colonies in stem/progenitor spermatogonial cultures and caused higher proliferation by WST-1 (p = 0.009) compared with the control on day 7. The EC50 was calculated as 114 ng/ml for leptin by RTCA. Proliferative dose of leptin induced increased expression of p-ERK1/2 (p = 0.009) and p-STAT3 (p = 0.023) on stem/progenitor spermatogonia when compared with the untreated group.

CONCLUSION

The results indicated that leptin supplementation exhibited a dose- and time-dependent proliferative effect on stem/progenitor spermatogonia that was associated with increased expression of ERK1/2 and STAT3 pathways while maintaining their undifferentiated state. This output presents a new agent that may help to expand the stem/progenitor spermatogonia pool from the neonatal testis in order to autotransplant after cancer treatment.

A bioenergetic shift is required for spermatogonial differentiation

A bioenergetic balance between glycolysis and mitochondrial respiration is particularly important for stem cell fate specification. It however remains to be determined whether undifferentiated spermatogonia switch their preference for bioenergy production during differentiation. In this study, we found that ATP generation in spermatogonia was gradually increased upon retinoic acid (RA)-induced differentiation. To accommodate this elevated energy demand, RA signaling concomitantly switched ATP production in spermatogonia from glycolysis to mitochondrial respiration, accompanied by increased levels of reactive oxygen species. Disrupting mitochondrial respiration significantly blocked spermatogonial differentiation. Inhibition of glucose conversion to glucose-6-phosphate or pentose phosphate pathway also repressed the formation of c-Kit+ differentiating germ cells, suggesting that metabolites produced from glycolysis are required for spermatogonial differentiation. We further demonstrated that the expression levels of several metabolic regulators and enzymes were significantly altered upon RA-induced differentiation, with both RNA-seq and quantitative proteomic analyses. Taken together, our data unveil a critically regulated bioenergetic balance between glycolysis and mitochondrial respiration that is required for spermatogonial proliferation and differentiation.

Antioxidant vitamins and lysophospholipids are critical for inducing mouse spermatogenesis under organ culture conditions

In vitro mouse spermatogenesis using a classical organ culture method became possible by supplementing basal culture medium with only the product of bovine serum albumin purified by chromatography (AlbuMAX), which indicated that AlbuMAX contained every chemical factor necessary for mouse spermatogenesis. However, since the identity of these factors was unclear, improvements in culture media and our understanding of the nutritional and signal substances required for spermatogenesis were hindered. In the present study, chemically defined media (CDM) without AlbuMAX was used to evaluate each supplementary factor and their combinations for the induction of spermatogenesis. Similar to in vivo conditions, retinoic acid, triiodothyronine (T₃ ), and testosterone (T) were needed. Based on differences in spermatogenic competence between AlbuMAX, fetal bovine serum, and adult bovine serum, we identified α-tocopherol, which strongly promoted spermatogenesis when combined with ascorbic acid and glutathione. Differences were also observed in the abilities of lipids extracted from AlbuMAX using two different methods to induce spermatogenesis. This led to the identification of lysophospholipids, particularly lysophosphatidylcholine, lysophosphatidic acid, and lysophosphatidylserine, as important molecules for spermatogenesis. New CDM formulated based on these results induced and promoted spermatogenesis as efficiently as AlbuMAX-containing medium. In vitro spermatogenesis with CDM may provide a unique experimental system for research on spermatogenesis that cannot be performed in in vivo experiments.

Live-cell imaging and ultrastructural analysis reveal remarkable features of cultured porcine gonocytes

Gonocytes in the neonatal testis have male germline stem cell potential. The objective of the present study was to examine the behavior and ultrastructure of gonocytes in culture. Neonatal porcine testis cells were cultured for 4 weeks and underwent live-cell imaging to explore real-time interactions among cultured cells. This included imaging every 1 h from day 0 to day 3, every 2 h from day 4 to day 7, and every 1 h for 24 h at days 14, 21, and 28. Samples also underwent scanning electron microscopy, transmission electron microscopy, morphometric evaluations, immunofluorescence, and RT-PCR. Live-cell imaging revealed an active amoeboid-like movement of gonocytes, assisted by the formation of extensive cytoplasmic projections, which, using scanning electron microscopy, were categorized into spike-like filopodia, leaf-like lamellipodia, membrane ruffles, and cytoplasmic blebs. In the first week of culture, gonocytes formed loose attachments on top of a somatic cell monolayer and, in week 2, formed grape-like clusters, which, over time, grew in cell number. Starting at week 3 of culture, some of the gonocyte clusters transformed into large multinucleated embryoid body-like colonies (EBLCs) that expressed both gonocyte- and pluripotent-specific markers. The number and diameter of individual gonocytes, the number and density of organelles within gonocytes, as well as the number and diameter of the EBLCs increased over time (P < 0.05). In conclusion, cultured porcine gonocytes displayed extensive migratory behavior facilitated by their various cytoplasmic projections, propagated, and transformed into EBLCs that increased in size and complexity over time.

Spermatogonial Stem Cell Culture in Oncofertility

Infertility caused by chemotherapy or radiation treatments negatively impacts patient-survivor quality of life. The only fertility preservation option available to prepubertal boys who are not making sperm is cryopreservation of testicular tissues that contain spermatogonial stem cells (SSCs) with potential to produce sperm and/or restore fertility. SSC transplantation to regenerate spermatogenesis in infertile adult survivors of childhood cancers is a mature technology. However, the number of SSCs obtained in a biopsy of a prepubertal testis may be small. Therefore, methods to expand SSC numbers in culture before transplantation are needed. Here we review progress with human SSC culture.

Spermatogonial Stem Cells in Fish: Characterization, Isolation, Enrichment, and Recent Advances of In Vitro Culture Systems

Spermatogenesis is a continuous and dynamic developmental process, in which a single diploid spermatogonial stem cell (SSC) proliferates and differentiates to form a mature spermatozoon. Herein, we summarize the accumulated knowledge of SSCs and their distribution in the testes of teleosts. We also reviewed the primary endocrine and paracrine influence on spermatogonium self-renewal vs. differentiation in fish. To provide insight into techniques and research related to SSCs, we review available protocols and advances in enriching undifferentiated spermatogonia based on their unique physiochemical and biochemical properties, such as size, density, and differential expression of specific surface markers. We summarize in vitro germ cell culture conditions developed to maintain proliferation and survival of spermatogonia in selected fish species. In traditional culture systems, sera and feeder cells were considered to be essential for SSC self-renewal, in contrast to recently developed systems with well-defined media and growth factors to induce either SSC self-renewal or differentiation in long-term cultures. The establishment of a germ cell culture contributes to efficient SSC propagation in rare, endangered, or commercially cultured fish species for use in biotechnological manipulation, such as cryopreservation and transplantation. Finally, we discuss organ culture and three-dimensional models for in vitro investigation of fish spermatogenesis.

Differentiation of neonate mouse spermatogonial stem cells on three-dimensional agar/polyvinyl alcohol nanofiber scaffold

Electrospun nanofiber matrices sufficiently mimic the structural morphology of natural extracellular matrix. In this study, we aimed to examine the effects of agar/polyvinyl alcohol nanofiber (PVA) scaffold on the proliferation efficiency and differentiation potential of neonate mouse spermatogonial stem cells (SCCs). Testicular cells were isolated from testes of 40 mouse pups and were seeded in: 1) 2D cell culture plates in the absence (2D/-GF) or presence (2D/+GF) of growth factors and 2) onto agar/PVA scaffold in the absence (3D/-GF) or presence (3D/+GF) of growth factors. The cells were subsequently cultured for 4 weeks. First 2 weeks were dedicated to proliferative phase, whereas the next 2 weeks emphasized the differentiation phase. The identity of the SCCs was investigated at different time-points by flow cytometry and quantitative reverse transcription PCR (qRT-PCR) analyses against the germ cell markers, including PLZF, Id-4, Gfrα-1, Tekt-1, and Sycp-3. After 2 weeks of culture, the 3D/+GF group showed the highest percentage of PLZF-positive cells among culture systems (P < 0.05). The expression levels of pre-meiotic markers (Id-4 and Gfrα-1) decreased significantly in all groups, particularly in 3D/+GF group after 28 days of culture. Additionally, the cells in the 3D/+GF group displayed the highest expression of meiotic (Sycp-3) and post-meiotic markers (Tekt-1) 14 days after differentiation induction. Seemingly, the combination of the agar/PVA scaffold and growth factor-supplemented medium synergistically increased the differentiation rate of mouse SSCs into meiotic and post-meiotic cells. Thus, agar/PVA nanofiber scaffolds may have the potential for applications in the restoration of infertility, especially in azoospermic males.

ABBREVIATIONS

2D: two dimentional; 3D: three dimentional; bFGF: basic fibroblast growth factor; BMP-4: bone morphogenetic protein 4; DMEM: Dulbecco's modified Eagle's medium; ECM: extracellular matrix; FCS: fetal calf serum; FTIR: Fourier-transform infrared spectroscopy; GDNF: glial cell line-derived neurotrophic factor; GF: growth factors; Gfrα-1, GDNF family co-receptor α1; Id-4, Inhibitor of DNA Binding 4; MTT: methylthiazoltetrazolium; PLZF: promyelocytic leukemia zinc finger; PVA: polyvinyl alcohol; qRT-PCR: quantitative reverse transcription PCR; RA: retinoic acid; SACS: soft agar culture system; SD: standard deviation; SEM: scanning electron microscope; SSCs: spermatogonial stem cells; Sycp-3, Synaptonemal complex protein 3; Tekt-1, Tektin 1.

Propagation of goat putative spermatogonial stem cells under growth factors defined serum-free culture conditions

In the present study, we used a serum-free culture media to propagate goat putative spermatogonial stem cells (SSCs) and evaluated the effect of crucial growth factors on relative expression of some SSC markers and self-renewal related genes. The enriched SSCs were cultured on a homologous Sertoli cell feeder layer in KO-DMEM supplemented with 10% KOSR. Putative SSC colonies emerged between day 6 and 10 which were then characterized by the expression of numerous spermatogonial and pluripotency related markers. After 15 days of subculture, the relative mRNA expression study revealed that 40 ng/mL concentration of Glial cell line-derived neurotrophic factor (GDNF) upregulated the expression of BCL6B, ID4, PLZF, and UCHL1. Moreover, the supplementation of GDNF + bFGF up-regulated the expression of PLZF and BCL6B. UCHL1 expression was higher after addition of GDNF + LIF while, THY1 overexpressed in response to the addition of GDNF + CSF1. These results demonstrated that the goat SSCs were efficiently propagated using a KOSR based serum-free media and the growth factor supplementation markedly influences their gene expression profile.

Low-dose ionizing irradiation triggers apoptosis of undifferentiated spermatogonia in vivo and in vitro

Background

The present study aimed to investigate the mechanism of low-dose ionizing radiation (IR) induced apoptosis of undifferentiated spermatogonia in vivo and in vitro.

Methods

Following 50 mGy IR, testicular tissues were collected from the adult DBA/2 mice at 1, 2 and 24 h; mice in the control group received pseudo-irradiation. Immunofluorescence (IF) staining and TUNEL were performed to assess DNA damage and apoptosis, respectively, in the irradiated testicular tissues. Furthermore, the spermatogonia were also irradiated in vitro, and the expression of apoptosis-related proteins was detected by Western blotting. TUNEL and flow cytometry were applied to assess cell apoptosis.

Results

γH2AX (a marker of DNA damage) was up-regulated in the seminiferous tubules at 1 and 2 h after IR, but it was reduced following the DNA repair. This was consistent with the finding that apoptosis of germline cells was present in the seminiferous tubules after IR, especially at 1 h (IF and TUNEL). Apoptosis was also present in the PLZF(+) spermatogonia, particularly at 1 h after IR. Apoptotic cells decreased with the increase in DNA repair time after IR. Moreover, the caspase-3 protein was expressed in the undifferentiated spermatogonia following IR. The expression of caspase-3, P53, Ku70 and DNA-PKcs in the cultured spermatogonia was also up-regulated following IR in vitro, but their expression decreased gradually over time after IR, which was supported by the findings from flow cytometry, and the apoptosis of spermatogonia peaked at 24 h post IR.

Conclusions

IR may induce the apoptosis of spermatogonia at early stage in vivo, but the apoptosis of spermatogonia secondary to IR occurs at a relatively later time point (24 h) in vitro mainly. The apoptosis of spermatogonia is improved over time after IR.

Glycylglycine plays critical roles in the proliferation of spermatogonial stem cells

Glial cell line‑derived neurotrophic factor (GDNF) is critical for the proliferation of spermatogonial stem cells (SSCs), but the underlying mechanisms remain poorly understood. In this study, an unbiased metabolomic analysis was performed to examine the metabolic modifications in SSCs following GDNF deprivation, and 11 metabolites were observed to decrease while three increased. Of the 11 decreased metabolites identified, glycylglycine was observed to significantly rescue the proliferation of the impaired SSCs, while no such effect was observed by adding sorbitol. However, the expression of self‑renewal genes, including B‑cell CLL/lymphoma 6 member B, ETS variant 5, GDNF family receptor α1 and early growth response protein 4 remained unaltered following glycylglycine treatment. This finding suggests that although glycylglycine serves an important role in the proliferation of SSCs, it is not required for the self‑renewal of SSCs.

Enrichment of transplantable germ cells in salmonids using a novel monoclonal antibody by magnetic-activated cell sorting

In the fish germ cell transplantation system, only type A spermatogonia (ASGs) and oogonia are known to be incorporated into the recipient genital ridges, where they undergo gametogenesis. Therefore, high colonization efficiency can be achieved by enriching undifferentiated germ cells out of whole testicular cells. In this study, we used magnetic-activated cell sorting (MACS) for enriching undifferentiated germ cells of rainbow trout using a monoclonal antibody that recognizes a specific antigen located on the germ cell membrane. We screened the antibodies to be used for MACS by performing immunohistochemistry on rainbow trout gonads. Two antibodies, nos. 172 and 189, showed strong signals for ASGs and oogonia. Next, we performed MACS with antibody no. 172 using gonadal cells isolated from vasa-gfp rainbow trout showing GFP in undifferentiated germ cells. We found that GFP-positive cells are highly enriched in antibody no. 172-positive fractions. Finally, to examine the transplantability of MACS-enriched cells, we intraperitoneally transplanted sorted or unsorted cells into recipient larvae. We observed that transplantability of sorted cells, particularly ovarian cells, were significantly higher than that of unsorted cells. Therefore, MACS with antibody no. 172 could enrich ASGs and oogonia and become a powerful tool to improve transplantation efficiency in salmonids.

Testicular endothelial cells promote self-renewal of spermatogonial stem cells in rats†

Spermatogonial stem cells (SSCs) are the basis of spermatogenesis in male due to their capability to multiply in numbers by self-renewal and subsequent meiotic processes. However, as SSCs are present in a very small proportion in the testis, in vitro proliferation of undifferentiated SSCs will facilitate the study of germ cell biology. In this study, we investigated the effectiveness of various cell lines as a feeder layer for rat SSCs. Germ cells enriched for SSCs were cultured on feeder layers including SIM mouse embryo-derived thioguanine and ouabain-resistant cells, C166 cells, and mouse and rat testicular endothelial cells (TECs) and their stem cell potential for generating donor-derived colonies and offspring was assessed by transplantation into recipient testes. Rat germ cells cultured on TECs showed increased mRNA and protein levels of undifferentiated spermatogonial markers. Rat SSCs derived from these germ cells underwent spermatogenesis and generated offspring when transplanted into recipients. Collectively, TECs can serve as an effective feeder layer that enhances the proliferative and self-renewal capacity of cultured rat SSCs while preserving their stemness properties.

Spermatogonial stem cells

Spermatogonial stem cells (SSCs) are the most primitive spermatogonia in the testis and have an essential role to maintain highly productive spermatogenesis by self-renewal and continuous generation of daughter spermatogonia that differentiate into spermatozoa, transmitting genetic information to the next generation. Since the 1950s, many experimental methods, including histology, immunostaining, whole-mount analyses, and pulse-chase labeling, had been used in attempts to identify SSCs, but without success. In 1994, a spermatogonial transplantation method was reported that established a quantitative functional assay to identify SSCs by evaluating their ability to both self-renew and differentiate to spermatozoa. The system was originally developed using mice and subsequently extended to nonrodents, including domestic animals and humans. Availability of the functional assay for SSCs has made it possible to develop culture systems for their ex vivo expansion, which dramatically advanced germ cell biology and allowed medical and agricultural applications. In coming years, SSCs will be increasingly used to understand their regulation, as well as in germline modification, including gene correction, enhancement of male fertility, and conversion of somatic cells to biologically competent male germline cells.

Evolution of an X-Linked miRNA Family Predominantly Expressed in Mammalian Male Germ Cells

MicroRNAs (miRNAs) are important posttranscriptional regulators of gene expression. However, comprehensive expression profiles of miRNAs during mammalian spermatogenesis are lacking. Herein, we sequenced small RNAs in highly purified mouse spermatogenic cells at different stages. We found that a family of X-linked miRNAs named spermatogenesis-related miRNAs (spermiRs) is predominantly expressed in the early meiotic phases and has a conserved testis-specific high expression pattern in different mammals. We identified one spermiR homolog in opossum; this homolog might originate from THER1, a retrotransposon that is active in marsupials but extinct in current placental mammals. SpermiRs have expanded rapidly with mammalian evolution and are diverged into two clades, spermiR-L and spermiR-R, which are likely to have been generated at least in part by tandem duplication mediated by flanking retrotransposable elements. Notably, despite having undergone highly frequent lineage-specific duplication events, the sequences encoding all spermiR family members are strictly located between two protein-coding genes, Slitrk2 and Fmr1. Moreover, spermiR-Ls and spermiR-Rs have evolved different expression patterns during spermatogenesis in different mammals. Intriguingly, the seed sequences of spermiRs, which are critical for the recognition of target genes, are highly divergent within and among mammals, whereas spermiR target genes largely overlap. When miR-741, the most highly expressed spermiR, is knocked out in cultured mouse spermatogonial stem cells (SSCs), another spermiR, miR-465a-5p, is dramatically upregulated and becomes the most abundant miRNA. Notably, miR-741^−/− SSCs grow normally, and the genome-wide expression levels of mRNAs remain unchanged. All these observations indicate functional compensation between spermiR family members and strong coevolution between spermiRs and their targets.

Manipulation of spermatogonial stem cells in livestock species

We are entering an exciting epoch in livestock biotechnology during which the fundamental approaches (such as transgenesis, spermatozoa cryopreservation and artificial insemination) will be enhanced based on the modern understanding of the biology of spermatogonial stem cells (SSCs) combined with the outstanding recent advances in genomic editing technologies and in vitro cell culture systems. The general aim of this review is to outline comprehensively the promising applications of SSC manipulation that could in the nearest future find practical application in livestock breeding. Here, we will focus on 1) the basics of mammalian SSC biology; 2) the approaches for SSC isolation and purification; 3) the available in vitro systems for the stable expansion of isolated SSCs; 4) a discussion of how the manipulation of SSCs can accelerate livestock transgenesis; 5) a thorough overview of the techniques of SSC transplantation in livestock species (including the preparation of recipients for SSC transplantation, the ultrasonographic-guided SSC transplantation technique in large farm animals, and the perspectives to improve further the SSC transplantation efficiency), and finally, 6) why SSC transplantation is valuable to extend the techniques of spermatozoa cryopreservation and/or artificial insemination. For situations where no reliable data have yet been obtained for a particular livestock species, we will rely on the data obtained from studies conducted in rodents because the knowledge gained from rodent research is translatable to livestock species to a great extent. On the other hand, we will draw special attention to situations where such translation is not possible.

A novel undifferentiated spermatogonia-specific surface protein 1 (USSP1) in neonatal mice

Mammalian spermatogenesis is maintained by a rare population of spermatogonial stem cells (SSCs), which are important for male fertility. SSCs remain a subset of undifferentiated spermatogonia, which can be isolated by a combination of surface markers. Specific markers to identify and isolate undifferentiated spermatogonia are lacking. Ussp1, a transcript previously annotated as long noncoding RNA (RIKEN cDNA 4933427D06, Gene ID: 232217), virtually encodes a membrane protein, USSP1, in a highly testis-specific manner in mouse. We demonstrate its expression on the membrane of undifferentiated spermatogonia by a homemade polyclonal rabbit antibody against the protein. In vivo, USSP1⁺ clusters consist mainly of As, Apr (GFRα1⁺) and Aal (PLZF⁺) cells. USSP1⁺ cells exhibit enrichment of undifferentiated spermatogonia, as shown by increased expression of SSC self-renewal molecular markers and the potential to form SSC clones in vitro and in vivo. However, Ussp1 knockout did not affect the number of SSCs or spermatogenesis in mice. Thy1⁺ cells from Ussp1 null mice did not show any defect in the SSC colony formation capacity, indicating that USSP1 is not essential for SSC self-renewal. Our data demonstrate that Ussp1 is specifically expressed in undifferentiated murine spermatogonia, indicating the potential to sort undifferentiated spermatogonia with USSP1 antibodies. Ussp1 might be a good maker for SSC enrichment in neonatal mice.

Optimization of In Vitro Culture Conditions of Sturgeon Germ Cells for Purpose of Surrogate Production

Simple Summary

The sturgeon is among the most ancient of actinopterygian fishes. Most species of sturgeon are listed as critically endangered due to habitat alteration caused by damming of rivers, pollution and overharvesting. Germ cell transplant is a useful tool to save these endangered species. To expand germ cell populations and sustain the supply for long periods for transplant, we established basal culture conditions for sturgeon germ cells. Germ cell mitotic activity has been enhanced by eliminating gonad somatic cells, supplementing with growth factor and using an alternative to fetal bovine serum. The optimal condition identified was purified germ cells cultured in serum-free medium supplemented with leukemia inhibitory factor (LIF) and glial cell line-derived neurotrophic factor (GDNF) at 21 °C. Cultured sterlet germ cells showed development after transplant into Russian sturgeon. The study provided useful information for sturgeon germ cell culture.

Abstract

To expand germ cell populations and provide a consistent supply for transplantation, we established basal culture conditions for sturgeon germ cells and subsequently increased their mitotic activity by eliminating gonad somatic cells, supplementing with growth factor, and replacing fetal bovine serum (FBS). The initial basal culture conditions were Leibovitz’s L-15 medium (pH 8.0) supplemented with 5% FBS (p < 0.001) at 21 °C. Proliferation of germ cells was significantly enhanced and maintained for longer periods by elimination of gonad somatic cells and culture under feeder-cell free conditions, with addition of leukemia inhibitory factor and glial-cell-derived neurotrophic factor (p < 0.001). A serum-free culture medium improved germ cell proliferation compared to the L-15 with FBS (p < 0.05). Morphology remained similar to that of fresh germ cells for at least 40 d culture. Germline-specific gene expression analysis revealed no significant changes to germ cells before and after culture. Sterlet Acipenser ruthenus germ cells cultured more than 40 days showed development after transplant into Russian sturgeon Acipenser gueldenstaedtii. Polymerase chain reaction showed 33.3% of recipient gonads to contain sterlet cells after four months. This study developed optimal culture condition for sturgeon germ cells. Germ cells after 40 d culture developed in recipient gonads. This study provided useful information for culture of sturgeon germ cells.