Clonal Origin of Germ Cell Colonies after Spermatogonial Transplantation in Mice1

Establishment of mud crab (Scylla paramamosain) spermatogonial stem cell line: A potential tool for immunological research

Spermatogonial stem cells (SSCs) can differentiate into sperm and are important for studying on genetic information transmission of animals. However, the establishment of the SSC line in crustaceans is still in its infancy. This study aimed to establish a method for the isolation, culture, and identification of SSCs derived from the gonad of a marine crustacean (mud crab, Scylla paramamosain), and evaluate their differentiation ability and potential application in immunological research, in vitro. SSCs showed robust growth, proliferation, and passaging ability (up to 35 passages) in germ cell culture medium. Proteomic analysis showed that the protein expression profile of SSC was closely related to the gonadal tissue. SSCs were found to be able to express male-specific and pluripotent markers, such as CD9, PIWI, DDX4, DAZL, NANOG, SOX2, and EPHA1. Furthermore, SSCs were differentiated into osteoblasts and adipocytes under in vitro induction. Green fluorescent protein (GFP), packaged by lentivirus, was able to be overexpressed in SSCs after infection. In addition, the infection of white spot syndrome virus (WSSV) simulated the expression of inflammation-associated factors, including TRAF6, TNF-α, MyD88, Dorsal, and Relish, and apoptosis-related genes (BAX and Bcl2) in SSCs. Thus, SSCs were initially isolated and characterized from mud crabs for the first time. Our results proved that SSCs can be used in reproduction technology, germplasm conservation, and immunological studies in crustaceans.

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.

Stage-specific regulation of undifferentiated spermatogonia by AKT1S1-mediated AKT-mTORC1 signaling during mouse spermatogenesis

Undifferentiated spermatogonia are composed of a heterogeneous cell population including spermatogonial stem cells (SSCs). Molecular mechanisms underlying the regulation of various spermatogonial cohorts during their self-renewal and differentiation are largely unclear. Here we show that AKT1S1, an AKT substrate and inhibitor of mTORC1, regulates the homeostasis of undifferentiated spermatogonia. Although deletion of Akt1s1 in mouse appears not grossly affecting steady-state spermatogenesis and male mice are fertile, the subset of differentiation-primed OCT4+ spermatogonia decreased significantly, whereas self-renewing GFRα1+ and proliferating PLZF+ spermatogonia were sustained. Both neonatal prospermatogonia and the first wave spermatogenesis were greatly reduced in Akt1s1-/- mice. Further analyses suggest that OCT4+ spermatogonia in Akt1s1-/- mice possess altered PI3K/AKT-mTORC1 signaling, gene expression and carbohydrate metabolism, leading to their functionally compromised developmental potential. Collectively, these results revealed an important role of AKT1S1 in mediating the stage-specific signals that regulate the self-renewal and differentiation of spermatogonia during mouse spermatogenesis.

Restoration of fertility in nonablated recipient mice after spermatogonial stem cell transplantation

Spermatogonial stem cell (SSC) transplantation is a valuable tool for studying stem cell-niche interaction. However, the conventional approach requires the removal of endogenous SSCs, causing damage to the niche. Here we introduce WIN18,446, an ALDH1A2 inhibitor, to enhance SSC colonization in nonablated recipients. Pre-transplantation treatment with WIN18,446 induced abnormal claudin protein expression, which comprises the blood-testis barrier and impedes SSC colonization. Consequently, WIN18,446 increased colonization efficiency by 4.6-fold compared with untreated host. WIN18,446-treated testes remained small despite the cessation of WIN18,446, suggesting its irreversible effect. Offspring were born by microinsemination using donor-derived sperm. While WIN18,446 was lethal to busulfan-treated mice, cyclophosphamide- or radiation-treated animals survived after WIN18,446 treatment. Although WIN18,446 is not applicable to humans due to toxicity, similar ALDH1A2 inhibitors may be useful for SSC transplantation into nonablated testes, shedding light on the role of retinoid metabolism on SSC-niche interactions and advancing SSC research in animal models and humans.

WIN18,446 enhances spermatogonial stem cell homing and fertility after germ cell transplantation by increasing blood-testis barrier permeability

Spermatogonial stem cells (SSCs) possess a unique ability to recolonize the seminiferous tubules. Upon microinjection into the adluminal compartment of the seminiferous tubules, SSCs transmigrate through the blood-testis barrier (BTB) to the basal compartment of the tubule and reinitiate spermatogenesis. It was recently discovered that inhibiting retinoic acid signaling with WIN18,446 enhances SSC colonization by transiently suppressing spermatogonia differentiation, thereby promoting fertility restoration. In this study, we report that WIN18,446 increases SSC colonization by disrupting the BTB. WIN18,446 altered the expression patterns of tight junction proteins (TJPs) and disrupted the BTB in busulfan-treated mice. WIN18,446 upregulated the expression of FGF2, one of the self-renewal factors for SSCs. While WIN18,446 enhanced SSC colonization in busulfan-treated wild-type mice, it did not increase colonization levels in busulfan-treated Cldn11-deficient mice, which lack the BTB, indicating that the enhancement of SSC colonization in wild-type testes depended on the loss of the BTB. Serial transplantation analysis revealed impaired self-renewal caused by WIN18,446, indicating that WIN18,446-mediated inhibition of retinoic acid signaling impaired SSC self-renewal. Strikingly, WIN18,446 administration resulted in the death of 45% of busulfan-treated recipient mice. These findings suggest that TJP modulation is the primary mechanism behind enhanced SSC homing by WIN18,446 and raise concerns regarding the use of WIN18,446 for human SSC transplantation.

Allogeneic offspring produced by induction of PD-L1 in spermatogonial stem cells via self-renewal stimulation

The testis is an immune-privileged organ. It is considered that the testis somatic microenvironment is responsible for immune suppression. However, immunological properties of spermatogonial stem cells (SSCs) have remained unknown. Here, we report the birth of allogeneic offspring by enhanced expression of immunosuppressive PD-L1 in SSCs. In vitro supplementation of GDNF and FGF2 increased expression of PD-L1 in SSCs. Cultured SSCs maintained allogeneic spermatogenesis that persisted for >1 year. However, depletion or gene editing of Pd-l1 family genes in SSCs prevented allogeneic spermatogenesis, which suggested that germ cells are responsible for suppression of the allogeneic response. PD-L1 was induced by activation of the MAPK14-BCL6B pathway, which drives self-renewal by reactive oxygen species (ROS) generation. By contrast, reduced ROS or Mapk14 deficiency downregulated PD-L1. Allogeneic offspring were born after SSC transplantation into congenitally infertile and chemically castrated mice. Thus, SSCs have unique immunological properties, which make allogeneic recipients into "surrogate fathers."

Spermatogonial Transplantation

Spermatogonial transplantation is the unequivocal method to detect spermatogonial stem cells (SSCs) based strictly on the functional definition of stem cells - the cells' regenerative capacity. This method further allows for SSC quantification. A weakness of spermatogonial transplantation is its time-consuming nature; it takes 2 months to confirm the production of terminally differentiated cells in spermatogenesis, spermatozoa, in mice, which gives the assay endpoint. Using the mouse as the model system, we here describe the basic techniques of spermatogonial transplantation and provide practical guidance to successfully carry out this technique and to interpret data generated.

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.

Regulation of male germline transmission patterns by the Trp53-Cdkn1a pathway

A small number of offspring are born from the numerous sperm generated from spermatogonial stem cells (SSCs). However, little is known regarding the rules and molecular mechanisms that govern germline transmission patterns. Here we report that the Trp53 tumor suppressor gene limits germline genetic diversity via Cdkn1a. Trp53-deficient SSCs outcompeted wild-type (WT) SSCs and produced significantly more progeny after co-transplantation into infertile mice. Lentivirus-mediated transgenerational lineage analysis showed that offspring bearing the same virus integration were repeatedly born in a non-random pattern from WT SSCs. However, SSCs lacking Trp53 or Cdkn1a sired transgenic offspring in random patterns with increased genetic diversity. Apoptosis of KIT⁺ differentiating germ cells was reduced in Trp53- or Cdkn1a-deficient mice. Reduced CDKN1A expression in Trp53-deficient spermatogonia suggested that Cdkn1a limits genetic diversity by supporting apoptosis of syncytial spermatogonial clones. Therefore, the TRP53-CDKN1A pathway regulates tumorigenesis and the germline transmission pattern.

The Field of Cell Competition Comes of Age: Semantics and Technological Synergy

Stem cells experience many selective pressures which shape their cellular populations, potentially pushing them to skew towards dominance of a few break-through clones. An evolutionarily conserved answer to curb these aberrant selective pressures is cell competition, the elimination of a subset of cells by their neighbours in a seemingly homogenous population. Cell competition in mammalian systems is a relatively recent discovery that has now been observed across many tissue systems, such as embryonic, haematopoietic, intestinal, and epithelial compartments. With this rapidly growing field, there is a need to revisit and standardize the terminology used, much of which has been co-opted from evolutionary biology. Further, the implications of cell competition across biological scales in organisms have been difficult to capture. In this review, we make three key points. One, we propose new nomenclature to standardize concepts across dispersed studies of different types of competition, each of which currently use the same terminology to describe different phenomena. Second, we highlight the challenges in capturing information flow across biological scales. Third, we challenge the field to incorporate next generation technologies into the cell competition toolkit to bridge these gaps. As the field of cell competition matures, synergy between cutting edge tools will help elucidate the molecular events which shape cellular growth and death dynamics, allowing a deeper examination of this evolutionarily conserved mechanism at the core of multicellularity.

Characterization and Survival of Human Infant Testicular Cells After Direct Xenotransplantation

BACKGROUND

Cryopreservation of prepubertal testicular tissue preserves spermatogonial stem cells (SSCs) that may be used to restore fertility in men at risk of infertility due to gonadotoxic treatments for either a malignant or non-malignant disease. Spermatogonial stem cell-based transplantation is a promising fertility restoration technique. Previously, we performed xenotransplantation of propagated SSCs from prepubertal testis and found human SSCs colonies within the recipient testes six weeks post-transplantation. In order to avoid the propagation step of SSCs in vitro that may cause genetic and epigenetic changes, we performed direct injection of single cell suspension in this study, which potentially may be safer and easier to be applied in future clinical applications.

METHODS

Testis biopsies were obtained from 11 infant boys (median age 1.3 years, range 0.5-3.5) with cryptorchidism. Following enzymatic digestion, dissociated single-cell suspensions were prelabeled with green fluorescent dye and directly transplanted into seminiferous tubules of busulfan-treated mice. Six to nine weeks post-transplantation, the presence of gonocytes and SSCs was determined by whole-mount immunofluorescence for a number of germ cell markers (MAGEA, GAGE, UCHL1, SALL4, UTF1, and LIN28), somatic cell markers (SOX9, CYP17A1).

RESULTS

Following xenotransplantation human infant germ cells, consisting of gonocytes and SSCs, were shown to settle on the basal membrane of the recipient seminiferous tubules and form SSC colonies with expression of MAGEA, GAGE, UCHL1, SALL4, UTF1, and LIN28. The colonization efficiency was approximately 6%. No human Sertoli cells were detected in the recipient mouse testes.

CONCLUSION

Xenotransplantation, without in vitro propagation, of testicular cell suspensions from infant boys with cryptorchidism resulted in colonization of mouse seminiferous tubules six to nine weeks post-transplantation. Spermatogonial stem cell-based transplantation could be a therapeutic treatment for infertility of prepubertal boys with cryptorchidism and boys diagnosed with cancer. However, more studies are required to investigate whether the low number of the transplanted SSC is sufficient to secure the presence of sperm in the ejaculate of those patients over time.

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.

Transient suppression of transplanted spermatogonial stem cell differentiation restores fertility in mice

A remarkable feature of tissue stem cells is their ability to regenerate the structure and function of host tissue following transplantation. However, the dynamics of donor stem cells during regeneration remains largely unknown. Here we conducted quantitative clonal fate studies of transplanted mouse spermatogonial stem cells in host seminiferous tubules. We found that, after a large population of donor spermatogonia settle in host testes, through stochastic fate choice, only a small fraction persist and regenerate over the long term, and the rest are lost through differentiation and cell death. Further, based on these insights, we showed how repopulation efficiency can be increased to a level where the fertility of infertile hosts is restored by transiently suppressing differentiation using a chemical inhibitor of retinoic acid synthesis. These findings unlock a range of potential applications of spermatogonial transplantation, from fertility restoration in individuals with cancer to conservation of biological diversity.

Strawberry Notch 1 (SBNO1) promotes proliferation of spermatogonial stem cells via the noncanonical Wnt pathway in mice

While it is known that spermatogonial stem cells (SSCs) initiate the production of male germ cells, the mechanisms of SSC self-renewal, proliferation, and differentiation remain poorly understood. We have previously identified Strawberry Notch 1 (SBNO1), a vertebrate strawberry notch family protein, in the proteome profile for mouse SSC maturation and differentiation, revealing SBNO1 is associated with neonatal testicular development. To explore further the location and function of SBNO1 in the testes, we performed Sbno1 gene knockdown in mice to study the effects of SBNO1 on neonatal testicular and SSC development. Our results revealed that SBNO1 is required for neonatal testicular and SSC development in mice. Particularly, in vitro Sbno1 gene knockdown with morpholino oligonucleotides caused a reduction of SSCs and inactivation of the noncanonical Wnt pathway, through Jun N-terminal kinases. Our study suggests SBNO1 maintains SSCs by promoting the noncanonical Wnt pathway.

Isolation, Cryopreservation, and Transplantation of Spermatogonial Stem Cells

Spermatogonial stem cell (SSC) culture and transplantation pave the way for clinical restoration of fertility in male prepubertal cancer survivors. In this chapter we detail the steps for isolating and freezing testicular tissue along with protocols for the subsequent recovery from cryopreservation and transplantation of cells into a recipient testis. Transplantation of cultured or thawed SSCs provides not only a functional assay for identification of stem cells, a critical tool for the study of the germline stem cell niche in model organisms, but also a framework for reconstitution of spermatogenesis in humans. As proof of concept, the outlined methods have been performed successfully in the murine model and have the potential to be translated to clinical environments.

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.

Heterogeneity of Spermatogonial Stem Cells

Germ cells transfer genetic materials from one generation to the next, which ensures the continuation of the species. Spermatogenesis, the process of male germ cell production, is one of the most productive systems in adult tissues. This high productivity depends on the well-coordinated differentiation cascade in spermatogonia, occurring via their synchronized cell division and proliferation. Spermatogonial stem cells (SSCs) are responsible for maintaining the spermatogonial population via self-renewal and the continuous generation of committed progenitor cells that differentiate into spermatozoa. Like other stem cells in the body, SSCs are defined by their self-renewal and differentiation abilities. A functional transplantation assay, in which these biological properties of SSCs can be quantitatively evaluated, was developed using mice, and the cell surface characteristics and intracellular marker gene expression of murine SSCs were successfully determined. Another approach to elucidate SSC identity is a cell lineage-tracing experiment using transgenic mice, which can track the SSC behavior in the testes. Recent studies using both these experimental approaches have revealed that the SSC identity changed depending upon the developmental, homeostatic, and regenerative circumstances. In addition, single-cell transcriptomic analyses have further indicated the instability of marker gene expression in SSCs. More studies are needed to unify the results of the determination of SSC identity based on the functional properties and accumulating transcriptomic data of SSCs, to elucidate the functional interaction between SSC behavior and gene products and illustrate the conserved features of SSCs amidst their heterogeneity. Furthermore, the deterministic roles of distinct SSC niches under different physiological conditions in the SSC heterogeneity and its causal regulators must also be clarified in future studies.

Biology and manipulation technologies of male germline stem cells in mammals

BACKGROUND

Spermatogonial stem cells (SSCs) are the origin of sperm and defined by their functions of "colonization in the testis" and "spermatogenesis". In vitro manipulation techniques of SSCs contribute to a wide variety of fields including reproductive medicine and molecular breeding. This review presents the recent progress of the biology and manipulation technologies of SSCs.

METHODS

Research articles regarding SSC biology and technologies were collected and summarized.

MAIN FINDINGS

Dr. Ralph Brinster developed the spermatogonial transplantation technique that enables SSC detection by functional markers. Using this technique, cultured SSCs, termed germline stem (GS) cells, were established from the mouse. GS cells provide the opportunity to produce genome-edited animals without using zygotes. In vitro spermatogenesis allows production of haploid germ cells from GS cells without spermatogonial transplantation. The recent advancement of pluripotent stem cell culture techniques has also achieved production of functional GS-like cells in addition to male/female germ cells.

CONCLUSION

Although in vitro manipulation techniques of GS cells have been developed for the mouse, it appears to be difficult to apply these techniques to other species. Understanding and control of interspecies barriers are required to extend this technology to nonrodent mammals.

Reversible inhibition of the blood-testis barrier protein improves stem cell homing in mouse testes

Stem cell homing is a complex phenomenon that involves multiple steps; thus far, attempts to increase homing efficiency have met with limited success. Spermatogonial stem cells (SSCs) migrate to the niche after microinjection into seminiferous tubules, but the homing efficiency is very low. Here we report that reversible disruption of the blood-testis barrier (BTB) between Sertoli cells enhances the homing efficiency of SSCs. We found that SSCs on a C57BL/6 background are triggered to proliferate in vitro when MHY1485, which stimulates MTORC, were added to culture medium. However, the cultured cells did not produce offspring by direct injection into the seminiferous tubules. When acyline, a gonadotropin-releasing hormone (GnRH) analogue, was administered into infertile recipients, SSC colonization increased by ~5-fold and the recipients sired offspring. In contrast, both untreated individuals and recipients that received leuprolide, another GnRH analogue, remained infertile. Acyline not only decreased CLDN5 expression but also impaired the BTB, suggesting that increased colonization was caused by efficient SSC migration through the BTB. Enhancement of stem cell homing by tight junction protein manipulation constitutes a new approach to improve homing efficiency, and similar strategy may be applicable to other self-renewing tissues.

Functional assessment of spermatogonial stem cell purity in experimental cell populations

Historically, research in spermatogonial biology has been hindered by a lack of validated approaches to identify and isolate pure populations of the various spermatogonial subsets for in-depth analysis. In particular, although a number of markers of the undifferentiated spermatogonial population have now been characterized, standardized methodology for assessing their specificity to the spermatogonial stem cell (SSC) and transit amplifying progenitor pools has been lacking. To date, SSC content within an undefined population of spermatogonia has been inferred using either lineage tracing or spermatogonial transplantation analyses which generate qualitative and quantitative data, respectively. Therefore, these techniques are not directly comparable, and are subject to variable interpretations as to a readout that is representative of a 'pure' SSC population. We propose standardization across the field for determining the SSC purity of a population via use of a limiting dilution transplantation assay that would eliminate subjectivity and help to minimize the generation of inconsistent data on 'SSC' populations. In the limiting dilution transplantation assay, a population of LacZ-expressing spermatogonia are selected based on a putative SSC marker, and a small, defined number of cells (i.e. 10 cells) are microinjected into the testis of a germ cell-deficient recipient mouse. Using colony counts and an estimated colonization efficiency of 5%; a quantitative value can be calculated that represents SSC purity in the starting population. The utilization of this technique would not only be useful to link functional relevance to novel markers that will be identified in the future, but also for providing validation of purity for marker-selected populations of spermatogonia that are commonly considered to be SSCs by many researchers in the field of spermatogenesis and stem cell biology.

A revised Asingle model to explain stem cell dynamics in the mouse male germline

Spermatogonial stem cells (SSCs) and progenitor spermatogonia encompass the undifferentiated spermatogonial pool in mammalian testes. In rodents, this population is comprised of A_single, A_paired and chains of 4-16 A_aligned spermatogonia. Although traditional models propose that the entire A_single pool represents SSCs, and formation of an A_paired syncytium symbolizes irreversible entry to a progenitor state destined for differentiation; recent models have emerged that suggest that the A_single pool is heterogeneous, and A_paired/A_aligned can fragment to produce new SSCs. In this review, we explore evidence from the literature for these differing models representing SSC dynamics, including the traditional 'A_single' and more recently formed 'fragmentation' models. Further, based on findings using a fluorescent reporter transgene (eGfp) that reflects expression of the SSC-specific transcription factor 'inhibitor of DNA binding 4' (Id4), we propose a revised version of the traditional model in which SSCs are a subset of the A_single population; the ID4-eGFP bright cells (SSC_ultimate). From the SSC_ultimate pool, other A_single and A_paired cohorts arise that are ID4-eGFP dim. Although the SSC_ultimate possess a transcriptome profile that reflects a self-renewing state, the transcriptome of the ID4-eGFP dim population resembles that of cells in transition (SSCtransitory) to a progenitor state. Accordingly, at the next mitotic division, these SSC_transitory are likely to join the progenitor pool and have lost stem cell capacity. This model supports the concept of a linear relationship between spermatogonial chain length and propensity for differentiation, while leaving open the possibility that the SSC_transitory (some A_single and potentially some A_paired spermatogonia), may contribute to the self-renewing pool rather than transition to a progenitor state in response to perturbations of steady-state conditions.

Systematic identification and comparison of expressed profiles of lncRNAs and circRNAs with associated co-expression and ceRNA networks in mouse germline stem cells

Accumulating evidence indicates that long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs) involve in germ cell development. However, little is known about the functions and mechanisms of lncRNAs and circRNAs in self-renewal and differentiation of germline stem cells. Therefore, we explored the expression profiles of mRNAs, lncRNAs, and circRNAs in male and female mouse germline stem cells by high-throughput sequencing. We identified 18573 novel lncRNAs and 18822 circRNAs in the germline stem cells and further confirmed the existence of these lncRNAs and circRNAs by RT-PCR. The results showed that male and female germline stem cells had similar GDNF signaling mechanism. Subsequently, 8115 mRNAs, 3996 lncRNAs, and 921 circRNAs exhibited sex-biased expression that may be associated with germline stem cell acquisition of the sex-specific properties required for differentiation into gametes. Gene Ontology (GO) and KEGG pathway enrichment analyses revealed different functions for these sex-biased lncRNAs and circRNAs. We further constructed correlated expression networks including coding–noncoding co-expression and competing endogenous RNAs with bioinformatics. Co-expression analysis showed hundreds of lncRNAs were correlated with sex differences in mouse germline stem cells, including lncRNA Gm11851, lncRNA Gm12840, lncRNA 4930405O22Rik, and lncRNA Atp10d. CeRNA network inferred that lncRNA Meg3 and cirRNA Igf1r could bind competitively with miRNA-15a-5p increasing target gene Inha, Acsl3, Kif21b, and Igfbp2 expressions. These findings provide novel perspectives on lncRNAs and circRNAs and lay a foundation for future research into the regulating mechanisms of lncRNAs and circRNAs in germline stem cells.

The Luteinizing Hormone-Testosterone Pathway Regulates Mouse Spermatogonial Stem Cell Self-Renewal by Suppressing WNT5A Expression in Sertoli Cells

Spermatogenesis originates from self-renewal of spermatogonial stem cells (SSCs). Previous studies have reported conflicting roles of gonadotropic pituitary hormones in SSC self-renewal. Here, we explored the role of hormonal regulation of SSCs using Fshb and Lhcgr knockout (KO) mice. Although follicle-stimulating hormone (FSH) is thought to promote self-renewal by glial cell line-derived neurotrophic factor (GDNF), no abnormalities were found in SSCs and their microenvironment. In contrast, SSCs were enriched in Lhcgr-deficient mice. Moreover, wild-type SSCs transplanted into Lhcgr-deficient mice showed enhanced self-renewal. Microarray analysis revealed that Lhcgr-deficient testes have enhanced WNT5A expression in Sertoli cells, which showed an immature phenotype. Since WNT5A was upregulated by anti-androgen treatment, testosterone produced by luteinizing hormone (LH) is required for Sertoli cell maturation. WNT5A promoted SSC activity both in vitro and in vivo. Therefore, FSH is not responsible for GDNF regulation, while LH negatively regulates SSC self-renewal by suppressing WNT5A via testosterone.

Myc/Mycn-mediated glycolysis enhances mouse spermatogonial stem cell self-renewal

Here, Kanatsu-Shinohara et al. investigated the mechanisms underlying Myc regulation of spermatogonial stem cell (SSC) fate. Their findings suggest that Myc-mediated glycolysis is an important factor that increases the frequency of SSC self-renewal division.

Myc plays critical roles in the self-renewal division of various stem cell types. In spermatogonial stem cells (SSCs), Myc controls SSC fate decisions because Myc overexpression induces enhanced self-renewal division, while depletion of Max, a Myc-binding partner, leads to meiotic induction. However, the mechanism by which Myc acts on SSC fate is unclear. Here we demonstrate a critical link between Myc/Mycn gene activity and glycolysis in SSC self-renewal. In SSCs, Myc/Mycn are regulated by Foxo1, whose deficiency impairs SSC self-renewal. Myc/Mycn-deficient SSCs not only undergo limited self-renewal division but also display diminished glycolytic activity. While inhibition of glycolysis decreased SSC activity, chemical stimulation of glycolysis or transfection of active Akt1 or Pdpk1 (phosphoinositide-dependent protein kinase 1 ) augmented self-renewal division, and long-term SSC cultures were derived from a nonpermissive strain that showed limited self-renewal division. These results suggested that Myc-mediated glycolysis is an important factor that increases the frequency of SSC self-renewal division.

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

ID4 levels dictate the stem cell state in mouse spermatogonia

Spermatogenesis is a classic model of cycling cell lineages that depend on a balance between stem cell self-renewal for continuity and the formation of progenitors as the initial step in the production of differentiated cells. The mechanisms that guide the continuum of spermatogonial stem cell (SSC) to progenitor spermatogonial transition and precise identifiers of subtypes in the process are undefined. Here we used an Id4-eGfp reporter mouse to discover that EGFP intensity is predictive of the subsets, with the ID4-EGFP^Bright population being mostly, if not purely, SSCs, whereas the ID4-EGFP^Dim population is in transition to the progenitor state. These subsets are also distinguishable by transcriptome signatures. Moreover, using a conditional overexpression mouse model, we found that transition from the stem cell to the immediate progenitor state requires downregulation of Id4 coincident with a major change in the transcriptome. Collectively, our results demonstrate that the level of ID4 is predictive of stem cell or progenitor capacity in spermatogonia and dictates the interface of transition between the different functional states.

Transplantation as a Quantitative Assay to Study Mammalian Male Germline Stem Cells

In mammals, the activities of spermatogonial stem cells (SSCs) provide the foundation for continual spermatogenesis throughout a male's reproductive lifetime. At present, the defining characteristics of SSCs and mechanisms controlling their fate decisions are not well understood. Transplantation is a definitive functional measure of stem cell capacity for male germ cells that can be used as an assay to provide an unequivocal quantification of the SSC content in an experimental cell population. Here, we discuss the procedure for mice and provide protocols for preparing donor germ cell suspensions from testes directly or primary cultures of spermatogonia for transplantation, enriching for SSCs, preparing recipient males, microinjection into recipient testes, and considerations for experimental design.

Avian Biotechnology

Primordial germ cells (PGCs) generate new individuals through differentiation, maturation and fertilization. This means that the manipulation of PGCs is directly linked to the manipulation of individuals, making PGCs attractive target cells in the animal biotechnology field. A unique biological property of avian PGCs is that they circulate temporarily in the vasculature during early development, and this allows us to access and manipulate avian germ lines. Following the development of a technique for transplantation, PGCs have become central to avian biotechnology, in contrast to the use of embryo manipulation and subsequent transfer to foster mothers, as in mammalian biotechnology. Today, avian PGC transplantation combined with recent advanced manipulation techniques, including cell purification, cryopreservation, depletion, and long-term culture in vitro, have enabled the establishment of genetically modified poultry lines and ex-situ conservation of poultry genetic resources. This chapter introduces the principles, history, and procedures of producing avian germline chimeras by transplantation of PGCs, and the current status of avian germline modification as well as germplasm cryopreservation. Other fundamental avian reproductive technologies are described, including artificial insemination and embryo culture, and perspectives of industrial applications in agriculture and pharmacy are considered, including poultry productivity improvement, egg modification, disease resistance impairment and poultry gene "pharming" as well as gene banking.

Spermatogonial Stem Cells: Implications for Genetic Disorders and Prevention

Spermatogonial stem cells (SSCs) propagate mammalian spermatogenesis throughout male reproductive life by continuously self-renewing and differentiating, ultimately, into sperm. SSCs can be cultured for long periods and restore spermatogenesis upon transplantation back into the native microenvironment in vivo. Conventionally, SSC research has been focused mainly on male infertility and, to a lesser extent, on cell reprogramming. With the advent of genome-wide sequencing technology, however, human studies have uncovered a wide range of pathogenic alleles that arise in the male germ line. A subset of de novo point mutations was shown to originate in SSCs and cause congenital disorders in children. This review describes both monogenic diseases (eg, Apert syndrome) and complex disorders that are either known or suspected to be driven by mutations in SSCs. We propose that SSC culture is a suitable model for studying the origin and mechanisms of these diseases. Lastly, we discuss strategies for future clinical implementation of SSC-based technology, from detecting mutation burden by sperm screening to gene correction in vitro.

Extrinsic and intrinsic factors controlling spermatogonial stem cell self-renewal and differentiation

Spermatogonial stem cells (SSCs), the stem cells responsible for male fertility, are one of a small number of cells with the abilities of both self-renewal and generation of large numbers of haploid cells. Technology improvements, most importantly, transplantation assays and in vitro culture systems have greatly expanded our understanding of SSC self-renewal and differentiation. Many important molecules crucial for the balance between self-renewal and differentiation have been recently identified although the exact mechanism(s) remain largely undefined. In this review, we give a brief introduction to SSCs, and then focus on extrinsic and intrinsic factors controlling SSCs self-renewal and differentiation.

Id4 Marks Spermatogonial Stem Cells in the Mouse Testis

Mammalian spermatogenesis is a classic adult stems cell–dependent process, supported by the self-renewal and differentiation of spermatogonial stem cells (SSCs). However, the identification of SSCs and elucidation of their behaviors in undisturbed testis has long been a big challenge. Here, we generated a knock-in mouse model, Id4-2A-CreERT2-2A-tdTomato, which allowed us to mark Id4-expressing (Id4⁺) cells at different time points in situ and track their behaviors across distinct developmental stages during steady-state and regenerating spermatogenesis. We found that Id4⁺ cells continue to produce spermatogonia, spermatocytes and sperm in mouse testis, showing they are capable of self-renewal and have differentiation potential. Consistent with these findings, ablation of Id4⁺ cells in mice results in a loss of spermatogenesis. Furthermore, developmental fate mapping reveals that Id4⁺ SSCs originate from neonate Id4⁺ gonocytes. Therefore, our results indicate that Id4 marks spermatogonial stem cells in the mouse testis.

Quantitative detection of human spermatogonia for optimization of spermatogonial stem cell culture

STUDY QUESTION

Can human spermatogonia be detected in long-term primary testicular cell cultures using validated, germ cell-specific markers of spermatogonia?

SUMMARY ANSWER

Germ cell-specific markers of spermatogonia/spermatogonial stem cells (SSCs) are detected in early (1-2 weeks) but not late (> 6 weeks) primary testicular cell cultures; somatic cell markers are detected in late primary testicular cell cultures.

WHAT IS KNOWN ALREADY

The development of conditions for human SSC culture is critically dependent on the ability to define cell types unequivocally and to quantify spermatogonia/SSCs. Growth by somatic cells presents a major challenge in the establishment of SSC cultures and therefore markers that define spermatogonia/SSCs, but are not also expressed by testicular somatic cells, are essential for accurate characterization of SSC cultures.

STUDY DESIGN, SIZE, DURATION

Testicular tissue from eight organ donors with normal spermatogenesis was used for assay validation and establishing primary testicular cell cultures.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Immunofluorescence analysis of normal human testicular tissue was used to validate antibodies (UTF1, SALL4, DAZL and VIM) and then the antibodies were used to demonstrate that primary testicular cells cultured in vitro for 1-2 weeks were composed of somatic cells and rare germ cells. Primary testicular cell cultures were further characterized by comparing to testicular somatic cell cultures using quantitative reverse transcriptase PCR (UTF1, FGFR3, ZBTB16, GPR125, DAZL, GATA4 and VIM) and flow cytometry (CD9 and SSEA4).

MAIN RESULTS AND THE ROLE OF CHANCE

UTF1, FGFR3, DAZL and ZBTB16 qRT-PCR and SSEA4 flow cytometry were validated for the sensitive, quantitative and specific detection of germ cells. In contrast, GPR125 mRNA and CD9 were found to be not specific to germ cells because they were also expressed in testicular somatic cell cultures. While the germ cell-specific markers were detected in early primary testicular cell cultures (1-2 weeks), their expression steadily declined over time in vitro. After 6 weeks in culture only somatic cells were detected.

LIMITATIONS, REASONS FOR CAUTION

Different groups attempting SSC culture have utilized different sources of human testes and minor differences in the preparation and maintenance of the testicular cell cultures. Differences in outcome may be explained by genetic background of the source tissue or technical differences.

WIDER IMPLICATIONS OF THE FINDINGS

The ability to propagate human SSCs in vitro is a prerequisite for proposed autologous transplantation therapy aimed at restoring fertility to men who have been treated for childhood cancer. By applying the assays validated here it will be possible to quantitatively compare human SSC culture conditions. The eventual development of conditions for long-term propagation of human SSCs in vitro will greatly facilitate learning about the basic biology of these cells and in turn the ability to use human SSCs in therapy.

STUDY FUNDING/COMPETING INTERESTS

The experiments presented in this manuscript were funded by a Project Development Team within the ICTSI NIH/NCRR Grant Number TR000006. The authors declare no competing interests.

TRIAL REGISTRATION NUMBER

Not applicable.

Fluorescence- and magnetic-activated cell sorting strategies to isolate and enrich human spermatogonial stem cells

OBJECTIVE

To determine the molecular characteristics of human spermatogonia and optimize methods to enrich spermatogonial stem cells (SSCs).

DESIGN

Laboratory study using human tissues.

SETTING

Research institute.

PATIENT(S)

Healthy adult human testicular tissue.

INTERVENTION(S)

Human testicular tissue was fixed or digested with enzymes to produce a cell suspension. Human testis cells were fractionated by fluorescence-activated cell sorting (FACS) and magnetic-activated cell sorting (MACS).

MAIN OUTCOME MEASURE(S)

Immunostaining for selected markers, human-to-nude mouse xenotransplantation assay.

RESULT(S)

Immunohistochemistry costaining revealed the relative expression patterns of SALL4, UTF1, ZBTB16, UCHL1, and ENO2 in human undifferentiated spermatogonia as well as the extent of overlap with the differentiation marker KIT. Whole mount analyses revealed that human undifferentiated spermatogonia (UCHL1+) were typically arranged in clones of one to four cells whereas differentiated spermatogonia (KIT+) were typically arranged in clones of eight or more cells. The ratio of undifferentiated-to-differentiated spermatogonia is greater in humans than in rodents. The SSC colonizing activity was enriched in the THY1dim and ITGA6+ fractions of human testes sorted by FACS. ITGA6 was effective for sorting human SSCs by MACS; THY1 and EPCAM were not.

CONCLUSION(S)

Human spermatogonial differentiation correlates with increased clone size and onset of KIT expression, similar to rodents. The undifferentiated-to-differentiated developmental dynamics in human spermatogonia is different than rodents. THY1, ITGA6, and EPCAM can be used to enrich human SSC colonizing activity by FACS, but only ITGA6 is amenable to high throughput sorting by MACS.

Spermatogonial stem cell self-renewal and development

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

THY1 as a reliable marker for enrichment of undifferentiated spermatogonia in the goat

Spermatogonial stem cells are unique cells of testes that can restore fertility upon transplantation into recipient testes. However, use of suitable markers for enrichment of these cells have important potential application. THY1, is an established conserved marker of spermatogonial stem cells in bovine, rodents, and primates, but there is no information available in goats. After three rounds of enzymatic digestion of prepubertal goat testicular tissues, undifferentiated spermatogonia positive for THY1 were isolated by magnetic-activated cell sorting and were used for immunocytochemistry, real-time polymerase chain reaction analysis for gene expression, protein expression, and transplantation into recipient mice. Immunocytochemical analyses showed that significantly higher percentage of THY1(+) cells were positive for PLZF and VASA when compared with unselected population. This result for PLZF was further confirmed at the protein level. Real-time polymerase chain reaction analysis revealed that expression of THY1, PLZF, VASA, BCL6B, and UCHL1 as SCCs characteristic genes in THY1(+) cells was significantly higher than in the initial population. Finally, transplantation of PKH26-labeled cells revealed that THY1(+) cells had higher capacity for colony formation when compared with unselected cells. In conclusion, the results provide indications that THY1 surface marker can be reliably used for enrichment of undifferentiated spermatogonial in the goats.

Eliminating malignant contamination from therapeutic human spermatogonial stem cells

Spermatogonial stem cell (SSC) transplantation has been shown to restore fertility in several species and may have application for treating some cases of male infertility (e.g., secondary to gonadotoxic therapy for cancer). To ensure safety of this fertility preservation strategy, methods are needed to isolate and enrich SSCs from human testis cell suspensions and also remove malignant contamination. We used flow cytometry to characterize cell surface antigen expression on human testicular cells and leukemic cells (MOLT-4 and TF-1a). We demonstrated via FACS that EpCAM is expressed by human spermatogonia but not MOLT-4 cells. In contrast, HLA-ABC and CD49e marked >95% of MOLT-4 cells but were not expressed on human spermatogonia. A multiparameter sort of MOLT-4-contaminated human testicular cell suspensions was performed to isolate EpCAM+/HLA-ABC-/CD49e- (putative spermatogonia) and EpCAM-/HLA-ABC+/CD49e+ (putative MOLT-4) cell fractions. The EpCAM+/HLA-ABC-/CD49e- fraction was enriched for spermatogonial colonizing activity and did not form tumors following human-to-nude mouse xenotransplantation. The EpCAM-/HLA-ABC+/CD49e+ fraction produced tumors following xenotransplantation. This approach could be generalized with slight modification to also remove contaminating TF-1a leukemia cells. Thus, FACS provides a method to isolate and enrich human spermatogonia and remove malignant contamination by exploiting differences in cell surface antigen expression.

Development of quantitative microscopy-based assays for evaluating dynamics of living cultures of mouse spermatogonial stem/progenitor cells

Spermatogonial stem cell (SSC) self-renewal and differentiation are required for continuous production of spermatozoa and long-term fertility. Studying SSCs in vivo remains challenging because SSCs are rare cells and definitive molecular markers for their identification are lacking. The development of a method for propagating SSCs in vitro greatly facilitated analysis of SSCs. The cultured cells grow as clusters of a dynamic mixture of "true" stem cells and differentiating progenitor cells. Cells in the stem/progenitor culture system share many properties with spermatogonia in vivo; however, to fully exploit it as a model for spermatogonial development, new assays are needed that account for the dynamic heterogeneity inherent in the culture system. Here, assays were developed for quantifying dynamics of cultures of stem/progenitor cells that expressed histone-green fluorescent protein (GFP). First, we built on published results showing that cluster formation in vitro reliably predicts the relative number of SSCs. The GFP-based in vitro cluster assay allows quantification of SSCs with significantly fewer resources than a transplantation assay. Second, we compared the dynamics of differentiation in two experimental paradigms by imaging over a 17-day time frame. Finally, we performed short-term live imaging and observed cell migration, coordinated cell proliferation, and cell death resembling that of spermatogonia in the testes. The methods that we present provide a foundation for the use of fluorescent reporters in future microscopy-based high-throughput screens by using living spermatogonial stem/progenitor cultures applicable to toxicology, contraceptive discovery, and identification of regulators of self-renewal and differentiation.

The germline stem cell niche unit in mammalian testes

This review addresses current understanding of the germline stem cell niche unit in mammalian testes. Spermatogenesis is a classic model of tissue-specific stem cell function relying on self-renewal and differentiation of spermatogonial stem cells (SSCs). These fate decisions are influenced by a niche microenvironment composed of a growth factor milieu that is provided by several testis somatic support cell populations. Investigations over the last two decades have identified key determinants of the SSC niche including cytokines that regulate SSC functions and support cells providing these factors, adhesion molecules that influence SSC homing, and developmental heterogeneity of the niche during postnatal aging. Emerging evidence suggests that Sertoli cells are a key support cell population influencing the formation and function of niches by secreting soluble factors and possibly orchestrating contributions of other support cells. Investigations with mice have shown that niche influence on SSC proliferation differs during early postnatal development and adulthood. Moreover, there is mounting evidence of an age-related decline in niche function, which is likely influenced by systemic factors. Defining the attributes of stem cell niches is key to developing methods to utilize these cells for regenerative medicine. The SSC population and associated niche comprise a valuable model system for study that provides fundamental knowledge about the biology of tissue-specific stem cells and their capacity to sustain homeostasis of regenerating tissue lineages. While the stem cell is essential for maintenance of all self-renewing tissues and has received considerable attention, the role of niche cells is at least as important and may prove to be more receptive to modification in regenerative medicine.