The pluripotency factor LIN28 in monkey and human testes: a marker for spermatogonial stem cells?

RNA-binding protein IGF2BP1 is required for spermatogenesis in an age-dependent manner

Post-transcriptional regulation mediated by RNA binding proteins is crucial for male germline development. Insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1), an RNA binding protein, is specifically expressed in human and mouse male gonads and is involved in manifold biological processes and tumorigenesis. However, the function of IGF2BP1 in mammalian spermatogenesis remains poorly understood. Herein, we generated an Igf2bp1 conditional knockout mouse model using Nanos3-Cre. Germ cell deficiency of Igf2bp1 in mice caused spermatogenic defects in an age-dependent manner, resulting in decreased numbers of undifferentiated spermatogonia and increased germ cell apoptosis. Immunoprecipitation-mass spectrometry analysis revealed that ELAV-like RNA binding protein 1, a well-recognized mRNA stabilizer, interacted with IGF2BP1. Single cell RNA-sequencing showed distinct mRNA profiles in spermatogonia from conditional knockout versus wide type mice. Further research showed that IGF2BP1 plays a vital role in the modulation of spermatogenesis by regulating Lin28a mRNA, which is essential for clonal expansion of undifferentiated spermatogonia. Thus, our results highlight the crucial effects of IGF2BP1 on spermatogonia for the long-term maintenance of spermatogenesis.

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.

Peptides from the croceine croaker (Larimichthys crocea) swim bladder attenuate busulfan-induced oligoasthenospermia in mice

CONTEXT

The swim bladder of the croceine croaker is believed to have a therapeutic effect on various diseases. However, there is no research about its effect on mammalian spermatogenesis.

OBJECTIVE

We investigated the swim bladder peptides (SBPs) effect on busulfan-induced oligoasthenospermia in mice.

MATERIALS AND METHODS

We first extracted SBP from protein hydrolysate of the croceine croaker swim bladder, and then five groups of ICR male mice were randomly assigned: control, control + SBP 60 mg/kg, busulfan, busulfan + SBP 30 mg/kg and busulfan + SBP 60 mg/kg. Mice received bilateral intratesticular injections of busulfan to establish oligoasthenospermia model. After treatment with SBP for 4 weeks, testis and epididymis were collected from all mice for further analysis.

RESULTS

After treatment with SBP 30-60 mg/kg for 4 weeks, epididymal sperm concentration and motility increased by 3.9-9.6- and 1.9-2.4-fold than those of oligoasthenospermia mice induced by busulfan. Meanwhile, histology showed that spermatogenic cells decreased, leading to increased lumen diameters and vacuolization in the busulfan group. These features were reversed by SBP treatment. RNA-sequencing analysis revealed that, compared with the busulfan group, Lin28b and Igf2bp1 expression related to germ cell proliferation, increased with a >1.5-fold change after SBP treatment. Additionally, PGK2 and Cfap69 mRNAs associated with sperm motility, also increased with a >1.5-fold change. Furthermore, these findings were validated by quantitative real-time PCR and Western blotting.

DISCUSSION AND CONCLUSIONS

This is the first reported evidence for the therapeutic effect of SBP on oligoasthenospermia. SBP may be a promising drug for oligoasthenospermia in humans.

LIN28 Family in Testis: Control of Cell Renewal, Maturation, Fertility and Aging

Male reproductive development starts early in the embryogenesis with somatic and germ cell differentiation in the testis. The LIN28 family of RNA-binding proteins promoting pluripotency has two members—LIN28A and LIN28B. Their function in the testis has been investigated but many questions about their exact role based on the expression patterns remain unclear. LIN28 expression is detected in the gonocytes and the migrating, mitotically active germ cells of the fetal testis. Postnatal expression of LIN28 A and B showed differential expression, with LIN28A expressed in the undifferentiated spermatogonia and LIN28B in the elongating spermatids and Leydig cells. LIN28 interferes with many signaling pathways, leading to cell proliferation, and it is involved in important testicular physiological processes, such as cell renewal, maturation, fertility, and aging. In addition, aberrant LIN28 expression is associated with testicular cancer and testicular disorders, such as hypogonadotropic hypogonadism and Klinefelter’s syndrome. This comprehensive review encompasses current knowledge of the function of LIN28 paralogs in testis and other tissues and cells because many studies suggest LIN28AB as a promising target for developing novel therapeutic agents.

Multiple 9-1-1 complexes promote homolog synapsis, DSB repair, and ATR signaling during mammalian meiosis

DNA damage response mechanisms have meiotic roles that ensure successful gamete formation. While completion of meiotic double-strand break (DSB) repair requires the canonical RAD9A-RAD1-HUS1 (9A-1-1) complex, mammalian meiocytes also express RAD9A and HUS1 paralogs, RAD9B and HUS1B, predicted to form alternative 9-1-1 complexes. The RAD1 subunit is shared by all predicted 9-1-1 complexes and localizes to meiotic chromosomes even in the absence of HUS1 and RAD9A. Here, we report that testis-specific disruption of RAD1 in mice resulted in impaired DSB repair, germ cell depletion, and infertility. Unlike Hus1 or Rad9a disruption, Rad1 loss in meiocytes also caused severe defects in homolog synapsis, impaired phosphorylation of ATR targets such as H2AX, CHK1, and HORMAD2, and compromised meiotic sex chromosome inactivation. Together, these results establish critical roles for both canonical and alternative 9-1-1 complexes in meiotic ATR activation and successful prophase I completion.

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.

Sexual Differentiation and Primordial Germ Cell Distribution in the Early Horse Fetus

It was the aim of this study to characterize the development of the gonads and genital ducts in the equine fetus around the time of sexual differentiation. This included the identification and localization of the primordial germ cell population. Equine fetuses between 45 and 60 days of gestation were evaluated using a combination of micro-computed tomography scanning, immunohistochemistry, and multiplex immunofluorescence. Fetal gonads increased in size 23-fold from 45 to 60 days of gestation, and an even greater increase was observed in the metanephros volume. Signs of mesonephros atrophy were detected during this time. Tubular structures of the fetal testes were present from day 50 onwards, whereas cell clusters dominated in the fetal ovary. The genital ducts were well-differentiated and presented a lumen in all samples. No sign of mesonephric or paramesonephric duct degeneration was detected. Expression of AMH was strong in the fetal testes but absent in ovaries. Irrespective of sex, primordial germ cells selectively expressed LIN28. Migration of primordial germ cells from the mesonephros to the gonad was detected at 45 days, but not at 60 days of development. Their number and distribution within the gonad were influenced (p < 0.05) by fetal sex. Most primordial germ cells (86.8 ± 3.2% in females and 84.6 ± 4.7% in males) were characterized as pluripotent according to co-localization with CD117. However, only a very small percentage of primordial germ cells were proliferating (7.5 ± 1.7% in females and 3.2 ± 1.2% in males) based on co-localization with Ki67. It can be concluded that gonadal sexual differentiation in the horse occurs asynchronously with regard to sex but already before 45 days of gestation.

RNA-binding protein ELAVL2 plays post-transcriptional roles in the regulation of spermatogonia proliferation and apoptosis

Objectives

RNA‐binding proteins (RBPs) play essential post‐transcriptional roles in regulating spermatogonial stem cells (SSCs) maintenance and differentiation. We identified a conserved and SSCs‐enriched RBP ELAVL2 from our single‐cell sequencing data, but its function and mechanism in SSCs were unclear.

Materials and methods

Expressions of ELAVL2 during human and mouse testis development were validated. Stable C18‐4 and TCam‐2 cell lines with overexpression and knockdown of ELAVL2 were established, which were applied to proliferation and apoptosis analysis. RNA immunoprecipitation and sequencing were used to identify ELAVL2 targets, and regulatory functions of ELAVL2 on target mRNAs were studied. Proteins interacting with ELAVL2 in human and mouse testes were identified using immunoprecipitation and mass spectrometric, which were validated by in vivo and in vitro experiments.

Results

ELAVL2 was testis‐enriched and preferentially expressed in human and mouse SSCs. ELAVL2 was down‐regulated in NOA patients. ELAVL2 promoted proliferation and inhibited apoptosis of C18‐4 and TCam‐2 cell lines via activating ERK and AKT pathways. ELAVL2 associated with mRNAs encoding essential regulators of SSCs proliferation and survival, and promoted their protein expression at post‐transcriptional level. ELAVL2 interacted with DAZL in vivo and in vitro in both human and mouse testes.

Conclusions

Taken together, these results indicate that ELAVL2 is a conserved SSCs‐enriched RBP that down‐regulated in NOA, which regulates spermatogonia proliferation and apoptosis by promoting protein expression of targets.

Postnatal germ cell development in cryptorchid boys

Cryptorchidism is associated with infertility in adulthood. Early orchiopexy is suggested to reduce the risk. Information is lacking on the potential link between infant germ cell maturation and the risk of future infertility. The objective of the study was to evaluate age-related germ cell development in cryptorchidism. Immunostaining for markers of germ cell development (octamer-binding transcription factor 3/4 [OCT3/4], placental alkaline phosphatase [PLAP], KIT proto-oncogene [C-KIT], podoplanin [D2-40], Lin-28 homolog A [LIN28], and G antigen 7 [GAGE-7]) was performed in testicular biopsies from 40 cryptorchid boys aged 4-35 months. Germ cell numbers and distributions were evaluated in cross sections of seminiferous tubules, with and without immunostaining. OCT3/4, D2-40, and LIN28 were generally expressed in the early stages of germ cell development, as shown by positive expression in germ cells in the central region of seminiferous tubules. In contrast, PLAP and GAGE-7 were expressed in both central and peripheral parts of the tubules in the early stages of development and expressed mainly in a peripheral position with advancing age. Germ cell maturation was delayed in this study population as compared with that observed in our previous study on germ cell markers in a healthy population. The number of GAGE-7-positive germ cells per tubular cross section obtained by immunostaining was significantly higher than that obtained by standard hematoxylin and eosin staining. Double immunostaining revealed heterogeneity in germ cell development in cryptorchid testes. These results shed light on the pathophysiology of germ cell development in boys with cryptorchidism.

The Progresses of Spermatogonial Stem Cells Sorting Using Fluorescence-Activated Cell Sorting

In recent years, the research on stem cells has been more and more in-depth, and many achievements have been made in application. However, due to the small number of spermatogonial stem cells (SSCs) and deficiency of efficient markers, it is difficult to obtain very pure SSCs, which results in the research on them being hindered. In fact, many methods have been developed to isolate and purify SSCs, but these methods have their shortcomings. Fluorescence-activated cell sorting (FACS), as a method to enrich SSCs with the help of specific surface markers, has the characteristics of high efficiency and accuracy in enrichment of SSCs, thus it is widely accepted as an effective method for purification of SSCs. This review summarizes the recent studies on the application of FACS in SSCs, and introduces some commonly used markers of effective SSCs sorting, aiming to further optimize the FACS sorting method for SSCs, so as to promote the research of germline stem cells and provide new ideas for the research of reproductive biology.

LIN28A binds to meiotic gene transcripts and modulates their translation in male germ cells

The RNA-binding protein LIN28A is required for maintaining tissue homeostasis, including in the reproductive system, but the underlying mechanisms on how LIN28A regulates germline progenitors remain unclear. Here, we dissected LIN28A-binding targets using high-throughput sequencing of RNAs isolated by crosslinking immunoprecipitation (HITS-CLIP) in the mouse testes. LIN28A preferentially binds to mRNA coding sequence (CDS) or 3'UTR regions at sites enriched with GGAG(A) sequences. Further investigation of Lin28a-null mouse testes indicated that meiosis-associated mRNAs bound by LIN28A were differentially expressed. Next, ribosome profiling revealed that the mRNA levels of these targets were significantly reduced in the polysome fractions, and their protein expression levels decreased, in Lin28a-null mouse testes, even when meiotic arrest in the null mouse testes was not apparent. Collectively, these findings provide a set of LIN28A-regulated target mRNAs, and show that LIN28A binding might be a mechanism through which LIN28A acts to regulate undifferentiated spermatogonia fates and male fertility in mammals.

Lin28 gene and mammalian puberty

Lin28a and Lin28b, homologs of the Caenorhabditis elegans Lin28 gene, play important roles in cell pluripotency, reprogramming, and tumorigenicity. Recently, genome-wide association and transgenic studies showed that Lin28a and/or Lin28b gene were involved in the onset of mammalian puberty, the stage representing the attainment of reproduction capacity; however, the detailed mechanism of these genes in mammalian puberty remains largely unknown. The present paper reviews the research progress on the roles of Lin28a/b genes in the onset of mammalian puberty by analyzing the results coming from gene expression patterns, mutations, and transgenic studies, and put forward possible pathways for further studies on their roles in animal reproduction.

Spermatogonial Stem Cells for In Vitro Spermatogenesis and In Vivo Restoration of Fertility

Spermatogonial stem cells (SSCs) are the only adult stem cells capable of passing genes onto the next generation. SSCs also have the potential to provide important knowledge about stem cells in general and to offer critical in vitro and in vivo applications in assisted reproductive technologies. After century-long research, proof-of-principle culture systems have been introduced to support the in vitro differentiation of SSCs from rodent models into haploid male germ cells. Despite recent progress in organotypic testicular tissue culture and two-dimensional or three-dimensional cell culture systems, to achieve complete in vitro spermatogenesis (IVS) using non-rodent species remains challenging. Successful in vitro production of human haploid male germ cells will foster hopes of preserving the fertility potential of prepubertal cancer patients who frequently face infertility due to the gonadotoxic side-effects of cancer treatment. Moreover, the development of optimal systems for IVS would allow designing experiments that are otherwise difficult or impossible to be performed directly in vivo, such as genetic manipulation of germ cells or correction of genetic disorders. This review outlines the recent progress in the use of SSCs for IVS and potential in vivo applications for the restoration of fertility.

Irisin is expressed by undifferentiated spermatogonia and modulates gene expression in organotypic primate testis cultures

The molecular mechanisms regulating undifferentiated spermatogonial cell proliferation and differentiation are still not fully understood. Irisin is an exercise-induced hormone, which is a cleaved and secreted fragment of the fibronectin type III repeat containing 5 (FNDC5) transmembrane protein. Recent studies have demonstrated the role of irisin in cell proliferation and differentiation in various tissues. However, testicular irisin expression and its potential action have not been analyzed. Here, we demonstrate expression of irisin in undifferentiated spermatogonia of primates and in the tree shrew, a bridging species between primates and insectivores. Rhesus monkeys are seasonal breeders with annual phases of high and low testicular activity and germ cell proliferation. Interestingly, expression of both FNDC5 mRNA and irisin is altered between breeding (high spermatogenesis) and nonbreeding seasons (low spermatogenesis). Organotypic testis culture in the presence of irisin increased the expression levels of the Sertoli cell (GDNF) and spermatogonial transcripts Kruppel-like factor 4 (KLF4), Inhibitor of differentiation 4 (ID4), Cluster of differentiation 117 (cKIT), and SALL4, compared to untreated controls, while irisin suppressed its own FNDC5 mRNA. Our data suggest that irisin is a novel endocrine factor involved in the regulation of spermatogonial activities in the testes of primates.

Propagation of Spermatogonial Stem Cell-Like Cells From Infant Boys

Background

Gonadotoxic treatment of malignant diseases as well as some non-malignant conditions such as cryptorchidism in young boys may result in infertility and failure to father children later in life. As a fertility preserving strategy, several centers collect testicular biopsies to cryopreserve spermatogonial stem cells (SSCs) world-wide. One of the most promising therapeutic strategies is to transplant SSCs back into the seminiferous tubules to initiate endogenous spermatogenesis. However, to obtain sufficient numbers of SSC to warrant transplantation, in vitro propagation of cells is needed together with proper validation of their stem cell identity.

Materials and Methods

A minute amount of testicular biopsies (between 5 mg and 10 mg) were processed by mechanical and enzymatic digestion. SSCs were enriched by differential plating method in StemPro-34 medium supplemented with several growth factors. SSC-like cell clusters (SSCLCs) were passaged five times. SSCLCs were identified by immunohistochemical and immunofluorescence staining, using protein expression patterns in testis biopsies as reference. Quantitative polymerase chain reaction analysis of SSC markers LIN-28 homolog A (LIN28A), G antigen 1 (GAGE1), promyelocytic leukemia zinc finger protein (PLZF), integrin alpha 6 (ITGA6), ubiquitin carboxy-terminal hydrolase L1 (UCHL1) and integrin beta 1 (ITGB1) were also used to validate the SSC-like cell identity.

Results

Proliferation of SSCLCs was achieved. The presence of SSCs in SSCLCs was confirmed by positive immunostaining of LIN28, UCHL1 and quantitative polymerase chain reaction for LIN28A, UCHL1, PLZF, ITGA6, and ITGB1, respectively.

Conclusion

This study has demonstrated that SSCs from infant boys possess the capacity for in vitro proliferation and advance a fertility preservation strategy for pre-pubertal boys who may otherwise lose their fertility.

Regulation of long non-coding RNAs and circular RNAs in spermatogonial stem cells

Spermatogonial stem cells (SSCs) are one of the most significant stem cells with the potentials of self-renewal, differentiation, transdifferentiation and dedifferentiation, and thus, they have important applications in reproductive and regenerative medicine. They can transmit the genetic and epigenetic information across generations, which highlights the importance of the correct establishment and maintenance of epigenetic marks. Accurate transcriptional and post-transcriptional regulation is required to support the highly coordinated expression of specific genes for each step of spermatogenesis. Increasing evidence indicates that non-coding RNAs (ncRNAs), including long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), play essential roles in controlling gene expression and fate determination of male germ cells. These ncRNA molecules have distinct characteristics and biological functions, and they independently or cooperatively modulate the proliferation, apoptosis and differentiation of SSCs. In this review, we summarized the features, biological function and fate of mouse and human SSCs, and we compared the characteristics of lncRNAs and circRNAs. We also addressed the roles and mechanisms of lncRNAs and circRNAs in regulating mouse and human SSCs, which would add novel insights into the epigenetic mechanisms underlying mammalian spermatogenesis and provide new approaches to treat male infertility.

Both Gfrα1a and Gfrα1b Are Involved in the Self-renewal and Maintenance of Spermatogonial Stem Cells in Medaka

Glial cell-derived neurotrophic factor family receptor alpha-1 (GFRα1) plays a crucial role in the self-renewal and maintenance of spermatogonial stem cells (SSCs) from mammals. However, to date, our knowledge about its role in fish SSCs is limited. In the present study, the medaka (Oryzias latipes) gfrα1 duplicate genes, Olgfrα1a and Olgfrα1b, were cloned and characterized. Furthermore, their expression profile and biological activity were investigated. OlGfrα1a and OlGfrα1b predict 524 and 466 amino acid residues, respectively. Both are orthologous to mammalian Gfrα1 by sequence analyses and appear high in spermatogonia by in situ hybridization assay. The knockdown of OlGfrα1a and/or OlGfrα1b via Vivo-Morpholino oligos significantly inhibited the self-renewal and maintenance of SSCs, as evidenced by the decreased proliferation activity of SG3 cells (a spermatogonial stem cell line derived from adult medaka testis) as well as spermatogonia in the testicular organ culture and by the decreased survival rate and expression levels of pluripotency-related genes (klf4, lin28b, bcl6b, and etv5) in SG3 cells. Additionally, our study indicates that OlGfrα1a might function by binding either Gdnfa or Gdnfb (the two medaka Gdnf homologs), whereas OlGfrα1b function by binding Gdnfa not Gdnfb. Taken together, our study indicates that both OlGfrα1a and OlGfrα1b are involved in the self-renewal and maintenance of SSCs by binding Gdnfa and/or Gdnfb, respectively. These findings suggest that the GDNF/GFRα1 signaling pathway might be conserved from mammals to fish species.

The C-X-C signalling system in the rodent vs primate testis: impact on germ cell niche interaction

In zebrafish, action of the chemokine Cxcl12 is mediated through its G-protein-coupled seven-transmembrane domain receptor Cxcr4 and the atypical receptor Cxcr7. Employing this animal model, it was revealed that this Cxcl12 signalling system plays a crucial role for directed migration of primordial germ cells (PGC) during early testicular development. Importantly, subsequent studies indicated that this regulatory mechanism is evolutionarily conserved also in mice. What is more, the functional role of the CXCL12 system does not seem to be limited to early phases of testicular development. Data from mouse studies rather demonstrate that CXCL12 and its receptors are also involved in the homing process of gonocytes into their niches at the basal membrane of the seminiferous tubules. Intriguingly, even the spermatogonial stem cells (SSCs) present in the adult mouse testis appear to maintain the ability to migrate towards a CXCL12 gradient as demonstrated by functional in vitro migration assays and in vivo germ cell transplantation assays. These findings not only indicate a role of the CXCL12 system throughout male germ cell development in mice but also suggest that this system may be evolutionarily conserved. In this review, we take into account the available literature focusing on the localization patterns of the CXCL12 system not only in rodents but also in primates, including the human. Based on these data, we discuss whether the CXCL12 system is also conserved between rodents and primates and discuss the known and potential functional consequences.

Spermatogonial stem cells and spermatogenesis in mice, monkeys and men

Continuous spermatogenesis in post-pubertal mammals is dependent on spermatogonial stem cells (SSCs), which balance self-renewing divisions that maintain stem cell pool with differentiating divisions that sustain continuous sperm production. Rodent stem and progenitor spermatogonia are described by their clonal arrangement in the seminiferous epithelium (e.g., A_single, A_paired or A_aligned spermatogonia), molecular markers (e.g., ID4, GFRA1, PLZF, SALL4 and others) and most importantly by their biological potential to produce and maintain spermatogenesis when transplanted into recipient testes. In contrast, stem cells in the testes of higher primates (nonhuman and human) are defined by description of their nuclear morphology and staining with hematoxylin as A_dark and A_pale spermatogonia. There is limited information about how dark and pale descriptions of nuclear morphology in higher primates correspond with clone size, molecular markers or transplant potential. Do the apparent differences in stem cells and spermatogenic lineage development between rodents and primates represent true biological differences or simply differences in the volume of research and the vocabulary that has developed over the past half century? This review will provide an overview of stem, progenitor and differentiating spermatogonia that support spermatogenesis; identifying parallels between rodents and primates where they exist as well as features unique to higher primates.

The ubiquitin ligase c-CBL is expressed in undifferentiated marmoset monkey pluripotent stem cells but is not a general stem cell marker

The protein c-CBL is a ubiquitin ligase. It catalyzes the last step of the transfer of ubiquitin to target proteins. Upon completion of polyubiquitination, the target proteins are degraded. Clinically, it is important that c-CBL is mutated in a subset of patients who develop myeloid malignancies, which are diseases of the hematopoietic stem or progenitor cells. c-CBL has also been shown to be expressed by human spermatogonia. The whole spermatogonial cell population possesses a subset that comprises also the spermatogonial stem cells. Based on these findings we hypothesized that c-CBL might be a general stem cell marker. To test this, we first validated the antibody using marmoset bone marrow and adult testis. In both tissues, the expected staining pattern was observed. Western blot analysis revealed only one band of the expected size. Then, we examined the expression of c-CBL in marmoset monkey embryonic stem (ES) cells, induced pluripotent stem (iPS) cells and adult stem cells. We found that c-CBL is strongly expressed in undifferentiated marmoset iPS cells and ES cells. However, adult stem cells in the gut and the stomach did not express c-CBL, indicating that c-CBL is not a general stem cell marker. In summary, c-CBL is strongly expressed in pluripotent stem cells of the marmoset monkey as well as in selected adult stem cell types. Future studies will define the function of c-CBL in pluripotent stem cells.

Human spermatogonial markers

In this review, we provide an up-to-date compilation of published human spermatogonial markers, with focus on the three nuclear subtypes A_dark, A_pale and B. In addition, we have extended our recently published list of putative spermatogonial markers with protein expression and RNA-sequencing data from the Human Protein Atlas and supported these by literature evidence. Most importantly, we have put substantial effort in acquiring a comprehensive list of new and potentially interesting markers by refiltering the raw data of 15 published germ cell expression datasets (four human, eleven rodent) and subsequent building of intersections to acquire a robust, cross-species set of spermatogonia-enriched or -specific transcripts.

Male germline stem cells in non-human primates

Over the past few decades, several studies have attempted to decipher the biology of mammalian germline stem cells (GSCs). These studies provide evidence that regulatory mechanisms for germ cell specification and migration are evolutionarily conserved across species. The characteristics and functions of primate GSCs are highly distinct from rodent species; therefore the findings from rodent models cannot be extrapolated to primates. Due to limited availability of human embryonic and testicular samples for research purposes, two non-human primate models (marmoset and macaque monkeys) are extensively employed to understand human germline development and differentiation. This review provides a broader introduction to the in vivo and in vitro germline stem cell terminology from primordial to differentiating germ cells. Primordial germ cells (PGCs) are the most immature germ cells colonizing the gonad prior to sex differentiation into testes or ovaries. PGC specification and migratory patterns among different primate species are compared in the review. It also reports the distinctions and similarities in expression patterns of pluripotency markers (OCT4A, NANOG, SALL4 and LIN28) during embryonic developmental stages, among marmosets, macaques and humans. This review presents a comparative summary with immunohistochemical and molecular evidence of germ cell marker expression patterns during postnatal developmental stages, among humans and non-human primates. Furthermore, it reports findings from the recent literature investigating the plasticity behavior of germ cells and stem cells in other organs of humans and monkeys. The use of non-human primate models would enable bridging the knowledge gap in primate GSC research and understanding the mechanisms involved in germline development. Reported similarities in regulatory mechanisms and germ cell expression profile in primates demonstrate the preclinical significance of monkey models for development of human fertility preservation strategies.

Undifferentiated embryonic cell transcription factor 1 (UTF1) and deleted in azoospermia-like (DAZL) expression in the testes of donkeys

Putative markers for each specific germ cell stage can be a useful tool to study the fate and functions of these cells. Undifferentiated embryonic cell transcription factor 1 (UTF1) is a putative marker for undifferentiated spermatogonia in humans, rats and horses. The deleted in azoospermia-like (DAZL) protein is also expressed by differentiated spermatogonia and primary spermatocytes in several species. However, whether the expression patterns of these molecular markers are identical and applicable to donkeys remains to be elucidated. The objective of this study was to investigate the expression patterns of UTF1 and DAZL in donkey testicular tissue, using immunohistochemistry (IHC). Testicular samples were collected from routine field castration of donkeys in Korea. The reproductive stages (pre- or post-puberty) of the testes were determined from the morphological characteristics of cross-sections of the seminiferous tubules. For IHC, the UTF1 and DAZL primary antibodies were diluted at 1:100 and 1:200, respectively. The immunolabelling revealed that UTF1 was expressed in approximately 50% of spermatogonia in the pre-pubertal stage, whereas its expression was limited to an early subset of spermatogonia in the post-pubertal stage. DAZL was expressed in some, but not all, spermatogonia in the pre-pubertal spermatogonia, and interestingly, its expression was also observed in spermatogonia and primary spermatocytes in the post-pubertal stage. Co-immunolabelling of the germ cells with both UTF1 and DAZL revealed three types of protein expression patterns at both reproductive stages, namely UTF1 only, DAZL only and both UTF1 and DAZL. These protein molecules were not expressed in Sertoli and Leydig cells. In conclusion, a co-immunolabelling system with UTF1 and DAZL antibodies may be used to identify undifferentiated (UTF1 only), differentiating (UTF1 and DAZL), and differentiated spermatogonia (DAZL only) in donkey testes.

Lin28a promotes self-renewal and proliferation of dairy goat spermatogonial stem cells (SSCs) through regulation of mTOR and PI3K/AKT

Lin28a is a conserved RNA-binding protein that plays an important role in development, pluripotency, stemness maintenance, proliferation and self-renewal. Early studies showed that Lin28a serves as a marker of spermatogonial stem cells (SSCs) and promotes the proliferation capacity of mouse SSCs. However, there is little information about Lin28a in livestock SSCs. In this study, we cloned Capra hircus Lin28a CDS and found that it is evolutionarily conserved. Lin28a is widely expressed in different tissues of Capra hircus, but is expressed at a high level in the testis. Lin28a is specifically located in the cytoplasm of Capra hircus spermatogonial stem cells and may also be a marker of dairy goat spermatogonial stem cells. Lin28a promoted proliferation and maintained the self-renewal of GmGSCs-I-SB in vivo and in vitro. Lin28a-overexpressing GmGSCs-I-SB showed an enhanced proliferation rate, which might be due to increased PCNA expression. Moreover, Lin28a maintained the self-renewal of GmGSCs-I-SB by up-regulating the expression of OCT4, SOX2, GFRA1, PLZF and ETV5. Furthermore, we found that Lin28a may activate the AKT, ERK, and mTOR signaling pathways to promote the proliferation and maintain the self-renewal of GmGSCs-I-SB.

The Lin28 Expression in Stallion Testes

The molecular markers for specific germ cell stages can be utilized for identifying, monitoring, and separating a particular stage of germ cells. The RNA-binding protein Lin28 is expressed in gonocytes of human fetal testes. The Lin28 expression is restricted to a very small population of spermatogonial cells in human, mice, and monkey. The main objective of this study was to investigate the expression pattern of Lin28 in stallion testes at different reproductive stages. Based on the presence or absence of full spermatogenesis and lumina in seminiferous tubules, the testicular samples were categorized into two reproductive stages pre-pubertal and post-pubertal. We performed a reverse transcription polymerase chain reaction to confirm the presence of Lin28 mRNA in the testicular tissues and a western blot analysis to verify the cross-reactivity of rabbit Lin28 antibody with horse testicular tissue. For immunohistochemistry, Lin28 (rabbit anti-human), GATA4 (goat anti-human) or DAZL (goat anti-human) antibodies were used. The results of RT-PCR confirmed the expression of Lin28 mRNA in the stallion testes. The western blot analysis showed that the expression of 28 kDa Lin28 protein was localized in the cytoplasm of spermatogonia at both reproductive stages. The numbers of Lin28-positive germ cells per 1000 Sertoli cells in pre- and post-pubertal stages were 253 ± 8.66 and 29.67 ± 2.18, respectively. At both reproductive stages, all Lin28 positive cells showed no co-stained with GATA4 antibody, whereas only some of the Lin28-positive germ cells showed co-staining with DAZL antibody. The results from whole-mount staining showed that the Lin28 expression was limited to Asingle (As) and Apaired (Apr) spermatogonia. In conclusion, Lin28 might be utilized as a molecular marker for undifferentiated spermatogonial stem cells when used with DAZL antibody.

De novo methylation in male germ cells of the common marmoset monkey occurs during postnatal development and is maintained in vitro

The timing of de novo DNA methylation in male germ cells during human testicular development is yet unsolved. Apart from that, the stability of established imprinting patterns in vitro is controversially discussed. This study aimed at determining the timing of DNA de novo methylation and at assessing the stability of the methylation status in vitro. We employed the marmoset monkey (Callithrix jacchus) as it is considered the best non-human primate model for human testicular development. We selected neonatal, pre-pubertal, pubertal, and adult animals (n = 3, each) and assessed germ cell global DNA methylation levels by 5-methyl cytosine staining, and Alu elements and gene-specific methylation (H19, LIT1, SNRPN, MEST, OCT4, MAGE-A4, and DDX-4) by pyrosequencing. De novo methylation is progressively established during postnatal primate development and continues until adulthood, a process that is different in most other species. Importantly, once established, methylation patterns remained stable, as demonstrated using in vitro cultures. Thus, the marmoset monkey is a unique model for the study of postnatal DNA methylation mechanisms in germ cells and for the identification of epimutations and their causes.

Spermatogonial cells: mouse, monkey and man comparison

In all mammals, spermatogonia are defined as constituting the mitotic compartment of spermatogenesis including stem, undifferentiated and differentiating cell types, possessing distinct morphological and molecular characteristics. Even though the real nature of the spermatogonial stem cell and its regulation is still debated the general consensus holds that in steady-state spermatogenesis the stem cell compartment needs to balance differentiation versus self-renewal. This review highlights current understanding of spermatogonial biology, the kinetics of amplification and the signals directing spermatogonial differentiation in mammals. The focus will be on relevant similarities and differences between rodents and non human and human primates.

Long-Term Oocyte-Like Cell Development in Cultures Derived from Neonatal Marmoset Monkey Ovary

We use the common marmoset monkey (Callithrix jacchus) as a preclinical nonhuman primate model to study reproductive and stem cell biology. The neonatal marmoset monkey ovary contains numerous primitive premeiotic germ cells (oogonia) expressing pluripotent stem cell markers including OCT4A (POU5F1). This is a peculiarity compared to neonatal human and rodent ovaries. Here, we aimed at culturing marmoset oogonia from neonatal ovaries. We established a culture system being stable for more than 20 passages and 5 months. Importantly, comparative transcriptome analysis of the cultured cells with neonatal ovary, embryonic stem cells, and fibroblasts revealed a lack of germ cell and pluripotency genes indicating the complete loss of oogonia upon initiation of the culture. From passage 4 onwards, however, the cultured cells produced large spherical, free-floating cells resembling oocyte-like cells (OLCs). OLCs strongly expressed several germ cell genes and may derive from the ovarian surface epithelium. In summary, our novel primate ovarian cell culture initially lacked detectable germ cells but then produced OLCs over a long period of time. This culture system may allow a deeper analysis of early phases of female primate germ cell development and-after significant refinement-possibly also the production of monkey oocytes.

Testicular expression of the Lin28/let-7 system: Hormonal regulation and changes during postnatal maturation and after manipulations of puberty

The Lin28/let-7 system, which includes the RNA-binding proteins, Lin28a/Lin28b, and let-7 miRNAs, has emerged as putative regulator of puberty and male gametogenesis; yet, its expression pattern and regulation in postnatal testis remain ill defined. We report herein expression profiles of Lin28 and let-7 members, and related mir-145 and mir-132, in rat testis during postnatal maturation and in models of altered puberty and hormonal deregulation. Neonatal expression of Lin28a and Lin28b was low and rose markedly during the infantile period; yet, expression patterns diverged thereafter, with persistently elevated levels only for Lin28b, which peaked at puberty. Let-7a, let-7b, mir-132 and mir-145 showed profiles opposite to Lin28b. In fact, let-7b and mir-145 were abundant in pachytene spermatocytes, but absent in elongating spermatids, where high expression of Lin28b was previously reported. Perturbation of puberty by neonatal estrogenization reverted the Lin28/let-7 expression ratio; expression changes were also detected in other models of delayed puberty, due to early photoperiod or nutritional manipulations. In addition, hypophysectomy or growth hormone (GH) deficiency revealed regulation of this system by gonadotropins and GH. Our data document the expression profiles of the Lin28/let-7 system in rat testis along postnatal/pubertal maturation, and their perturbation in models of pubertal and hormonal manipulation.

Mammalian gonocyte and spermatogonia differentiation: recent advances and remaining challenges

The production of spermatozoa relies on a pool of spermatogonial stem cells (SSCs), formed in infancy from the differentiation of their precursor cells, the gonocytes. Throughout adult life, SSCs will either self-renew or differentiate, in order to maintain a stem cell reserve while providing cells to the spermatogenic cycle. By contrast, gonocytes represent a transient and finite phase of development leading to the formation of SSCs or spermatogonia of the first spermatogenic wave. Gonocyte development involves phases of quiescence, cell proliferation, migration, and differentiation. Spermatogonia, on the other hand, remain located at the basement membrane of the seminiferous tubules throughout their successive phases of proliferation and differentiation. Apoptosis is an integral part of both developmental phases, allowing for the removal of defective cells and the maintenance of proper germ–Sertoli cell ratios. While gonocytes and spermatogonia mitosis are regulated by distinct factors, they both undergo differentiation in response to retinoic acid. In contrast to postpubertal spermatogenesis, the early steps of germ cell development have only recently attracted attention, unveiling genes and pathways regulating SSC self-renewal and proliferation. Yet, less is known on the mechanisms regulating differentiation. The processes leading from gonocytes to spermatogonia have been seldom investigated. While the formation of abnormal gonocytes or SSCs could lead to infertility, defective gonocyte differentiation might be at the origin of testicular germ cell tumors. Thus, it is important to better understand the molecular mechanisms regulating these processes. This review summarizes and compares the present knowledge on the mechanisms regulating mammalian gonocyte and spermatogonial differentiation.

LIN28A marks the spermatogonial progenitor population and regulates its cyclic expansion

One of the hallmarks of highly proliferative adult tissues is the presence of a stem cell population that produces progenitor cells bound for differentiation. Progenitor cells undergo multiple transit amplifying (TA) divisions before initiating terminal differentiation. In the adult male germline, daughter cells arising from the spermatogonial stem cells undergo multiple rounds of TA divisions to produce undifferentiated clones of interconnected 2, 4, 8, and 16 cells, collectively termed A(undifferentiated) (A(undiff)) spermatogonia, before entering a stereotypic differentiation cascade. Although the number of TA divisions markedly affects the tissue output both at steady state and during regeneration, mechanisms regulating the expansion of the TA cell population are poorly understood in mammals. Here, we show that mice with a conditional deletion of Lin28a in the adult male germline, display impaired clonal expansion of the progenitor TA A(undiff) spermatogonia. The in vivo proliferative activity of Au(ndiff) spermatogonial cells as indicated by BrdU incorporation during S-phase was reduced in the absence of LIN28A. Thus, contrary to the role of LIN28A as a key determinant of cell fate signals in multiple stem cell lineages, in the adult male germline it functions as an intrinsic regulator of proliferation in the population of A(undiff) TA spermatogonia. In addition, neither precocious differentiation nor diminished capacity for self-renewal potential as assessed by transplantation was observed, suggesting that neither LIN28A itself nor the pool of Aal progenitor cells substantially contribute to the functional stem cell compartment.

Gene expression ontogeny of spermatogenesis in the marmoset uncovers primate characteristics during testicular development

Mammalian spermatogenesis has been investigated extensively in rodents and a strictly controlled developmental process has been defined at cellular and molecular levels. In comparison, primate spermatogenesis has been far less well characterized. However, important differences between primate and rodent spermatogenesis are emerging so it is not always accurate to extrapolate findings in rodents to primate systems. Here, we performed an extensive immunofluorescence study of spermatogenesis in neonatal, juvenile, and adult testes in the common marmoset (Callithrix jacchus) to determine primate-specific patterns of gene expression that underpin primate germ cell development. Initially we characterized adult spermatogonia into two main classes; mitotically active C-KIT(+)Ki67(+) cells and mitotically quiescent SALL4(+)PLZF(+)LIN28(+)DPPA4(+) cells. We then explored the expression of a set of markers, including PIWIL1/MARWI, VASA, DAZL, CLGN, RanBPM, SYCP1 and HAPRIN, during germ cell differentiation from early spermatocytes through round and elongating spermatids, and a clear program of gene expression changes was determined as development proceeded. We then examined the juvenile marmoset testis. Markers of gonocytes demonstrated two populations; one that migrates to the basal membrane where they form the SALL4(+) or C-KIT(+) spermatogonia, and another that remains in the lumen of the seminiferous tubule. This later population, historically identified as pre-spermatogonia, expressed meiotic and apoptotic markers and were eliminated because they appear to have failed to correctly migrate. Our findings provide the first platform of gene expression dynamics in adult and developing germ cells of the common marmoset. Although we have characterized a limited number of genes, these results will facilitate primate spermatogenesis research and understanding of human reproduction.

microRNA in Human Reproduction

microRNAs constitute a large family of approximately 21-nucleotide-long, noncoding RNAs. They emerged more than 20 years ago as key posttranscriptional regulators of gene expression. The regulatory role of these small RNA molecules has recently begun to be explored in the human reproductive system. microRNAs have been shown to play an important role in control of reproductive functions, especially in the processes of oocyte maturation, folliculogenesis, corpus luteum function, implantation, and early embryonic development. Knockout of Dicer, the cytoplasmic enzyme that cleaves the pre-miRNA to its mature form, results in postimplantation embryonic lethality in several animal models, attributing to these small RNA vital functions in reproduction and development. Another intriguing characteristic of microRNAs is their presence in body fluids in a remarkably stable form that is protected from endogenous RNase activity. In this chapter we will describe the current knowledge on microRNAs, specifically relating to human gonadal cells. We will focus on their role in the ovarian physiologic process and ovulation dysfunction, regulation of spermatogenesis and male fertility, and putative involvement in human normal and aberrant trophoblast differentiation and invasion through the process of placentation.

Primordial germ cell development in the marmoset monkey as revealed by pluripotency factor expression: suggestion of a novel model of embryonic germ cell translocation

Primordial germ cells (PGCs) are the embryonic progenitors of sperm and egg cells. Mammalian PGCs are thought to actively migrate from the yolk sac endoderm over long distances across the embryo to reach the somatic genital ridges. The general principles of mammalian PGC development were discovered in mice. In contrast, little is known about PGC development in primates due to extremely limited access to primate embryos. Here, we analyzed 12 well preserved marmoset monkey (Callithrix jacchus) embryos covering the phase from PGC emergence in the endoderm to the formation of the sexually differentiated gonad (embryonic day (E) 50 to E95). We show using immunohistochemistry that the pluripotency factors OCT4A and NANOG specifically mark PGCs throughout the period studied. In contrast, SALL4 and LIN28 were first expressed ubiquitously and only later down-regulated in somatic tissues. We further show, for the first time, that PGCs are located in the endoderm in E50 embryos in close spatial proximity to the prospective genital ridge, making a long-range migration of PGCs dispensable. At E65, PGCs are already present in the primitive gonad, while significantly later embryonic stages still exhibit PGCs at their original endodermal site, revealing a wide spatio-temporal window of PGC distribution. Our findings challenge the ‘dogma’ of active long-range PGC migration from the endoderm to the gonads. We therefore favor an alternative model based primarily on passive translocation of PGCs from the mesenchyme that surrounds the gut to the prospective gonad through the intercalar expansion of mesenchymal tissue which contains the PGCs. In summary, we (i) show differential pluripotency factor expression during primate embryo development and (ii) provide a schematic model for embryonic PGC translocation.

No evidence for neo-oogenesis may link to ovarian senescence in adult monkey

Female germline or oogonial stem cells transiently residing in fetal ovaries are analogous to the spermatogonial stem cells or germline stem cells (GSCs) in adult testes where GSCs and meiosis continuously renew. Oocytes can be generated in vitro from embryonic stem cells and induced pluripotent stem cells, but the existence of GSCs and neo-oogenesis in adult mammalian ovaries is less clear. Preliminary findings of GSCs and neo-oogenesis in mice and humans have not been consistently reproducible. Monkeys provide the most relevant model of human ovarian biology. We searched for GSCs and neo-meiosis in ovaries of adult monkeys at various ages, and compared them with GSCs from adult monkey testis, which are characterized by cytoplasmic staining for the germ cell marker DAZL and nuclear expression of the proliferative markers PCNA and KI67, and pluripotency-associated genes LIN28 and SOX2, and lack of nuclear LAMIN A, a marker for cell differentiation. Early meiocytes undergo homologous pairing at prophase I distinguished by synaptonemal complex lateral filaments with telomere perinuclear distribution. By exhaustive searching using comprehensive experimental approaches, we show that proliferative GSCs and neo-meiocytes by these specific criteria were undetectable in adult mouse and monkey ovaries. However, we found proliferative nongermline somatic stem cells that do not express LAMIN A and germ cell markers in the adult ovaries, notably in the cortex and granulosa cells of growing follicles. These data support the paradigm that adult ovaries do not undergo germ cell renewal, which may contribute significantly to ovarian senescence that occurs with age.

The neonatal marmoset monkey ovary is very primitive exhibiting many oogonia

Oogonia are characterized by diploidy and mitotic proliferation. Human and mouse oogonia express several factors such as OCT4, which are characteristic of pluripotent cells. In human, almost all oogonia enter meiosis between weeks 9 and 22 of prenatal development or undergo mitotic arrest and subsequent elimination from the ovary. As a consequence, neonatal human ovaries generally lack oogonia. The same was found in neonatal ovaries of the rhesus monkey, a representative of the old world monkeys (Catarrhini). By contrast, proliferating oogonia were found in adult prosimians (now called Strepsirrhini), which is a group of 'lower' primates. The common marmoset monkey (Callithrix jacchus) belongs to the new world monkeys (Platyrrhini) and is increasingly used in reproductive biology and stem cell research. However, ovarian development in the marmoset monkey has not been widely investigated. Herein, we show that the neonatal marmoset ovary has an extremely immature histological appearance compared with the human ovary. It contains numerous oogonia expressing the pluripotency factors OCT4A, SALL4, and LIN28A (LIN28). The pluripotency factor-positive germ cells also express the proliferation marker MKI67 (Ki-67), which has previously been shown in the human ovary to be restricted to premeiotic germ cells. Together, the data demonstrate the primitiveness of the neonatal marmoset ovary compared with human. This study may introduce the marmoset monkey as a non-human primate model to experimentally study the aspects of primate primitive gonad development, follicle assembly, and germ cell biology in vivo.

MicroRNAs and spermatogenesis

In mammals, male gametes are produced inside the testis by spermatogenesis, which has three phases: mitotic proliferation of spermatogonia, meiosis of spermatocytes, and haploid differentiation of spermatids. The genome of male germ cells is actively transcribed to produce phase-specific gene expression patterns. Male germ cells have a complex transcriptome. In addition to protein-coding messenger RNAs, many noncoding RNAs, including microRNAs (miRNAs), are produced. The miRNAs are important regulators of gene expression. They function mainly post-transcriptionally to control the stability or translation of their target messenger RNAs. The miRNAs are expressed in a cell-specific manner during spermatogenesis to participate in the control of each step of male germ cell differentiation. Genetically modified mouse models have demonstrated the importance of miRNA pathways for normal spermatogenesis, and functional studies have been designed to dissect the roles of specific miRNAs in distinct cell types. Clinical studies have exploited the well-defined expression profiles of miRNAs, and human spermatozoal or seminal plasma miRNAs have been explored as potential biomarkers for male factor infertility. This review article discusses the current findings that support the central role of miRNAs in the regulation of spermatogenesis and male fertility.

Separation of somatic and germ cells is required to establish primate spermatogonial cultures

STUDY QUESTION

Can primate spermatogonial cultures be optimized by application of separation steps and well defined culture conditions?

SUMMARY ANSWER

We identified the cell fraction which provides the best source for primate spermatogonia when prolonged culture is desired.

WHAT IS KNOWN ALREADY

Man and marmoset show similar characteristics in regard to germ cell development and function. Several protocols for isolation and culture of human testis-derived germline stem cells have been described. Subsequent analysis revealed doubts on the germline origin of these cells and characterized them as mesenchymal stem cells or fibroblasts. Studies using marmosets as preclinical model confirmed that the published isolation protocols did not lead to propagation of germline cells.

STUDY DESIGN, SIZE, DURATION

Testicular cells derived from nine adult marmoset monkeys (Callithrix jacchus) were cultured for 1, 3, 6 and 11 days and consecutively analyzed for the presence of spermatogonia, differentiating germ cells and testicular somatic cells.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Testicular tissue of nine adult marmoset monkeys was enzymatically dissociated and subjected to two different cell culture approaches. In the first approach all cells were kept in the same dish (non-separate culture, n = 5). In the second approach the supernatant cells were transferred into a new dish 24 h after seeding and subsequently supernatant and attached cells were cultured separately (separate culture, n = 4). Real-time quantitative PCR and immunofluorescence were used to analyze the expression of reliable germ cell and somatic markers throughout the culture period. Germ cell transplantation assays and subsequent wholemount analyses were performed to functionally evaluate the colonization of spermatogonial cells.

MAIN RESULTS AND THE ROLE OF CHANCE

This is the first report revealing an efficient isolation and culture of putative marmoset spermatogonial stem cells with colonization ability. Our results indicate that a separation of spermatogonia from testicular somatic cells is a crucial step during cell preparation. We identified the overgrowth of more rapidly expanding somatic cells to be a major problem when establishing spermatogonial cultures. Initiating germ cell cultures from the supernatant and maintaining germ cells in suspension cultures minimized the somatic cell contamination and provided enriched germ cell fractions which displayed after 11 days of culture a significantly higher expression of germ cell markers genes (DDX-4, MAGE A-4; P < 0.05) compared with separately cultured attached cells. Additionally, germ cell transplantation experiments demonstrated a significantly higher absolute number of cells with colonization ability (P < 0.001) in supernatant cells after 11 days of separate culture.

LIMITATIONS, REASONS FOR CAUTION

This study presents a relevant aspect for the successful setup of spermatogonial cultures but provides limited data regarding the question of whether the long-term maintenance of spermatogonia can be achieved. Transfer of these preclinical data to man may require modifications of the protocol.

WIDER IMPLICATIONS OF THE FINDINGS

Spermatogonial cultures from rodents have become important and innovative tools for basic and applied research in reproductive biology and veterinary medicine. It is expected that spermatogonia-based strategies will be transformed into clinical applications for the treatment of male infertility. Our data in the marmoset monkey may be highly relevant to establish spermatogonial cultures of human testes.

STUDY FUNDING/COMPETING INTERESTS

Funding was provided by the DFG-Research Unit FOR 1041 Germ Cell Potential (SCHL394/11-2) and by the Graduate Program Cell Dynamics and Disease (CEDAD) together with the International Max Planck Research School - Molecular Biomedicine (IMPRS-MBM). The authors declare that there is no conflict of interest.

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.

Germ cell loss is associated with fading Lin28a expression in a mouse model for Klinefelter's syndrome

Klinefelter's syndrome is a male sex-chromosomal disorder (47,XXY), causing hypogonadism, cognitive and metabolic deficits. The majority of patients are infertile due to complete germ cell loss after puberty. As the depletion occurs during development, the possibilities to study the underlying causes in humans are limited. In this study, we used the 41,XXY* mouse model to characterise the germ line postnatally. We examined marker expression of testicular cells focusing on the spermatogonial stem cells (SSCs) and found that the number of germ cells was approximately reduced fivefold at day 1pp in the 41,XXY* mice, indicating the loss to start prenatally. Concurrently, immunohistochemical SSC markers LIN28A and PGP9.5 also showed decreased expression on day 1pp in the 41,XXY* mice (48.5 and 38.9% of all germ cells were positive), which dropped to 7.8 and 7.3% on 3dpp, and were no longer detectable on days 5 and 10pp respectively. The differences in PCNA-positive proliferating cells in XY* and XXY* mice dramatically increased towards day 10pp. The mRNA expression of the germ cell markers Lin28a (Lin28), Pou5f1 (Oct4), Utf1, Ddx4 (Vasa), Dazl, and Fapb1 (Sycp3) was reduced and the Lin28a regulating miRNAs were deregulated in the 41,XXY* mice. We suggest a model for the course of germ cell loss starting during the intrauterine period. Neonatally, SSC marker expression by the already lowered number of spermatogonia is reduced and continues fading during the first postnatal week, indicating the surviving cells of the SSC population to be disturbed in their stem cell characteristics. Subsequently, the entire germ line is then generally lost when entering meiosis.

Promyelocytic leukaemia zinc finger maintains self-renewal of male germline stem cells (mGSCs) and its expression pattern in dairy goat testis

OBJECTIVES

Previous studies have shown that promyelocytic leukaemia zinc finger (PLZF) is a spermatogonia-specific transcription factor in the testis, required to regulate self-renewal and maintenance of the spermatogonia stem cell. Up to now, expression and function of PLZF in the goat testis has not been known. The objectives of this study were to investigate PLZF expression pattern in the dairy goat and its effect on male goat germline stem cell (mGSC) self-renewal and differentiation.

MATERIALS AND METHODS

Testis development and expression patterns of PLZF in the dairy goat were analysed by haematoxylin and eosin staining, immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR). Furthermore, effects of PLZF overexpression on mGSC self-renewal and differentiation were evaluated by quantitative RT-PCR (QRT-PCR), immunofluorescence and BrdU incorporation assay.

RESULTS

Promyelocytic leukaemia zinc finger was essential for dairy goat testis development and expression of several proliferation and pluripotency-associated proteins including OCT4, C-MYC were upregulated by PLZF overexpression. The study demonstrated that PLZF played a key role in maintaining self-renewal of mGSCs and its overexpression enhanced expression of proliferation-associated genes.

CONCLUSIONS

Promyelocytic leukaemia zinc finger could function in the dairy goat as well as in other species in maintaining self-renewal of germline stem cells and this study provides a model to study the mechanism on self-renewal and differentiation of mGSCs in livestock.

The Lin28/Let-7 system in early human embryonic tissue and ectopic pregnancy

Our objective was to determine the expression of the elements of the Lin28/Let-7 system, and related microRNAs (miRNAs), in early stages of human placentation and ectopic pregnancy, as a means to assess the potential role of this molecular hub in the pathogenesis of ectopic gestation. Seventeen patients suffering from tubal ectopic pregnancy (cases) and forty-three women with normal on-going gestation that desired voluntary termination of pregnancy (VTOP; controls) were recruited for the study. Embryonic tissues were subjected to RNA extraction and quantitative PCR analyses for LIN28B, Let-7a, miR-132, miR-145 and mir-323-3p were performed. Our results demonstrate that the expression of LIN28B mRNA was barely detectable in embryonic tissue from early stages of gestation and sharply increased thereafter to plateau between gestational weeks 7–9. In contrast, expression levels of Let-7, mir-132 and mir-145 were high in embryonic tissue from early gestations (≤6-weeks) and abruptly declined thereafter, especially for Let-7. Opposite trends were detected for mir-323-3p. Embryonic expression of LIN28B mRNA was higher in early stages (≤6-weeks) of ectopic pregnancy than in normal gestation. In contrast, Let-7a expression was significantly lower in early ectopic pregnancies, while miR-132 and miR-145 levels were not altered. Expression of mir-323-3p was also suppressed in ectopic embryonic tissue. We are the first to document reciprocal changes in the expression profiles of the gene encoding the RNA-binding protein, LIN28B, and the related miRNAs, Let-7a, mir-132 and mir-145, in early stages of human placentation. This finding suggests the potential involvement of LIN28B/Let-7 (de)regulated pathways in the pathophysiology of ectopic pregnancy in humans.

Germline stem cells: toward the regeneration of spermatogenesis

Improved therapies for cancer and other conditions have resulted in a growing population of long-term survivors. Infertility is an unfortunate side effect of some cancer therapies that impacts the quality of life of survivors who are in their reproductive or prereproductive years. Some of these patients have the opportunity to preserve their fertility using standard technologies that include sperm, egg, or embryo banking, followed by IVF and/or ET. However, these options are not available to all patients, especially the prepubertal patients who are not yet producing mature gametes. For these patients, there are several stem cell technologies in the research pipeline that may give rise to new fertility options and allow infertile patients to have their own biological children. We will review the role of stem cells in normal spermatogenesis as well as experimental stem cell-based techniques that may have potential to generate or regenerate spermatogenesis and sperm. We will present these technologies in the context of the fertility preservation paradigm, but we anticipate that they will have broad implications for the assisted reproduction field.

Distinct expression patterns predict differential roles of the miRNA-binding proteins, Lin28 and Lin28b, in the mouse testis: studies during postnatal development and in a model of hypogonadotropic hypogonadism

Lin28 (also termed Lin28a) and Lin28b are related RNA-binding proteins, involved in the control of microRNA synthesis, especially of the let-7 family, with putative functions in early (embryo) development. However, their roles during postnatal maturation remain ill defined. Despite the general assumption that Lin28 and Lin28b share similar targets and functions, conclusive demonstration of such redundancy is still missing. In addition, recent observations suggest a role of Lin28 proteins in mammalian reproduction, which is yet to be defined. We document herein the patterns of RNA expression and protein distribution of Lin28 and Lin28b in mouse testis during postnatal development and in a model of hypogonadotropic hypogonadism as a result of inactivation of the kisspeptin receptor, Gpr54. Lin28 and Lin28b mRNAs were expressed in mouse testis across postnatal maturation, but their levels disparately varied between neonatal and pubertal periods, with peak Lin28 levels in infantile testes and sustained elevation of Lin28b mRNA in young adult male gonads, where relative levels of let-7a and let-7b miRNAs were significantly suppressed. In addition, Lin28 peptides displayed totally different patterns of cellular distribution in mouse testis: Lin28 was located in undifferentiated and type-A1 spermatogonia, whereas Lin28b was confined to spermatids and interstitial Leydig cells. These profiles were perturbed in Gpr54 null mouse testis, which showed preserved but irregular Lin28 signal and absence of Lin28b peptide, which was rescued by administration of gonadotropins, mainly hCG (as super-agonist of LH). In addition, increased relative levels of Lin28, but not Lin28b, mRNA and of let-7a/let-7b miRNAs were observed in Gpr54 KO mouse testes. Altogether, our data are the first to document the divergent patterns of cellular distribution and mRNA expression of Lin28 and Lin28b in the mouse testis along postnatal maturation and their alteration in a model of congenital hypogonadotropic hypogonadism. Our findings suggest distinct functional roles of these two related, but not overlapping, miRNA-binding proteins in the male gonad.

The rise of testicular germ cell tumours: the search for causes, risk factors and novel therapeutic targets

Since the beginning of the 20th century there has been a decline in the reproductive vitality of men within the Western world. The declining sperm quantity and quality has been associated with increased overt disorders of sexual development including hypospadias, undescended testes and type II testicular germ cell tumours (TGCTs). The increase in TGCTs cannot be accounted for by genetic changes in the population. Therefore exposure to environmental toxicants appears to be a major contributor to the aetiology of TGCTs and men with a genetic predisposition are particularly vulnerable. In particular, Type II TGCTs have been identified to arise from a precursor lesion Carcinoma in situ (CIS), identified as a dysfunctional gonocyte; however, the exact triggers for CIS development are currently unknown. Therefore the transition from gonocytes into spermatogonia is key to those studying TGCTs. Recently we have identified seven miRNA molecules (including members of the miR-290 family and miR-136, 463* and 743a) to be significantly changed over this transition period. These miRNA molecules are predicted to have targets within the CXCR4, PTEN, DHH, RAC and PDGF pathways, all of which have important roles in germ cell migration, proliferation and homing to the spermatogonial stem cell niche. Given the plethora of potential targets affected by each miRNA molecule, subtle changes in miRNA expression could have significant consequences e.g. tumourigenesis. The role of non-traditional oncogenes and tumour suppressors such as miRNA in TGCT is highlighted by the fact that the majority of these tumours express wild type p53, a pivotal tumour suppressor usually inactivated in cancer. While treatment of TGCTs is highly successful, the impact of these treatments on fertility means that identification of exact triggers, earlier diagnosis and alternate treatments are essential. This review examines the genetic factors and possible triggers of type II TGCT to highlight target areas for potential new treatments.

Importance of the pluripotency factor LIN28 in the mammalian nucleolus during early embryonic development

The maternal nucleolus is required for proper activation of the embryonic genome (EGA) and early embryonic development. Nucleologenesis is characterized by the transformation of a nucleolar precursor body (NPB) to a mature nucleolus during preimplantation development. However, the function of NPBs and the involved molecular factors are unknown. We uncover a novel role for the pluripotency factor LIN28, the biological significance of which was previously demonstrated in the reprogramming of human somatic cells to induced pluripotent stem (iPS) cells. Here, we show that LIN28 accumulates at the NPB and the mature nucleolus in mouse preimplantation embryos and embryonic stem cells (ESCs), where it colocalizes with the nucleolar marker B23 (nucleophosmin 1). LIN28 has nucleolar localization in non-human primate (NHP) preimplantation embryos, but is cytoplasmic in NHP ESCs. Lin28 transcripts show a striking decline before mouse EGA, whereas LIN28 protein localizes to NPBs at the time of EGA. Following knockdown with a Lin28 morpholino, the majority of embryos arrest between the 2- and 4-cell stages and never develop to morula or blastocyst. Lin28 morpholino-injected embryos arrested at the 2-cell stage were not enriched with nucleophosmin at presumptive NPB sites, indicating that functional NPBs were not assembled. Based on these results, we propose that LIN28 is an essential factor of nucleologenesis during early embryonic development.