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

Spermatogonial stem cells in the 129 inbred strain exhibit unique requirements for self-renewal

Spermatogonial stem cells (SSCs) undergo self-renewal division to sustain spermatogenesis. Although it is possible to derive SSC cultures in most mouse strains, SSCs from a 129 background never proliferate under the same culture conditions, suggesting they have distinct self-renewal requirements. Here, we established long-term culture conditions for SSCs from mice of the 129 background (129 mice). An analysis of 129 testes showed significant reduction of GDNF and CXCL12, whereas FGF2, INHBA and INHBB were higher than in testes of C57BL/6 mice. An analysis of undifferentiated spermatogonia in 129 mice showed higher expression of Chrna4, which encodes an acetylcholine (Ach) receptor component. By supplementing medium with INHBA and Ach, SSC cultures were derived from 129 mice. Following lentivirus transduction for marking donor cells, transplanted cells re-initiated spermatogenesis in infertile mouse testes and produced transgenic offspring. These results suggest that the requirements of SSC self-renewal in mice are diverse, which has important implications for understanding self-renewal mechanisms in various animal species.

Impact of the hypoxic microenvironment on spermatogonial stem cells in culture

The stem cell niche plays a crucial role in the decision to either self-renew or differentiate. Recent observations lead to the hypothesis that O2 supply by blood and local O2 tension could be key components of the testicular niche of spermatogonial stem cells (SSCs). In this study, we investigated the impact of different hypoxic conditions (3.5%, 1%, and 0.1% O2 tension) on murine and human SSCs in culture. We observed a deleterious effect of severe hypoxia (1% O2 and 0.1% O2) on the capacity of murine SSCs to form germ cell clusters when plated at low density. Severe effects on SSCs proliferation occur at an O2 tension ≤1% and hypoxia was shown to induce a slight differentiation bias under 1% and 0.1% O2 conditions. Exposure to hypoxia did not appear to change the mitochondrial mass and the potential of membrane of mitochondria in SSCs, but induced the generation of mitochondrial ROS at 3.5% and 1% O2. In 3.5% O2 conditions, the capacity of SSCs to form colonies was maintained at the level of 21% O2 at low cell density, but it was impossible to amplify and maintain stem cell number in high cell density culture. In addition, we observed that 3.5% hypoxia did not improve the maintenance and propagation of human SSCs. Finally, our data tend to show that the transcription factors HIF-1α and HIF-2α are not involved in the SSCs cell autonomous response to hypoxia.

Characterization and enrichment of spermatogonial stem cells of common carp (Cyprinus carpio)

Spermatogenesis is a systematically organized process that ensures uninterrupted sperm production in which the spermatogonial stem cells (SSCs) play a crucial role. However, the existing absence of teleost-specific molecular markers for SSCs presents a notable challenge. Herein we characterized phenotypically the spermatogonial stem cells using specific molecular markers and transmission electron microscopy. Moreover, we also describe a simple method to suppress common carp spermatogenesis using the combination of Busulfan and thermo-chemical treatment, and finally, we isolate and enrich the undifferentiated spermatogonial fraction. Our results showed that C-kit, GFRα1, and POU2 proteins were expressed by germ cells, meanwhile, undifferentiated spermatogonial populations preferentially expressed GFRα1 and POU2. Moreover, the combination of high temperature (35 °C) and Busulfan (40 mg/kg/BW) effectively suppressed the spermatogenesis of common carp males. Additionally, the amh expression analysis showed differences between the control (26 °C) when compared to 35 °C with a single or two Busulfan doses, confirming that the testes were depleted by the association of Busulfan at high temperatures. In an attempt to isolate the undifferentiated spermatogonial fraction, we used the Percoll discontinuous density gradient. Thus, we successfully dissociated the carp whole testes in different cellular fractions; subsequently, we isolated and enriched the undifferentiated spermatogonial population. Therefore, our results suggest that probably both GFRα-1 and POU2 are highly conserved factors expressed in common carp germinative epithelium and that these molecules were well conserved along the evolutionary process. Furthermore, the enriched undifferentiated spermatogonial population developed here can be used in further germ cell transplantation experiments to preserve and propagate valued and endangered fish species.

Intracytoplasmic sperm injection induces transgenerational abnormalities in mice

In vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) are 2 major assisted reproductive techniques (ARTs) used widely to treat infertility. Recently, spermatogonial transplantation emerged as a new ART to restore fertility to young patients with cancer after cancer therapy. To examine the influence of germ cell manipulation on behavior of offspring, we produced F1 offspring by a combination of two ARTs, spermatogonial transplantation and ICSI. When these animals were compared with F1 offspring produced by ICSI using fresh wild-type sperm, not only spermatogonial transplantation-ICSI mice but also ICSI-only control mice exhibited behavioral abnormalities, which persisted in the F2 generation. Furthermore, although these F1 offspring appeared normal, F2 offspring produced by IVF using F1 sperm and wild-type oocytes showed various types of congenital abnormalities, including anophthalmia, hydrocephalus, and missing limbs. Therefore, ARTs can induce morphological and functional defects in mice, some of which become evident only after germline transmission.

Glutamine protects mouse spermatogonial stem cells against NOX1-derived ROS for sustaining self-renewal division in vitro

Reactive oxygen species (ROS) are generated from NADPH oxidases and mitochondria; they are generally harmful for stem cells. Spermatogonial stem cells (SSCs) are unique among tissue-stem cells because they undergo ROS-dependent self-renewal via NOX1 activation. However, the mechanism by which SSCs are protected from ROS remains unknown. Here, we demonstrate a crucial role for Gln in ROS protection using cultured SSCs derived from immature testes. Measurements of amino acids required for SSC cultures revealed the indispensable role of Gln in SSC survival. Gln induced Myc expression to drive SSC self-renewal in vitro, whereas Gln deprivation triggered Trp53-dependent apoptosis and impaired SSC activity. However, apoptosis was attenuated in cultured SSCs that lacked NOX1. In contrast, cultured SSCs lacking Top1mt mitochondria-specific topoisomerase exhibited poor mitochondrial ROS production and underwent apoptosis. Gln deprivation reduced glutathione production; supra-molar Asn supplementation allowed offspring production from SSCs cultured without Gln. Therefore, Gln ensures ROS-dependent SSC-self-renewal by providing protection against NOX1 and inducing Myc.

New Insights on Sperm Function in Male Infertility of Unknown Origin: A Multimodal Approach

The global trend of rising (male) infertility is concerning, and the unidentifiable causes in half of the cases, the so-called unknown origin male infertility (UOMI), demands a better understanding and assessment of both external/internal factors and mechanisms potentially involved. In this work, it was our aim to obtain new insight on UOMI, specifically on idiopathic (ID) and Unexplained male infertility (UMI), relying on a detailed evaluation of the male gamete, including functional, metabolic and proteomic aspects. For this purpose, 1114 semen samples, from males in couples seeking infertility treatment, were collected at the Reproductive Medicine Unit from the Centro Hospitalar e Universitário de Coimbra (CHUC), from July 2018-July 2022. Based on the couples' clinical data, seminal/hormonal analysis, and strict eligibility criteria, samples were categorized in 3 groups, control (CTRL), ID and UMI. Lifestyle factors and anxiety/depression symptoms were assessed via survey. Sperm samples were evaluated functionally, mitochondrially and using proteomics. The results of Assisted Reproduction Techniques were assessed whenever available. According to our results, ID patients presented the worst sperm functional profile, while UMI patients were similar to controls. The proteomic analysis revealed 145 differentially expressed proteins, 8 of which were specifically altered in ID and UMI samples. Acrosin (ACRO) and sperm acrosome membrane-associated protein 4 (SACA4) were downregulated in ID patients while laminin subunit beta-2 (LAMB2), mannose 6-phosphate isomerase (MPI), ATP-dependent 6-phosphofructokinase liver type (PFKAL), STAR domain-containing protein 10 (STA10), serotransferrin (TRFE) and exportin-2 (XPO2) were downregulated in UMI patients. Using random forest analysis, SACA4 and LAMB2 were identified as the sperm proteins with a higher chance of distinguishing ID and UMI patients, and their function and expression variation were in accordance with the functional results. No alterations were observed in terms of lifestyle and psychological factors among the 3 groups. These findings obtained in an experimental setting based on 3 well-defined groups of subjects, might help to validate new biomarkers for unknown origin male infertility (ID and UMI) that, in the future, can be used to improve diagnostics and treatments.

Transcription factor E4F1 dictates spermatogonial stem cell fate decisions by regulating mitochondrial functions and cell cycle progression

BACKGROUND

Spermatogonial stem cells (SSCs) provide a foundation for robust and continual spermatogenesis in mammals. SSCs self-renew to maintain a functional stem cell pool and differentiate to supply committed progenitors. Metabolism acts as a crucial determinant of stem cell fates; however, factors linking metabolic programs to SSC development and maintenance are poorly understood.

RESULTS

We analyzed the chromatin accessibility of undifferentiated spermatogonia at the single-cell level and identified 37 positive TF regulators that may have potential roles in dictating SSC fates. The transcription factor E4F1 is expressed in spermatogonia, and its conditional deletion in mouse germ cells results in progressive loss of the entire undifferentiated spermatogonial pool. Single-cell RNA-seq analysis of control and E4f1-deficient spermatogonia revealed that E4F1 acts as a key regulator of mitochondrial function. E4F1 binds to promotors of genes that encode components of the mitochondrial respiratory chain, including Ndufs5, Cox7a2, Cox6c, and Dnajc19. Loss of E4f1 function caused abnormal mitochondrial morphology and defects in fatty acid metabolism; as a result, undifferentiated spermatogonia were gradually lost due to cell cycle arrest and elevated apoptosis. Deletion of p53 in E4f1-deficient germ cells only temporarily prevented spermatogonial loss but did not rescue the defects in SSC maintenance.

CONCLUSIONS

Emerging evidence indicates that metabolic signals dictate stem cell fate decisions. In this study, we identified a list of transcription regulators that have potential roles in the fate transitions of undifferentiated spermatogonia in mice. Functional experiments demonstrated that the E4F1-mediated transcription program is a crucial regulator of metabolism and SSC fate decisions in mammals.

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

BACKGROUND

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

AIM

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

RESULTS AND CONCLUSIONS

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

Pancreatic lipase-related protein 2 is selectively expressed by peritubular myoid cells in the murine testis and sustains long-term spermatogenesis

Spermatogenesis is a complicated process of germ cell differentiation that occurs within the seminiferous tubule in the testis. Peritubular myoid cells (PTMCs) produce major components of the basement membrane that separates and ensures the structural integrity of seminiferous tubules. These cells secrete niche factors to promote spermatogonial stem cell (SSC) maintenance and mediate androgen signals to direct spermatid development. However, the regulatory mechanisms underlying the identity and function of PTMCs have not been fully elucidated. In the present study, we showed that the expression of pancreatic lipase-related protein 2 (Pnliprp2) was restricted in PTMCs in the testis and that its genetic ablation caused age-dependent defects in spermatogenesis. The fertility of Pnliprp2 knockout animals (Pnliprp2-/-) was normal at a young age but declined sharply beginning at 9 months. Pnliprp2 deletion impaired the homeostasis of undifferentiated spermatogonia and severely disrupted the development and function of spermatids. Integrated analyses of single-cell RNA-seq and metabolomics data revealed that glyceride metabolism was changed in PTMCs from Pnliprp2-/- mice. Further analysis found that 60 metabolites were altered in the sperm of the Pnliprp2-/- animals; notably, lipid metabolism was significantly dysregulated. Collectively, these results revealed that Pnliprp2 was exclusively expressed in PTMCs in the testis and played a novel role in supporting continual spermatogenesis in mice. The outcomes of these findings highlight the function of lipid metabolism in reproduction and provide new insights into the regulation of PTMCs in mammals.

Developmental regulators moonlighting as transposons defense factors

BACKGROUND

The germline perpetuates genetic information across generations. To maintain the integrity of the germline, transposable elements in the genome must be silenced, as these mobile elements would otherwise engender widespread mutations passed on to subsequent generations. There are several well-established mechanisms that are dedicated to providing defense against transposable elements, including DNA methylation, RNA interference, and the PIWI-interacting RNA pathway.

OBJECTIVES

Recently, several studies have provided evidence that transposon defense is not only provided by factors dedicated to this purpose but also factors with other roles, including in germline development. Many of these are transcription factors. Our objective is to summarize what is known about these "bi-functional" transcriptional regulators.

MATERIALS AND METHODS

Literature search.

RESULTS AND CONCLUSION

We summarize the evidence that six transcriptional regulators-GLIS3, MYBL1, RB1, RHOX10, SETDB1, and ZBTB16-are both developmental regulators and transposable element-defense factors. These factors act at different stages of germ cell development, including in pro-spermatogonia, spermatogonial stem cells, and spermatocytes. Collectively, the data suggest a model in which specific key transcriptional regulators have acquired multiple functions over evolutionary time to influence developmental decisions and safeguard transgenerational genetic information. It remains to be determined whether their developmental roles were primordial and their transposon defense roles were co-opted, or vice versa.

Evolution and implications of de novo genes in humans

Genes and translated open reading frames (ORFs) that emerged de novo from previously non-coding sequences provide species with opportunities for adaptation. When aberrantly activated, some human-specific de novo genes and ORFs have disease-promoting properties-for instance, driving tumour growth. Thousands of putative de novo coding sequences have been described in humans, but we still do not know what fraction of those ORFs has readily acquired a function. Here, we discuss the challenges and controversies surrounding the detection, mechanisms of origin, annotation, validation and characterization of de novo genes and ORFs. Through manual curation of literature and databases, we provide a thorough table with most de novo genes reported for humans to date. We re-evaluate each locus by tracing the enabling mutations and list proposed disease associations, protein characteristics and supporting evidence for translation and protein detection. This work will support future explorations of de novo genes and ORFs in humans.

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."

Ubiquitin protein E3 ligase ASB9 suppresses proliferation and promotes apoptosis in human spermatogonial stem cell line by inducing HIF1AN degradation

BACKGROUND

Spermatogonial stem cells (SSCs) are critical for sustaining spermatogenesis. Even though several regulators of SSC have been identified in rodents, the regulatory mechanism of SSC in humans has yet to be discovered.

METHODS

To explore the regulatory mechanisms of human SSCs, we analyzed publicly available human testicular single-cell sequencing data and found that Ankyrin repeat and SOCS box protein 9 (ASB9) is highly expressed in SSCs. We examined the expression localization of ASB9 using immunohistochemistry and overexpressed ASB9 in human SSC lines to explore its role in SSC proliferation and apoptosis. Meanwhile, we used immunoprecipitation to find the target protein of ASB9 and verified its functions. In addition, we examined the changes in the distribution of ASB9 in non-obstructive azoospermia (NOA) patients using Western blot and immunofluorescence.

RESULTS

The results of uniform manifold approximation and projection (UMAP) clustering and pseudotime analysis showed that ASB9 was highly expressed in SSCs, and its expression gradually increased during development. The immunohistochemical and dual-color immunofluorescence results displayed that ASB9 was mainly expressed in nonproliferating SSCs. Overexpression of ASB9 in the SSC line revealed significant inhibition of cell proliferation and increased apoptosis. We predicted the target proteins of ASB9 and verified that hypoxia-inducible factor 1-alpha inhibitor (HIF1AN), but not creatine kinase B-type (CKB), has a direct interaction with ASB9 in human SSC line using protein immunoprecipitation experiments. Subsequently, we re-expressed HIF1AN in ASB9 overexpressing cells and found that HIF1AN reversed the proliferative and apoptotic changes induced by ASB9 overexpression. In addition, we found that ABS9 was significantly downregulated in some NOA patients, implying a correlation between ASB9 dysregulation and impaired spermatogenesis.

CONCLUSION

ASB9 is predominantly expressed in human SSCs, it affects the proliferation and apoptotic process of the SSC line through HIF1AN, and its abnormal expression may be associated with NOA.

SPOC domain-containing protein 1 regulates the proliferation and apoptosis of human spermatogonial stem cells through adenylate kinase 4

BACKGROUND

Spermatogonial stem cells (SSCs) are the origin of male spermatogenesis, which can reconstruct germ cell lineage in mice. However, the application of SSCs for male fertility restoration is hindered due to the unclear mechanisms of proliferation and self-renewal in humans.

AIM

To investigate the role and mechanism of SPOC domain-containing protein 1 (SPOCD1) in human SSC proliferation.

METHODS

We analyzed publicly available human testis single-cell RNA sequencing (RNA-seq) data and found that SPOCD1 is predominantly expressed in SSCs in the early developmental stages. Small interfering RNA was applied to suppress SPOCD1 expression to detect the impacts of SPOCD1 inhibition on SSC proliferation and apoptosis. Subsequently, we explored the target genes of SPOCD1 using RNA-seq and confirmed their role by restoring the expression of the target genes. In addition, we examined SPOCD1 expression in some non-obstructive azoospermia (NOA) patients to explore the correlation between SPOCD1 and NOA.

RESULTS

The uniform manifold approximation and projection clustering and pseudotime analysis showed that SPOCD1 was highly expressed in the early stages of SSC, and immunohistological results showed that SPOCD1 was mainly localized in glial cell line-derived neurotrophic factor family receptor alpha-1 positive SSCs. SPOCD1 knockdown significantly inhibited cell proliferation and promoted apoptosis. RNA-seq results showed that SPOCD1 knockdown significantly downregulated genes such as adenylate kinase 4 (AK4). Overexpression of AK4 in SPOCD1 knockdown cells partially reversed the phenotypic changes, indicating that AK4 is a functional target gene of SPOCD1. In addition, we found a significant downregulation of SPOCD1 expression in some NOA patients, suggesting that the downregulation of SPOCD1 may be relevant for NOA.

CONCLUSION

Our study broadens the understanding of human SSC fate determination and may offer new theories on the etiology of male infertility.

Metabolic transitions define spermatogonial stem cell maturation

STUDY QUESTION

Do spermatogonia, including spermatogonial stem cells (SSCs), undergo metabolic changes during prepubertal development?

SUMMARY ANSWER

Here, we show that the metabolic phenotype of prepubertal human spermatogonia is distinct from that of adult spermatogonia and that SSC development is characterized by distinct metabolic transitions from oxidative phosphorylation (OXPHOS) to anaerobic metabolism.

WHAT IS KNOWN ALREADY

Maintenance of both mouse and human adult SSCs relies on glycolysis, while embryonic SSC precursors, primordial germ cells (PGCs), exhibit an elevated dependence on OXPHOS. Neonatal porcine SSC precursors reportedly initiate a transition to an adult SSC metabolic phenotype at 2 months of development. However, when and if such a metabolic transition occurs in humans is ambiguous.

STUDY DESIGN, SIZE, DURATION

To address our research questions: (i) we performed a meta-analysis of publicly available and newly generated (current study) single-cell RNA sequencing (scRNA-Seq) datasets in order to establish a roadmap of SSC metabolic development from embryonic stages (embryonic week 6) to adulthood in humans (25 years of age) with a total of ten groups; (ii) in parallel, we analyzed single-cell RNA sequencing datasets of isolated pup (n = 3) and adult (n = 2) murine spermatogonia to determine whether a similar metabolic switch occurs; and (iii) we characterized the mechanisms that regulate these metabolic transitions during SSC maturation by conducting quantitative proteomic analysis using two different ages of prepubertal pig spermatogonia as a model, each with four independently collected cell populations.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Single testicular cells collected from 1-year, 2-year and 7-year-old human males and sorted spermatogonia isolated from 6- to 8-day (n = 3) and 4-month (n = 2) old mice were subjected to scRNA-Seq. The human sequences were individually processed and then merged with the publicly available datasets for a meta-analysis using Seurat V4 package. We then performed a pairwise differential gene expression analysis between groups of age, followed by pathways enrichment analysis using gene set enrichment analysis (cutoff of false discovery rate < 0.05). The sequences from mice were subjected to a similar workflow as described for humans. Early (1-week-old) and late (8-week-old) prepubertal pig spermatogonia were analyzed to reveal underlying cellular mechanisms of the metabolic shift using immunohistochemistry, western blot, qRT-PCR, quantitative proteomics, and culture experiments.

MAIN RESULTS AND THE ROLE OF CHANCE

Human PGCs and prepubertal human spermatogonia show an enrichment of OXPHOS-associated genes, which is downregulated at the onset of puberty (P < 0.0001). Furthermore, we demonstrate that similar metabolic changes between pup and adult spermatogonia are detectable in the mouse (P < 0.0001). In humans, the metabolic transition at puberty is also preceded by a drastic change in SSC shape at 11 years of age (P < 0.0001). Using a pig model, we reveal that this metabolic shift could be regulated by an insulin growth factor-1 dependent signaling pathway via mammalian target of rapamycin and proteasome inhibition.

LARGE SCALE DATA

New single-cell RNA sequencing datasets obtained from this study are freely available through NCBI GEO with accession number GSE196819.

LIMITATIONS, REASONS FOR CAUTION

Human prepubertal tissue samples are scarce, which led to the investigation of a low number of samples per age. Gene enrichment analysis gives only an indication about the functional state of the cells. Due to limited numbers of prepubertal human spermatogonia, porcine spermatogonia were used for further proteomic and in vitro analyses.

WIDER IMPLICATIONS OF THE FINDINGS

We show that prepubertal human spermatogonia exhibit high OXHPOS and switch to an adult-like metabolism only after 11 years of age. Prepubescent cancer survivors often suffer from infertility in adulthood. SSC transplantation could provide a powerful tool for the treatment of infertility; however, it requires high cell numbers. This work provides key insight into the dynamic metabolic requirements of human SSCs across development that would be critical in establishing ex vivo systems to support expansion and sustained function of SSCs toward clinical use.

STUDY FUNDING/COMPETING INTEREST(S)

This work was funded by the NIH/NICHD R01 HD091068 and NIH/ORIP R01 OD016575 to I.D. K.E.O. was supported by R01 HD100197. S.K.M. was supported by T32 HD087194 and F31 HD101323. The authors declare no conflict of interest.

Open reading frame dominance indicates protein‐coding potential of RNAs

Recent studies have identified numerous RNAs with both coding and noncoding functions. However, the sequence characteristics that determine this bifunctionality remain largely unknown. In the present study, we develop and test the open reading frame (ORF) dominance score, which we define as the fraction of the longest ORF in the sum of all putative ORF lengths. This score correlates with translation efficiency in coding transcripts and with translation of noncoding RNAs. In bacteria and archaea, coding and noncoding transcripts have narrow distributions of high and low ORF dominance, respectively, whereas those of eukaryotes show relatively broader ORF dominance distributions, with considerable overlap between coding and noncoding transcripts. The extent of overlap positively and negatively correlates with the mutation rate of genomes and the effective population size of species, respectively. Tissue‐specific transcripts show higher ORF dominance than ubiquitously expressed transcripts, and the majority of tissue‐specific transcripts are expressed in mature testes. These data suggest that the decrease in population size and the emergence of testes in eukaryotic organisms allowed for the evolution of potentially bifunctional RNAs.

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

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

Transcriptional profiling of β-2M-SPα-6+THY1+ spermatogonial stem cells in human spermatogenesis

Male infertility is responsible for approximately half of all cases of reproductive issues. Spermatogenesis originates in a small pool of spermatogonial stem cells (SSCs), which are of interest for therapy of infertility but remain not well defined in humans. Using multiparametric analysis of the side population (SP) phenotype and the α-6 integrin, THY1, and β-2 microglobulin cell markers, we identified a population of human primitive undifferentiated spermatogonia with the phenotype β-2 microglobulin (β-2M)⁻SPα-6⁺THY1⁺, which is highly enriched in stem cells. By analyzing the expression signatures of this SSC-enriched population along with other germinal progenitors, we established an exhaustive transcriptome of human spermatogenesis. Transcriptome profiling of the human β-2M⁻SPα-6⁺THY1⁺ population and comparison with the profile of mouse undifferentiated spermatogonia provide insights into the molecular networks and key transcriptional regulators regulating human SSCs, including the basic-helix-loop-helix (bHLH) transcriptional repressor HES1, which we show to be implicated in maintenance of SSCs in vitro.

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Human β-2M⁻SPα-6⁺THY1⁺ undifferentiated spermatogonia are enriched in stem cells

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Comparative transcriptomics analysis of human and murine spermatogonia

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HES1 is involved in the physiology of SSCs in vitro

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

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

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

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

Metabolic Control of Germline Formation and Differentiation in Mammals

BACKGROUND

The germ cell lineage involves dynamic epigenetic changes during its formation and differentiation that are completely different from those of the somatic cell lineage. Metabolites and metabolic pathways have been reported as key factors related to the regulation of epigenetics as cofactors and substrates. However, our knowledge about the metabolic characteristics of germ cells, especially during the fetal stage, and their transition during differentiation is quite limited due to the rarity of the cells. Nevertheless, recent developments in omics technologies have made it possible to extract comprehensive metabolomic features of germ cells.

SUMMARY

In this review, we present the latest researches on the metabolic properties of germ cells in 4 stages: primordial germ cell specification, fetal germ cell differentiation, spermatogenesis, and oogenesis. At every stage, extensive published data has been accumulated on energy metabolism, and it is possible to describe its changes during germ cell differentiation in detail. As pluripotent stem cells differentiate into germ cells, energy metabolism shifts from glycolysis to oxidative phosphorylation; however, in spermatogenesis, glycolytic pathways are also temporarily dominant in spermatogonial stem cells. Although the significance of metabolic pathways other than energy metabolism in germ cell differentiation is largely unknown, the relation of the pentose phosphate pathway and Ser-Gly-one-carbon metabolism with germ cell properties has been suggested at various stages. We further discuss the relationship between these characteristic metabolic pathways and epigenetic regulation during germ cell specification and differentiation. Finally, the relevance of dietary and supplemental interventions on germ cell function and epigenomic regulation is also discussed. Key Messages: Comprehensive elucidation of metabolic features and metabolism-epigenome crosstalk in germ cells is important to reveal how the characteristic metabolic pathways are involved in the germ cell regulation. The accumulation of such insights would lead to suggestions for optimal diets and supplements to maintain reproductive health through modulating metabolic and epigenetic status of germ cells.

OUP accepted manuscript

The adult mammalian heart is recalcitrant to regeneration after injury, in part due to the postmitotic nature of cardiomyocytes. Accumulating evidence suggests that cardiomyocyte proliferation in fetal or neonatal mammals and in regenerative non-mammalian models depends on a conducive metabolic state. Results from numerous studies in adult hearts indicate that conditions of relatively low fatty acid oxidation, low reactive oxygen species generation, and high glycolysis are required for induction of cardiomyocyte proliferation. Glycolysis appears particularly important because it provides branchpoint metabolites for several biosynthetic pathways that are essential for synthesis of nucleotides and nucleotide sugars, amino acids, and glycerophospholipids, all of which are required for daughter cell formation. In addition, the proliferative cardiomyocyte phenotype is supported in part by relatively low oxygen tensions and through the actions of critical transcription factors, coactivators, and signaling pathways that promote a more glycolytic and proliferative cardiomyocyte phenotype, such as hypoxia inducible factor 1α (Hif1α), Yes-associated protein (Yap), and ErbB2. Interventions that inhibit glycolysis or its integrated biosynthetic pathways almost universally impair cardiomyocyte proliferative capacity. Furthermore, metabolic enzymes that augment biosynthetic capacity such as phosphoenolpyruvate carboxykinase 2 and pyruvate kinase M2 appear to be amplifiers of cardiomyocyte proliferation. Collectively, these studies suggest that acquisition of a glycolytic and biosynthetic metabolic phenotype is a sine qua non of cardiomyocyte proliferation. Further knowledge of the regulatory mechanisms that control substrate partitioning to coordinate biosynthesis with energy provision could be leveraged to prompt or augment cardiomyocyte division and to promote cardiac repair.

Metabolic pathways regulating the development and non-genomic heritable traits of germ cells

Metabolism is an important cellular process necessary not only for producing energy and building blocks for cells, but also for regulating various cell functions, including intracellular signaling, epigenomic effects, and transcription. The regulatory roles of metabolism have been extensively studied in somatic cells, including stem cells and cancer cells, but data regarding germ cells are limited. Because germ cells produce individuals of subsequent generations, understanding the role of metabolism and its regulatory functions in germ cells is important. Although limited information concerning the specific role of metabolism in germ cells is available, recent advances in related research have revealed specific metabolic states of undifferentiated germ cells in embryos as well as in germ cells undergoing oogenesis and spermatogenesis. Studies have also elucidated the functions of some metabolic pathways associated with germ cell development and the non-genomic heritable machinery of germ cells. In this review, we summarized all the available knowledge on the characteristic metabolic pathways in germ cells, focusing on their regulatory functions, while discussing the issues that need to be addressed to enhance the understanding of germ cell metabolism.

The Proliferation of Pre-Pubertal Porcine Spermatogonia in Stirred Suspension Bioreactors Is Partially Mediated by the Wnt/β-Catenin Pathway

Male survivors of childhood cancer are at risk of suffering from infertility in adulthood because of gonadotoxic chemotherapies. For adult men, sperm collection and preservation are routine procedures prior to treatment; however, this is not an option for pre-pubertal children. From young boys, a small biopsy may be taken before chemotherapy, and spermatogonia may be propagated in vitro for future transplantation to restore fertility. A robust system that allows for scalable expansion of spermatogonia within a controlled environment is therefore required. Stirred suspension culture has been applied to different types of stem cells but has so far not been explored for spermatogonia. Here, we report that pre-pubertal porcine spermatogonia proliferate more in bioreactor suspension culture, compared with static culture. Interestingly, oxygen tension provides an avenue to modulate spermatogonia status, with culture under 10% oxygen retaining a more undifferentiated state and reducing proliferation in comparison with the conventional approach of culturing under ambient oxygen levels. Spermatogonia grown in bioreactors upregulate the Wnt/ β-catenin pathway, which, along with enhanced gas and nutrient exchange observed in bioreactor culture, may synergistically account for higher spermatogonia proliferation. Therefore, stirred suspension bioreactors provide novel platforms to culture spermatogonia in a scalable manner and with minimal handling.

The glucose-sensing transcription factor MLX balances metabolism and stress to suppress apoptosis and maintain spermatogenesis

Male germ cell (GC) production is a metabolically driven and apoptosis-prone process. Here, we show that the glucose-sensing transcription factor (TF) MAX-Like protein X (MLX) and its binding partner MondoA are both required for male fertility in the mouse, as well as survival of human tumor cells derived from the male germ line. Loss of Mlx results in altered metabolism as well as activation of multiple stress pathways and GC apoptosis in the testes. This is concomitant with dysregulation of the expression of male-specific GC transcripts and proteins. Our genomic and functional analyses identify loci directly bound by MLX involved in these processes, including metabolic targets, obligate components of male-specific GC development, and apoptotic effectors. These in vivo and in vitro studies implicate MLX and other members of the proximal MYC network, such as MNT, in regulation of metabolism and differentiation, as well as in suppression of intrinsic and extrinsic death signaling pathways in both spermatogenesis and male germ cell tumors (MGCTs).

Loss of Ubiquitin Carboxy-Terminal Hydrolase L1 Impairs Long-Term Differentiation Competence and Metabolic Regulation in Murine Spermatogonial Stem Cells

Spermatogonia are stem and progenitor cells responsible for maintaining mammalian spermatogenesis. Preserving the balance between self-renewal of spermatogonial stem cells (SSCs) and differentiation is critical for spermatogenesis and fertility. Ubiquitin carboxy-terminal hydrolase-L1 (UCH-L1) is highly expressed in spermatogonia of many species; however, its functional role has not been identified. Here, we aimed to understand the role of UCH-L1 in murine spermatogonia using a Uch-l1^−/− mouse model. We confirmed that UCH-L1 is expressed in undifferentiated and early-differentiating spermatogonia in the post-natal mammalian testis. The Uch-l1^−/− mice showed reduced testis weight and progressive degeneration of seminiferous tubules. Single-cell transcriptome analysis detected a dysregulated metabolic profile in spermatogonia of Uch-l1^−/− compared to wild-type mice. Furthermore, cultured Uch-l1^−/− SSCs had decreased capacity in regenerating full spermatogenesis after transplantation in vivo and accelerated oxidative phosphorylation (OXPHOS) during maintenance in vitro. Together, these results indicate that the absence of UCH-L1 impacts the maintenance of SSC homeostasis and metabolism and impacts the differentiation competence. Metabolic perturbations associated with loss of UCH-L1 appear to underlie a reduced capacity for supporting spermatogenesis and fertility with age. This work is one step further in understanding the complex regulatory circuits underlying SSC function.

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

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

NANOS2 is a sequence-specific mRNA-binding protein that promotes transcript degradation in spermatogonial stem cells

Spermatogonial stem cells (SSCs) sustain spermatogenesis and fertility throughout adult male life. The conserved RNA-binding protein NANOS2 is essential for the maintenance of SSCs, but its targets and mechanisms of function are not fully understood. Here, we generated a fully functional epitope-tagged Nanos2 mouse allele and applied the highly stringent cross-linking and analysis of cDNAs to define NANOS2 RNA occupancy in SSC lines. NANOS2 recognizes the AUKAAWU consensus motif, mostly found in the 3' untranslated region of defined messenger RNAs (mRNAs). We find that NANOS2 is a regulator of key signaling and metabolic pathways whose dosage or activity are known to be critical for SSC maintenance. NANOS2 interacts with components of CCR4-NOT deadenylase complex in SSC lines, and consequently, NANOS2 binding reduces the half-lives of target transcripts. In summary, NANOS2 contributes to SSC maintenance through the regulation of target mRNA stability and key self-renewal pathways.

Metabolism and Sex Differentiation in Animals from a Starvation Perspective

Animals determine their sex genetically (GSD: genetic sex determination) and/or environmentally (ESD: environmental sex determination). Medaka (Oryzias latipes) employ a XX/XY GSD system, however, they display female-to-male sex reversal in response to various environmental changes such as temperature, hypoxia, and green light. Interestingly, we found that 5 days of starvation during sex differentiation caused female-to-male sex reversal. In this situation, the metabolism of pantothenate and fatty acid synthesis plays an important role in sex reversal. Metabolism is associated with other biological factors such as germ cells, HPG axis, lipids, and epigenetics, and supplys substances and acts as signal transducers. In this review, we discuss the importance of metabolism during sex differentiation and how metabolism contributes to sex differentiation.

Spermatogonial stem cell transplantation into nonablated mouse recipient testes

Spermatogonial transplantation has been used as a standard assay for spermatogonial stem cells (SSCs). After transplantation into the seminiferous tubules, SSCs transmigrate through the blood-testis barrier (BTB) between Sertoli cells and settle in a niche. Unlike in the repair of other self-renewing systems, SSC transplantation is generally performed after complete destruction of endogenous spermatogenesis. Here, we examined the impacts of recipient conditioning on SSC homing. Germ cell ablation downregulated the expression of glial cell line-derived neurotrophic factor, which has been shown to attract SSCs to niches, implying that nonablated niches would attract SSCs more efficiently. As expected, SSCs colonized nonablated testes when transplanted into recipients with the same genetic background. Moreover, although spermatogenesis was arrested at the spermatocyte stage in Cldn11-deficient mice without a BTB, transplantation not only enhanced donor colonization but also restored normal spermatogenesis. The results show promise for the development of a new transplantation strategy to overcome male infertility.

Expression and functional analyses of ephrin type-A receptor 2 in mouse spermatogonial stem cells†

Spermatogonial stem cells (SSCs) undergo continuous self-renewal division in response to self-renewal factors. The present study identified ephrin type-A receptor 2 (EPHA2) on mouse SSCs and showed that supplementation of glial cell-derived neurotrophic factor (GDNF) and fibroblast growth factor 2 (FGF2), which are both SSC self-renewal factors, induced EPHA2 expression in cultured SSCs. Spermatogonial transplantation combined with magnetic-activated cell sorting or fluorescence-activated cell sorting also revealed that EPHA2 was expressed in SSCs. Additionally, ret proto-oncogene (RET) phosphorylation levels decreased following the knockdown (KD) of Epha2 expression via short hairpin ribonucleic acid (RNA). Although the present immunoprecipitation experiments did not reveal an association between RET with EPHA2, RET interacted with FGFR2. The Epha2 KD decreased the proliferation of cultured SSCs and inhibited the binding of cultured SSCs to laminin-coated plates. The Epha2 KD also significantly reduced the colonization of testis cells by spermatogonial transplantation. EPHA2 was also expressed in human GDNF family receptor alpha 1-positive spermatogonia. The present results indicate that SSCs express EPHA2 and suggest that it is a critical modifier of self-renewal signals in SSCs.

Integrated Proteomic and Metabolomic Analysis of the Testes Characterizes BDE-47-Induced Reproductive Toxicity in Mice

A representative congener of polybrominated diphenyl ethers in the environment, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), is associated with male reproductive toxicity, yet the underlying mechanisms remain largely unclear. In this study, mice were administered environmentally relevant concentrations of BDE-47 for six weeks. Histopathological observations showed that BDE-47 induced inflammatory reactions and damaged the testes. By conducting an integrated proteomic and metabolomic analysis coupled with a bioinformatic analysis using ingenuity pathway analysis (IPA) methods, we found that BDE-47 mainly affected the molecules involved in free radical scavenging, cell death and survival, neurological disease, and inflammatory response. IPA canonical pathways showed inflammatory and apoptosis pathways, including hepatic fibrosis/hepatic stellate cell activation, the GP6 signaling pathway, tight junction signaling, acute phase response signaling, LXR/RXR activation, unfolded protein response, and FXR/RXR activation, which are related to male reproductive toxicity. Key transcriptional regulator networks were activated via a focus on upstream regulator analysis. The expression of MYC and Clu as the core transcriptional factor and targeted protein, respectively, was verified. It is further proposed that MYC may contribute to the etiology of male reproductive toxicity. These findings will improve our understanding of the mechanisms responsible for BDE-47-induced male reproductive toxicity, which may promote the discovery of useful biomarkers indicative of BDE-47 exposure.

Enhanced Myc Expression in Silkworm Silk Gland Promotes DNA Replication and Silk Production

Simple Summary

Based on a transgenic approach, enhancing Myc expression in the silkworm posterior silk gland (PSG), which was driven by the promoter of the fibroin heavy chain (FibH) gene, was performed for investigating the biological functions of Myc in silk gland. Enhanced Myc expression elevated the cocoon size. This elevation might be resulted from the increasing of FibH expression and DNA content in the PSG cells by promoting the transcription of the genes that are involved in DNA replication.

Abstract

Silkworm is an economically important insect that synthetizes silk proteins for silk production in silk gland, and silk gland cells undergo endoreplication during larval period. Transcription factor Myc is essential for cell growth and proliferation. Although silkworm Myc gene has been identified previously, its biological functions in silkworm silk gland are still largely unknown. In this study, we examined whether enhanced Myc expression in silk gland could facilitate cell growth and silk production. Based on a transgenic approach, Myc was driven by the promoter of the fibroin heavy chain (FibH) gene to be successfully overexpressed in posterior silk gland. Enhanced Myc expression in the PSG elevated FibH expression by about 20% compared to the control, and also increased the weight and shell rate of the cocoon shell. Further investigation confirmed that Myc overexpression increased nucleus size and DNA content of the PSG cells by promoting the transcription of the genes involved in DNA replication. Therefore, we conclude that enhanced Myc expression promotes DNA replication and silk protein expression in endoreplicating silk gland cells, which subsequently raises silk yield.

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

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

Spermatogonial Gene Networks Selectively Couple to Glutathione and Pentose Phosphate Metabolism but Not Cysteine Biosynthesis

In adult males, spermatogonia maintain lifelong spermatozoa production for oocyte fertilization. To understand spermatogonial metabolism we compared gene profiles in rat spermatogonia to publicly available mouse, monkey, and human spermatogonial gene profiles. Interestingly, rat spermatogonia expressed metabolic control factors Foxa1, Foxa2, and Foxa3. Germline Foxa2 was enriched in Gfra1Hi and Gfra1Low undifferentiated A-single spermatogonia. Foxa2-bound loci in spermatogonial chromatin were overrepresented by conserved stemness genes (Dusp6, Gfra1, Etv5, Rest, Nanos2, Foxp1) that intersect bioinformatically with conserved glutathione/pentose phosphate metabolism genes (Tkt, Gss, Gc l c , Gc l m, Gpx1, Gpx4, Fth), marking elevated spermatogonial GSH:GSSG. Cystine-uptake and intracellular conversion to cysteine typically couple glutathione biosynthesis to pentose phosphate metabolism. Rat spermatogonia, curiously, displayed poor germline stem cell viability in cystine-containing media, and, like primate spermatogonia, exhibited reduced transsulfuration pathway markers. Exogenous cysteine, cysteine-like mercaptans, somatic testis cells, and ferroptosis inhibitors counteracted the cysteine-starvation-induced spermatogonial death and stimulated spermatogonial growth factor activity in vitro.

An interplay of NOX1-derived ROS and oxygen determines the spermatogonial stem cell self-renewal efficiency under hypoxia

Reactive oxygen species (ROS) produced by NADPH1 oxidase 1 (NOX1) are thought to drive spermatogonial stem cell (SSC) self-renewal through feed-forward production of ROS by the ROS-BCL6B-NOX1 pathway. Here we report the critical role of oxygen on ROS-induced self-renewal. Cultured SSCs proliferated poorly and lacked BCL6B expression under hypoxia despite increase in mitochondria-derived ROS. Due to lack of ROS amplification under hypoxia, NOX1-derived ROS were significantly reduced, and Nox1-deficient SSCs proliferated poorly under hypoxia but normally under normoxia. NOX1-derived ROS also influenced hypoxic response in vivo because Nox1-deficient undifferentiated spermatogonia showed significantly reduced expression of HIF1A, a master transcription factor for hypoxic response. Hypoxia-induced poor proliferation occurred despite activation of MYC and suppression of CDKN1A by HIF1A, whose deficiency exacerbated self-renewal efficiency. Impaired proliferation of Nox1- or Hif1a-deficient SSCs under hypoxia was rescued by Cdkn1a depletion. Consistent with these observations, Cdkn1a-deficient SSCs proliferated actively only under hypoxia but not under normoxia. On the other hand, chemical suppression of mitochondria-derived ROS or Top1mt mitochondria-specific topoisomerase deficiency did not influence SSC fate, suggesting that NOX1-derived ROS play a more important role in SSCs than mitochondria-derived ROS. These results underscore the importance of ROS origin and oxygen tension on SSC self-renewal.

CD2 is a surface marker for mouse and rat spermatogonial stem cells

The spermatogonial stem cell (SSC) population in testis is small, and the lack of SSC markers has severely handicapped research on these cells. During our attempt to identify genes involved in SSC aging, we found that CD2 is expressed in cultured SSCs. Flow cytometric analysis and spermatogonial transplantation experiments showed that CD2 is expressed in SSCs from mature adult mouse testes. Cultured SSCs transfected with short hairpin RNAs (shRNAs) against CD2 proliferated poorly and showed an increased frequency of apoptosis. Moreover, functional analysis of transfected cells revealed impairment of SSC activity. Fluorescence activated cell sorting and spermatogonial transplantation experiments showed that CD2 is expressed not only in mouse but also in rat SSCs. The results indicate that CD2 is a novel SSC surface marker conserved between mouse and rat SSCs.

Regulation of GDNF expression in Sertoli cells

Sertoli cells regulate male germ cell proliferation and differentiation and are a critical component of the spermatogonial stem cell (SSC) niche, where homeostasis is maintained by the interplay of several signaling pathways and growth factors. These factors are secreted by Sertoli cells located within the seminiferous epithelium, and by interstitial cells residing between the seminiferous tubules. Sertoli cells and peritubular myoid cells produce glial cell line-derived neurotrophic factor (GDNF), which binds to the RET/GFRA1 receptor complex at the surface of undifferentiated spermatogonia. GDNF is known for its ability to drive SSC self-renewal and proliferation of their direct cell progeny. Even though the effects of GDNF are well studied, our understanding of the regulation its expression is still limited. The purpose of this review is to discuss how GDNF expression in Sertoli cells is modulated within the niche, and how these mechanisms impact germ cell homeostasis.

A bioenergetic shift is required for spermatogonial differentiation

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

Metabolic Changes Accompanying Spermatogonial Stem Cell Differentiation

Male fertility is driven by spermatogonial stem cells (SSCs) that self-renew while also giving rise to differentiating spermatogonia. Spermatogonial transitions are accompanied by a shift in gene expression, however, whether equivalent changes in metabolism occur remains unexplored. In this review, we mined recently published scRNA-seq databases from mouse and human testes to compare expression profiles of spermatogonial subsets, focusing on metabolism. Comparisons revealed a conserved upregulation of genes involved in mitochondrial function, biogenesis, and oxidative phosphorylation in differentiating spermatogonia, while gene expression in SSCs reflected a glycolytic cell. Here, we also discuss the relationship between metabolism and the external microenvironment within which spermatogonia reside.

Proteomic and metabolomic analyses uncover sex-specific regulatory pathways in mouse fetal germline differentiation†

Regulatory mechanisms of germline differentiation have generally been explained via the function of signaling pathways, transcription factors, and epigenetic regulation; however, little is known regarding proteomic and metabolomic regulation and their contribution to germ cell development. Here, we conducted integrated proteomic and metabolomic analyses of fetal germ cells in mice on embryonic day (E)13.5 and E18.5 and demonstrate sex- and developmental stage-dependent changes in these processes. In male germ cells, RNA processing, translation, oxidative phosphorylation, and nucleotide synthesis are dominant in E13.5 and then decline until E18.5, which corresponds to the prolonged cell division and more enhanced hyper-transcription/translation in male primordial germ cells and their subsequent repression. Tricarboxylic acid cycle and one-carbon pathway are consistently upregulated in fetal male germ cells, suggesting their involvement in epigenetic changes preceding in males. Increased protein stability and oxidative phosphorylation during female germ cell differentiation suggests an upregulation of aerobic energy metabolism, which likely contributes to the proteostasis required for oocyte maturation in subsequent stages. The features elucidated in this study shed light on the unrevealed mechanisms of germ cell development.

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.

Histone Post-Translational Modifications and CircRNAs in Mouse and Human Spermatozoa: Potential Epigenetic Marks to Assess Human Sperm Quality

Spermatozoa (SPZ) are motile cells, characterized by a cargo of epigenetic information including histone post-translational modifications (histone PTMs) and non-coding RNAs. Specific histone PTMs are present in developing germ cells, with a key role in spermatogenic events such as self-renewal and commitment of spermatogonia (SPG), meiotic recombination, nuclear condensation in spermatids (SPT). Nuclear condensation is related to chromatin remodeling events and requires a massive histone-to-protamine exchange. After this event a small percentage of chromatin is condensed by histones and SPZ contain nucleoprotamines and a small fraction of nucleohistone chromatin carrying a landascape of histone PTMs. Circular RNAs (circRNAs), a new class of non-coding RNAs, characterized by a nonlinear back-spliced junction, able to play as microRNA (miRNA) sponges, protein scaffolds and translation templates, have been recently characterized in both human and mouse SPZ. Since their abundance in eukaryote tissues, it is challenging to deepen their biological function, especially in the field of reproduction. Here we review the critical role of histone PTMs in male germ cells and the profile of circRNAs in mouse and human SPZ. Furthermore, we discuss their suggested role as novel epigenetic biomarkers to assess sperm quality and improve artificial insemination procedure.

The expression characteristics of FBXW7 in human testis suggest its function is different from that in mice

F-box and WD domain protein 7 (FBXW7) is reported to bind with c-Myc in mouse spermatogonial stem cells, regulating self-renewal; however, the pattern and stage of expression of FBXW7 in human testes are unclear. In the present study, we examined the expression of human FBXW7 in adult testis, and analyzed fixed sections from adult testes and fetal testes to determine the cell type-specific expression pattern of FBXW7. The results showed that FBXW7α and FBXW7β genes are expressed in the testis; however, only FBXW7α protein could be detected. FBXW7 was not detected in human spermatogonial stem cells. Interestingly, FBXW7 was mainly expressed in the cell nuclei of later stage germ cells and differentiated somatic cells. We also observed high FBXW7 expression in human fetal germ cells, particularly in prespermatogonia. Our results raised the possibility that FBXW7 has different functions in humans and mice. The cell type-specific expression pattern of FBXW7 suggests that it performs regulatory functions during the late stage of human spermatogenesis instead of being involved in the self-renewal of spermatogonial stem cells.

Myotubularin related protein 7 is essential for the spermatogonial stem cell homeostasis via PI3K/AKT signaling

Myotubularin related protein 7 (MTMR7), a key member of the MTMR family, depicts phosphatase activity and is involved in myogenesis and tumor growth. We have previously identified MTMR7 in the proteomic profile of mouse spermatogonial stem cell (SSC) maturation and differentiation, implying that MTMR7 is associated with neonatal testicular development. In this study, to further explore the distribution and function of MTMR7 in mouse testis, we studied the effect of Mtmr7 knockdown on neonatal testicular development by testicular and SSC culture methods. Our results revealed that MTMR7 is exclusively located in early germ cells. Deficiency of MTMR7 by morpholino in neonatal testis caused excessive SSC proliferation, which was attributable to the aberrant PI3K/AKT signaling activation. Altogether, our study demonstrates that MTMR7 maintains SSC homeostasis by inhibiting PI3K/AKT signaling activation.

Aging of spermatogonial stem cells by Jnk-mediated glycolysis activation

Because spermatogonial stem cells (SSCs) are immortal by serial transplantation, SSC aging in intact testes is considered to be caused by a deteriorated microenvironment. Here, we report a cell-intrinsic mode of SSC aging by glycolysis activation. Using cultured SSCs, we found that aged SSCs proliferated more actively than young SSCs and showed enhanced glycolytic activity. Moreover, they remained euploid and exhibited stable androgenetic imprinting patterns with robust SSC activity despite having shortened telomeres. Aged SSCs showed increased Wnt7b expression, which was associated with decreased Polycomb complex 2 activity. Our results suggest that aberrant Wnt7b expression activated c-jun N-terminal kinase (JNK), which down-regulated mitochondria numbers by suppressing Ppargc1a Down-regulation of Ppargc1a probably decreased reactive oxygen species and enhanced glycolysis. Analyses of the Klotho-deficient aging mouse model and 2-y-old aged rats confirmed JNK hyperactivation and increased glycolysis. Therefore, not only microenvironment but also intrinsic activation of JNK-mediated glycolysis contributes to SSC aging.

Single cell RNA-sequencing identified Dec2 as a suppressive factor for spermatogonial differentiation by inhibiting Sohlh1 expression

Gonocyte-to-spermatogonia transition is a critical fate determination process to initiate sperm production throughout the lifecycle. However, the molecular dynamics of this process has not been fully elucidated mainly due to the asynchronized differentiation stages of neonatal germ cells. In this study, we employed single cell RNA sequencing analyses of P1.5–5.5 germ cells to clarify the temporal dynamics of gene expression during gonocyte-to-spermatogonia transition. The analyses identified transcriptional modules, one of which regulates spermatogonial gene network in neonatal germ cells. Among them, we identified Dec2, a bHLH-type transcription factor, as a transcriptional repressor for a spermatogonial differentiation factor Sohlh1. Deficiency of Dec2 in mice induces significant reduction of undifferentiated spermatogonia, and transplantation assay using Dec2-depleted cells also demonstrated the impaired efficiency of engraftment, suggesting its role in maintaining spermatogonial stem cells (SSCs). Collectively, this study revealed the intrinsic role of a new SSC factor Dec2, which protects germ cells from inadequate differentiation during neonatal testis development.

Sendai virus-mediated transduction of mammalian spermatogonial stem cells†

Spermatogonial stem cells (SSCs) provide the foundation of spermatogenesis. However, because of their small number and slow self-renewal, transfection of SSCs has met with limited success. Although several viral vectors can infect SSCs, genome integration and an inability to maintain long-term gene expression have hampered studies on SSCs. Here we report successful SSC infection by Sendai virus (SV), an RNA virus in the Paramyxoviridae. The SV efficiently transduced germline stem (GS) cells, cultured spermatogonia with enriched SSC activity, and maintained gene expression for at least 5 months. It also infected freshly isolated SSCs from adult testes. The transfected GS cells reinitiated spermatogenesis following spermatogonial transplantation into seminiferous tubules of infertile mice, suggesting that SV transfection does not interfere with spermatogenesis progression. On the other hand, microinjection of SV into the seminiferous tubules of immature mice transduced SSCs and Sertoli cells, but did not transduce Leydig or peritubular cells by interstitial virus injection. SV-infected hamster GS cells, and freshly isolated rabbit or monkey SSC-like cells were identified following xenogeneic spermatogonial transplantation, suggesting that SV transduces SSCs from several mammalian species. Thus, SV is a useful vector that can transduce both SSCs and Sertoli cells and overcome problems associated with other viral vectors.

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.