Spermatogonial stem cell functions in physiological and pathological conditions

DIS3 ribonuclease is essential for spermatogenesis and male fertility in mice

Spermatogonial stem cell (SSC) self-renewal and differentiation provide foundational support for long-term, steady-state spermatogenesis in mammals. Here, we have investigated the essential role of RNA exosome associated DIS3 ribonuclease in maintaining spermatogonial homeostasis and facilitating germ cell differentiation. We have established male germ-cell Dis3 conditional knockout (cKO) mice in which the first and subsequent waves of spermatogenesis are disrupted. This leads to a Sertoli cell-only phenotype and sterility in adult male mice. Bulk RNA-seq documents that Dis3 deficiency partially abolishes RNA degradation and causes significant increases in the abundance of transcripts. This also includes pervasively transcribed PROMoter uPstream Transcripts (PROMPTs), which accumulate robustly in Dis3 cKO testes. In addition, scRNA-seq analysis indicates that Dis3 deficiency in spermatogonia significantly disrupts RNA metabolism and gene expression, and impairs early germline cell development. Overall, we document that exosome-associated DIS3 ribonuclease plays crucial roles in maintaining early male germ cell lineage in mice.

SRSF2 is required for mRNA splicing during spermatogenesis

BACKGROUND

RNA splicing plays significant roles in fundamental biological activities. However, our knowledge about the roles of alternative splicing and underlying mechanisms during spermatogenesis is limited.

RESULTS

Here, we report that Serine/arginine-rich splicing factor 2 (SRSF2), also known as SC35, plays critical roles in alternative splicing and male reproduction. Male germ cell-specific deletion of Srsf2 by Stra8-Cre caused complete infertility and defective spermatogenesis. Further analyses revealed that deletion of Srsf2 disrupted differentiation and meiosis initiation of spermatogonia. Mechanistically, by combining RNA-seq data with LACE-seq data, we showed that SRSF2 regulatory networks play critical roles in several major events including reproductive development, spermatogenesis, meiotic cell cycle, synapse organization, DNA recombination, chromosome segregation, and male sex differentiation. Furthermore, SRSF2 affected expression and alternative splicing of Stra8, Stag3 and Atr encoding critical factors for spermatogenesis in a direct manner.

CONCLUSIONS

Taken together, our results demonstrate that SRSF2 has important functions in spermatogenesis and male fertility by regulating alternative splicing.

ARHGEF15 is expressed in undifferentiated spermatogonia but is not required for spermatogenesis in mice

Spermatogenesis is a continual process that relies on the activities of undifferentiated spermatogonia, which contain spermatogonial stem cells (SSCs) that serve as the basis of spermatogenesis. The gene expression pattern and molecular control of fate decisions of undifferentiated spermatogonia are not well understood. Rho guanine nucleotide exchange factor 15 (ARHGEF15, also known as EPHEXIN5) is a guanine nucleotide-exchange factor (GEF) that activates the Rho protein. Here, we reported that ARHGEF15 was expressed in undifferentiated spermatogonia and spermatocytes in mouse testes; however, its deletion did not affect spermatogenesis. Arhgef15-/- mice were fertile, and histological examination of the seminiferous tubules of Arhgef15-/- mice revealed complete spermatogenesis with the presence of all types of spermatogenic cells. Proliferation and differentiation of the undifferentiated spermatogonia were not impacted; however, further analysis showed that Arhgef15 deletion resulted in decreased expression of Nanos2, Lin28a and Ddx4. Together, these findings suggest that ARHGEF15 was specifically enriched in undifferentiated spermatogonia and regulated gene expression but dispensable for spermatogenesis in mice.

Throughout in vitro first spermatogenic wave: Next-generation sequencing gene expression patterns of fresh and cryopreserved prepubertal mice testicular tissue explants

INTRODUCTION

Suitable cryopreservation procedures of pre-pubertal testicular tissue associated with efficient culture conditions are crucial in the fields of fertility preservation and restoration. In vitro spermatogenesis remains a challenging technical procedure to undergo a complete spermatogenesis.The number of haploid cells and more specifically the spermatic yield produced in vitro in mice is still extremely low compared to age-matched in vivo controls and this procedure has never yet been successfully transferred to humans.

METHODS

To evaluate the impact of in vitro culture and freezing procedure, pre-pubertal testicular mice testes were directly cultured until day 4 (D4), D16 and D30 or cryopreserved by controlled slow freezing then cultured until D30. Testes composed of a panel of 6.5 dpp (days postpartum), 10.5 dpp, 22.5 dpp, and 36.5 dpp mice were used as in vivo controls. Testicular tissues were assessed by histological (HES) and immunofluorescence (stimulated by retinoic acid gene 8, STRA8) analyses. Moreover, a detailed transcriptome evaluation study has been carried out to study the gene expression patterns throughout the first in vitro spermatogenic wave.

RESULTS

Transcriptomic analyses reveal that cultured tissues expression profiles are almost comparable between D16 and D30; highlighting an abnormal kinetic throughout the second half of the first spermatogenesis during in vitro cultures. In addition, testicular explants have shown dysregulation of their transcriptomic profile compared to controls with genes related to inflammation response, insulin-like growth factor and genes involved in steroidogenesis.

DISCUSSION

The present work first shows that cryopreservation had very little impact on gene expression in testicular tissue, either directly after thawing or after 30 days in culture. Transcriptomic analysis of testis tissue samples is highly informative due to the large number of expressed genes and identified isoforms. This study provides a very valuable basis for future studies concerning in vitro spermatogenesis in mice.

Modeling mammalian spermatogonial differentiation and meiotic initiation in vitro

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

Hypobaric hypoxia exposure alters transcriptome in mouse testis and impairs spermatogenesis in offspring

Male fertility relies on continual and robust spermatogenesis. Environmental hypoxia adversely affects reproductive health in humans and animal studies provide compelling evidences that hypoxia impairs spermatogenesis in directly exposed individuals. However, a detail examination of hypoxia induced changes in testicular gene expression is still lacking and spermatogenesis in offspring of hypoxia exposed animals of awaits investigation. In this study, a hypobaric hypoxic chamber was used to simulate hypoxic conditions in mice and effects of hypoxia on spermatogenesis, fertility and testicular gene expression were evaluated. The results showed that hypoxia exposure reduced the number of undifferentiated spermatogonia but did not change the regenerative capacity of spermatogonial stem cells (SSCs) after transplantation. Hypoxia significantly increased the percent of abnormal sperm and these defects were recovered 2 months after returning to the normoxia. Transcriptome analysis of testicular tissues from control and hypoxia treated animals revealed that 766 genes were up-regulated and 965 genes were down-regulated. Surprisingly, expressions of genes that regulate epigenetic modifications were altered, indicating hypoxia-induced damage to spermatogenesis may be intergenerational. Indeed, animals that were sired by hypoxia exposed males exhibited impaired spermatogenesis. Together, these findings suggest that hypoxia exposure alters testicular gene expression and causes long-lasting damage to spermatogenesis.

Action and Interaction between Retinoic Acid Signaling and Blood–Testis Barrier Function in the Spermatogenesis Cycle

Spermatogenesis is a complex process occurring in mammalian testes, and constant sperm production depends on the exact regulation of the microenvironment in the testes. Many studies have indicated the crucial role of blood–testis barrier (BTB) junctions and retinoic acid (RA) signaling in the spermatogenesis process. The BTB consists of junctions between adjacent Sertoli cells, comprised mainly of tight junctions and gap junctions. In vitamin A-deficient mice, halted spermatogenesis could be rebooted by RA or vitamin A administration, indicating that RA is absolutely required for spermatogenesis. Accordingly, this manuscript will review and discuss how RA and the BTB regulate spermatogenesis and the interaction between RA signaling and BTB function.

The cell biology of fertilization: Gamete attachment and fusion

Fertilization is defined as the union of two gametes. During fertilization, sperm and egg fuse to form a diploid zygote to initiate prenatal development. In mammals, fertilization involves multiple ordered steps, including the acrosome reaction, zona pellucida penetration, sperm-egg attachment, and membrane fusion. Given the success of in vitro fertilization, one would think that the mechanisms of fertilization are understood; however, the precise details for many of the steps in fertilization remain a mystery. Recent studies using genetic knockout mouse models and structural biology are providing valuable insight into the molecular basis of sperm-egg attachment and fusion. Here, we review the cell biology of fertilization, specifically summarizing data from recent structural and functional studies that provide insights into the interactions involved in human gamete attachment and fusion.

Interaction between DMRT1 and PLZF protein regulates self-renewal and proliferation in male germline stem cells

Double sex and mab-3 related transcription factor 1 (DMRT1) encodes a double sex/mab-3 (DM) domain, which is the most conserved structure that involved in sex determination both in vertebrates and invertebrates. This study revealed important roles of DMRT1 in maintaining self-renewal of male germline stem cells (mGSCs). Our results showed that insufficient expression of DMRT1 in mice testes resulted in decreased number of spermatogonial cells and collapse of testicular niche in vivo. Self-renewal and proliferation of mGSCs were inhibited. Based on the bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (co-IP) assay, it was finally revealed that the interaction between DMRT1 and promyelocytic leukemia zinc finger (PLZF) protein was essential for maintaining self-renewal of mGSCs. Moreover, BTB domain of PLZF, DM and DMRT1 domain of DMRT1 were indispensable in mGSC, which were responsible for preserving the quantity of germ cells. Our research provided a new scientific basis for studying the mechanism of self-renewal and spermatogenesis in goat mGSCs.

Expression of paired box protein PAX7 in prepubertal boar testicular gonocytes

Spermatogenesis involves mitosis, meiosis, growth, and differentiation of spermatogonial stem cells (SSCs), which are capable of self-renewal and differentiation into spermatozoa. Markers of spermatogonia and other spermatogenic cells have been extensively studied in rodents, whereas physiological characteristics and stage-specific markers of germ cells remain largely unknown in large domestic animals. In rodents, paired box protein 7 (PAX7) is known to be a specific marker of a rare spermatogonial subpopulation in adult testes, while being expressed by a large proportion of neonatal testicular germ cells. However, the expression of PAX7 has not yet been investigated in domestic animals. The objective of this study was to characterize PAX7 expression during boar testis development and in in vitro cultured porcine SSCs (pSSCs). Notably, the expression of PAX7 was positively correlated with that of a known boar testis spermatogonial and gonocyte marker, protein gene product 9.5 (PGP9.5), in prepubertal (5-day-old) boar testes but was not observed during or following puberty. Furthermore, the early-stage spermatogonial markers GDNF family receptor alpha-1 (GFRα1) and Sal-like protein 4 (SALL4) were coexpressed in PAX7⁺ testicular cells from 5-day-old boars. PAX7 expression was also maintained in in vitro cultured undifferentiated porcine spermatogonia, with both PAX7 and PGP9.5 strongly expressed in pSSC colonies but not in feeder cells (testicular somatic cells). These data demonstrated that PAX7 expression only occurred in boar testes during prepuberty and was mainly restricted to very early-stage spermatogonial germ cells, such as gonocytes, which implies that PAX7 can be used as a boar gonocyte marker.

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.

The rapamycin analog Everolimus reversibly impairs male germ cell differentiation and fertility in the mouse†

Sirolimus, also known as rapamycin, and its closely related rapamycin analog (rapalog) Everolimus inhibit "mammalian target of rapamycin complex 1" (mTORC1), whose activity is required for spermatogenesis. Everolimus is Food and Drug Administration approved for treating human patients to slow growth of aggressive cancers and preventing organ transplant rejection. Here, we test the hypothesis that rapalog inhibition of mTORC1 activity has a negative, but reversible, impact upon spermatogenesis. Juvenile (P20) or adult (P>60) mice received daily injections of sirolimus or Everolimus for 30 days, and tissues were examined at completion of treatment or following a recovery period. Rapalog treatments reduced body and testis weights, testis weight/body weight ratios, cauda epididymal sperm counts, and seminal vesicle weights in animals of both ages. Following rapalog treatment, numbers of differentiating spermatogonia were reduced, with concomitant increases in the ratio of undifferentiated spermatogonia to total number of remaining germ cells. To determine if even low doses of Everolimus can inhibit spermatogenesis, an additional group of adult mice received a dose of Everolimus ∼6-fold lower than a human clinical dose used to treat cancer. In these animals, only testis weights, testis weight/body weight ratios, and tubule diameters were reduced. Return to control values following a recovery period was variable for each of the measured parameters and was duration and dose dependent. Together, these data indicate rapalogs exerted a dose-dependent restriction on overall growth of juvenile and adult mice and negative impact upon spermatogenesis that were largely reversed; following treatment cessation, males from all treatment groups were able to sire offspring.

MFN2 Plays a Distinct Role from MFN1 in Regulating Spermatogonial Differentiation

Although mitochondrial morphology is well-known for its role in cellular homeostasis, there is surprisingly little knowledge on whether mitochondrial remodeling is required for postnatal germ cell development. In this study, we investigated the functions of MFN1 and MFN2, two GTPases in mitochondrial fusion, during early spermatogenesis. We observed increased MFN expressions along with increased mitochondrial and endoplasmic reticulum (ER) activities during spermatogonial differentiation. Deletion of either Mfn led to DNA oxidation and apoptosis specifically in differentiating spermatogonia and spermatocytes, which in turn caused male infertility. We further found MFN2 regulated spermatogenesis by modulating both mitochondrial and ER functions, a mechanism distinct from that of MFN1. Defects of germ cell development in MFN2 mutants were corrected by MFN2 at either mitochondria or ER but not by MFN1. Our study thus reveals an essential requirement of MFN-mediated mitochondrial and ER coordination in spermatogenesis, providing critical insights into mitochondrial determinants of male fertility.

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Mitochondrial and ER activities increase during spermatogonial differentiation

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Mfn deletions specifically impair differentiating spermatogonia and spermatocytes

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MFN2 impacts male fertility via regulating mitochondrial fusion and ER homeostasis

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MFN2 functions non-redundantly from MFN1 in spermatogenesis

Sources of all-trans retinal oxidation independent of the aldehyde dehydrogenase 1A isozymes exist in the postnatal testis†

Despite the essential role of the active metabolite of vitamin A, all-trans retinoic acid (atRA) in spermatogenesis, the enzymes, and cellular populations responsible for its synthesis in the postnatal testis remain largely unknown. The aldehyde dehydrogenase 1A (ALDH1A) family of enzymes residing within Sertoli cells is responsible for the synthesis of atRA, driving the first round of spermatogenesis. Those studies also revealed that the atRA required to drive subsequent rounds of spermatogenesis is possibly derived from the ALDH1A enzymes residing within the meiotic and post-meiotic germ cells. Three ALDH1A isozymes (ALDH1A1, ALDH1A2, and ALDH1A3) are present in the testis. Although, ALDH1A1 is expressed in adult Sertoli cells and is suggested to contribute to the atRA required for the pre-meiotic transitions, ALDH1A2 is proposed to be the essential isomer involved in testicular atRA biosynthesis. In this report, we first examine the requirement for ALDH1A2 via the generation and analysis of a conditional Aldh1a2 germ cell knockout and a tamoxifen-induced Aldh1a2 knockout model. We then utilized the pan-ALDH1A inhibitor (WIN 18446) to test the collective contribution of the ALDH1A enzymes to atRA biosynthesis following the first round of spermatogenesis. Collectively, our data provide the first in vivo evidence demonstrating that animals severely deficient in ALDH1A2 postnatally proceed normally through spermatogenesis. Our studies with a pan-ALDH1A inhibitor (WIN 18446) also suggest that an alternative source of atRA biosynthesis independent of the ALDH1A enzymes becomes available to maintain atRA levels for several spermatogenic cycles following an initial atRA injection.

Connective tissue growth factor mediates mouse spermatogonial migration associated with differentiation

Spermatogonia migrate to the microenvironment during the establishment from gonocytes and leave it when they differentiate. However, the mechanisms underlying the regulation of spermatogonial differentiation-associated migration remain mostly unknown. In this study, we show that spermatogonial differentiation induced by retinoic acid (RA) was accompanied with increased migration ability and elevated expression of connective tissue growth factor (CTGF), a member of the CCN family. CTGF was mainly expressed in the testicular somatic cells and committed spermatogonial progenitors. Recombinant CTGF (rCTGF) promoted the spermatogonial migration and silencing of endogenous CTGF suppressed the migration of homogenous spermatogonial cell lines. Moreover, depletion of CTGF by neutralizing antibody inhibited the elevated migration ability induced by RA, suggesting both the paracrine and autocrine roles of CTGF in spermatogonial migration associated with differentiation. Finally, CTGF interacted with β1-integrin and regulated its level in spermatogonial cell lines. Together, our study provides novel insights into the regulation of spermatogonial migration by CTGF, which may shed light on the diagnosis and treatment of male infertility.

Ring 1 and YY1 Binding Protein Expressed in Murine Spermatocytes but Dispensable for Spermatogenesis

Spermatogenesis is a complex cellular-differentiation process that relies on the precise regulation of gene expression in spermatogonia, meiotic, and postmeiotic germ cells. The Ring 1 and YY1 binding protein (Rybp) is a member of the mammalian polycomb-group (PcG) protein family that plays multifunctional roles in development. Previous findings indicate that Rybp may function as an important regulator of meiosis. However, its expression in the testes and function in spermatogenesis have not been examined. In this study, we investigated Rybp expression in postnatal mouse testes using qRT-PCR and immunohistochemistry. We also examined the function of Rybp in spermatogenesis by using a conditional-knockout approach. Results showed that the relative expression of Rybp mRNA was significantly upregulated in the testes of postnatal day (PD) 6 mice. Immunofluorescent staining revealed that Rybp was enriched in the spermatocytes. Surprisingly, a conditional deletion of Rybp in fetal germ cells did not affect the fertility or normal development of spermatogenic cells. Further analysis revealed that Rybp deletion resulted in a decreased expression of meiosis-related genes, but that meiosis progression was normal. Together, these findings suggest that Rybp expression was enriched in spermatocytes, but that it was not required for spermatogenesis.

The expression of histone methyltransferases and distribution of selected histone methylations in testes of yak and cattle-yak hybrid

Interspecies hybridization exists widely in nature and plays an important role in animal evolution and adaptation. It is commonly recognized that male offspring of interspecies hybrid are often sterile, which presents a crucial way of reproductive isolation. Currently, the mechanisms underlying interspecies hybrid male sterility are not well understood. Cattle-yak, progeny of yak (Bos grunniens) and cattle (Bos taurus) cross, is a unique animal model for investigating hybrid male sterility. Because histone modifications are vital for spermatogenesis, herein, we examined expressions of histone methyltransferases (HMTs) and distributions of histone methylations in the yak and cattle-yak testis. Histological examination of seminiferous tubules revealed that gonocytes and spermatocytes were established normally, however, spermatogenesis was arrested at the meiosis phase began at 10 months after birth in the hybrids. SUV420H1 was the only HMT examined showing a significant enrichment in cattle-yak testes at 3 months. Relative expressions of MLL5, SETDB1 and SUV420H1 were increased while SETDB2 and EZH2 were decreased in cattle-yak testes at 10 months. Relative concentrations of MLL5 and SUV420H1 were again increased while EHMT2 and PRDM9 expressions were decreased at 24 months. Immunofluorescent detection of selected histone methylations in cross-sections of testicular tissues or meiotic chromosomes demonstrated that depletion of H3K4me3 and significant enrichment of H3K27me3 and H4K20me3 were observed in Sertoli cells of cattle-yak. Levels and localizations of H3K4me3, H3K9me1, H3K9me3 and H4K20me3 were strikingly different in meiotic chromosomes of cattle-yak spermatocytes. These results highlighted the potential roles of histone methylations in spermatogenic failure and hybrid male sterility.

CDK2 kinase activity is a regulator of male germ cell fate

The ability of men to remain fertile throughout their lives depends upon establishment of a spermatogonial stem cell (SSC) pool from gonocyte progenitors, and thereafter balancing SSC renewal versus terminal differentiation. Here, we report that precise regulation of the cell cycle is crucial for this balance. Whereas cyclin-dependent kinase 2 (Cdk2) is not necessary for mouse viability or gametogenesis stages prior to meiotic prophase I, mice bearing a deregulated allele (Cdk2Y15S ) are severely deficient in spermatogonial differentiation. This allele disrupts an inhibitory phosphorylation site (Tyr15) for the kinase WEE1. Remarkably, Cdk2Y15S/Y15S mice possess abnormal clusters of mitotically active SSC-like cells, but these are eventually removed by apoptosis after failing to differentiate properly. Analyses of lineage markers, germ cell proliferation over time, and single cell RNA-seq data revealed delayed and defective differentiation of gonocytes into SSCs. Biochemical and genetic data demonstrated that Cdk2Y15S is a gain-of-function allele causing elevated kinase activity, which underlies these differentiation defects. Our results demonstrate that precise regulation of CDK2 kinase activity in male germ cell development is crucial for the gonocyte-to-spermatogonia transition and long-term spermatogenic homeostasis.

Gene expression dynamics during the gonocyte to spermatogonia transition and spermatogenesis in the domestic yak

Background

Spermatogenesis is a cellular differentiation process that includes three major events: mitosis of spermatogonia, meiosis of spermatocytes and spermiogenesis. Steady-state spermatogenesis relies on functions of spermatogonial stem cells (SSCs). Establishing and maintaining a foundational SSC pool is essential for continued spermatogenesis in mammals. Currently, our knowledge about SSC and spermatogenesis is severely limited in domestic animals.

Results

In the present study, we examined transcriptomes of testes from domestic yaks at four different stages (3, 5, 8 and 24 months of age) and attempted to identify genes that are associated with key developmental events of spermatogenesis. Histological analyses showed that the most advanced germ cells within seminiferous tubules of testes from 3, 5, 8 and 24 months old yaks were gonocytes, spermatogonia, spermatocytes and elongated spermatids, respectively. RNA-sequencing (RNA-seq) analyses revealed that 11904, 4381 and 2459 genes were differentially expressed during the gonocyte to spermatogonia transition, the mitosis to meiosis transition and the meiosis to post-meiosis transition. Further analyses identified a list of candidate genes than may regulate these important cellular processes. CXCR4, a previously identified SSC niche factor in mouse, was one of the up-regulated genes in the 5 months old yak testis. Results of immunohistochemical staining confirmed that CXCR4 was exclusively expressed in gonocytes and a subpopulation of spermatogonia in the yak testis.

Conclusions

Together, these findings demonstrated histological changes of postnatal testis development in the domestic yak. During development of spermatogonial lineage, meiotic and haploid germ cells are supported by dynamic transcriptional regulation of gene expression. Our transcriptomic analyses provided a list of candidate genes that potentially play crucial roles in directing the establishment of SSC and spermatogenesis in yak.

Electronic supplementary material

The online version of this article (10.1186/s40104-019-0360-7) contains supplementary material, which is available to authorized users.

Developmental kinetics and transcriptome dynamics of stem cell specification in the spermatogenic lineage

Continuity, robustness, and regeneration of cell lineages relies on stem cell pools that are established during development. For the mammalian spermatogenic lineage, a foundational spermatogonial stem cell (SSC) pool arises from prospermatogonial precursors during neonatal life via mechanisms that remain undefined. Here, we mapped the kinetics of this process in vivo using a multi-transgenic reporter mouse model, in silico with single-cell RNA sequencing, and functionally with transplantation analyses to define the SSC trajectory from prospermatogonia. Outcomes revealed that a heterogeneous prospermatogonial population undergoes dynamic changes during late fetal and neonatal development. Differential transcriptome profiles predicted divergent developmental trajectories from fetal prospermatogonia to descendant postnatal spermatogonia. Furthermore, transplantation analyses demonstrated that a defined subset of fetal prospermatogonia is fated to function as SSCs. Collectively, these findings suggest that SSC fate is preprogrammed within a subset of fetal prospermatogonia prior to building of the foundational pool during early neonatal development.

In neonatal testes, prospermatogonia generate both spermatogonia for the first wave of spermatogenesis and spermatogonial stem cells (SSCs) for maintenance of spermatogenesis in males. Here the authors characterize the development of mouse SSCs from prospermatogonia using single-cell RNA-seq and transplantation assays.

Pluripotent Very Small Embryonic-Like Stem Cells in Adult Testes - An Alternate Premise to Explain Testicular Germ Cell Tumors

Developmental exposure to endocrine disruptors has resulted in the increased incidence of infertility and testicular germ cell tumors (T2GCT) in young men residing in developed countries. Unlike T1GCT (infants and young children) and T3GCT (aged men), T2GCT arise from CIS/GCNIS that develops from pre-CIS. Pre-CIS represents undifferentiated, growth-arrested gonocytes that persist in fetal testes due to endocrine disruption. However, whether pre-CIS truly exist, do CIS develop into T2GCT, why no CIS in T1GCT/T3GCT, why germ cell tumors (GCT) also occur along midline at extra-gonadal sites, why T1GCT show partial erasure and T2GCT show complete erasure of genomic imprints are open questions that are awaiting answers. We propose that rather than pre-CIS, pluripotent, very small embryonic-like stem cells (VSELs) get affected by exposure to endocrine disruption. Since VSELs are developmentally equivalent to primordial germ cells (PGCs), T2GCT cells show complete erasure of genomic imprints and CIS represents growth-arrested clonally expanding stem/progenitor cells. PGCs/VSELs migrate along the midline to various organs and this explains why GCT occur along the midline, T1GCT show partial erasure of imprints as they develop from migrating PGCs. T3GCT possibly reflects effects of aging due to compromised differentiation and expansion of pre-meiotic spermatocytes. Absent spermatogenesis in pre-pubertal and aged testes explains absence of CIS in T1GCT and T3GCT. Endocrine disruptors possibly alter epigenetic state of VSELs and thus rather than maintaining normal tissue homeostasis, VSELs undergo increased proliferation and compromised differentiation resulting in reduced sperm count, infertility and TGCT. This newly emerging understanding offers alternate premise to explain TGCT and warrants further exploration.

Roles of microRNAs in mammalian reproduction: from the commitment of germ cells to peri-implantation embryos

MicroRNAs (miRNAs) are active regulators of numerous biological and physiological processes including most of the events of mammalian reproduction. Understanding the biological functions of miRNAs in the context of mammalian reproduction will allow a better and comparative understanding of fertility and sterility in male and female mammals. Herein, we summarize recent progress in miRNA‐mediated regulation of mammalian reproduction and highlight the significance of miRNAs in different aspects of mammalian reproduction including the biogenesis of germ cells, the functionality of reproductive organs, and the development of early embryos. Furthermore, we focus on the gene expression regulatory feedback loops involving hormones and miRNA expression to increase our understanding of germ cell commitment and the functioning of reproductive organs. Finally, we discuss the influence of miRNAs on male and female reproductive failure, and provide perspectives for future studies on this topic.

Dynamic cytoplasmic projections connect mammalian spermatogonia in vivo

Throughout the male reproductive lifespan, spermatogonial stem cells (SSCs) produce committed progenitors that proliferate and then remain physically connected in growing clones via short cylindrical intercellular bridges (ICBs). These ICBs, which enlarge in meiotic spermatocytes, have been demonstrated to provide a conduit for postmeiotic haploid spermatids to share sex chromosome-derived gene products. In addition to ICBs, spermatogonia exhibit multiple thin cytoplasmic projections. Here, we have explored the nature of these projections in mice and find that they are dynamic, span considerable distances from their cell body (≥25 μm), either terminate or physically connect multiple adjacent spermatogonia, and allow for sharing of macromolecules. Our results extend the current model that subsets of spermatogonia exist as isolated cells or clones, and support a model in which spermatogonia of similar developmental fates are functionally connected through a shared dynamic cytoplasm mediated by thin cytoplasmic projections.

Cell-autonomous requirement for mammalian target of rapamycin (Mtor) in spermatogonial proliferation and differentiation in the mouse†

Spermatogonial stem cells must balance self-renewal with production of transit-amplifying progenitors that differentiate in response to retinoic acid (RA) before entering meiosis. This self-renewal vs. differentiation fate decision is critical for maintaining tissue homeostasis, as imbalances cause defects that can lead to human testicular cancer or infertility. Little is currently known about the program of differentiation initiated by RA, and the pathways and proteins involved are poorly defined. We recently found that RA stimulation of the Phosphatidylinositol 3-kinase (PI3K)/AKT/Mammalian target of rapamycin (mTOR) kinase signaling pathway is required for differentiation, and that short-term inhibition of mTOR complex 1 (mTORC1) by rapamycin blocked spermatogonial differentiation in vivo and prevented RA-induced translational activation. Since this phenotype resulted from global inhibition of mTORC1, we created conditional germ cell knockout mice to investigate the germ cell-autonomous role of MTOR in spermatogonial differentiation. MTOR germ cell KO mice were viable and healthy, but testes from neonatal (postnatal day (P)8), juvenile (P18), and adult (P > 60) KO mice were smaller than littermate controls, and no sperm were produced in adult testes. Histological and immunostaining analyses revealed that spermatogonial differentiation was blocked, and no spermatocytes were formed at any of the ages examined. Although spermatogonial proliferation was reduced in the neonatal testis, it was blocked altogether in the juvenile and adult testis. Importantly, a small population of self-renewing undifferentiated spermatogonia remained in adult testes. Taken together, these results reveal that MTOR is dispensable for the maintenance of undifferentiated spermatogonia, but is cell autonomously required for their proliferation and differentiation.

Advanced immunostaining approaches to study early male germ cell development

Mammalian male germ cell development takes place in the testis under the influence of a variety of somatic cells and an incompletely defined paracrine and endocrine influences. Since it is not recapitulated well in vitro, researchers studying spermatogenesis often manipulate the germline by creating transgenic or knockout mice or by administering pharmaceutical agonists/antagonists or inhibitors. The effects of these types of manipulations on germline development can often be determined following microscopic imaging, both of stained and immunostained testis sections. Here, we describe approaches for microscopic analysis of the developing male germline, provide detailed protocols for a variety of immunostaining approaches, and discuss transgenic fluorescent reporter lines for studying the early stages of spermatogenesis.

In Vitro Modeling of Human Germ Cell Development Using Pluripotent Stem Cells

Due to differences across species, the mechanisms of cell fate decisions determined in mice cannot be readily extrapolated to humans. In this study, we developed a feeder- and xeno-free culture protocol that efficiently induced human pluripotent stem cells (iPSCs) into PLZF+/GPR125+/CD90+ spermatogonium-like cells (SLCs). These SLCs were enriched with key genes in germ cell development such as MVH, DAZL, GFRα1, NANOS3, and DMRT1. In addition, a small fraction of SLCs went through meiosis in vitro to develop into haploid cells. We further demonstrated that this chemically defined induction protocol faithfully recapitulated the features of compromised germ cell development of PSCs with NANOS3 deficiency or iPSC lines established from patients with non-obstructive azoospermia. Taken together, we established a powerful experimental platform to investigate human germ cell development and pathology related to male infertility.

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SLCs and haploid cells are formed from PSCs via a feeder- and xeno-free condition

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In-vitro-developed SLCs are enriched with germ cell-specific genes

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NANOS3 deficiency compromises SLC derivation from PSCs

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iPSCs derived from NOA patients display disturbed germ cell development

Germ cell-intrinsic requirement for the homeodomain transcription factor PKnox1/Prep1 in adult spermatogenesis

PKnox1 (also known as Prep1) belongs to the TALE family of homeodomain transcription factors that are critical for regulating growth and differentiation during embryonic and postnatal development in vertebrates. We demonstrate here that PKnox1 is required for adult spermatogenesis in a germ cell-intrinsic manner. Tamoxifen-mediated PKnox1 loss in the adult testes, as well as its germ cell-specific ablation, causes testis hypotrophy with germ cell apoptosis and, as a consequence, compromised spermatogenesis. In PKnox1-deficient testes, spermatogenesis was arrested at the c-Kit⁺ spermatogonia stage, with a complete loss of the meiotic spermatocytes, and was accompanied by compromised differentiation of the c-Kit⁺ spermatogonia. Taken together, these results indicate that PKnox1 is a critical regulator of maintenance and subsequent differentiation of the c-Kit⁺ stage of spermatogonia in the adult testes.

Germ cell tumors: Insights from the Drosophila ovary and the mouse testis

Ovarian and testicular germ cell tumors of young adults are thought to arise from defects in germ cell development, but the molecular mechanisms underlying malignant transformation are poorly understood. In this review, we focus on the biology of germ cell tumor formation in the Drosophila ovary and the mouse testis, for which evidence supports common underlying mechanisms, such as blocking initiation into the differentiation pathway, impaired lineage progression, and sexual identity instability. We then discuss how these concepts inform our understanding of the disease in humans. Mol. Reprod. Dev. 84: 200-211, 2017. © 2017 Wiley Periodicals, Inc.

The roles of microRNAs in regulation of mammalian spermatogenesis

Mammalian spermatogenesis contains three continuous and organized processes, by which spermatogonia undergo mitosis and differentiate to spermatocytes, follow on meiosis to form haploid spermatids and ultimately transform into spermatozoa. These processes require an accurately, spatially and temporally regulated gene expression patterns. The microRNAs are a novel class of post-transcriptional regulators. Cumulating evidences have demonstrated that microRNAs are expressed in a cell-specific or stage-specific manner during spermatogenesis. In this review, we focus on the roles of microRNAs in spermatogenesis. We highlight that N6-methyladenosine (m6A) is involved in the biogenesis of microRNAs and miRNA regulates the m6A modification on mRNA, and that specific miRNAs have been exploited as potential biomarkers for the male factor infertility, which will provide insightful understanding of microRNA roles in spermatogenesis.

Visualization and Lineage Tracing of Pax7+ Spermatogonial Stem Cells in the Mouse

The precise identity of spermatogonial stem cells-the germline stem cell of the adult testis-remains a controversial topic. Technical limitations have included the lack of specific markers and methods for lineage tracing of A_single spermatogonia and their subsets. Immunolocalization of proteins in tissue sections has been a standard tool for the in situ identification and visualization of rare cellular subsets. However, these studies are limited by the need for faithful and reliable protein markers to define these cell types, as well as the availability of specific antibodies to these markers. Here we describe the use of a monoclonal antibody to Pax7 as a means to detect spermatogonial stem cells (SSCs) both in tissue sections and in intact seminiferous tubules. Furthermore, we describe methods for lineage tracing as an alternative method to visualize Pax7⁺ spermatogonial stem cells and their progeny.

TSPAN8 Expression Distinguishes Spermatogonial Stem Cells in the Prepubertal Mouse Testis

Precise separation of spermatogonial stem cells (SSCs) from progenitor spermatogonia that lack stem cell activity and are committed to differentiation remains a challenge. To distinguish between these spermatogonial subtypes, we identified genes that exhibited bimodal mRNA levels at the single-cell level among undifferentiated spermatogonia from Postnatal Day 6 mouse testes, including Tspan8, Epha2, and Pvr, each of which encode cell surface proteins useful for cell selection. Transplantation studies provided definitive evidence that a TSPAN8-high subpopulation is enriched for SSCs. RNA-seq analyses identified genes differentially expressed between TSPAN8-high and -low subpopulations that clustered into multiple biological pathways potentially involved in SSC renewal or differentiation, respectively. Methyl-seq analysis identified hypomethylated domains in the promoters of these genes in both subpopulations that colocalized with peaks of histone modifications defined by ChIP-seq analysis. Taken together, these results demonstrate functional heterogeneity among mouse undifferentiated spermatogonia and point to key biological characteristics that distinguish SSCs from progenitor spermatogonia.

Rhox13 is required for a quantitatively normal first wave of spermatogenesis in mice

We previously described a novel germ cell-specific X-linked reproductive homeobox gene (Rhox13) that is upregulated at the level of translation in response to retinoic acid (RA) in differentiating spermatogonia and preleptotene spermatocytes. We hypothesize that RHOX13 plays an essential role in male germ cell differentiation, and have tested this by creating a Rhox13 gene knockout (KO) mouse. Rhox13 KO mice are born in expected Mendelian ratios, and adults have slightly reduced testis weights, yet a full complement of spermatogenic cell types. Young KO mice (at ~7-8 weeks of age) have a ≈50% reduction in epididymal sperm counts, but numbers increased to WT levels as the mice reach ~17 weeks of age. Histological analysis of testes from juvenile KO mice reveals a number of defects during the first wave of spermatogenesis. These include increased apoptosis, delayed appearance of round spermatids and disruption of the precise stage-specific association of germ cells within the seminiferous tubules. Breeding studies reveal that both young and aged KO males produce normal-sized litters. Taken together, our results indicate that RHOX13 is not essential for mouse fertility in a controlled laboratory setting, but that it is required for optimal development of differentiating germ cells and progression of the first wave of spermatogenesis.

Transcription Factor GLIS3: A New and Critical Regulator of Postnatal Stages of Mouse Spermatogenesis

In this study, we identify a novel and essential role for the Krüppel-like zinc finger transcription factor GLI-similar 3 (GLIS3) in the regulation of postnatal spermatogenesis. We show that GLIS3 is expressed in gonocytes, spermatogonial stem cells (SSCs) and spermatogonial progenitors (SPCs), but not in differentiated spermatogonia and later stages of spermatogenesis or in somatic cells. Spermatogenesis is greatly impaired in GLIS3 knockout mice. Loss of GLIS3 function causes a moderate reduction in the number of gonocytes, but greatly affects the generation of SSCs/SPCs, and as a consequence the development of spermatocytes. Gene expression profiling demonstrated that the expression of genes associated with undifferentiated spermatogonia was dramatically decreased in GLIS3-deficient mice and that the cytoplasmic-to-nuclear translocation of FOXO1, which marks the gonocyte-to-SSC transition and is necessary for SSC self-renewal, is inhibited. These observations suggest that GLIS3 promotes the gonocyte-to-SSC transition and is a critical regulator of the dynamics of early postnatal spermatogenesis. Stem Cells 2016;34:2772-2783.

Germline Stem Cell Competition, Mutation Hot Spots, Genetic Disorders, and Older Fathers

Some de novo human mutations arise at frequencies far exceeding the genome average mutation rate. Examples include the common mutations at one or a few sites in the genes that cause achondroplasia, Apert syndrome, multiple endocrine neoplasia type 2B, and Noonan syndrome. These mutations are recurrent, provide a gain of function, are paternally derived, and are more likely to be transmitted as the father ages. Recent experiments have tested whether the high mutation frequencies are due to an elevated mutation rate per cell division, as expected, or to an advantage of the mutant spermatogonial stem cells over wild-type stem cells. The evidence, which includes the surprising discovery of testis mutation clusters, rules out the former model but not the latter. We propose how the mutations might alter spermatogonial stem cell function and discuss how germline selection contributes to the paternal age effect, the human mutational load, and adaptive evolution.

Targeting the Gdnf Gene in peritubular myoid cells disrupts undifferentiated spermatogonial cell development

Spermatogonial stem cells (SSCs) are a subpopulation of undifferentiated spermatogonia located in a niche at the base of the seminiferous epithelium delimited by Sertoli cells and peritubular myoid (PM) cells. SSCs self-renew or differentiate into spermatogonia that proliferate to give rise to spermatocytes and maintain spermatogenesis. Glial cell line-derived neurotrophic factor (GDNF) is essential for this process. Sertoli cells produce GDNF and other growth factors and are commonly thought to be responsible for regulating SSC development, but limited attention has been paid to the role of PM cells in this process. A conditional knockout (cKO) of the androgen receptor gene in PM cells resulted in male infertility. We found that testosterone (T) induces GDNF expression in mouse PM cells in vitro and neonatal spermatogonia (including SSCs) co-cultured with T-treated PM cells were able to colonize testes of germ cell-depleted mice after transplantation. This strongly suggested that T-regulated production of GDNF by PM cells is required for spermatogonial development, but PM cells might produce other factors in vitro that are responsible. In this study, we tested the hypothesis that production of GDNF by PM cells is essential for spermatogonial development by generating mice with a cKO of the Gdnf gene in PM cells. The cKO males sired up to two litters but became infertile due to collapse of spermatogenesis and loss of undifferentiated spermatogonia. These studies show for the first time, to our knowledge, that the production of GDNF by PM cells is essential for undifferentiated spermatogonial cell development in vivo.

Chemokine (C-X-C) Ligand 12 Facilitates Trafficking of Donor Spermatogonial Stem Cells

The chemokine (C-X-C) receptor type 4 (CXCR4) is an early marker of primordial germ cells (PGCs) essential for their migration and colonization of the gonads. In spermatogonial stem cells (SSCs), the expression of CXCR4 is promoted by the self-renewal factor, glial cell line-derived neurotrophic factor (GDNF). Here, we demonstrate an important role of CXCR4 during donor mouse SSCs reoccupation of the endogenous niche in recipient testis. Silencing of CXCR4 expression in mouse SSCs dramatically reduced the number of donor stem cell-derived colonies, whereas colony morphology and spermatogenesis were comparable to controls. Inhibition of CXCR4 signaling using a small molecule inhibitor (AMD3100) during the critical window of homing also significantly lowered the efficiency of donor-derived SSCs to establish spermatogenic colonies in recipient mice; however, the self-renewal of SSCs was not affected by exposure to AMD3100. Rather, in vitro migration assays demonstrate the influence of CXCR4-CXCL12 signaling in promoting germ cell migration. Together, these studies suggest that CXCR4-CXCL12 signaling functions to promote homing of SSCs towards the stem cell niche and plays a critical role in reestablishing spermatogenesis.

Mechanisms Regulating Spermatogonial Differentiation

Mammalian spermatogenesis is a complex and highly ordered process by which male germ cells proceed through a series of differentiation steps to produce haploid flagellated spermatozoa. Underlying this process is a pool of adult stem cells, the spermatogonial stem cells (SSCs), which commence the spermatogenic lineage by undertaking a differentiation fate decision to become progenitor spermatogonia. Subsequently, progenitors acquire a differentiating spermatogonia phenotype and undergo a series of amplifying mitoses while becoming competent to enter meiosis. After spermatocytes complete meiosis, post-meiotic spermatids must then undergo a remarkable transformation from small round spermatids to a flagellated spermatozoa with extremely compacted nuclei. This chapter reviews the current literature pertaining to spermatogonial differentiation with an emphasis on the mechanisms controlling stem cell fate decisions and early differentiation events in the life of a spermatogonium.

Marker expression reveals heterogeneity of spermatogonia in the neonatal mouse testis

Prospermatogonia transition to type A spermatogonia, which provide the source for the spermatogonial stem cell (SSC) pool. A percentage of these type A spermatogonia then differentiate to enter meiosis as spermatocytes by ∼P10. It is currently unclear as to when these distinct populations are initially formed in the neonatal testis, and when the expression of markers both characteristic of and required for the adult undifferentiated and differentiating states is established. In this study, we compared expression of known spermatogonial cell fate markers during normal development and in response to the differentiation signal provided by retinoic acid (RA). We found that some markers for the undifferentiated state (ZBTB16/PLZF and CDH1) were expressed in nearly all spermatogonia from P1 through P7. In contrast, differentiation markers (STRA8 and KIT) appeared in a subset of spermatogonia at P4, coincident with the onset of RA signaling. GFRA1, which was present in nearly all prospermatogonia at P1, was only retained in STRA8/KIT- spermatogonia. From P4 through P10, there was a great deal of heterogeneity in the male germ cell population in terms of expression of markers, as markers characteristic of the undifferentiated (except GFRA1) and differentiating states were co-expressed through this interval. After P10, these fate markers diverged to mark distinct populations of undifferentiated and differentiating spermatogonia, and this pattern was maintained in juvenile (P18) and adult (P>60) testes. Taken together, these results reveal that the spermatogonia population is heterogeneous during the first wave of spermatogenesis, and indicate that neonatal spermatogonia may not serve as an ideal substitute for studying the function of adult spermatogonia.

The Role of Retinoic Acid (RA) in Spermatogonial Differentiation

Retinoic acid (RA) directs the sequential, but distinct, programs of spermatogonial differentiation and meiotic differentiation that are both essential for the generation of functional spermatozoa. These processes are functionally and temporally decoupled, as they occur in distinct cell types that arise over a week apart, both in the neonatal and adult testis. However, our understanding is limited in terms of what cellular and molecular changes occur downstream of RA exposure that prepare differentiating spermatogonia for meiotic initiation. In this review, we describe the process of spermatogonial differentiation and summarize the current state of knowledge regarding RA signaling in spermatogonia.

Application of three-dimensional culture systems to study mammalian spermatogenesis, with an emphasis on the rhesus monkey (Macaca mulatta)

In vitro culture of spermatogonial stem cells (SSCs) has generally been performed using two-dimensional (2D) culture systems; however, such cultures have not led to the development of complete spermatogenesis. It seems that 2D systems do not replicate optimal conditions of the seminiferous tubules (including those generated by the SSC niche) and necessary for spermatogenesis. Recently, one of our laboratories has been able to induce proliferation and differentiation of mouse testicular germ cells to meiotic and postmeiotic stages including generation of sperm in a 3D soft agar culture system (SACS) and a 3D methylcellulose culture system (MCS). It was suggested that SACS and MCS form a special 3D microenvironment that mimics germ cell niche formation in the seminiferous tubules, and thus permits mouse spermatogenesis in vitro. In this review, we (1) provide a brief overview of the differences in spermatogenesis in rodents and primates, (2) summarize data related to attempts to generate sperm in vitro, (3) report for the first time formation of colonies/clusters of cells and differentiation of meiotic (expression of CREM-1) and postmeiotic (expression of acrosin) germ cells from undifferentiated spermatogonia isolated from the testis of prepubertal rhesus monkeys and cultured in SACS and MCS, and (4) indicate research needed to optimize 3D systems for in vitro primate spermatogenesis and for possible future application to man.

RNA Binding Protein Ptbp2 Is Essential for Male Germ Cell Development

RNA binding proteins (RBPs) are increasingly recognized as essential factors in tissue development and homeostasis. The polypyrimidine tract binding (PTB) protein family of RBPs are important posttranscriptional regulators of gene expression. In the nervous system, the function and importance of PTB protein 2 (Ptbp2) as a key alternative splicing regulator is well established. Ptbp2 is also abundantly expressed during spermatogenesis, but its role in this developmental program has not been explored. Additionally, the importance of alternative splicing regulation in spermatogenesis is unclear. Here, we demonstrate that Ptbp2 is essential for spermatogenesis. We also describe an improved dual fluorescence flow cytometry strategy to discriminate, quantify, and collect germ cells in different stages of development. Using this approach, in combination with traditional histological methods, we show that Ptbp2 ablation results in germ cell loss due to increased apoptosis of meiotic spermatocytes and postmeiotic arrest of spermatid differentiation. Furthermore, we show that Ptbp2 is required for alternative splicing regulation in the testis, as in brain. Strikingly, not all of the alternatively spliced RNAs examined were sensitive to Ptbp2 loss in both tissues. Collectively, the data provide evidence for an important role for alternative splicing regulation in germ cell development and a central role for Ptbp2 in this process.

The histone demethylase KDM1A is essential for the maintenance and differentiation of spermatogonial stem cells and progenitors

Little is known of the fundamental processes governed by epigenetic mechanisms in the supplier cells of spermatogenesis, the spermatogonial stem cells (SSCs). The histone H3 lysine demethylase KDM1A is expressed in spermatogonia. We hypothesized that KDM1A serves in transcriptional regulation of SSCs and fertility. Using a conditional deletion of Kdm1a [conditional knockout (cKO)] in mouse spermatogonia, we determined that Kdm1a is essential for spermatogenesis as adult cKO males completely lack germ cells. Analysis of postnatal testis development revealed that undifferentiated and differentiating spermatogonial populations form in Kdm1a-cKO animals, yet the majority fail to enter meiosis. Loss of germ cells in the cKO was rapid with none remaining by postnatal day (PND) 21. To gain insight into the mechanistic implications of Kdm1a ablation, we isolated PND 6 spermatogonia enriched for SSCs and analyzed their transcriptome by RNA sequencing. Loss of Kdm1a was associated with altered transcription of 1206 genes. Importantly, differentially expressed genes between control and Kdm1a-cKO animals included those that are essential for SSC and progenitor maintenance and spermatogonial differentiation. The complete loss of fertility and failure to establish spermatogenesis indicate that Kdm1a is a master controller of gene transcription in spermatogonia and is required for SSC and progenitor maintenance and differentiation.

Inactivation of Retinoblastoma Protein (Rb1) in the Oocyte: Evidence That Dysregulated Follicle Growth Drives Ovarian Teratoma Formation in Mice

The origin of most ovarian tumors is undefined. Here, we report development of a novel mouse model in which conditional inactivation of the tumor suppressor gene Rb1 in oocytes leads to the formation of ovarian teratomas (OTs). While parthenogenetically activated ooctyes are a known source of OT in some mutant mouse models, enhanced parthenogenetic propensity in vitro was not observed for Rb1-deficient oocytes. Further analyses revealed that follicle recruitment and growth is disrupted in ovaries of mice with conditional inactivation of Rb1, leading to abnormal accumulation of secondary/preantral follicles. These findings underpin the concept that miscues between the germ cell and somatic compartments cause premature oocyte activation and the formation of OTs. Furthermore, these results suggest that defects in folliculogenesis and a permissive genetic background are sufficient to drive OT development, even in the absence of enhanced parthenogenetic activation. Thus, we have discovered a novel role of Rb1 in regulating the entry of primordial oocytes into the pool of growing follicles and signaling between the oocyte and granulosa cells during the protracted process of oocyte growth. Our findings, coupled with data from studies of other OT models, suggest that defects in the coordinated regulation between growth of the oocyte and somatic components in follicles are an underlying cause of OT formation.