The Histone Demethylase FBXL10 Regulates the Proliferation of Spermatogonia and Ensures Long-Term Sustainable Spermatogenesis in Mice

The emerging and diverse roles of F-box proteins in spermatogenesis and male infertility

F-box proteins play essential roles in various cellular processes of spermatogenesis by means of ubiquitylation and subsequent target protein degradation. They are the substrate-recognition subunits of SKP1-cullin 1-F-box protein (SCF) E3 ligase complexes. Dysregulation of F‑box protein‑mediated proteolysis could lead to male infertility in humans and mice. The emerging studies revealed the physiological function, pathological evidence, and biochemical substrates of F-box proteins in the development of male germ cells, which urging us to review the current understanding of how F‑box proteins contribute to spermatogenesis. More functional and mechanistic study will be helpful to define the roles of F-box protein in spermatogenesis, which will pave the way for the logical design of F-box protein-targeted diagnosis and therapies for male infertility, as the spermatogenic role of many F-box proteins remains elusive.

Age-related decline in spermatogenic activity accompanied with endothelial cell senescence in male mice

Male fertility decreases with aging, with spermatogenic decline being one of its causes. Altered testis environment is suggested as a cause of the phenotype; however, the associated mechanisms remain unclear. Herein, we investigated the age-related changes in testicular somatic cells on spermatogenic activity. The number and proliferation of spermatogonia significantly reduced with aging in mice. Interestingly, senescence-associated β-galactosidase-positive cells appeared in testicular endothelial cell (EC) populations, but not in germ cell populations, with aging. Transcriptome analysis of ECs indicated that senescence occurred in the ECs of aged mice. Furthermore, the support capacity of ECs for spermatogonial proliferation significantly decreased with aging; however, the senolytic-induced removal of senescent cells from aged ECs restored their supporting capacity to a comparable level as that of young ECs. Our results suggest that the accumulation of senescent ECs in the testis is a potential factor contributing to the age-related decline in spermatogenic activity.

Testis-specific knockout of Kdm2a reveals nonessential roles in male fertility but partially compromises spermatogenesis

Lysine-specific histone demethylase 2 (Kdm2a) is a regulatory factor of histone modifications that participates in gametogenesis and embryonic development. The mis-regulation of Kdm2a can lead to aberrant gene expression, thereby contributing to abnormal cell proliferation, differentiation, apoptosis, and tumorigenesis. However, due to the potential confounding effects that are secondary to the loss of Kdm2a function from the soma in existing whole-animal mutants, the in vivo function of Kdm2a in spermatogenesis for male fertility remains unknown. Herein, we focus on exploring the spatiotemporal expression profile and biological functions of Kdm2a in the spermatogenesis and fertility of male mice. A testis-specific knockout Kdm2a model (Kdm2a cKO) was established by using the Stra8-Cre/loxP recombinase system to explore the roles of Kdm2a in male fertility. Our results showed that Kdm2a was ubiquitously expressed and dynamically distributed in multiple tissues and cell types in the testis of mice. Surprisingly, Kdm2a-deficient adult males were completely fertile and comparable with their control (Kdm2aflox/flox) counterparts. Despite the significantly reduced total number of sperm and density of seminiferous tubules in Kdm2a cKO testis accompanied by the degeneration of spermatogenesis, the fertilization ability and embryonic developmental competence of the Kdm2a cKO were comparable with those of their control littermates, suggesting that Kdm2a disruption did not markedly affect male fertility, at least during younger ages. Furthermore, Kdm2a homozygous mutants exhibited a lower total number and motility of sperm than the control group and showed notably affected serum 17β-estradiol concentration. Interestingly, the transcriptome sequencing revealed that the loss of Kdm2a remarkably upregulated the expression level of Kdm2b. This effect, in turn, may induce compensative effects in the case of Kdm2a deficiency to maintain normal male reproduction. Together, our results reveal that Kdm2a shows spatiotemporal expression during testicular development and that its loss is insufficient to compromise the production of spermatozoa completely. The homologous Kdm2b gene might compensate for the loss of Kdm2a. Our work provides a novel Kdm2a cKO mouse allowing for the efficient deletion of Kdm2a in a testis-specific manner, and further investigated the biological function of Kdm2a and the compensatory effects of Kdm2b. Our study will advance our understanding of underlying mechanisms in spermatogenesis and male fertility.

Epigenetic markers in the embryonal germ cell development and spermatogenesis

Spermatogenesis is the process of generation of male reproductive cells from spermatogonial stem cells in the seminiferous epithelium of the testis. During spermatogenesis, key spermatogenic events such as stem cell self-renewal and commitment to meiosis, meiotic recombination, meiotic sex chromosome inactivation, followed by cellular and chromatin remodeling of elongating spermatids occur, leading to sperm cell production. All the mentioned events are at least partially controlled by the epigenetic modifications of DNA and histones. Additionally, during embryonal development in primordial germ cells, global epigenetic reprogramming of DNA occurs. In this review, we summarized the most important epigenetic modifications in the particular stages of germ cell development, in DNA and histone proteins, starting from primordial germ cells, during embryonal development, and ending with histone-to-protamine transition during spermiogenesis.

Biological Functions of the KDM2 Family of Histone Demethylases

The histone lysine demethylase 2 (KDM2) family of α-Ketoglutarate-Fe++-dependent dioxygenases were the first Jumonji-domain-containing proteins reported to harbor demethylase activity. This landmark discovery paved the way for the characterization of more than 25 enzymes capable of demethylating lysine residues on histones-an epigenetic modification previously thought to be irreversible. The KDM2 family is comprised of KDM2A and KDM2B which share significant structural similarities and demethylate lysine 36 on histone H3. However, they exert distinct cellular functions and are frequently deregulated in a broad spectrum of human cancers. With the advent of next generation sequencing and development of genetically engineered mouse models, it was shown that KDM2A and KDM2B play critical roles in stem cell biology, somatic cell reprograming, and organismal development by regulating cell fate and lineage commitment decisions. Thus, understanding the biochemistry and elucidating the context-dependent function of these enzymes is an emerging new frontier for the development of small molecule inhibitors to treat cancer and other diseases.

Oocyte-Specific Knockout of Histone Lysine Demethylase KDM2a Compromises Fertility by Blocking the Development of Follicles and Oocytes

The methylation status of histones plays a crucial role in many cellular processes, including follicular and oocyte development. Lysine-specific demethylase 2a (KDM2a) has been reported to be closely associated with gametogenesis and reproductive performance, but the specific function and regulatory mechanism have been poorly characterized in vivo. We found KDM2a to be highly expressed in growing follicles and oocytes of mice in this study. To elucidate the physiological role of Kdm2a, the zona pellucida 3-Cre (Zp3-Cre)/LoxP system was used to generate an oocyte Kdm2a conditional knockout (Zp3-Cre; Kdm2a^flox/flox, termed Kdm2a cKO) model. Our results showed that the number of pups was reduced by approximately 50% in adult Kdm2a cKO female mice mating with wildtype males than that of the control (Kdm2a^flox/flox) group. To analyze the potential causes, the ovaries of Kdm2a cKO mice were subjected to histological examination, and results indicated an obvious difference in follicular development between Kdm2a cKO and control female mice and partial arrest at the primary antral follicle stage. The GVBD and matured rates of oocytes were also compromised after conditional knockout Kdm2a, and the morphological abnormal oocytes increased. Furthermore, the level of 17β-estradiol of Kdm2a cKO mice was only 60% of that in the counterparts, and hormone sensitivity decreased as the total number of ovulated and matured oocytes decreased after superovulation. After deletion of Kdm2a, the patterns of H3K36me2/3 in GVBD-stage oocytes were remarkedly changed. Transcriptome sequencing showed that the mRNA expression profiles in Kdm2a cKO oocytes were significantly different, and numerous differentially expressed genes were involved in pathways regulating follicular and oocyte development. Taken together, these results indicated that the oocyte-specific knockout Kdm2a gene led to female subfertility, suggesting the crucial role of Kdm2a in epigenetic modification and follicular and oocyte development.

Genome-wide identification, evolution of histone lysine demethylases (KDM) genes and their expression during gonadal development in Nile tilapia

Histone lysine demethylases (KDM) are responsible for histone demethylation and are involved in gene expression regulation. Previous studies have shown that histone lysine demethylation plays an important role in gonadal development of vertebrates. The KDM family consists of eight subfamilies, i.e., kdm1, kdm2, kdm3, kdm4, kdm5, kdm6, kdm7 and JmjC-only subfamily. In this study, 13 to 63 KDM genes in 23 representative species were identified based on the available version of genome assembly. Phylogenetic relationships, domain architecture, and synteny of these genes were comprehensively analyzed and the results suggested KDM genes probably originated from the early diverging metazoan and significantly expanded in vertebrates with multiple whole genome duplication, especially in the third-round whole genome duplication (3R-WGD) and polyploidization of teleosts. The subfamilies of kdm2, kdm3, kdm4, kdm5, kdm6 and kdm7 were duplicated with 1R-2R events, and duplicates of kdm2a, kdm4a, kdm5b and kdm6b were resulted from 3R-WGD. Based on transcriptome data, the KDM genes were found to be dominantly expressed in the ovary and testis. More than 80% of KDM genes displayed sexual dimorphic expression, with 15 genes dominantly expressed in ovaries, and 12 genes dominantly expressed in testes. Importantly, from transcriptome data, qRT-PCR and fluorescence in situ hybridization during sex reversal, genes with higher expression in ovary than testis, such as kdm1b and two JmjC-only subfamily members hspbap1 and riox1, were downregulated, while other genes, such as kdm3c, kdm5bb, kdm6ba, kdm6bb and kdm7b, with higher expression in testis than ovary, were upregulated in ovotestis, indicating these genes play critical roles in the gonadal development and sex reversal. This study provided new insights into the evolution of the KDM genes and a fundamental clue for understanding their important roles in sex differentiation and gonadal development in teleosts.

The histone demethylase KDM2B regulates human primordial germ cell-like cells specification

Germline specification is a fundamental step for human reproduction and this biological phenomenon possesses technical challenges to study in vivo as it occurs immediately after blastocyst implantation. The establishment of in vitro human primordial germ cell-like cells (hPGCLCs) induction system allows sophisticated characterization of human primordial germ cells (hPGCs) development. However, the underlying molecular mechanisms of hPGCLC specification are not fully elucidated. Here, we observed particularly high expression of the histone demethylase KDM2B in male fetal germ cells (FGCs) but not in male somatic cells. Besides, KDM2B shared similar expression pattern with hPGC marker genes in hPGCLCs, suggesting an important role of KDM2B in germ cell development. Although deletion of KDM2B had no significant effects on human embryonic stem cell (hESC)'s pluripotency, loss of KDM2B dramatically impaired hPGCLCs differentiation whereas ectopically expressed KDM2B could efficiently rescue such defect, indicating this defect was due to KDM2B's loss in hPGCLC specification. Mechanistically, as revealed by the transcriptional profiling, KDM2B suppressed the expression of somatic genes thus inhibited somatic differentiation during hPGCLC specification. These data collectively indicate that KDM2B is an indispensable epigenetic regulator for hPGCLC specification, shedding lights on how epigenetic regulations orchestrate transcriptional events in hPGC development for future investigation.

Epigenetic Regulation of Spermatogonial Stem Cell Homeostasis: From DNA Methylation to Histone Modification

Spermatogonial stem cells(SSCs)are the ultimate germline stem cells with the potential of self-renewal and differentiation, and a dynamic balance of SSCs play an essential role in spermatogenesis. During the gene expression process, genomic DNA and nuclear protein, working together, contribute to SSC homeostasis. Recently, emerging studies have shown that epigenome-related molecules such as chromatin modifiers play an important role in SSC homeostasis through regulating target gene expression. Here, we focus on two types of epigenetic events, including DNA methylation and histone modification, and summarize their function in SSC homeostasis. Understanding the molecular mechanism during SSC homeostasis will promote the recognition of epigenetic biomarkers in male infertility, and bring light into therapies of infertile patients.Graphical Abstract.

RNA-binding protein Ptbp1 regulates alternative splicing and transcriptome in spermatogonia and maintains spermatogenesis in concert with Nanos3

PTBP1, a well-conserved RNA-binding protein, regulates cellular development by tuning posttranscriptional mRNA modification such as alternative splicing (AS) or mRNA stabilization. We previously revealed that the loss of Ptbp1 in spermatogonia causes the dysregulation of spermatogenesis, but the molecular mechanisms by which PTBP1 regulates spermatogonium homeostasis are unclear. In this study, changes of AS or transcriptome in Ptbp1-knockout (KO) germline stem cells (GSC), an in vitro model of proliferating spermatogonia, was determined by next generation sequencing. We identified more than 200 differentially expressed genes, as well as 85 genes with altered AS due to the loss of PTBP1. Surprisingly, no differentially expressed genes overlapped with different AS genes in Ptbp1-KO GSC. In addition, we observed that the mRNA expression of Nanos3, an essential gene for normal spermatogenesis, was significantly decreased in Ptbp1-KO spermatogonia. We also revealed that PTBP1 protein binds to Nanos3 mRNA in spermatogonia. Furthermore, Nanos3^+/−;Ptbp1^+/− mice exhibited abnormal spermatogenesis, which resembled the effects of germ cell-specific Ptbp1 KO, whereas no significant abnormality was observed in mice heterozygous for either gene alone. These data implied that PTBP1 regulates alternative splicing and transcriptome in spermatogonia under different molecular pathways, and contributes spermatogenesis, at least in part, in concert with NANOS3.

Emerging of lysine demethylases (KDMs): From pathophysiological insights to novel therapeutic opportunities

In recent years, there have been remarkable scientific advancements in the understanding of lysine demethylases (KDMs) because of their demethylation of diverse substrates, including nucleic acids and proteins. Novel structural architectures, physiological roles in the gene expression regulation, and ability to modify protein functions made KDMs the topic of interest in biomedical research. These structural diversities allow them to exert their function either alone or in complex with numerous other bio-macromolecules. Impressive number of studies have demonstrated that KDMs are localized dynamically across the cellular and tissue microenvironment. Their dysregulation is often associated with human diseases, such as cancer, immune disorders, neurological disorders, and developmental abnormalities. Advancements in the knowledge of the underlying biochemistry and disease associations have led to the development of a series of modulators and technical compounds. Given the distinct biophysical and biochemical properties of KDMs, in this review we have focused on advances related to the structure, function, disease association, and therapeutic targeting of KDMs highlighting improvements in both the specificity and efficacy of KDM modulation.

Expression profiling of the Kdm genes in scallop Patinopecten yessoensis suggests involvement of histone demethylation in regulation of early development and gametogenesis

Histone demethylation modification is an important means of gene expression regulation and is widely involved in biological processes such as animal reproduction and development. Histone lysine demethylases (Kdm) plays an important role in the demethylation of histones. To understand the role of histone demethylation in scallops, we identified the Kdm gene family of the Yesso scallop Patinopecten yessoensis, and analyzed its expression during the gonad and early development. The results showed that the P. yessoensis has a complete Kdm family including seventeen members that belong to sixteen subfamilies (Hif1an, Hspbap1, Jarid2, Jmjd4, Jmjd6, Jmjd7, Jmjd8, Kdm1, Kdm2, Kdm3, Kdm4, Kdm5, Kdm6, Kdm7, Kdm8 and Kdm9). The Kdm genes showed five different expression patterns in the early development of scallop, with some of them (e.g. Jmjd7, Jmjd8 and Kdm8) being highly expressed in only one or two stage and the others (e.g. Kdm1A, Kdm9, Jmjd4 and Jmjd6) in several consecutive stages. During gonadal development, the Kdm genes also display various expression patterns. Some genes (e.g. Kdm2, Kdm4 and Jmjd7) display preferential expression in the testis, and the others have no obvious sex bias but show stage preference (resting, proliferative, growing or maturation stage). These results suggest that various histone demethylation modifications (e.g. H3K4, H3K9 and H3K27) may participate in the regulation of gametogenesis and early development of Yesso scallop. It will facilitate a better understanding of the epigenetic contributions to mollusk development.

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.

PTBP1 contributes to spermatogenesis through regulation of proliferation in spermatogonia

Polypyrimidine tract-binding protein 1 (PTBP1) is a highly conserved RNA-binding protein that is a well-known regulator of alternative splicing. Testicular tissue is one of the richest tissues with respect to the number of alternative splicing mRNA isoforms, but the molecular role(s) of PTBP1 in the regulation of these isoforms during spermatogenesis is still unclear. Here, we developed a germ cell–specific Ptbp1 conditional knockout (cKO) mouse model by using the Cre-loxP system to investigate the role of PTBP1 in spermatogenesis. Testis weight in Ptbp1 cKO mice was comparable to that in age-matched controls until 3 weeks of age; at ≥ 2 months old, testis weight was significantly lighter in cKO mice than in age-matched controls. Sperm count in Ptbp1 cKO mice at 2 months old was comparable to that in controls, whereas sperm count significantly decreased at 6 months old. Seminiferous tubules that exhibited degeneration in spermatogenic function were more evident in the 2-month-old Ptbp1 cKO mice than in controls. In addition, the early neonatal proliferation of spermatogonia, during postnatal days 1–5, was significantly retarded in Ptbp1 cKO mice compared with that in controls. An in vitro spermatogonia culture model (germline stem cells) revealed that hydroxytamoxifen-induced deletion of PTBP1 from germline stem cells caused severe proliferation arrest accompanied by an increase of apoptotic cell death. These data suggest that PTBP1 contributes to spermatogenesis through regulation of spermatogonia proliferation.

Noncoding RNAs: Potential players in the self-renewal of mammalian spermatogonial stem cells

Spermatogonial stem cells (SSCs), a unique population of male germ cells with self-renewal ability, are the foundation for maintenance of spermatogenesis throughout the life of the male. Although many regulatory molecules essential for SSC self-renewal have been identified, the fundamental mechanism underlying how SSCs acquire and maintain their self-renewal activity remains largely to be elucidated. In recent years, many types of noncoding RNAs (ncRNAs) have been suggested to regulate the SSC self-renewal through multiple ways, indicating ncRNAs play crucial roles in SSC self-renewal. In this paper, we mainly focus on four types of ncRNAs including microRNA, long ncRNA, piwi-interacting RNA, as well as circular RNAs, and reviewed their potential roles in SSC self-renewal that discovered recently to help us gain a better understanding of molecular mechanisms by which ncRNAs perform their function in regulating SSC self-renewal.

Histone lysine demethylases in mammalian embryonic development

Post-translational modifications, such as methylation, acetylation and phosphorylation, of histone proteins play important roles in regulating dynamic chromatin structure. Histone demethylation has become one of the most active research areas of epigenetics in the past decade. To date, with the exception of histone H3 lysine 79 methylation, the demethylases for all major lysine methylation sites have been discovered. These enzymes have been shown to be involved in various biological processes, with embryonic development being an exciting emerging area. This review will primarily discuss the involvement of these demethylases in the regulation of mammalian embryonic development, including their roles in embryonic stem cell pluripotency, primordial germ cell (PGC) formation and maternal-to-zygotic transition.