A haploid and a diploid cell coexist in an in vitro immortalized spermatogenic cell line

Regulation of Miwi-mediated mRNA stabilization by Ck137956/Tssa is essential for male fertility

BACKGROUND

Sperm is formed through spermiogenesis, a highly complex process involving chromatin condensation that results in cessation of transcription. mRNAs required for spermiogenesis are transcribed at earlier stages and translated in a delayed fashion during spermatid formation. However, it remains unknown that how these repressed mRNAs are stabilized.

RESULTS

Here we report a Miwi-interacting testis-specific and spermiogenic arrest protein, Ck137956, which we rename Tssa. Deletion of Tssa led to male sterility and absence of sperm formation. The spermiogenesis arrested at the round spermatid stage and numerous spermiogenic mRNAs were down-regulated in Tssa^-/- mice. Deletion of Tssa disrupted the localization of Miwi to chromatoid body, a specialized assembly of cytoplasmic messenger ribonucleoproteins (mRNPs) foci present in germ cells. We found that Tssa interacted with Miwi in repressed mRNPs and stabilized Miwi-interacting spermiogenesis-essential mRNAs.

CONCLUSIONS

Our findings indicate that Tssa is indispensable in male fertility and has critical roles in post-transcriptional regulations by interacting with Miwi during spermiogenesis.

Transcriptomic landscape of GC-2spd(ts) cell in response to the downregulation of piR-1207/2107

Spermatogenesis is a highly orchestrated dynamic developmental process, during which piRNAs play an indispensable role. Our previous studies have confirmed that the levels of piR-1207 and piR-2107 were significantly decreased in spermatozoa and seminal plasma of patients with asthenozoospermia, compared with the fertile controls. In order to explore the function of piR-1207 and piR-2107 in human spermatogenesis and their potential regulatory downstream genes, we examined the transcriptomic landscape in mouse spermatocyte cell line GC-2spd(ts) transfected with anti-piR-1207 and anti-piR-2107 by RNA sequencing. The result showed that 86 and 75 differential expression genes (DEGs) in anti-piR-1207 and anti-piR-2107 group, respectively. Among the DEGs, three genes including Pbp2, Pde3a and Cage1 were identified as potential key genes playing important roles in mediating sperm motility and morphology in anti-piR-1207 or anti-piR-2107 transfected transcriptomic response. Next, the expression levels of Pbp2, Pde3a and Cage1 were confirmed by qRT-PCR. These results showed that Pbp2, Pde3a and Cage1 may serve as the potential regulatory genes of piR-1207 or piR-2107. In summary, piR-1207 and piR-2107 might have an implication in modulating the process of spermatogenesis through regulating the expression of potential genes.

Kinesin-5 Eg5 mediates centrosome separation to control spindle assembly in spermatocytes

Timely and accurate centrosome separation is critical for bipolar spindle organization and faithful chromosome segregation during cell division. Kinesin-5 Eg5 is essential for centrosome separation and spindle organization in somatic cells; however, the detailed functions and mechanisms of Eg5 in spermatocytes remain unclear. In this study, we show that Eg5 proteins are located at spindle microtubules and centrosomes in spermatocytes both in vivo and in vitro. We reveal that the spermatocytes are arrested at metaphase I in seminiferous tubules after Eg5 inhibition. Eg5 ablation results in cell cycle arrest, the formation of monopolar spindle, and chromosome misalignment in cultured GC-2 spd cells. Importantly, we find that the long-term inhibition of Eg5 results in an increased number of centrosomes and chromosomal instability in spermatocytes. Our findings indicate that Eg5 mediates centrosome separation to control spindle assembly and chromosome alignment in spermatocytes, which finally contribute to chromosome stability and faithful cell division of the spermatocytes.

PHB regulates meiotic recombination via JAK2-mediated histone modifications in spermatogenesis

Previously, we have shown that human sperm Prohibitin (PHB) expression is significantly negatively correlated with mitochondrial ROS levels but positively correlated with mitochondrial membrane potential and motility. However, the possible role of PHB in mammalian spermatogenesis has not been investigated. Here we document the presence of PHB in spermatocytes and its functional roles in meiosis by generating the first male germ cell-specific Phb-cKO mouse. Loss of PHB in spermatocytes resulted in complete male infertility, associated with not only meiotic pachytene arrest with accompanying apoptosis, but also apoptosis resulting from mitochondrial morphology and function impairment. Our mechanistic studies show that PHB in spermatocytes regulates the expression of STAG3, a key component of the meiotic cohesin complex, via a non-canonical JAK/STAT pathway, and consequently promotes meiotic DSB repair and homologous recombination. Furthermore, the PHB/JAK2 axis was found as a novel mechanism in the maintenance of stabilization of meiotic STAG3 cohesin complex and the modulation of heterochromatin formation in spermatocytes during meiosis. The observed JAK2-mediated epigenetic changes in histone modifications, reflected in a reduction of histone 3 tyrosine 41 phosphorylation (H3Y41ph) and a retention of H3K9me3 at the Stag3 locus, could be responsible for Stag3 dysregulation in spermatocytes with the loss of PHB.

A Translation-Activating Function of MIWI/piRNA during Mouse Spermiogenesis

Spermiogenesis is a highly orchestrated developmental process during which chromatin condensation decouples transcription from translation. Spermiogenic mRNAs are transcribed earlier and stored in a translationally inert state until needed for translation; however, it remains largely unclear how such repressed mRNAs become activated during spermiogenesis. We previously reported that the MIWI/piRNA machinery is responsible for mRNA elimination during late spermiogenesis in preparation for spermatozoa production. Here we unexpectedly discover that the same machinery is also responsible for activating translation of a subset of spermiogenic mRNAs to coordinate with morphological transformation into spermatozoa. Such action requires specific base-pairing interactions of piRNAs with target mRNAs in their 3' UTRs, which activates translation through coupling with cis-acting AU-rich elements to nucleate the formation of a MIWI/piRNA/eIF3f/HuR super-complex in a developmental stage-specific manner. These findings reveal a critical role of the piRNA system in translation activation, which we show is functionally required for spermatid development.

Mitochondrial fusion is required for spermatogonial differentiation and meiosis

Differentiating cells tailor their metabolism to fulfill their specialized functions. We examined whether mitochondrial fusion is important for metabolic tailoring during spermatogenesis. Acutely after depletion of mitofusins Mfn1 and Mfn2, spermatogenesis arrests due to failure to accomplish a metabolic shift during meiosis. This metabolic shift includes increased mitochondrial content, mitochondrial elongation, and upregulation of oxidative phosphorylation (OXPHOS). With long-term mitofusin loss, all differentiating germ cell types are depleted, but proliferation of stem-like undifferentiated spermatogonia remains unaffected. Thus, compared with undifferentiated spermatogonia, differentiating spermatogonia and meiotic spermatocytes have cell physiologies that require high levels of mitochondrial fusion. Proteomics in fibroblasts reveals that mitofusin-null cells downregulate respiratory chain complexes and mitochondrial ribosomal subunits. Similarly, mitofusin depletion in immortalized spermatocytes or germ cells in vivo results in reduced OXPHOS subunits and activity. We reveal that by promoting OXPHOS, mitofusins enable spermatogonial differentiation and a metabolic shift during meiosis.

Ablation of Ggnbp2 impairs meiotic DNA double-strand break repair during spermatogenesis in mice

Gametogenetin (GGN) binding protein 2 (GGNBP2) is a zinc finger protein expressed abundantly in spermatocytes and spermatids. We previously discovered that Ggnbp2 resection caused metamorphotic defects during spermatid differentiation and resulted in an absence of mature spermatozoa in mice. However, whether GGNBP2 affects meiotic progression of spermatocytes remains to be established. In this study, flow cytometric analyses showed a decrease in haploid, while an increase in tetraploid spermatogenic cells in both 30‐ and 60‐day‐old Ggnbp2 knockout testes. In spread spermatocyte nuclei, Ggnbp2 loss increased DNA double‐strand breaks (DSB), compromised DSB repair and reduced crossovers. Further investigations demonstrated that GGNBP2 co‐immunoprecipitated with a testis‐enriched protein GGN1. Immunofluorescent staining revealed that both GGNBP2 and GGN1 had the same subcellular localizations in spermatocyte, spermatid and spermatozoa. Ggnbp2 loss suppressed Ggn expression and nuclear accumulation. Furthermore, deletion of either Ggnbp2 or Ggn in GC‐2spd cells inhibited their differentiation into haploid cells in vitro. Overexpression of Ggnbp2 in Ggnbp2 null but not in Ggn null GC‐2spd cells partially rescued the defect coinciding with a restoration of Ggn expression. Together, these data suggest that GGNBP2, likely mediated by its interaction with GGN1, plays a role in DSB repair during meiotic progression of spermatocytes.

Identification and characterization of methylation-dependent/independent DNA regulatory elements in the human SLC9B1 gene

The human NHEDC1 (hNHEDC1) protein is thought to be essential for sperm motility and fertility however the mechanisms regulating its gene expression are largely unknown. In this study we have identified multiple DNA regulatory elements in the 5' end of the gene encoding hNHEDC1 (SLC9B1) and have explored the role that DNA methylation at these elements plays in the regulation of its expression. We first show that the full-length hNHEDC1 protein is testis-specific for the tissues that we tested and that it localizes to the cells of the seminiferous tubules. In silico analysis of the SLC9B1 gene locus identified two putative promoters (P1 and P2) and two CpG islands - CpGI (overlapping with P1) and CpGII (intragenic) - at the 5' end of the gene. By deletion analysis of P1, we show that the region from -23 bp to +200 bp relative to the transcription start site (TSS) is sufficient for optimal promoter activity in a germ cell line. Additionally, in vitro methylation of the P1 (the -500 bp to +200 bp region relative to the TSS) abolishes its activity in germ cells and somatic cells strongly suggesting that DNA methylation at this promoter could regulate SLC9B1 expression. Furthermore, bisulfite-sequencing analysis of the P1/CpGI uncovered reduced methylation in the testis vs. lung whereas CpGII displayed no differences in methylation between these two tissues. Additionally, treatment of HEK 293 cells with 5-aza-2-Deoxycytidine led to upregulation of NHEDC1 transcript and reduced methylation in the promoter CpGI. Finally, we have uncovered both enhancer and silencer functions of the intragenic SLC9B1 CpGII. In all, our data suggests that SLC9B1 gene expression could be regulated via a concerted action of DNA methylation-dependent and independent mechanisms mediated by these multiple DNA regulatory elements.

MIWI and piRNA-mediated cleavage of messenger RNAs in mouse testes

The piRNA machinery is known for its role in mediating epigenetic silencing of transposons. Recent studies suggest that this function also involves piRNA-guided cleavage of transposon-derived transcripts. As many piRNAs also appear to have the capacity to target diverse mRNAs, this raises the intriguing possibility that piRNAs may act extensively as siRNAs to degrade specific mRNAs. To directly test this hypothesis, we compared mouse PIWI (MIWI)-associated piRNAs with experimentally identified cleaved mRNA fragments from mouse testes, and observed cleavage sites that predominantly occur at position 10 from the 5' end of putative targeting piRNAs. We also noted strong biases for U and A residues at nucleotide positions 1 and 10, respectively, in both piRNAs and mRNA fragments, features that resemble the pattern of piRNA amplification by the 'ping-pong' cycle. Through mapping of MIWI-RNA interactions by CLIP-seq and gene expression profiling, we found that many potential piRNA-targeted mRNAs directly interact with MIWI and show elevated expression levels in the testes of Miwi catalytic mutant mice. Reporter-based assays further revealed the importance of base pairing between piRNAs and mRNA targets and the requirement for both the slicer activity and piRNA-loading ability of MIWI in piRNA-mediated target repression. Importantly, we demonstrated that proper turnover of certain key piRNA targets is essential for sperm formation. Together, these findings reveal the siRNA-like function of the piRNA machinery in mouse testes and its central requirement for male germ cell development and maturation.

Pachytene piRNAs instruct massive mRNA elimination during late spermiogenesis

Spermatogenesis in mammals is characterized by two waves of piRNA expression: one corresponds to classic piRNAs responsible for silencing retrotransponsons and the second wave is predominantly derived from nontransposon intergenic regions in pachytene spermatocytes, but the function of these pachytene piRNAs is largely unknown. Here, we report the involvement of pachytene piRNAs in instructing massive mRNA elimination in mouse elongating spermatids (ES). We demonstrate that a piRNA-induced silencing complex (pi-RISC) containing murine PIWI (MIWI) and deadenylase CAF1 is selectively assembled in ES, which is responsible for inducing mRNA deadenylation and decay via a mechanism that resembles the action of miRNAs in somatic cells. Such a highly orchestrated program appears to take full advantage of the enormous repertoire of diversified targeting capacity of pachytene piRNAs derived from nontransposon intergenic regions. These findings suggest that pachytene piRNAs are responsible for inactivating vast cellular programs in preparation for sperm production from ES.

c-kit expression profile and regulatory factors during spermatogonial stem cell differentiation

Background

It has been proven that c-kit is crucial for proliferation, migration, survival and maturation of spermatogenic cells. A periodic expression of c-kit is observed from primordial germ cells (PGCs) to spermatogenetic stem cells (SSCs), However, the expression profile of c-kit during the entire spermatogenesis process is still unclear. This study aims to reveal and compare c-kit expression profiles in the SSCs before and after the anticipated differentiation, as well as to examine its relationship with retinoic acid (RA) stimulation.

Results

We have found that there are more than 4 transcripts of c-kit expressed in the cell lines and in the testes. The transcripts can be divided into short and long categories. The long transcripts include the full-length canonical c-kit transcript and the 3′ end short transcript. Short transcripts include the 3.4 kb short transcript and several truncated transcripts (1.9-3.2 kb). In addition, the 3.4 kb transcript (starting from intron 9 and covering exons 10 ~ 21) is discovered to be specifically expressed in the spermatogonia. The extracellular domain of Kit is obtained in the spermatogonia stage, but the intracellular domain (50 kDa) is constantly expressed in both SSCs and spermatogonia. The c-kit expression profiles in the testis and the spermatogonial stem cell lines vary after RA stimulation. The wave-like changes of the quantitative expression pattern of c-kit (increase initially and decrease afterwards) during the induction process are similar to that of the in vivo male germ cell development process.

Conclusions

There are dynamic transcription and translation changes of c-kit before and after SSCs’ anticipated differentiation and most importantly, RA is a significant upstream regulatory factor for c-kit expression.

Basigin null mutant male mice are sterile and exhibit impaired interactions between germ cells and Sertoli cells

Basigin (BSG) is a multifunctional glycoprotein that plays an important role in male reproduction since male knockout (KO) mice are sterile. The Bsg KO testis lacks elongated spermatids and mature spermatozoa, a phenotype similar to that of alpha-mannosidase IIx (MX) KO mice. MX regulates formation of N-acetylglucosamine (GlcNAc) terminated N-glycans that participate in germ cell-Sertoli cell adhesion. Results showed that Bsg KO spermatocytes displayed normal homologous chromosome synapsis and progression through meiosis. However, only punctate expression of the round spermatid marker SP-10 in the acrosomal granule of germ cells of Bsg KO mice was detected indicating that spermatogenesis in Bsg KO mice was arrested at the early round spermatid stages. We observed a large increase in the number of germ cells undergoing apoptosis in Bsg KO testes. Using lectin blotting, we determined that GlcNAc terminated N-glycans are linked to BSG. GlcNAc terminated N-glycans were significantly reduced in Bsg KO testes. These observations indicate that BSG may act as a germ cell-Sertoli cell attachment molecule. Loss of BSG significantly reduced adhesion between GC-2 and SF7 cells. Moreover, wild type testes showed strong expression of N-cadherin (CDH2) while expression was greatly reduced in the testes of Bsg KO mice. In addition, the integrity of the blood-testis barrier (BTB) was compromised in Bsg KO testes. In conclusion, although some Bsg KO spermatogonia can undergo normal progression to the spermatocyte stage, BSG-mediated germ cell-Sertoli cell interactions appear to be necessary for integrity of the BTB and spermatocyte progression to mature spermatozoa.

Importin alpha2-interacting proteins with nuclear roles during mammalian spermatogenesis

Spermatogenesis, the process of generating haploid sperm capable of fertilizing the female gamete, requires the timely transport into the nucleus of transcription and chromatin-remodeling factors, mediated by members of the importin (IMP) superfamily. Previous IMP expression profiling implies a role for IMPalpha2 in testicular germ cells late in spermatogenesis. To identify interacting proteins of IMPalpha2 that are potential drivers of germ cell development, we performed yeast two-hybrid screening of an adult mouse testis library. IMPalpha2 interactions were verified by coimmunoprecipitation approaches, whereas immunohistochemical staining of testis sections confirmed their coexpression with IMPalpha2 in specific testicular cell types. Key interactors identified were a novel isoform of a cysteine and histidine rich protein (Chrp), a protein inhibitor of activated STAT (PIAS) family member involved in transcriptional regulation and sumoylation, Androgen receptor interacting protein 3 (Arip3), and Homologous protein 2 (Hop2), known to be involved in homologous chromosome pairing and recombination, all of which are highly expressed in the testis and show mRNA expression profiles similar to that of IMPalpha2 throughout testicular development. This is the first study to identify binding partners of IMPalpha2 in the developmental context of germ line development, and we propose that the regulated expression and timely IMPalpha2-mediated nuclear transport of these proteins may coordinate events during spermatogenesis, with IMPalpha2-mediated nuclear localization representing a potentially critical developmental switch in the testis.

TSEG-1, a novel member of histone H2A variants, participates in spermatogenesis via promoting apoptosis of spermatogenic cells

A novel variant of histone H2A, named as testis specific expressed gene 1 (TSEG-1, approved symbol: H2afb1), was identified from adult mouse testis. The TSEG-1 gene is 610-bp in length and consists of one exon. TSEG-1 transcript was robustly and exclusively expressed in adult mouse testis, mainly in spermatocytes. In developmental testis, the TSEG-1 transcript was robustly expressed since postnatal day (P) 21, peaked at P30, and gradually decreased in the testis of aging mouse. The surgical cryptorchidism mouse model showed an increase in the TSEG-1 expression, accompanied by enhanced apoptosis of spermatogenic cells. The EGFP-tagged TSEG-1 protein is located in the nuclei of cultured spermatocytes (GC-2spd cells). Transfection of TSEG-1 into GC-2spd cells resulted in suppressed cell viabilities, increased apoptosis, and decreased mitochondrial membrane potential. Intratesticular injection of TSEG-1 resulted in increased apoptosis of spermatogenic cells in vivo. These results suggest that TSEG-1 may participate in the spermatogenesis via regulating the apoptosis of spermatogenic cells.

Genomic organization and structure of the 5'-flanking region of the TEX101 gene: alternative promoter usage and splicing generate transcript variants with distinct 5'-untranslated region

A novel germ cell-specific antigen, TEX101 (TES101-reactive protein), was previously identified using a monoclonal antibody directed against mouse testicular cells. TEX101 is specifically located on the plasma membrane of germ cells, and its expression in gonadal organs is sexually dimorphic. To understand the fundamental mechanism directing gene expression, the genomic organization of TEX101 was studied. The gene consists of five translated exons (exons 2-6) and three 5'-untranslated exons (exon 1a, 1b, and 1c), respectively. TEX101 forms three major transcripts classified by usage of the three 5'-untranslated exons. One form of TEX101 mRNA is transcribed from exon 1c and spliced to the common acceptor site in exon 2. In the second form of the transcript, exon 1a is spliced to exon 1b and exon 2 in a sequential manner. Splicing from exon 1a to exon 2, arises the third form of transcript. Reverse Transcription (RT)-polymerase chain reaction (PCR) analysis demonstrated differential expression pattern of the TEX101 transcripts between testis and ovary. Whereas the expression of transcript-1 is constitutive in male and female gonads, the transcript-2 and -3 are detected only after starting of the spermatogenesis. Luciferase reporter assays using GC-2spd(ts) cells, a cell line from immortalized mouse testicular cells, showed that the 5'-flanking sequence of exon 1c has higher promoter activity than exon 1a. Deletion analysis of the chimeric structures indicated that sequences essential to gene expression are present on the 5'-flanking region between -3186 and +14, where the cluster of five CAAT boxes is located. Taken together, these findings should facilitate an understanding of the regulation of TEX101 expression during gametogenesis.

The transcription factor CREMtau and cAMP regulate promoter activity of the Na,K-ATPase alpha4 isoform

The Na,K-ATPase is an essential enzyme of the plasma membrane that plays a key role in numerous cell processes that depend on the transcellular gradients of Na(+) and K(+). Among the various isoforms of the catalytic subunit of the Na,K-ATPase, alpha4 exhibits the most limited pattern of expression, being restricted to male germ cells. Activity of alpha4 is essential for sperm function, and alpha4 is upregulated during spermatogenesis. The present study addressed the transcriptional control of the human Na,K-ATPase alpha4 gene, ATP1A4. We describe that a 5' untranslated region of the ATP1A4 gene (designated -339/+480 based on the ATP1A4 transcription initiation site) has promoter activity in luciferase reporter assays. Computer analysis of this promoter region revealed consensus sites (CRE) for the cyclic AMP (cAMP) response element modulator (CREM). Accordingly, dibutyryl cAMP (db-cAMP) and ectopic expression of CREMtau, a testis specific splice variant of CREM were able to activate the ATP1A4 promoter driven expression of luciferase in HEK 293 T, JEG-3 and GC-1 cells. Further characterization of the effect of db-cAMP and CREMtau on deleted constructs of the ATP1A4 promoter (-339/+80, and +25/+480), and on the -339/+480 region carrying mutations in the CRE sites showed that db-cAMP and CREMtau effect required the CRE motif located 263 bp upstream the transcription initiation site. EMSA experiments confirmed the CRE sequence as a bonafide CREMtau binding site. These results constitute the first demonstration of the transcriptional control of ATP1A4 gene expression by cAMP and by CREMtau, a transcription factor essential for male germ cell gene expression.

Reciprocal regulation of the mouse protamine genes by the orphan nuclear receptor germ cell nuclear factor and CREMtau

Germ cell nuclear factor (GCNF) is a member of the nuclear receptor superfamily, which is expressed in the adult predominantly in the male and female germ cells. In the male, GCNF is expressed in spermatogenic cells. GCNF binds as a homodimer to direct repeat response elements of the consensus half-site sequence, AGGTCA, with 0 bp spacing (DR0). Using this information, a search of genomic databases was performed to identify candidate GCNF responsive, spermatogenic-specific, genes that contain DR0 sequences. The mouse protamine genes are the strongest candidates identified to date, as they are post-meiotically expressed in round spermatids and contain DR0 elements in their proximal promoters. Previous work has shown that both recombinant and endogenous GCNF bind to DR0 elements in the mouse protamine 1 and 2 (Prm 1 and Prm 2) promoters with high affinity and specificity. The present work shows that in transient transfection assays in GC-1 and JEG-3 cells, co-transfection of a GCNF-VP16 expression plasmid with reporter plasmids containing either the wild type Prm 1 or Prm 2 promoter established that GCNF-VP16 is able to regulate transcription from both promoters in a DR0-dependent manner. Wild type GCNF, in contrast, acts as a repressor of basal transcription on both the Prm 1 and Prm 2 promoters in a DR0-dependent manner. Furthermore, CREMtau activation of these promoters is also repressed by wild-type GCNF, indicating that GCNF also acts as a repressor of activated transcription. GCNF therefore defines a novel nuclear receptor-signaling pathway that may regulate a subset of genes involved in the terminal differentiation process of spermatogenesis, exemplified by the protamines.

Usp9y (ubiquitin-specific protease 9 gene on the Y) is associated with a functional promoter and encodes an intact open reading frame homologous to Usp9x that is under selective constraint

Sequences complementary to the X-linked ubiquitin-specific protease gene Usp9x (Dffrx) have been shown to map to the Sxr(b) interval of the mouse Y Chromosome (chr) and to be expressed in a testis-specific manner. In humans, ubiquitously expressed functional homologues (USP9Y and USP9X DFFRY/DFFRX) are present on both sex chromosomes, whereas in mouse it remains to be demonstrated that the Y-linked sequences encode a functional protein. In this paper, it is shown that the Usp9y gene encodes a potentially functional ubiquitin-specific protease possessing a core promoter region that shares several features characteristic of other testis-specific genes. Analysis of synonymous and nonsynonymous nucleotide changes suggests that there is constraint on the amino acid sequence of both the mouse Usp9x and Usp9y genes, a finding that mirrors similar analysis of the human orthologs. Thus, in both mouse and human, selection is acting to maintain the amino acid sequence of the X and Y-linked genes. This indicates that in both species the genes on each sex chromosome continue to encode an important function.