Primary piRNA biogenesis: caught up in a Maelstrom

Decabromodiphenyl ether induces the chromosome association disorders of spermatocytes and deformation failures of spermatids in mice

The environmental presence of decabromodiphenyl ether (BDE-209), which is toxic to the male reproductive system, is widespread. The current study investigated its mechanism of toxicity in mice. The results showed, that BDE-209 induced DNA damage, decreased the expression of the promoter of meiosis spermatogenesis- and oogenesis-specific basic helix-loop-helix 1 (Sohlh1), meiosis related-factors Lethal (3) malignant brain tumor like 2 (L3MBTL2), PIWI-like protein 2 (MILI), Cyclin-dependent kinase 2 (CDK2), Cyclin A, synaptonemal complex protein 1 (SYCP1) and synaptonemal complex protein 3 (SYCP3), and caused spermatogenic cell apoptosis, resulting in a decrease in sperm quantity and quality. Furthermore, BDE-209 downregulated the levels of anaphase-promoting complex/cyclosome (APC/C), increased the expression of PIWI-like protein 1 (MIWI) in the cytoplasm of elongating spermatids, and decreased the nuclear levels of RING finger protein 8 (RNF8), ubiquitinated (ub)-H2A/ub-H2B, and Protamine 1 (PRM1)/Protamine 2 (PRM2), while increasing H2A/H2B nuclear levels in spermatids. The reproductive toxicity was persistent for 50 days following the withdrawal of BDE-209 exposure. The results suggested that BDE-209 inhibits the initiation of meiosis by decreasing the expression of Sohlh1. Furthermore, the reduced expression of L3MBTL2 inhibited the formation of chromosomal synaptonemal complexes by depressing the expression of meiosis regulators affecting the meiotic progression and also inhibited histone ubiquitination preventing the replacement of histones by protamines, by preventing RNF8 from entering nuclei, which affected the evolution of spermatids into mature sperm.

The MAEL expression in mitochondria of human spermatozoa and the association with asthenozoospermia

PURPOSE

The maelstrom spermatogenic transposon silencer (MAEL) function in postmeiotic germ cells remains unclear, and its protein localization in human testis and spermatozoa awaits determination. This study aims to clarify the MAEL expression in human spermatogenesis and to explore its role in sperm function.

MATERIALS AND METHODS

Twenty-seven asthenozoospermic men, 40 normozoospermic controls, and three obstructive azoospermic men were enrolled. The transcripts of MAEL in the seminiferous epithelium and MAEL downstream targets were identified by bioinformatics analysis. MAEL protein expression in human testis and ejaculated sperms were examined by immunohistochemical and immunogold staining, respectively. The roles of MAEL in mitochondria function were investigated by siRNA knockdown in human H358 cells. The association between MAEL protein levels and clinical sperm features was evaluated.

RESULTS

Abundant MAEL was expressed in spermatid and spermatozoa of the human testis. Remarkably, MAEL was located in the mitochondria of ejaculated sperm, and bioinformatics analysis identified GPX4 and UBL4B as MAEL's downstream targets. Knockdown of MAEL sabotaged mitochondria function and reduced adenosine triphosphate (ATP) production in H358 cells. MAEL, GPX4, and UBL4B expression levels were significantly decreased in asthenozoospermic sperms than in controls. The MAEL protein levels were positively correlated with GPX4 and UBL4B in human sperm. Total motile sperm count (TMSC) was positively correlated with protein levels of MAEL, GPX4, and UBL4B in ejaculated sperms.

CONCLUSIONS

We highlight prominent MAEL expression in the intratesticular spermatid and the mitochondria of ejaculated spermatozoa. MAEL directly binds to GPX4 and UBL4B, and loss of MAEL induces mitochondrial dysfunction. MAEL-mitochondrial function-motility relationship might advance our understanding of the causes of asthenozoospermia.

Cis- and trans-regulation by histone H4 basic patch R17/R19 in metazoan development

The histone H4 basic patch is critical for chromatin structure and regulation of the chromatin machinery. However, the biological roles of these positively charged residues and the mechanisms by which they regulate gene expression remain unclear. In this study, we used histone mutagenesis to investigate the physiological function and downstream regulatory genes of H4 residues R17 and R19 in Drosophila. We found all histone mutations including R17A/E/H and R19A/E/H (R17 and R19 of H4 are substituted by A, E and H respectively) result in a range of growth defects and abnormalities in chromosomal high-order structures, whereas R17E mutation is embryonic lethal. RNA-seq demonstrates that downregulated genes in both R17A and R19A show significant overlap and are enriched in development-related pathways. In addition, Western and cytological analyses showed that the R17A mutation resulted in a significant reduction in H4K16 acetylation and male offspring, implying that the R17 may be involved in male dosage compensation mechanisms. R19 mutation on the other hand strongly affect Gpp (Dot1 homologue in flies)-mediated H3K79 methylation, possibly through histone crosstalk. Together these results provide insights into the differential impacts of positive charges of H4 basic patch R17/R19 on regulation of gene transcription during developmental processes.

Knockout Gene-Based Evidence for PIWI-Interacting RNA Pathway in Mammals

The PIWI-interacting RNA (piRNA) pathway mainly consists of evolutionarily conserved protein factors. Intriguingly, many mutations of piRNA pathway factors lead to meiotic arrest during spermatogenesis. The majority of piRNA factor-knockout animals show arrested meiosis in spermatogenesis, and only a few show post-meiosis male germ cell arrest. It is still unclear whether the majority of piRNA factors expressed in spermatids are involved in long interspersed nuclear element-1 repression after meiosis, but future conditional knockout research is expected to resolve this. In addition, recent hamster knockout studies showed that a piRNA factor is necessary for oocytes-in complete contrast to the findings in mice. This species discrepancy allows researchers to reexamine the function of piRNA in female germ cells. This mini-review focuses on the current knowledge of protein factors derived from mammalian knockout studies and summarizes their roles in the biogenesis and function of piRNAs.

Of rodents and ruminants: a comparison of small noncoding RNA requirements in mouse and bovine reproduction

Ruminants are major producers of meat and milk, thus managing their reproductive potential is a key element in cost-effective, safe, and efficient food production. Of particular concern, defects in male germ cells and female germ cells may lead to significantly reduced live births relative to fertilization. However, the underlying molecular drivers of these defects are unclear. Small noncoding RNAs, such as piRNAs and miRNAs, are known to be important regulators of germ-cell physiology in mouse (the best-studied mammalian model organism) and emerging evidence suggests that this is also the case in a range of ruminant species, in particular bovine. Similarities exist between mouse and bovids, especially in the case of meiotic and postmeiotic male germ cells. However, fundamental differences in small RNA abundance and metabolism between these species have been observed in the female germ cell, differences that likely have profound impacts on their physiology. Further, parentally derived small noncoding RNAs are known to influence early embryos and significant species-specific differences in germ-cell born small noncoding RNAs have been observed. These findings demonstrate the mouse to be an imperfect model for understanding germ-cell small noncoding RNA biology in ruminants and highlight the need to increase research efforts in this underappreciated aspect of animal reproduction.

Effect of ovarian stimulation on the expression of piRNA pathway proteins

PIWI-interacting RNAs (piRNAs) play an important role in gametogenesis, fertility and embryonic development. The current study investigated the effect of different doses of pregnant mare serum gonadotrophin/human chorionic gonadotrophin (PMSG/hCG) and repeated ovarian stimulation (OS) on the expression of the Mili, Miwi, Mael, Tdrd1, Tdrd9, qnd Mitopld genes, which have crucial roles in the biogenesis and function of piRNAs. Here, we found that after treatment with 7.5 I.U. PMSG/hCG and two repeated rounds of OS, both the mRNA and protein levels of Tdrd9, Tdrd1 and Mael showed the greatest decrease in the ovarian tissue, but the plasma E₂ levels showed the strongest increases (p<0.05). However, we found that the Mitopld, Miwi and Mili gene levels were decreased significantly after treatment with 12.5 I.U. PMSG/hCG. Our results suggested that exogenous gonadotropin administration leads to a significant decrease in the expression of the Mili, Miwi, Mael, Tdrd1, Tdrd9 and Mitopld genes, which are critically important in the piRNA pathway, and the changes in the expression levels of Tdrd9, Tdrd1 and Mael may be associated with plasma E₂ levels. New comprehensive studies are needed to reduce the potential effects of OS on the piRNA pathway, which silences transposable elements and maintains genome integrity, and to contribute to the safety of OS.

PIWI-Interacting RNA in Drosophila: Biogenesis, Transposon Regulation, and Beyond

PIWI-interacting RNAs (piRNAs) are germline-enriched small RNAs that control transposons to maintain genome integrity. To achieve this, upon being processed from piRNA precursors, most of which are transcripts of intergenic piRNA clusters, piRNAs bind PIWI proteins, germline-specific Argonaute proteins, to form effector complexes. The mechanism of this piRNA-mediated transposon silencing pathway is fundamentally similar to that of siRNA/miRNA-dependent gene silencing in that a small RNA guides its partner Argonaute protein to target gene transcripts for repression via RNA-RNA base pairing. However, the uniqueness of this piRNA pathway has emerged through intensive genetic, biochemical, bioinformatic, and structural investigations. Here, we review the studies that elucidated the piRNA pathway, mainly in Drosophila, by describing both historical and recent progress. Studies in other species that have made important contributions to the field are also described.

Critical Role of Hepatic Cyp450s in the Testis-Specific Toxicity of (5R)-5-Hydroxytriptolide in C57BL/6 Mice

Low solubility, tissue accumulation, and toxicity are chief obstacles to developing triptolide derivatives, so a better understanding of the pharmacokinetics and toxicity of triptolide derivatives will help with these limitations. To address this, we studied pharmacokinetics and toxicity of (5R)-5-hydroxytriptolide (LLDT-8), a novel triptolide derivative immunosuppressant in a conditional knockout (KO) mouse model with liver-specific deletion of CYP450 reductase. Compared to wild type (WT) mice, after LLDT-8 treatment, KO mice suffered severe testicular toxicity (decreased testicular weight, spermatocytes apoptosis) unlike WT mice. Moreover, KO mice had greater LLDT-8 exposure as confirmed with elevated AUC and Cmax, increased drug half-life, and greater tissue distribution. γ-H2AX, a marker of meiosis process, its localization and protein level in testis showed a distinct meiosis block induced by LLDT-8. RNA polymerase II (Pol II), an essential factor for RNA storage and synapsis in spermatogenesis, decreased in testes of KO mice after LLDT-8 treatment. Germ-cell line based assays confirmed that LLDT-8 selectively inhibited Pol II in spermatocyte-like cells. Importantly, the analysis of androgen receptor (AR) related genes showed that LLDT-8 did not change AR-related signaling in testes. Thus, hepatic CYP450s were responsible for in vivo metabolism and clearance of LLDT-8 and aggravated testicular injury may be due to increased LLDT-8 exposure in testis and subsequent Pol II reduction.

Small non-coding RNAs and their associated proteins in spermatogenesis

The importance of the gene regulation roles of small non-coding RNAs and their protein partners is of increasing focus. In this paper, we reviewed three main small RNA species which appear to affect spermatogenesis. MicroRNAs (miRNAs) are single stand RNAs derived from transcripts containing stem-loops and hairpins which target corresponding mRNAs and affect their stability or translation. Many miRNA species have been found to be related to normal male germ cell development. The biogenesis of piRNAs is still largely unknown but several models have been proposed. Some piRNAs and PIWIs target transposable elements and it is these that may be active in regulating translation or stem cell maintenance. endo-siRNAs may also participate in sperm development. Some possible interactions between different kinds of small RNAs have even been suggested. We also show that male germ granules are seen to have a close relationship with a considerable number of mRNAs and small RNAs. Those special structures may also participate in sperm development.