An essential role for PNLDC1 in piRNA 3' end trimming and male fertility in mice

Factories without walls: The molecular architecture and functions of non-membrane organelles in small RNA-guided genome protection

Non-membrane organelles, Yb body and nuage, play an essential role in piRNA-guided genome defense in Drosophila gonad by mediating piRNA biogenesis and transposon silencing. Yb body, found in somatic follicle cells, is responsible for primary piRNA processing, while nuage, located in germline cells, facilitates the ping-pong cycle to amplify the piRNAs corresponding to both sense and antisense strands of the expressed transposons. These organelles are assembled by liquid-liquid phase separation (LLPS) and protein-protein interactions, integrating RNA helicases (Vasa, Armitage), Tudor domain-containing proteins (Krimper, Tejas, Qin/Kumo), and proteins containing both domains (Yb, SoYb, Spn-E). Within these condensates, we summarize the protein-protein interactions experimentally validated and predicted by AlphaFold3, providing new structural insights into the non-membrane organelle assembly. This review highlights how the dynamic organization of Yb body and nuage enables efficient RNA processing, ensuring transposon suppression and genome stability.

Insights into the role of hnRNPK in spermatogenesis via the piRNA pathway

Deletion of hnRNPK in mouse spermatogonia leads to male sterility due to arrest permatogenesis, yet the underlying molecular mechanisms remain elusive. This study investigated the testicular proteome on postnatal day 28 (P28) to elucidate the infertility associated with Hnrnpk deficiency, identifying 791 proteins with altered expression: 256 were upregulated, and 535 were downregulated. Pathway enrichment analysis demonstrated that the downregulated proteins are primarily involved in spermatogenesis, fertilization, and piRNA metabolic processes. In Hnrnpk cKO mice, key proteins essential for piRNA metabolism, such as PIWIL1, TDRD7, DDX4, and MAEL, exhibited reduced expression, resulting in impaired piRNA production. Mechanistic studies employing RNA immunoprecipitation (RIP), dual-luciferase reporter assays, and fluorescence in situ hybridization/immunofluorescence (FISH/IF) assays demonstrated that hnRNPK directly interacts with the 3'UTR of piRNA pathway transcripts, enhancing their translational efficiency. These results establish that Hnrnpk deficiency disrupts the piRNA pathway by diminishing the expression of essential regulatory proteins, thereby impairing piRNA production and spermatogenesis. Our findings elucidate a novel molecular basis for infertility linked to hnRNPK dysfunction and advance understanding of post-transcriptional regulation in male germ cell development.

Identified Candidate Genes of Semen Trait in Three Pig Breeds Through Weighted GWAS and Multi-Tissue Transcriptome Analysis

High-quality semen is an essential factor for the success of artificial insemination, and revealing the genetic structure of pig semen traits helps improve semen quality. This study aimed to identify candidate genes associated with semen traits in three pig breeds (Duroc, Landrace, and Yorkshire) through weighted GWAS and multi-tissue transcriptome analysis. In this study, to identify candidate genes associated with semen traits in Duroc, Landrace, and Yorkshire, we performed weighted GWAS in four traits (sperm motility, sperm progressive motility, sperm abnormality rate, and total sperm count) using 936 pigs and multi-tissue transcriptome analysis using 34 tissues RNA-seq data of 5457 pigs from FarmGTEx. It was found that 16, 9, and 12 significant SNPs associated with semen traits were identified in Duroc, Landrace, and Yorkshire, with corresponding 7, 5, and 7 candidate genes in these three breeds, respectively, which may be involved in mammal spermatogenesis, testicular function, and male fertility. Moreover, we not only found the same candidate gene DNAI2 as in previous studies but also found two new candidate genes PNLDC1 and RSPH3, which were identified simultaneously in both Landrace and Yorkshire. By integrating the GWAS and multi-tissue transcriptome analysis results, we found that candidate genes associated with semen traits of three pig breeds were highly expressed in the testis tissue. The three genotypes of rs320928244 had significant effects on the expression of the DYNLT1 gene in the testis tissue of Landrace. These results together showed that these candidate genes were mainly related to sperm motility defects. This study helps deepen the understanding of the genetic basis of semen traits and provides a theoretical foundation for improving the semen quality of Duroc, Landrace, and Yorkshire breeds.

ASB1 engages with ELOB to facilitate SQOR ubiquitination and H2S homeostasis during spermiogenesis

Male infertility, frequently driven by oxidative stress, impacts half of infertile couples globally. Despite its significance, the precise mechanisms governing this process remain elusive. In this study, we demonstrate that ASB1, the substrate recognition subunit of a ubiquitin ligase, is highly expressed in the mouse testis. Mice lacking the Asb1 gene exhibit severe fertility impairment, characterized by oligoasthenoteratozoospermia. Subsequent investigations unveiled that Asb1 knockout (Asb1-KO) mice encountered excessive oxidative stress and decreased hydrogen sulfide (H2S) levels in their testes, and severe sperm DNA damage. Notably, the compromised fertility and sperm quality in Asb1-KO mice was significantly ameliorated by administering NaHS, a H2S donor. Mechanistically, ASB1 interacts with ELOB to induce the instability of sulfide-quinone oxidoreductase (SQOR) by enhancing its K48-linked ubiquitination on residues K207 and K344, consequently triggering proteasomal degradation. This process is crucial for preserving H2S homeostasis and redox balance. Overall, our findings offer valuable insights into the role of ASB1 during spermiogenesis and propose H2S supplementation as a promising therapeutic approach for oxidative stress-related male infertility.

piRNA processing within non-membrane structures is governed by constituent proteins and their functional motifs

Discovered two decades ago, PIWI-interacting RNAs (piRNAs) are crucial for silencing transposable elements (TEs) in animal gonads, thereby protecting the germline genome from harmful transposition, and ensuring species continuity. Silencing of TEs is achieved through transcriptional and post-transcriptional suppression by piRNAs and the PIWI clade of Argonaute proteins within non-membrane structured organelle. These structures are composed of proteins involved in piRNA processing, including PIWIs and other proteins by distinct functional motifs such as the Tudor domain, LOTUS, and intrinsic disordered regions (IDRs). This review highlights recent advances in understanding the roles of these conserved proteins and structural motifs in piRNA biogenesis. We explore the molecular mechanisms of piRNA biogenesis, with a primary focus on Drosophila as a model organism, identifying common themes and species-specific variations. Additionally, we extend the discussion to the roles of these components in nongonadal tissues.

Functions and applications of RNA interference and small regulatory RNAs

Small regulatory RNAs play a variety of crucial roles in eukaryotes, influencing gene regulation, developmental timing, antiviral defense, and genome integrity via a process termed RNA interference (RNAi). This process involves Argonaute/small RNA (AGO/sRNA) complexes that target transcripts via sequence complementarity and modulate gene expression and epigenetic modifications. RNAi is a highly conserved gene regulatory phenomenon that recognizes self- and non-self nucleic acids, thereby defending against invasive sequences. Since its discovery, RNAi has been widely applied in functional genomic studies and a range of practical applications. In this review, we focus on the current understanding of the biological roles of the RNAi pathway in transposon silencing, fertility, developmental regulation, immunity, stress responses, and acquired transgenerational inheritance. Additionally, we provide an overview of the applications of RNAi technology in biomedical research, agriculture, and therapeutics.

Advances in PIWI-piRNA function in female reproduction in mammals

PIWI-interacting RNAs (piRNAs), which associate with PIWI clade Argonaute proteins to form piRNA-induced silencing complexes (piRISCs) in germline cells, are responsible for maintaining genomic integrity and reproductive function through transcriptional or post-transcriptional suppression of transposable elements and regulation of protein-coding genes. Recent discoveries of crucial PIWI-piRNA functions in oogenesis and embryogenesis in golden hamsters suggest an indispensable role in female fertility that has been obscured in the predominant mouse model of PIWI-piRNA pathway regulation. In particular, studies of piRNA expression dynamics, functional redundancies, and compositional variations across mammal species have advanced our understanding of piRNA functions in male and, especially, female reproduction. These findings further support the use of hamsters as a more representative model of piRNA biology in mammals. In addition to discussing these new perspectives, the current review also covers emerging directions for piRNA research, its implications for female fertility, and our fundamental understanding of reproductive mechanisms.

The functions and mechanisms of piRNAs in mediating mammalian spermatogenesis and their applications in reproductive medicine

As the most abundant small RNAs, piwi-interacting RNAs (piRNAs) have been identified as a new class of non-coding RNAs with 24-32 nucleotides in length, and they are expressed at high levels in male germ cells. PiRNAs have been implicated in the regulation of several biological processes, including cell differentiation, development, and male reproduction. In this review, we focused on the functions and molecular mechanisms of piRNAs in controlling spermatogenesis, including genome stability, regulation of gene expression, and male germ cell development. The piRNA pathways include two major pathways, namely the pre-pachytene piRNA pathway and the pachytene piRNA pathway. In the pre-pachytene stage, piRNAs are involved in chromosome remodeling and gene expression regulation to maintain genome stability by inhibiting transposon activity. In the pachytene stage, piRNAs mediate the development of male germ cells via regulating gene expression by binding to mRNA and RNA cleavage. We further discussed the correlations between the abnormalities of piRNAs and male infertility and the prospective of piRNAs' applications in reproductive medicine and future studies. This review provides novel insights into mechanisms underlying mammalian spermatogenesis and offers new targets for diagnosing and treating male infertility.

PNLDC1 catalysis and postnatal germline function are required for piRNA trimming, LINE1 silencing, and spermatogenesis in mice

PIWI-interacting RNAs (piRNAs) play critical and conserved roles in transposon silencing and gene regulation in the animal germline. Three distinct piRNA populations are present during mouse spermatogenesis: fetal piRNAs in fetal/perinatal testes, pre-pachytene and pachytene piRNAs in postnatal testes. PNLDC1 is required for piRNA 3' end maturation in multiple species. However, whether PNLDC1 is the bona fide piRNA trimmer and the physiological role of 3' trimming of different piRNA populations in spermatogenesis in mammals remain unclear. Here, by inactivating Pnldc1 exonuclease activity in vitro and in mice, we reveal that the PNLDC1 trimmer activity is essential for spermatogenesis and male fertility. PNLDC1 catalytic activity is required for both fetal and postnatal piRNA 3' end trimming. Despite this, postnatal piRNA trimming but not fetal piRNA trimming is critical for LINE1 transposon silencing. Furthermore, conditional inactivation of Pnldc1 in postnatal germ cells causes LINE1 transposon de-repression and spermatogenic arrest in mice, indicating that germline-specific postnatal piRNA trimming is essential for transposon silencing and germ cell development. Our findings highlight the germ cell-intrinsic role of PNLDC1 and piRNA trimming in mammals to safeguard the germline genome and promote fertility.

Inherited defects of piRNA biogenesis cause transposon de-repression, impaired spermatogenesis, and human male infertility

piRNAs are crucial for transposon silencing, germ cell maturation, and fertility in male mice. Here, we report on the genetic landscape of piRNA dysfunction in humans and present 39 infertile men carrying biallelic variants in 14 different piRNA pathway genes, including PIWIL1, GTSF1, GPAT2, MAEL, TDRD1, and DDX4. In some affected men, the testicular phenotypes differ from those of the respective knockout mice and range from complete germ cell loss to the production of a few morphologically abnormal sperm. A reduced number of pachytene piRNAs was detected in the testicular tissue of variant carriers, demonstrating impaired piRNA biogenesis. Furthermore, LINE1 expression in spermatogonia links impaired piRNA biogenesis to transposon de-silencing and serves to classify variants as functionally relevant. These results establish the disrupted piRNA pathway as a major cause of human spermatogenic failure and provide insights into transposon silencing in human male germ cells.

The dual role of Spn-E in supporting heterotypic ping-pong piRNA amplification in silkworms

The PIWI-interacting RNA (piRNA) pathway plays a crucial role in silencing transposons in the germline. piRNA-guided target cleavage by PIWI proteins triggers the biogenesis of new piRNAs from the cleaved RNA fragments. This process, known as the ping-pong cycle, is mediated by the two PIWI proteins, Siwi and BmAgo3, in silkworms. However, the detailed molecular mechanism of the ping-pong cycle remains largely unclear. Here, we show that Spindle-E (Spn-E), a putative ATP-dependent RNA helicase, is essential for BmAgo3-dependent production of Siwi-bound piRNAs in the ping-pong cycle and that this function of Spn-E requires its ATPase activity. Moreover, Spn-E acts to suppress homotypic Siwi-Siwi ping-pong, but this function of Spn-E is independent of its ATPase activity. These results highlight the dual role of Spn-E in facilitating proper heterotypic ping-pong in silkworms.

The burgeoning importance of PIWI-interacting RNAs in cancer progression

PIWI-interacting RNAs (piRNAs) are a class of small noncoding RNA molecules that specifically bind to piwi protein family members to exert regulatory functions in germ cells. Recent studies have found that piRNAs, as tissue-specific molecules, both play oncogenic and tumor suppressive roles in cancer progression, including cancer cell proliferation, metastasis, chemoresistance and stemness. Additionally, the atypical manifestation of piRNAs and PIWI proteins in various malignancies presents a promising strategy for the identification of novel biomarkers and therapeutic targets in the diagnosis and management of tumors. Nonetheless, the precise functions of piRNAs in cancer progression and their underlying mechanisms have yet to be fully comprehended. This review aims to examine current research on the biogenesis and functions of piRNA and its burgeoning importance in cancer progression, thereby offering novel perspectives on the potential utilization of piRNAs and piwi proteins in the management and treatment of advanced cancer.

piRNA loading triggers MIWI translocation from the intermitochondrial cement to chromatoid body during mouse spermatogenesis

The intermitochondrial cement (IMC) and chromatoid body (CB) are posited as central sites for piRNA activity in mice, with MIWI initially assembling in the IMC for piRNA processing before translocating to the CB for functional deployment. The regulatory mechanism underpinning MIWI translocation, however, has remained elusive. We unveil that piRNA loading is the trigger for MIWI translocation from the IMC to CB. Mechanistically, piRNA loading facilitates MIWI release from the IMC by weakening its ties with the mitochondria-anchored TDRKH. This, in turn, enables arginine methylation of MIWI, augmenting its binding affinity for TDRD6 and ensuring its integration within the CB. Notably, loss of piRNA-loading ability causes MIWI entrapment in the IMC and its destabilization in male germ cells, leading to defective spermatogenesis and male infertility in mice. Collectively, our findings establish the critical role of piRNA loading in MIWI translocation during spermatogenesis, offering new insights into piRNA biology in mammals.

Pre-piRNA trimming safeguards piRNAs against erroneous targeting by RNA-dependent RNA polymerase

The Piwi/Piwi-interacting RNA (piRNA) pathway protects genome integrity in animal germ lines. Maturation of piRNAs involves nucleolytic processing at both 5' and 3' ends. The ribonuclease PARN-1 and its orthologs mediate piRNA 3' trimming in worms, insects, and mammals. However, the significance of this evolutionarily conserved processing step is not fully understood. Employing C. elegans as a model, we recently discovered that 3' trimming protects piRNAs against non-templated nucleotide additions and degradation. Here, we find that worms lacking PARN-1 accumulate an uncharacterized RNA species termed anti-piRNAs, which are antisense to piRNAs. Anti-piRNAs associate with Piwi proteins, are 17-19 nucleotides long, and begin with 5' guanine or adenine. Untrimmed pre-piRNAs are misdirected by the terminal nucleotidyltransferase RDE-3 and RNA-dependent RNA polymerase EGO-1, leading to the formation of anti-piRNAs. This work identifies a class of small RNAs in parn-1 mutants and provides insight into the activities of RDE-3, EGO-1, and Piwi proteins.

Critical appraisal of the piRNA-PIWI axis in cancer and cancer stem cells

Small noncoding RNAs play an important role in various disease states, including cancer. PIWI proteins, a subfamily of Argonaute proteins, and PIWI-interacting RNAs (piRNAs) were originally described as germline-specific molecules that inhibit the deleterious activity of transposable elements. However, several studies have suggested a role for the piRNA-PIWI axis in somatic cells, including somatic stem cells. Dysregulated expression of piRNAs and PIWI proteins in human tumors implies that, analogously to their roles in undifferentiated cells under physiological conditions, these molecules may be important for cancer stem cells and thus contribute to cancer progression. We provide an overview of piRNA biogenesis and critically review the evidence for the role of piRNA-PIWI axis in cancer stem cells. In addition, we examine the potential of piRNAs and PIWI proteins to become biomarkers in cancer.

PNLDC1 catalysis and postnatal germline function are required for piRNA trimming, LINE1 silencing, and spermatogenesis in mice

PIWI-interacting RNAs (piRNAs) play critical and conserved roles in transposon silencing and gene regulation in the animal germline. Two distinct piRNA populations are present during mouse spermatogenesis: pre-pachytene piRNAs in fetal/neonatal testes and pachytene piRNAs in adult testes. PNLDC1 is required for both pre-pachytene piRNA and pachytene piRNA 3' end maturation in multiple species. However, whether PNLDC1 is the bona fide piRNA trimmer and the physiological role of 3' trimming of two distinct piRNA populations in spermatogenesis remain unclear. Here, by inactivating Pnldc1 exonuclease activity in vitro and in mice, we reveal that PNLDC1 trimmer activity is required for both pre-pachytene piRNA and pachytene piRNA 3' end trimming and male fertility. Furthermore, conditional inactivation of Pnldc1 in postnatal germ cells causes LINE1 transposon de-repression and spermatogenic arrest in mice. This indicates that pachytene piRNA trimming, but not pre-pachytene piRNA trimming, is essential for mouse germ cell development and transposon silencing. Our findings highlight the potential of inhibiting germline piRNA trimmer activity as a potential means for male contraception.

Pre-piRNA trimming safeguards piRNAs against erroneous targeting by RNA-dependent RNA Polymerase

In animal germ lines, The Piwi/piRNA pathway plays a crucial role in safeguarding genome integrity and promoting fertility. Following transcription from discrete genomic loci, piRNA precursors undergo nucleolytic processing at both 5' and 3' ends. The ribonuclease PARN-1 and its orthologs mediate piRNA 3' trimming in worms, insects and mammals. Yet, the significance of this evolutionarily conserved processing step is not well understood. Employing C. elegans as a model organism, our recent work has demonstrated that 3' trimming protects piRNAs against non-templated nucleotide additions and degradation. In this study, we present an unexpected finding that C. elegans deficient for PARN-1 accumulate a heretofore uncharacterized RNA species termed anti-piRNAs, which are antisense to piRNAs. These anti-piRNAs associate with Piwi proteins and display the propensity for a length of 17-19 nucleotides and 5' guanine and adenine residues. We show that untrimmed pre-piRNAs in parn-1 mutants are modified by the terminal nucleotidyltransferase RDE-3 and erroneously targeted by the RNA-dependent RNA polymerase EGO-1, thereby giving rise to anti-piRNAs. Taken together, our work identifies a previously unknown class of small RNAs upon loss of parn-1 and provides mechanistic insight to activities of RDE-3, EGO-1 and Piwi proteins.

Non-gonadal somatic piRNA pathways ensure sexual differentiation, larval growth, and wing development in silkworms

PIWI-interacting RNAs (piRNAs) guide PIWI proteins to target transposons in germline cells, thereby suppressing transposon activity to preserve genome integrity in metazoans' gonadal tissues. Piwi, one of three Drosophila PIWI proteins, is expressed in the nucleus and suppresses transposon activity by forming heterochromatin in an RNA cleavage-independent manner. Recently, Piwi was reported to control cell metabolism in Drosophila fat body, providing an example of piRNAs acting in non-gonadal somatic tissues. However, mutant flies of the other two PIWI proteins, Aubergine (Aub) and Argonaute3 (Ago3), show no apparent phenotype except for infertility, blurring the importance of the piRNA pathway in non-gonadal somatic tissues. The silkworm, Bombyx mori, possesses two PIWI proteins, Siwi (Aub homolog) and BmAgo3 (Ago3 homolog), whereas B. mori does not have a Piwi homolog. Siwi and BmAgo3 are mainly expressed in gonadal tissues and play a role in repressing transposon activity by cleaving transposon RNA in the cytoplasm. Here, we generated Siwi and BmAgo3 loss-of-function mutants of B. mori and found that they both showed delayed larval growth and failed to become adult moths. They also exhibited defects in wing development and sexual differentiation. Transcriptome analysis revealed that loss of somatic piRNA biogenesis pathways results in abnormal expression of not only transposons but also host genes, presumably causing severe growth defects. Our results highlight the roles of non-gonadal somatic piRNAs in B. mori development.

How genetic defects in piRNA trimming contribute to male infertility

In germ cells, small non-coding PIWI-interacting RNAs (piRNAs) work to silence harmful transposons to maintain genomic stability and regulate gene expression to ensure fertility. However, these piRNAs must undergo a series of steps during biogenesis to be properly loaded onto PIWI proteins and reach the correct nucleotide length. This review is focused on what we are learning about a crucial step in this process, piRNA trimming, in which pre-piRNAs are shortened to final lengths of 21-35 nucleotides. Recently, the 3'-5' exonuclease trimmer has been identified in various models as PNLDC1/PARN-1. Mutations of the piRNA trimmers in vivo lead to increased transposon expression, elevated levels of untrimmed pre-piRNAs, decreased piRNA stability, and male infertility. Here, we will discuss the role of piRNA trimmers in piRNA biogenesis and function, describe consequences of piRNA trimmer mutations using mammalian models and human patients, and examine future avenues of piRNA trimming-related study for clinical advancements for male infertility.

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.

Emerging roles and functional mechanisms of PIWI-interacting RNAs

PIWI-interacting RNAs (piRNAs) are a class of small non-coding RNAs that associate with proteins of the PIWI clade of the Argonaute family. First identified in animal germ line cells, piRNAs have essential roles in germ line development. The first function of PIWI-piRNA complexes to be described was the silencing of transposable elements, which is crucial for maintaining the integrity of the germ line genome. Later studies provided new insights into the functions of PIWI-piRNA complexes by demonstrating that they regulate protein-coding genes. Recent studies of piRNA biology, including in new model organisms such as golden hamsters, have deepened our understanding of both piRNA biogenesis and piRNA function. In this Review, we discuss the most recent advances in our understanding of piRNA biogenesis, the molecular mechanisms of piRNA function and the emerging roles of piRNAs in germ line development mainly in flies and mice, and in infertility, cancer and neurological diseases in humans.

Murine fertility and spermatogenesis are independent of the testis-specific Spdye4a gene

BACKGROUND

While many testis-enriched genes have been identified as important regulators of the spermatogenic process, the specific roles played by several of these genes and their functional importance has yet to be fully clarified.

METHODS

We employed a CRISPR/Cas9 approach to introduce a 5 bp in-frame deletion within the Spdye4a gene (Exon 2) of C57BL/6 mice (Spdye4a^-/-). Fertility and sperm counts were evaluated. Testes tissues and cell suspensions were analyzed via histological and immunofluorescence staining. mRNA and protein levels of candidate genes were assessed through qPCR and Western blotting. In vitro fertilization was used to assess the ability of sperm cells to bind to egg cells.

RESULTS

Spdye4a^-/- mice did not exhibit any reduction in fertility, and exhibited comparable sperm counts, morphology and motility to those of wildtype littermates. Functionally, Spdye4a^-/- sperm exhibited normal sperm-egg binding activity in vitro. Furthermore, the testes of Spdye4a^-/- mice exhibited a full range of germ cells from spermatogonia to mature spermatozoa. No differences in the progression of meiotic prophase I were observed when comparing Spdye4a^-/- and wildtype mice, indicating that the loss of Spdye4a had no adverse effect on spermatogenesis.

DISCUSSION

Spdye4a is dispensable in the context of mice fertility and spermatogenesis. This study will prevent other laboratories from expending repeated efforts to generate similar knockout mice.

The two Gtsf paralogs in silkworms orthogonally activate their partner PIWI proteins for target cleavage

The PIWI-interacting RNA (piRNA) pathway is a protection mechanism against transposons in animal germ cells. Most PIWI proteins possess piRNA-guided endonuclease activity, which is critical for silencing transposons and producing new piRNAs. Gametocyte-specific factor 1 (Gtsf1), an evolutionarily conserved zinc finger protein, promotes catalysis by PIWI proteins. Many animals have multiple Gtsf1 paralogs; however, their respective roles in the piRNA pathway are not fully understood. Here, we dissected the roles of Gtsf1 and its paralog Gtsf1-like (Gtsf1L) in the silkworm piRNA pathway. We found that Gtsf1 and Gtsf1L preferentially bind the two silkworm PIWI paralogs, Siwi and BmAgo3, respectively, and facilitate the endonuclease activity of each PIWI protein. This orthogonal activation effect was further supported by specific reduction of BmAgo3-bound Masculinizer piRNA and Siwi-bound Feminizer piRNA, the unique piRNA pair required for silkworm feminization, upon depletion of Gtsf1 and Gtsf1L, respectively. Our results indicate that the two Gtsf paralogs in silkworms activate their respective PIWI partners, thereby facilitating the amplification of piRNAs.

Testis-enriched Asb15 is not required for spermatogenesis and male fertility in mice

BACKGROUND

The function of Asb15, which encodes an ASB protein with ankyrin (ANK) repeats and a C-terminal suppressor of cytokine signaling (SOCS) box motif, in male germ cells is poorly understood. Because expression of Asb15 is enriched in mouse testis, it may have a role in spermatogenesis.

METHODS AND RESULTS

We used a computer-assisted sperm analysis (CASA) system to analyze sperm from Asb15 gene knockout (KO) mice that we generated using the clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein 9 (Cas9) technique. Histological staining and immunostaining were used to evaluate spermatogenesis in Asb15-KO mice. Asb15-KO and wild-type mice showed no differences in histology or in semen quality, fertility, or sperm apoptosis. Asb15- and Asb17-double KO (dKO) mice were generated to determine whether Asb17 compensated for the loss of Asb15. However, Asb15/17-dKO mice also showed normal fertility, except for an increase in giant cells in testicular tubules, suggesting a minor functional compensation between the two genes during spermatogenesis.

CONCLUSIONS

Our study suggests that Asb15 was individually not required for spermatogenesis or for fertility in mice. However, further investigation might be needed to reach a firm conclusion. These findings can prevent redundant research by other scientists and provides new information for further studies on the genetics of fertility in humans.

PIWI-interacting RNAs in cancer: Biogenesis, function, and clinical significance

PIWI-interacting RNAs (piRNAs) are a less-studied class of small non-coding RNAs approximately 24–31 nucleotides in length. They express in germline and somatic cells and form complexes with PIWI proteins to exert regulatory effects. New studies show that piRNAs are aberrantly expressed in various cancers. In this review, we focus on those piRNAs that are associated with cancer hallmarks such as proliferation, invasion, and chemoresistance and discuss their potential as biomarkers for cancer diagnosis and prognosis.

Global phosphoproteomic analysis identified key kinases regulating male meiosis in mouse

Meiosis, a highly conserved process in organisms from fungi to mammals, is subjected to protein phosphorylation regulation. Due to the low abundance of phosphorylation, there is a lack of systemic characterization of phosphorylation regulation of meiosis in mammals. Using the phosphoproteomic approach, we profiled large-scale phosphoproteome of purified primary spermatocytes undergoing meiosis I, and identified 14,660 phosphorylation sites in 4419 phosphoproteins. Kinase-substrate phosphorylation network analysis followed by in vitro meiosis study showed that CDK9 was essential for meiosis progression to metaphase I and had enriched substrate phosphorylation sites in proteins involved in meiotic cell cycle. In addition, histones and epigenetic factors were found to be widely phosphorylated. Among those, HASPIN was found to be essential for male fertility. Haspin knockout led to misalignment of chromosomes, apoptosis of metaphase spermatocytes and a decreased number of sperm by deregulation of H3T3ph, chromosomal passenger complex (CPC) and spindle assembly checkpoint (SAC). The complicated protein phosphorylation and its important regulatory functions in meiosis indicated that in-depth studies of phosphorylation-mediated signaling could help us elucidate the mechanisms of meiosis.

Unraveling mitochondrial piRNAs in mouse embryonic gonadal cells

Although mitochondria are widely studied organelles, the recent interest in the role of mitochondrial small noncoding RNAs (sncRNAs), miRNAs, and more recently, piRNAs, is providing new functional perspectives in germ cell development and differentiation. piRNAs (PIWI-interacting RNAs) are single-stranded sncRNAs of mostly about 20-35 nucleotides, generated from the processing of pre-piRNAs. We leverage next-generation sequencing data obtained from mouse primordial germ cells and somatic cells purified from early-differentiating embryonic ovaries and testis from 11.5 to 13.5 days postcoitum. Using bioinformatic tools, we elucidate (i) the origins of piRNAs as transcribed from mitochondrial DNA fragments inserted in the nucleus or from the mitochondrial genome; (ii) their levels of expression; and (iii) their potential roles, as well as their association with genomic regions encoding other sncRNAs (such as tRNAs and rRNAs) and the mitochondrial regulatory region (D-loop). Finally, our results suggest how nucleo-mitochondrial communication, both anterograde and retrograde signaling, may be mediated by mitochondria-associated piRNAs.

Hierarchical length and sequence preferences establish a single major piRNA 3'-end

PIWI-interacting RNAs (piRNAs) guard germline genomes against the deleterious action of mobile genetic elements. PiRNAs use extensive base-pairing to recognize their targets and variable 3'ends could change the specificity and efficacy of piRNA silencing. Here, we identify conserved rules that ensure the generation of a single major piRNA 3'end in flies and mice. Our data suggest that the PIWI proteins initially define a short interval on pre-piRNAs that grants access to the ZUC-processor complex. Within this Goldilocks zone, the preference to cut in front of Uridine determines the ultimate processing site. We observe a mouse-specific roadblock that relocates the Goldilocks zone and generates an opportunity for consecutive trimming. Our data reveal a conserved hierarchy between length and sequence preferences that controls the piRNA sequence space. The unanticipated precision of 3'end formation bolsters the emerging understanding that the functional piRNA sequence space is tightly controlled to ensure effective defense.

An evolutionarily conserved stop codon enrichment at the 5' ends of mammalian piRNAs

PIWI-interacting RNAs (piRNAs) are small RNAs required to recognize and silence transposable elements. The 5' ends of mature piRNAs are defined through cleavage of long precursor transcripts, primarily by Zucchini (Zuc). Zuc-dependent cleavage typically occurs immediately upstream of a uridine. However, Zuc lacks sequence preference in vitro, pointing towards additional unknown specificity factors. Here, we examine murine piRNAs and reveal a strong and specific enrichment of three sequences (UAA, UAG, UGA)-corresponding to stop codons-at piRNA 5' ends. Stop codon sequences are also enriched immediately after piRNA processing intermediates, reflecting their Zuc-dependent tail-to-head arrangement. Further analyses reveal that a Zuc in vivo cleavage preference at four sequences (UAA, UAG, UGA, UAC) promotes 5' end stop codons. This observation is conserved across mammals and possibly further. Our work provides new insights into Zuc-dependent cleavage and may point to a previously unrecognized connection between piRNA biogenesis and the translational machinery.

Noncanonical Functions of PIWIL1/piRNAs in animal male germ cells and human diseases

PIWI proteins and PIWI-interacting RNAs (piRNAs) are specifically expressed in animal germlines and play essential roles during gametogenesis in animals. The primary function of PIWI/piRNAs is known to silence transposable elements for protecting genome integrity in animal germlines, while their roles beyond silencing transposons are also documented by us and others. In particular, we show that mouse PIWIL1 (MIWI)/piRNAs play a dual role in regulating protein-coding genes in mouse spermatids through interacting with different protein factors in a developmental stage-dependent manner, including translationally activating a subset of ARE-containing mRNAs in round spermatids and inducing massive mRNA degradation in late spermatids. We further show that MIWI is eliminated through the ubiquitin-26S proteasome pathway during late spermiogenesis. By exploring the biological function of MIWI ubiquitination by APC/C, we identified ubiquitination-deficient mutations in human PIWIL1 of infertile men and further established their causative role in male infertility in mouse model, supporting PIWIL1 as a human male infertility-relevant gene. Additionally, we reported that PIWIL1, aberrantly induced in human tumors, functions as an oncoprotein in a piRNA-independent manner in cancer cells. In the current review, we summarize our latest findings regarding the roles and mechanisms of PIWIL1 and piRNAs in mouse spermatids and human diseases, and discuss the related works in the field.

Retrotransposons in the Mammalian Male Germline

Retrotransposons are a subset of DNA sequences that constitute a large part of the mammalian genome. They can translocate autonomously or non-autonomously, potentially jeopardizing the heritable germline genome. Retrotransposons coevolved with the host genome, and the germline is the prominent battlefield between retrotransposons and the host genome to maximize their mutual fitness. Host genomes have developed various mechanisms to suppress and control retrotransposons, including DNA methylation, histone modifications, and Piwi-interacting RNA (piRNA), for their own benefit. Thus, rapidly evolved retrotransposons often acquire positive functions, including gene regulation within the germline, conferring reproductive fitness in a species over the course of evolution. The male germline serves as an ideal model to examine the regulation and evolution of retrotransposons, resulting in genomic co-evolution with the host genome. In this review, we summarize and discuss the regulatory mechanisms of retrotransposons, stage-by-stage, during male germ cell development, with a particular focus on mice as an extensively studied mammalian model, highlighting suppression mechanisms and emerging functions of retrotransposons in the male germline.

PIWI-interacting RNAs and human testicular function

Small noncoding RNAs (sncRNAs) are pieces of RNA with a length below 200 bp and represent a diverse group of RNAs having many different biological functions. The best described subtype is the microRNAs which primarily function in posttranscriptional gene regulation and appear essential for most physiological processes. Of particular interest for the germline is the PIWI-interacting RNAs (piRNAs) which are a class of sncRNA of 21-35 bp in length that are almost exclusively found in germ cells. Recently, it has become clear that piRNAs are essential for testicular function, and in this perspective, we outline the current knowledge of piRNAs in humans. Although piRNAs appear unique to germ cells, they have also been described in various somatic cancers and biofluids. Here, we discuss the potential function of piRNAs in somatic tissues and whether detection in biofluids may be used as a biomarker for testicular function. This article is categorized under: Reproductive System Diseases > Genetics/Genomics/Epigenetics Reproductive System Diseases > Molecular and Cellular Physiology.

An introduction to PIWI-interacting RNAs (piRNAs) in the context of metazoan small RNA silencing pathways

PIWI proteins and their associated PIWI-interacting RNAs (piRNAs) constitute a small RNA-based adaptive immune system that restricts the deleterious activity of mobile genetic elements to protect genome integrity. Self/nonself discrimination is at the very core of successful defence and relies on complementary base-pairing in RNA-guided immunity. How the millions of piRNA sequences faithfully discriminate between self and nonself and how they adapt to novel genomic invaders remain key outstanding questions in genome biology. This review aims to introduce principles of piRNA silencing in the context of metazoan small RNA pathways. A distinct feature of piRNAs is their origin from single-stranded instead of double-stranded RNA precursors, and piRNAs require a unique set of processing factors. Novel nucleases, helicases and RNA binding proteins have been identified in piRNA biology, and while we are starting to understand some mechanisms of piRNA biogenesis and function, this diverse and prolific class of small RNAs remains full of surprises.

Terminal modification, sequence, length, and PIWI-protein identity determine piRNA stability

In animals, PIWI-interacting RNAs (piRNAs) silence transposons, fight viral infections, and regulate gene expression. piRNA biogenesis concludes with 3' terminal trimming and 2'-O-methylation. Both trimming and methylation influence piRNA stability. Our biochemical data show that multiple mechanisms destabilize unmethylated mouse piRNAs, depending on whether the piRNA 5' or 3' sequence is complementary to a trigger RNA. Unlike target-directed degradation of microRNAs, complementarity-dependent destabilization of piRNAs in mice and flies is blocked by 3' terminal 2'-O-methylation and does not require base pairing to both the piRNA seed and the 3' sequence. In flies, 2'-O-methylation also protects small interfering RNAs (siRNAs) from complementarity-dependent destruction. By contrast, pre-piRNA trimming protects mouse piRNAs from a degradation pathway unaffected by trigger complementarity. In testis lysate and in vivo, internal or 3' terminal uridine- or guanine-rich tracts accelerate pre-piRNA decay. Loss of both trimming and 2'-O-methylation causes the mouse piRNA pathway to collapse, demonstrating that these modifications collaborate to stabilize piRNAs.

Post-transcriptional regulation in spermatogenesis: all RNA pathways lead to healthy sperm

Multiple RNA pathways are required to produce functional sperm. Here, we review RNA post-transcriptional regulation during spermatogenesis with particular emphasis on the role of 3' end modifications. From early studies in the 1970s, it became clear that spermiogenesis transcripts could be stored for days only to be translated at advanced stages of spermatid differentiation. The transition between the translationally repressed and active states was observed to correlate with the shortening of the transcripts' poly(A) tail, establishing a link between RNA 3' end metabolism and male germ cell differentiation. Since then, numerous RNA metabolic pathways have been implicated not only in the progression through spermatogenesis, but also in the maintenance of genomic integrity. Recent studies have characterized the elusive 3' biogenesis of Piwi-interacting RNAs (piRNAs), identified a critical role for messenger RNA (mRNA) 3' uridylation in meiotic progression, established the mechanisms that destabilize transcripts with long 3' untranslated regions (3'UTRs) in post-mitotic cells, and defined the physiological relevance of RNA exonucleases and deadenylases in male germ cells. In this review, we discuss RNA processing in the male germline in the light of the most recent findings. A brief recollection of different RNA-processing events will aid future studies exploring post-transcriptional regulation in spermatogenesis.

PABPN1L assemble into "ring-like" aggregates in the cytoplasm of MII oocytes and is associated with female infertility†

Infertility affects 10% - 15% of families worldwide. However, the pathogenesis of female infertility caused by abnormal early embryonic development is not clear. A resent study showed that PABPN1L recruited BTG4 to mRNA 3'-poly(A) tails and was essential for maternal mRNA degradation. Here, we generated an PABPN1L-antibody and found "ring-like" PABPN1L aggregates in the cytoplasm of MII oocytes. PABPN1L-EGFP proteins spontaneously formed"ring-like" aggregates in vitro. This phenomenon is similar with CCR4-NOT catalytic subunit, CNOT7, when it starts deadenylation process in vitro. We constructed two mouse model (Pabpn1l  -/- and Pabpn1l  tm1a/tm1a) simulating the intron1-exon2 abnormality of human PABPN1L and found that the female was sterile and the male was fertile. Using RNA-Seq, we observed a large-scale up-regulation of RNA in zygotes derived from Pabpn1l-/- MII oocytes. We found that 9222 genes were up-regulated instead of being degraded in the Pabpn1l-♀/+♂zygote. Both the Btg4 and Cnot61 genes are necessary for the deadenylation process and Pabpn1l  -/- resembled both the Btg4 and Cnot6l knockouts, where 71.2% genes stabilized in the Btg4-♀/+♂ zygote and 84.2% genes stabilized in the Cnot6l-♀/+♂zygote were also stabilized in Pabpn1l-♀/+♂ zygote. BTG4/CNOT7/CNOT6L was partially co-located with PABPN1L in MII oocytes. The above results suggest that PABPN1L is widely associated with CCR4-NOT-mediated maternal mRNA degradation and PABPN1L variants on intron1-exon2 could be a genetic marker of female infertility. Summary sentence. "Ring-like" PABPN1L aggregates was found in the cytoplasm of MII oocytes and in vitro; intron1-exon2 abnormality of Pabpn1l leads female sterile in mice.

The birth of piRNAs: how mammalian piRNAs are produced, originated, and evolved

PIWI-interacting RNAs (piRNAs), small noncoding RNAs 24–35 nucleotides long, are essential for animal fertility. They play critical roles in a range of functions, including transposable element suppression, gene expression regulation, imprinting, and viral defense. In mammals, piRNAs are the most abundant small RNAs in adult testes and the only small RNAs that direct epigenetic modification of chromatin in the nucleus. The production of piRNAs is a complex process from transcription to post-transcription, requiring unique machinery often distinct from the biogenesis of other RNAs. In mice, piRNA biogenesis occurs in specialized subcellular locations, involves dynamic developmental regulation, and displays sexual dimorphism. Furthermore, the genomic loci and sequences of piRNAs evolve much more rapidly than most of the genomic regions. Understanding piRNA biogenesis should reveal novel RNA regulations recognizing and processing piRNA precursors and the forces driving the gain and loss of piRNAs during animal evolution. Such findings may provide the basis for the development of engineered piRNAs capable of modulating epigenetic regulation, thereby offering possible single-dose RNA therapy without changing the genomic DNA. In this review, we focus on the biogenesis of piRNAs in mammalian adult testes that are derived from long non-coding RNAs. Although piRNA biogenesis is believed to be evolutionarily conserved from fruit flies to humans, recent studies argue for the existence of diverse, mammalian-specific RNA-processing pathways that convert precursor RNAs into piRNAs, perhaps associated with the unique features of mammalian piRNAs or germ cell development. We end with the discussion of major questions in the field, including substrate recognition and the birth of new piRNAs.

MORC3, a novel MIWI2 association partner, as an epigenetic regulator of piRNA dependent transposon silencing in male germ cells

The PIWI (P-element-induced wimpy testis)-interacting-RNA (piRNA) pathway plays a crucial role in the repression of TE (transposable element) expression via de novo DNA methylation in mouse embryonic male germ cells. Various proteins, including MIWI2 are involved in the process. TE silencing is ensured by piRNA-guided MIWI2 that recruits some effector proteins of the DNA methylation machinery to TE regions. However, the molecular mechanism underlying the methylation is complex and has not been fully elucidated. Here, we identified MORC3 as a novel associating partner of MIWI2 and also a nuclear effector of retrotransposon silencing via piRNA-dependent de novo DNA methylation in embryonic testis. Moreover, we show that MORC3 is important for transcription of piRNA precursors and subsequently affects piRNA production. Thus, we provide the first mechanistic insights into the role of this effector protein in the first stage of piRNA biogenesis in embryonic TE silencing mechanism.

Coupled protein synthesis and ribosome-guided piRNA processing on mRNAs

PIWI-interacting small RNAs (piRNAs) protect the germline genome and are essential for fertility. piRNAs originate from transposable element (TE) RNAs, long non-coding RNAs, or 3´ untranslated regions (3´UTRs) of protein-coding messenger genes, with the last being the least characterized of the three piRNA classes. Here, we demonstrate that the precursors of 3´UTR piRNAs are full-length mRNAs and that post-termination 80S ribosomes guide piRNA production on 3´UTRs in mice and chickens. At the pachytene stage, when other co-translational RNA surveillance pathways are sequestered, piRNA biogenesis degrades mRNAs right after pioneer rounds of translation and fine-tunes protein production from mRNAs. Although 3´UTR piRNA precursor mRNAs code for distinct proteins in mice and chickens, they all harbor embedded TEs and produce piRNAs that cleave TEs. Altogether, we discover a function of the piRNA pathway in fine-tuning protein production and reveal a conserved piRNA biogenesis mechanism that recognizes translating RNAs in amniotes.

Factors Regulating the Activity of LINE1 Retrotransposons

LINE-1 (L1) is a class of autonomous mobile genetic elements that form somatic mosaicisms in various tissues of the organism. The activity of L1 retrotransposons is strictly controlled by many factors in somatic and germ cells at all stages of ontogenesis. Alteration of L1 activity was noted in a number of diseases: in neuropsychiatric and autoimmune diseases, as well as in various forms of cancer. Altered activity of L1 retrotransposons for some pathologies is associated with epigenetic changes and defects in the genes involved in their repression. This review discusses the molecular genetic mechanisms of the retrotransposition and regulation of the activity of L1 elements. The contribution of various factors controlling the expression and distribution of L1 elements in the genome occurs at all stages of the retrotransposition. The regulation of L1 elements at the transcriptional, post-transcriptional and integration into the genome stages is described in detail. Finally, this review also focuses on the evolutionary aspects of L1 accumulation and their interplay with the host regulation system.

PiRNA pathway in the cardiovascular system: a novel regulator of cardiac differentiation, repair and regeneration

Piwi-interacting RNAs (piRNAs) are a novel group of small non-coding RNA molecules with lengths of 21-35 nucleotides, first identified from the germline. PiRNAs and their associated PIWI clade Argonaute proteins constitute a key part of the piRNA pathway, with the best-known biological function to silence transposable elements in germ cells. The piRNA pathway, in fact, is not exclusive to the germline. Somatic functions of piRNAs have been recorded since their first discovery. To date, involvement of the piRNA pathway has been identified within the biological functions of genome rearrangement, epigenetic regulation, protein regulation in the germline and/or the soma transcriptionally or post-transcriptionally. Emerging evidence has shown that the piRNA pathway is essential for the normal function of the cardiovascular system and that its abnormal expression is correlated with cardiovascular dysfunction, although comprehensive roles of the piRNA pathway in the cardiovascular system and underlying mechanisms remain unclear. In this review, we discuss current findings of piRNA pathway expression in cardiac cell types and their potential functions in cardiac differentiation, repair and regeneration, thus providing new insights into cardiovascular disease development associated with the piRNA pathway.

pre-piRNA trimming and 2'-O-methylation protect piRNAs from 3' tailing and degradation in C. elegans

The Piwi-interacting RNA (piRNA) pathway suppresses transposable elements and promotes fertility in diverse organisms. Maturation of piRNAs involves pre-piRNA trimming followed by 2'-O-methylation at their 3' termini. Here, we report that the 3' termini of Caenorhabditis elegans piRNAs are subject to nontemplated nucleotide addition, and piRNAs with 3' addition exhibit extensive base-pairing interaction with their target RNAs. Animals deficient for PARN-1 (pre-piRNA trimmer) and HENN-1 (2'-O-methyltransferase) accumulate piRNAs with 3' nontemplated nucleotides. In henn-1 mutants, piRNAs are shortened prior to 3' addition, whereas long isoforms of untrimmed piRNAs are preferentially modified in parn-1 mutant animals. Loss of either PARN-1 or HENN-1 results in modest reduction in steady-state levels of piRNAs. Deletion of both enzymes leads to depletion of piRNAs, desilenced piRNA targets, and impaired fecundity. Together, our findings suggest that pre-piRNA trimming and 2'-O-methylation act collaboratively to protect piRNAs from tailing and degradation.

Variant PNLDC1, Defective piRNA Processing, and Azoospermia

BACKGROUND

P-element-induced wimpy testis (PIWI)-interacting RNAs (piRNAs) are short (21 to 35 nucleotides in length) and noncoding and are found almost exclusively in germ cells, where they regulate aberrant expression of transposable elements and postmeiotic gene expression. Critical to the processing of piRNAs is the protein poly(A)-specific RNase-like domain containing 1 (PNLDC1), which trims their 3' ends and, when disrupted in mice, causes azoospermia and male infertility.

METHODS

We performed exome sequencing on DNA samples from 924 men who had received a diagnosis of nonobstructive azoospermia. Testicular-biopsy samples were analyzed by means of histologic and immunohistochemical tests, in situ hybridization, reverse-transcriptase-quantitative-polymerase-chain-reaction assay, and small-RNA sequencing.

RESULTS

Four unrelated men of Middle Eastern descent who had nonobstructive azoospermia were found to carry mutations in PNLDC1: the first patient had a biallelic stop-gain mutation, p.R452Ter (rs200629089; minor allele frequency, 0.00004); the second, a novel biallelic missense variant, p.P84S; the third, two compound heterozygous mutations consisting of p.M259T (rs141903829; minor allele frequency, 0.0007) and p.L35PfsTer3 (rs754159168; minor allele frequency, 0.00004); and the fourth, a novel biallelic canonical splice acceptor site variant, c.607-2A→T. Testicular histologic findings consistently showed error-prone meiosis and spermatogenic arrest with round spermatids of type Sa as the most advanced population of germ cells. Gene and protein expression of PNLDC1, as well as the piRNA-processing proteins PIWIL1, PIWIL4, MYBL1, and TDRKH, were greatly diminished in cells of the testes. Furthermore, the length distribution of piRNAs and the number of pachytene piRNAs was significantly altered in men carrying PNLDC1 mutations.

CONCLUSIONS

Our results suggest a direct mechanistic effect of faulty piRNA processing on meiosis and spermatogenesis in men, ultimately leading to male infertility. (Funded by Innovation Fund Denmark and others.).

RNase κ promotes robust piRNA production by generating 2',3'-cyclic phosphate-containing precursors

In animal germlines, PIWI proteins and the associated PIWI-interacting RNAs (piRNAs) protect genome integrity by silencing transposons. Here we report the extensive sequence and quantitative correlations between 2',3'-cyclic phosphate-containing RNAs (cP-RNAs), identified using cP-RNA-seq, and piRNAs in the Bombyx germ cell line and mouse testes. The cP-RNAs containing 5'-phosphate (P-cP-RNAs) identified by P-cP-RNA-seq harbor highly consistent 5'-end positions as the piRNAs and are loaded onto PIWI protein, suggesting their direct utilization as piRNA precursors. We identified Bombyx RNase Kappa (BmRNase κ) as a mitochondria-associated endoribonuclease which produces cP-RNAs during piRNA biogenesis. BmRNase κ-depletion elevated transposon levels and disrupted a piRNA-mediated sex determination in Bombyx embryos, indicating the crucial roles of BmRNase κ in piRNA biogenesis and embryonic development. Our results reveal a BmRNase κ-engaged piRNA biogenesis pathway, in which the generation of cP-RNAs promotes robust piRNA production.

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.

Eef2k is not required for fertility in male mice

Background

Eukaryotic elongation factor-2 kinase (Eef2k) is a protein kinase associated with the calmodulin-induced signaling pathway and an atypical alpha-kinase family member. Eef2k-mediated phosphorylation of eukaryotic translation elongation factor 2 (Eef2) can inhibit the functionality of this protein, altering protein translation. Prior work suggests Eef2k to be overexpressed in breast, pancreatic, brain, and lung cancers wherein it may control key processes associated with apoptosis, autophagy, and cell cycle progression. The functional importance of Eef2k in the testes of male mice, however, has yet to be clarified.

Methods

A CRISPR/Cas9 approach was used to generate male Eef2k-knockout mice, which were evaluated for phenotypic changes in epididymal or testicular tissues through histological and immunofluorescent staining assays. In addition, TUNEL staining was conducted to assess the apoptotic death of cells in the testis. Fertility, sperm counts, and sperm motility were further assessed.

Results

Male Eef2k-knockout mice were successfully generated, and exhibited normal fertility and development. No apparent differences were observed with respect to spermatogenesis, sperm counts, or germ cell apoptosis when comparing male Eef2k ^-/- and Eef2k ^+/+ mice.

Conclusions

Male Eef2k-knockout mice remained fertile and were free of any evident developmental or spermatogenic abnormalities, suggesting Eef2k to be dispensable in the context of male fertility.

yama, a mutant allele of Mov10l1, disrupts retrotransposon silencing and piRNA biogenesis

Piwi-interacting RNAs (piRNAs) play critical roles in protecting germline genome integrity and promoting normal spermiogenic differentiation. In mammals, there are two populations of piRNAs: pre-pachytene and pachytene. Transposon-rich pre-pachytene piRNAs are expressed in fetal and perinatal germ cells and are required for retrotransposon silencing, whereas transposon-poor pachytene piRNAs are expressed in spermatocytes and round spermatids and regulate mRNA transcript levels. MOV10L1, a germ cell-specific RNA helicase, is essential for the production of both populations of piRNAs. Although the requirement of the RNA helicase domain located in the MOV10L1 C-terminal region for piRNA biogenesis is well known, its large N-terminal region remains mysterious. Here we report a novel Mov10l1 mutation, named yama, in the Mov10l1 N-terminal region. The yama mutation results in a single amino acid substitution V229E. The yama mutation causes meiotic arrest, de-repression of transposable elements, and male sterility because of defects in pre-pachytene piRNA biogenesis. Moreover, restricting the Mov10l1 mutation effects to later stages in germ cell development by combining with a postnatal conditional deletion of a complementing wild-type allele causes absence of pachytene piRNAs, accumulation of piRNA precursors, polar conglomeration of piRNA pathway proteins in spermatocytes, and spermiogenic arrest. Mechanistically, the V229E substitution in MOV10L1 reduces its interaction with PLD6, an endonuclease that generates the 5′ ends of piRNA intermediates. Our results uncover an important role for the MOV10L1-PLD6 interaction in piRNA biogenesis throughout male germ cell development.

Small non-coding RNAs play critical roles in silencing of exogenous viruses, endogenous retroviruses, and transposable elements, and also play multifaceted roles in controlling gene expression. Piwi-interacting RNAs (piRNAs) are found in gonads in diverse species from flies to humans. An evolutionarily conserved function of piRNAs is to silence transposable elements through an adaptive mechanism and thus to protect germline genome integrity. In mammals, piRNAs also provide a poorly understood function to regulate postmeiotic differentiation of spermatids. More than two dozen proteins are involved in the piRNA pathway. MOV10L1, a germ-cell-specific RNA helicase, binds to piRNA precursors to initiate piRNA biogenesis. Here we have identified a single amino acid substitution (V229E) in MOV10L1 in the yama mouse mutant. When constitutively expressed as the only source of MOV10L1 throughout germ cell development, the yama mutation abolishes piRNA biogenesis, de-silences transposable elements, and causes meiotic arrest. When the mutant phenotype is instead revealed only later in germ cell development by conditionally inactivating a wild-type copy of the gene, the point mutant abolishes formation of later classes of piRNAs and again disrupts germ cell development. Point mutations in MOV10L1 may thus contribute to male infertility in humans.

The testis-specifically expressed gene Trim69 is not essential for fertility in mice

Protein ubiquitination is essential for diverse cellular functions including spermatogenesis. The tripartite motif (TRIM) family proteins, most of which have E3 ubiquitin ligase activity, are highly conserved in mammals. They are involved in important cellular processes such as embryonic development, immunity, and fertility. Our previous studies indicated that Trim69, a testis-specific expressed TRIM family gene, potentially participates in the spermatogenesis by mediating testicular cells apoptosis. In this study, we investigated the biological functions of Trim69 in male mice by established Trim69 knockout mice with CRISPR/Cas9 genomic editing technology. Here, we reported that the male Trim69 knockout mice had normal fertility. The adult knockout mice have shown that the appearance of testes, testis/body weight ratios, testicular histomorphology, and the number and quality of sperm were consistent with wild-type mice. These results indicated that the E3 ubiquitin ligase protein Trim69 was not essential for male mouse fertility, and it might be compensated by other TRIM family members such as Trim58 in Trim69-deficiency testis. This study would help to elucidate the functions of tripartite motif protein family and the regulation of spermatogenesis.