Spermatozoa lacking Fertilization Influencing Membrane Protein (FIMP) fail to fuse with oocytes in mice.

Sperm-oocyte fusion is a critical event in mammalian fertilization, categorized by three indispensable proteins. Sperm membrane protein IZUMO1 and its counterpart oocyte membrane protein JUNO make a protein complex allowing sperm to interact with the oocyte, and subsequent sperm-oocyte fusion. Oocyte tetraspanin protein CD9 also contributes to sperm-oocyte fusion. However, the fusion process cannot be explained solely by these three essential factors. In this study, we focused on analyzing a testis-specific gene 4930451I11Rik and generated mutant mice using the CRISPR/Cas9 system. Although IZUMO1 remained in 4930451I11Rik knockout (KO) spermatozoa, the KO spermatozoa were unable to fuse with oocytes and the KO males were severely subfertile. 4930451I11Rik encodes two isoforms: a transmembrane (TM) form and a secreted form. Both CRISPR/Cas9-mediated TM deletion and transgenic (Tg) rescue with the TM form revealed that only the TM form plays a critical role in sperm-oocyte fusion. Thus, we renamed this TM form Fertilization Influencing Membrane Protein (FIMP). The mCherry-tagged FIMP TM form was localized to the sperm equatorial segment where the sperm-oocyte fusion event occurs. Thus, FIMP is a sperm-specific transmembrane protein that is necessary for the sperm-oocyte fusion process.

Identification of multiple male reproductive tract-specific proteins that regulate sperm migration through the oviduct in mice.

CRISPR/Cas9-mediated genome editing technology enables researchers to efficiently generate and analyze genetically modified animals. We have taken advantage of this game-changing technology to uncover essential factors for fertility. In this study, we generated knockouts (KOs) of multiple male reproductive organ-specific genes and performed phenotypic screening of these null mutant mice to attempt to identify proteins essential for male fertility. We focused on making large deletions (dels) within 2 gene clusters encoding cystatin (CST) and prostate and testis expressed (PATE) proteins and individual gene mutations in 2 other gene families encoding glycerophosphodiester phosphodiesterase domain (GDPD) containing and lymphocyte antigen 6 (Ly6)/Plaur domain (LYPD) containing proteins. These gene families were chosen because many of the genes demonstrate male reproductive tract-specific expression. Although Gdpd1 and Gdpd4 mutant mice were fertile, disruptions of Cst and Pate gene clusters and Lypd4 resulted in male sterility or severe fertility defects secondary to impaired sperm migration through the oviduct. While absence of the epididymal protein families CST and PATE affect the localization of the sperm membrane protein A disintegrin and metallopeptidase domain 3 (ADAM3), the sperm acrosomal membrane protein LYPD4 regulates sperm fertilizing ability via an ADAM3-independent pathway. Thus, use of CRISPR/Cas9 technologies has allowed us to quickly rule in and rule out proteins required for male fertility and expand our list of male-specific proteins that function in sperm migration through the oviduct.

Co-expression of sperm membrane proteins CMTM2A and CMTM2B is essential for ADAM3 localization and male fertility in mice.

Chemokines are signaling proteins that are secreted to induce chemotaxis during an immunological response. However, the functions of transmembrane-type chemokine-like factor (CKLF) and the CMTM (CKLF-like MARVEL transmembrane domain containing) protein family remain to be determined. In this study, we focused on the testis-specific mouse CMTM gene cluster (Cmtm1, Cmtm2a and Cmtm2b) and generated CRISPR/Cas9-mediated mutant mice to examine their physiological functions. Although Cmtm1 mutant mice were fertile, Cmtm2a and Cmtm2b double mutant mice had defects in male fertility due to impaired sperm function. We found that co-expression of sperm membrane proteins CMTM2A and CMTM2B is required for male fertility and affects the localization of the sperm membrane protein ADAM3 in regulating sperm fertilizing ability.

MVH in piRNA processing and gene silencing of retrotransposons.

VASA is an evolutionarily conserved RNA helicase essential for germ cell development. The mouse PIWI family proteins MILI and MIWI2 are involved in production of Piwi-interacting RNAs (piRNAs) in fetal male germ cells through a ping-pong amplification cycle. Expression of retrotransposons is elevated in MILI- and MIWI2-deficient male germ cells due to defective de novo DNA methylation, which is presumably caused by impaired piRNA expression. Here, we report that essentially the same abnormalities are observed in MVH (mouse VASA homolog)-deficient mice. Comprehensive analysis of piRNAs in MVH-deficient fetal male germ cells showed that MVH plays crucial roles in the early phase of the ping-pong amplification cycle.

Human Globozoospermia-Related Gene Spata16 Is Required for Sperm Formation Revealed by CRISPR/Cas9-Mediated Mouse Models.

A recent genetic analysis of infertile globozoospermic patients identified causative mutations in three genes: a protein interacting with C kinase 1 (PICK1), dpy 19-like 2 (DPY19L2), and spermatogenesis associated 16 (SPATA16). Although mouse models have clarified the physiological functions of Pick1 and Dpy19l2 during spermatogenesis, Spata16 remains to be determined. Globozoospermic patients carried a homozygous point mutation in SPATA16 at 848G→A/R283Q. We generated CRISPR/Cas9-mediated mutant mice with the same amino acid substitution in the fourth exon of Spata16 to analyze the mutation site at R284Q, which corresponded with R283Q of mutated human SPATA16. We found that the point mutation in Spata16 was not essential for male fertility; however, deletion of the fourth exon of Spata16 resulted in infertile male mice due to spermiogenic arrest but not globozoospermia. This study demonstrates that Spata16 is indispensable for male fertility in mice, as well as in humans, as revealed by CRISPR/Cas9-mediated mouse models.

GPAT2, a mitochondrial outer membrane protein, in piRNA biogenesis in germline stem cells.

piRNA (PIWI-interacting RNA) is a germ cell-specific small RNA in which biogenesis PIWI (P-element wimpy testis) family proteins play crucial roles. MILI (mouse Piwi-like), one of the three mouse PIWI family members, is indispensable for piRNA production, DNA methylation of retrotransposons presumably through the piRNA, and spermatogenesis. The biogenesis of piRNA has been divided into primary and secondary processing pathways; in both of these MILI is involved in mice. To analyze the molecular function of MILI in piRNA biogenesis, we utilized germline stem (GS) cells, which are derived from testicular stem cells and possess a spermatogonial phenotype. We established MILI-null GS cell lines and their revertant, MILI-rescued GS cells, by introducing the Mili gene with Sendai virus vector. Comparison of wild-type, MILI-null, and MILI-rescued GS cells revealed that GS cells were quite useful for analyzing the molecular mechanisms of piRNA production, especially the primary processing pathway. We found that glycerol-3-phosphate acyltransferase 2 (GPAT2), a mitochondrial outer membrane protein for lysophosphatidic acid, bound to MILI using the cells and that gene knockdown of GPAT2 brought about impaired piRNA production in GS cells. GPAT2 is not only one of the MILI bound proteins but also a protein essential for primary piRNA biogenesis.

ADAD2 functions in spermiogenesis and piRNA biogenesis in mice.

BACKGROUND: Adenosine deaminase domain containing 2 (ADAD2) is a testis-specific protein composed of a double-stranded RNA binding domain and a non-catalytic adenosine deaminase domain. A recent study showed that ADAD2 is indispensable for the male reproduction in mice. However, the detailed functions of ADAD2 remain elusive. OBJECTIVES: This study aimed to investigate the cause of male sterility in Adad2 mutant mice and to understand the molecular functions of ADAD2. MATERIALS AND METHODS: Adad2 homozygous mutant mouse lines, Adad2-/- and Adad2Δ/Δ , were generated by CRISPR/Cas9. Western blotting and immunohistochemistry were used to reveal the expression and subcellular localization of ADAD2. Co-immunoprecipitation tandem mass spectrometry was employed to determine the ADAD2-interacting proteins in mouse testes. RNA-sequencing analyses were carried out to analyze the transcriptome and PIWI-interacting RNA (piRNA) populations in wildtype and Adad2 mutant testes. RESULTS: Adad2-/- and Adad2Δ/Δ mice exhibit male-specific sterility because of abnormal spermiogenesis. ADAD2 interacts with multiple RNA-binding proteins involved in piRNA biogenesis, including MILI, MIWI, RNF17, and YTHDC2. ADAD2 co-localizes and forms novel granules with RNF17 in spermatocytes. Ablation of ADAD2 impairs the formation of RNF17 granules, decreases the number of cluster-derived pachytene piRNAs, and increases expression of ping-pong-derived piRNAs. DISCUSSION AND CONCLUSION: In collaboration with RNF17 and other RNA-binding proteins in spermatocytes, ADAD2 directly or indirectly functions in piRNA biogenesis.

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.

MIWI2 as an Effector of DNA Methylation and Gene Silencing in Embryonic Male Germ Cells.

During the development of mammalian embryonic germ cells, global demethylation and de novo DNA methylation take place. In mouse embryonic germ cells, two PIWI family proteins, MILI and MIWI2, are essential for the de novo DNA methylation of retrotransposons, presumably through PIWI-interacting RNAs (piRNAs). Although piRNA-associated MIWI2 has been reported to play critical roles in the process, its molecular mechanisms have remained unclear. To identify the mechanism, transgenic mice were produced; they contained a fusion protein of MIWI2 and a zinc finger (ZF) that recognized the promoter region of a type A LINE-1 gene. The ZF-MIWI2 fusion protein brought about DNA methylation, suppression of the type A LINE-1 gene, and a partial rescue of the impaired spermatogenesis of MILI-null mice. In addition, ZF-MIWI2 was associated with the proteins involved in DNA methylation. These data indicate that MIWI2 functions as an effector of de novo DNA methylation of the retrotransposon.

Identification of MIWI-associated Poly(A) RNAs by immunoprecipitation with an anti-MIWI monoclonal antibody.

MIWI is one of the PIWI subfamily of proteins mainly expressed in mouse germ cells, and associates with pachytene piRNAs. MIWI has been thought to play an essential role in spermatogenesis and spermiogenesis via biogenesis and/or stability of pachytene piRNAs, retrotransposon silencing, and post-transcriptional regulation of target mRNAs. However, MIWI's detailed role and function are not well understood. In this study, we produced an anti-MIWI mouse monoclonal antibody and identified MIWI-associated poly(A) RNAs by immunoprecipitation from adult mouse testes lysates. Approximately 70% of the MIWI-associated poly(A) RNAs were known mRNAs and 30% of them were unknown non-coding RNAs. These poly(A) RNAs contained piRNA-encoding RNAs transcribed from piRNA cluster regions and piRNA-encoding mRNA, such as Aym1 mRNA. Mature piRNAs specifically encoded in these piRNA-encoding RNAs were generated in pachytene spermatocytes and not detected in Miwi-deficient (Miwi-/-) testes. Moreover, MIWI associated with a large number of known mRNAs whose expression levels were increased in pachytene spermatocytes, and the expression of these mRNAs was decreased in Miwi-/- testes at 20 days postpartum when pachytene spermatocytes were most abundant. These results strongly suggest that MIWI is involved in pachytene piRNA biogenesis and the positive regulation of target mRNA metabolism in pachytene spermatocytes via association with pachytene piRNA precursors and target mRNAs.