GASZ and mitofusin‐mediated mitochondrial functions are crucial for spermatogenesis

Nuage is an electron‐dense cytoplasmic structure in germ cells that contains ribonucleoproteins and participates in piRNA biosynthesis. Despite the observation that clustered mitochondria are associated with a specific type of nuage called intermitochondrial cement (pi‐body), the importance of mitochondrial functions in nuage formation and spermatogenesis is yet to be determined. We show that a germ cell‐specific protein GASZ contains a functional mitochondrial targeting signal and is largely localized at mitochondria both endogenously in germ cells and in somatic cells when ectopically expressed. In addition, GASZ interacts with itself at the outer membrane of mitochondria and promotes mitofusion in a mitofusin/MFN‐dependent manner. In mice, deletion of the mitochondrial targeting signal reveals that mitochondrial localization of GASZ is essential for nuage formation, mitochondrial clustering, transposon repression, and spermatogenesis. MFN1 deficiency also leads to defects in mitochondrial activity and male infertility. Our data thus reveal a requirement for GASZ and MFN‐mediated mitofusion during spermatogenesis.     

Expression of MAEL in nuage and non-nuage compartments of rat spermatogenic cells and colocalization with DDX4, DDX25 and MIWI

The functions of MAELSTROM protein (MAEL) in spermatogenesis are gradually being identified but the precise distribution of MAEL in spermatogenic cells during spermatogenesis has not yet been mapped. We studied the expression of MAEL in rat testis by immunofluorescence and immunoelectron microscopy (IEM). Immunofluorescence staining showed that MAEL was localized in intermitochondrial cement, irregularly-shaped perinuclear granules and satellite bodies of pachytene spermatocytes, and in chromatoid bodies of spermatids. The SBs appeared exclusively in pachytene spermatocytes at stages IX–X and were stained strongly for MAEL. In step 12–19 spermatids, many granules stained for MAEL but not DDX4. These granules were confirmed to be non-nuage structures, including mitochondria-associated granules, reticulated body, granulated body by IEM. In the neck region of late spermatids and sperm, MAEL-positive small granules were found. MAEL is colocalized with MIWI in nuage and non-nuage. The results suggest that MAEL seems to function in nuage and non-nuage structures and interacts with MIWI.     

Argonaute2 Protein in Rat Spermatogenic Cells Is Localized to Nuage Structures and LAMP2-Positive Vesicles Surrounding Chromatoid Bodies

Localization of Argonaute2 (AGO2) protein—an essential component for the processing of small interfering RNA (siRNA)-directed RNA interference (RNAi) in RNA-induced silencing complex (RISC) in nuage of rat spermatogenic cells—was evaluated by immunofluorescence microscopy (IFM) and immunoelectron microscopy (IEM). AGO2 was shown, for the first time, to be localized to four previously classified types of nuage: irregularly shaped perinuclear granules (ISPGs), intermitochondrial cement (IMC), satellite bodies (SBs), and chromatoid bodies (CBs). Dual IEM staining for AGO2/Maelstrom (MAEL) protein or AGO2/MIWI protein demonstrated that AGO2 is colocalized with MAEL or MIWI proteins in these types of nuage. Dual IFM and IEM staining of AGO2/lysosomal-associated membrane protein 2 (LAMP2) showed that CB in round spermatids are in contact with and surrounded by LAMP2-positive vesicles, whereas nuage in pachytene spermatocytes are not. Taken together, our findings indicate that: (i) AGO2 in pachytene spermatocytes functions in ISPGs, IMC, and SBs; (ii) AGO2 in round spermatids functions in CBs, and that CBs are associated with lysosomal compartments.     

PNLDC1, mouse pre‐piRNA Trimmer,is required for meiotic and post‐meiotic male germ cell development

PIWI‐interacting RNAs (piRNAs) are germ cell‐specific small RNAs essential for retrotransposon gene silencing and male germ cell development. In piRNA biogenesis, the endonuclease MitoPLD/Zucchini cleaves long, single‐stranded RNAs to generate 5′ termini of precursor piRNAs (pre‐piRNAs) that are consecutively loaded into PIWI‐family proteins. Subsequently, these pre‐piRNAs are trimmed at their 3′‐end by an exonuclease called Trimmer. Recently, poly(A)‐specific ribonuclease‐like domain‐containing 1 (PNLDC1) was identified as the pre‐piRNA Trimmer in silkworms. However, the function of PNLDC1 in other species remains unknown. Here, we generate Pnldc1 mutant mice and analyze small RNAs in their testes. Our results demonstrate that mouse PNLDC1 functions in the trimming of both embryonic and post‐natal pre‐piRNAs. In addition, piRNA trimming defects in embryonic and post‐natal testes cause impaired DNA methylation and reduced MIWI expression, respectively. Phenotypically, both meiotic and post‐meiotic arrests are evident in the same individual Pnldc1 mutant mouse. The former and latter phenotypes are similar to those of MILI and MIWI mutant mice, respectively. Thus, PNLDC1‐mediated piRNA trimming is indispensable for the function of piRNAs throughout mouse spermatogenesis.     

Tudor-related proteins TDRD1/MTR-1, TDRD6 and TDRD7/TRAP: domain composition, intracellular localization, and function in male germ cells in mice

The germ-line cells of many animals possess a characteristic cytoplasmic structure termed nuage or germinal granules. In mice, nuage that is prominent in postnatal male germ cells is also called intermitochondrial cement or chromatoid bodies. TDRD1/MTR-1, which contains Tudor domain repeats, is a specific component of the mouse nuage, analogously to Drosophila Tudor, a constituent of polar granules/nuage in oocytes and embryos. We show that TDRD6 and TDRD7/TRAP, which also contain multiple Tudor domains, specifically localize to nuage and form a ribonucleoprotein complex together with TDRD1/MTR-1. The characteristic co-localization of TDRD1, 6 and 7 was disrupted in a mutant of mouse vasa homologue/DEAD box polypeptide 4 (Mvh/Ddx4), which encodes another evolutionarily conserved component of nuage. In vivo over-expression experiments of the TDRD proteins and truncated forms during male germ cell differentiation showed that a single Tudor domain is a structural unit that localizes or accumulates to nuage, but the expression of the truncated, putative dominant negative forms is detrimental to meiotic spermatocytes. These results indicate that the Tudor-related proteins, which contain multiple repeats of the Tudor domain, constitute an evolutionarily conserved class of nuage components in the germ-line, and their localization or accumulation to nuage is likely conferred by a Tudor domain structure and downstream of Mvh, while the characteristic repeated architecture of the domain is functionally essential for the differentiation of germ cells.     

Sm proteins, the constituents of the spliceosome, are components of nuage and mitochondrial cement in Xenopus oocytes

A conserved feature of germ cells in many animal species is the presence of perinuclear electron-dense material called the "nuage" that is believed to be a precursor of germinal (or polar or P) granules. In Xenopus oogenesis the nuage is first observed near the nuclear envelope and subsequently in close contact with mitochondria, at which stage it is called the mitochondrial cement. In this study, we found that, in Xenopus pre-stage I and stage I oocytes, nuage and mitochondrial cement contain the spliceosomal Sm proteins, Xcat2 mRNA, and DEAD-box RNA helicase XVLG1. Other components of Cajal bodies or splicing machinery such as coilin, SMN protein, and snRNAs are absent from the nuage and mitochondrial cement. We suggest that Xenopus Sm proteins have adapted to a role independent of pre-mRNA splicing and that instead of binding to their traditional spliceosomal partner such as snRNA, they bind mRNAs that are the components of germinal granules (i.e., Xcat2 mRNA) and facilitate the transport of these mRNAs from the nucleus to the nuage that is a precursor of germinal granules. In addition, the presence of Vasa-like DEAD-box helicase in Xenopus nuage suggests involvement of nuage in the microRNA and/or RNAi pathway, similar to the role of nuage in Drosophila.     

Two mouse piwi-related genes: miwi and mili

Genes belonging to the piwi family are required for stem cell self-renewal in diverse organisms. We cloned mouse homologues of piwi by RT-PCR using degenerative primers. The deduced amino acid sequences of mouse homologues MIWI and MILI showed that each contains a well-conserved C-terminal PIWI domain and that each shares significant homology with PIWI and their human counterparts HIWI. Both miwi and mili were found in germ cells of adult testis by in situ hybridization, suggesting that these genes may function in spermatogenesis. Furthermore, mili was expressed in primordial germ cells (PGCs) of developing mouse embryos and may therefore play a role during germ cell formation. MIWI may be involved in RNA processing or translational regulation, since MIWI was found to possess RNA binding activity. Our data suggest that miwi and mili regulate spermatogenesis and primordial germ cell production.     

Light and electron microscopic analyses of Vasa expression in adult germ cells of the fish medaka

Germ cells of diverse animal species have a unique membrane-less organelle called germ plasm (GP). GP is usually associated with mitochondria and contains RNA binding proteins and mRNAs of germ genes such as vasa. GP has been described as the mitochondrial cloud (MC), intermitochondrial cement (IC) and chromatoid body (CB). The mechanism underlying varying GP structures has remained incompletely understood. Here we report the analysis of GP through light and electron microscopy by using Vasa as a marker in adult male germ cells of the fish medaka (Oryzias latipes). Immunofluorescence light microscopy revealed germ cell-specific Vasa expression. Vasa is the most abundant in mitotic germ cells (oogonia and spermatogonia) and reduced in meiotic germ cells. Vasa in round spermatids exist as a spherical structure reminiscent of CB. Nanogold immunoelectron microscopy revealed subcellular Vasa redistribution in male germ cells. Vasa in spermatogonia concentrates in small areas of the cytoplasm and is surrounded by mitochondria, which is reminiscent of MC. Vasa is intermixed with mitochondria to form IC in primary spermatocytes, appears as the free cement (FC) via separation from mitochondria in secondary spermatocyte and becomes condensed in CB at the caudal pole of round spermatids. During spermatid morphogenesis, Vasa redistributes and forms a second CB that is a ring-like structure surrounding the dense fiber of the flagellum in the midpiece. These structures resemble those described for GP in various species. Thus, Vasa identifies GP and adopts varying structures via dynamic reorganization at different stages of germ cell development.     

piRNA biogenesis during adult spermatogenesis in mice is independent of the ping-pong mechanism

piRNAs, a class of small non-coding RNAs associated with PIWI proteins, have broad functions in germline development, transposon silencing, and epigenetic regulation. In diverse organisms, a subset of piRNAs derived from repeat sequences are produced via the interplay between two PIWI proteins. This mechanism, termed “ping-pong” cycle, operates among the PIWI proteins of the primordial mouse testis; however, its involvement in postnatal testes remains elusive. Here we show that adult testicular piRNAs are produced independent of the ping-pong mechanism. We identified and characterized large populations of piRNAs in the adult and postnatal developing testes associated with MILI and MIWI, the only PIWI proteins detectable in these testes. No interaction between MILI and MIWI or sequence feature for the ping-pong mechanism among their piRNAs was detected in the adult testis. The majority of MILI- and MIWI-associated piRNAs originate from the same DNA strands within the same loci. Both populations of piRNAs are biased for 5′ Uracil but not for Adenine on the 10th nucleotide position, and display no complementarity. Furthermore, in Miwi mutants, MILI-associated piRNAs are not downregulated, but instead upregulated. These results indicate that the adult testicular piRNAs are predominantly, if not exclusively, produced by a primary processing mechanism instead of the ping-pong mechanism. In this primary pathway, biogenesis of MILI- and MIWI-associated piRNAs may compete for the same precursors; the types of piRNAs produced tend to be non-selectively dictated by the available precursors in the cell; and precursors with introns tend to be spliced before processed into piRNAs.     

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.     

Expression of DDX25 in nuage components of mammalian spermatogenic cells: immunofluorescence and immunoelectron microscopic study

The localization of DDX25/GRTH and gonadotropin-stimulated RNA helicase was studied in the spermatogenic cells of rat, mouse, and guinea pig by immunofluorescence and immunoelectron microscopy (IEM). Immunofluorescence studies identified four kinds of granular staining: (1) fine particles observed in meiotic cells; (2) small granules associated with a mitochondrial marker, appearing in pachytene spermatocytes after stage V; (3) short strands lacking the mitochondrial marker in late spermatocytes; and, (4) large irregularly shaped granules in round spermatids. IEM identified DDX25 signals in nine compartments: (1) fine dense particles in the meiotic cells; (2) intermitochondrial cement; (3) loose aggregates of 70–90 nm particles; (4) chromatoid bodies; (5) late chromatoid bodies; (6) satellite bodies; (7) granulated bodies; (8) mitochondria-associated granules; and, (9) reticulated bodies. Compartments (1) to (6) were previously classified into nuage while (7) to (9) were classified as nuage components by the present study. The results suggest that DDX25 functions in these nine compartments.     

Nuage proteins: their localization in subcellular structures of spermatogenic cells as revealed by immunoelectron microscopy

Chromatoid body (CB) was identified as granules stained by basic dye 130 years ago and called by various names. Electron microscopy revealed that the CB belonged to nuage (cloud in French) specific for germ cells. We described the localization of several proteins, including RNA helicases, in the nuage compartments classified into six types and in several spermatogenic cell-specific structures. All the proteins examined were detected in the nuage, including the CB with different staining intensities. Several proteins were localized to non-nuage structures, suggesting that these nuage proteins structures are related to nuage function.     

RNF17, a component of the mammalian germ cell nuage, is essential for spermiogenesis

Nuages are found in the germ cells of diverse organisms. However, nuages in postnatal male germ cells of mice are poorly studied. Previously, we cloned a germ cell-specific gene named Rnf17, which encodes a protein containing both a RING finger and tudor domains. Here, we report that RNF17 is a component of a novel nuage in male germ cells--the RNF17 granule, which is an electron-dense non-membrane bound spherical organelle with a diameter of 0.5 mum. RNF17 granules are prominent in late pachytene and diplotene spermatocytes, and in elongating spermatids. RNF17 granules are distinguishable from other known nuages, such as chromatoid bodies. RNF17 is able to form dimers or polymers both in vitro and in vivo, indicating that it may play a role in the assembly of RNF17 granules. Rnf17-deficient male mice were sterile and exhibited a complete arrest in round spermatids, demonstrating that Rnf17 encodes a novel key regulator of spermiogenesis. Rnf17-null round spermatids advanced to step 4 but failed to produce sperm. These results have shown that RNF17 is a component of a novel germ cell nuage and is required for differentiation of male germ cells.     

Targeting and anchoring Tudor in the pole plasm of the Drosophila oocyte

Background

Germline formation is a highly regulated process in all organisms. In Drosophila embryos germ cells are specified by the pole plasm, a specialized cytoplasmic region containing polar granules. Components of these granules are also present in the perinuclear ring surrounding nurse cells, the nuage. Two such molecules are the Vasa and Tudor proteins. How Tudor localizes and is maintained in the pole plasm is, however, not known.

Methodology/Principal Findings

Here, the process of Tudor localization in nuage and pole plasm was analyzed. The initial positioning of Tudor at the posterior pole of stage 9 oocytes was found to occur in the absence of a structurally detectable nuage. However, in mutants for genes encoding components of the nuage, including vasa, aubergine, maelstrom, and krimper, Tudor was detached from the posterior cortex in stage 10 oocytes, suggesting a prior passage in the nuage for its stability in the pole plasm. Further studies indicated that Valois, which was previously shown to bind in vitro to Tudor, mediates the localization of Tudor in the pole plasm by physically interacting with Oskar, the polar granule organizer. An association between Tudor and Vasa mediated by RNA was also detected in ovarian extracts.

Conclusions/Significance

The present data challenge the view that the assembly of the polar granules occurs in a stepwise and hierarchical manner and, consequently, a revised model of polar granule assembly is proposed. In this model Oskar recruits two downstream components of the polar granules, Vasa and Tudor, independently from each other: Vasa directly interacts with Oskar while Valois mediates the recruitment of Tudor by interacting with Oskar and Tudor.