A Role for Vasa in Regulating Mitotic Chromosome Condensation in Drosophila

Deletion of ddx4 Ovary-Specific Transcript Causes Dysfunction of Meiosis and Derepress of DNA Transposons in Zebrafish Ovaries

Alternative splicing of ddx4 (DEAD-box helicase 4), a key germline marker gene, has been reported to generate sex-specific transcripts in zebrafish gonads. The biological functions and regulatory mechanisms of the ddx4 ovary-specific transcript (ddx4-L) during oogenesis remain unclear. In this study, we found that ddx4-L mutants, in which ddx4-L was specifically deleted, had enlarged ovaries but laid fewer eggs, along with having a lower fertilization rate compared to WT controls. RNA-seq analysis was performed to detect the changes in gene expression between WT and ddx4-L mutant ovaries. A total of 524 upregulated and 610 downregulated DEGs were identified. GO and GSEA enrichment analyses showed that genes involved in fertilization and reproduction biological processes were significantly downregulated. More specifically, we observed a remarkable reduction in Sycp1, a core component of synaptonemal complex, in ddx4-L mutant ovaries at both the mRNA and protein levels. In addition, the expressions of transposon elements, as well as the events of alternative splicing, alternative polyadenylation, and RNA editing, were analyzed based on the RNA-seq data. We found that the deletion of ddx4-L resulted in derepression of DNA transposons in zebrafish ovaries, possibly causing genome instability. In conclusion, our work demonstrates that the ovary-specific ddx4 transcript plays important roles in oocyte meiosis and DNA transposon repression, which extends our understanding of the biological functions and regulatory mechanisms of sex-specific alternative splicing in zebrafish oogenesis and reproduction.

The TOP-2/condensin II axis silences transcription during germline specification in C. elegans

In C. elegans, the germline is specified via a preformation mechanism that relies on the PIE-1 protein's ability to globally silence mRNA transcription in germline precursor cells, also known as the P lineage. Recent work from our group has identified additional genome silencing events in C. elegans during oogenesis and in starved L1 larvae, and these require the condensin II complex, topoisomerase II (TOP-2), and components of the H3K9me/heterochromatin pathway. Interestingly, silencing in oocytes also requires PIE-1, but this is not the case in starved L1s. Here, we ask if additional genome silencing components besides PIE-1 are required to repress gene expression in the P lineage of early embryos, and we find that condensin II and TOP-2 are required and the H3K9me/heterochromatin pathway is not. We show that depletion of TOP-2/condensin II activates the normally suppressed RNA polymerase II to inappropriately transcribe somatic genes in the P lineage. We also present evidence that while both PIE-1 and TOP-2/condensin II are required for genome silencing in the P lineage, PIE-1 can silence transcription independently of TOP-2/condensin II when misexpressed in somatic cells. Thus, in oocytes, all three genome silencing systems (TOP-2/condensin II, H3K9me, and PIE-1) are operational while in both early embryos and starved L1s two of the three are active. Our data show that multiple, redundantly acting genome silencing mechanisms act in a mix and match manner to repress transcription at different developmental stages in the C. elegans germline.

Analysis of a rare progeria variant of Barrier-to-autointegration factor in Drosophila connects centromere function to tissue homeostasis

Barrier-to-autointegration factor (BAF/BANF) is a nuclear lamina protein essential for nuclear integrity, chromatin structure, and genome stability. Whereas complete loss of BAF causes lethality in multiple organisms, the A12T missense mutation of the BANF1 gene in humans causes a premature aging syndrome, called Néstor-Guillermo Progeria Syndrome (NGPS). Here, we report the first in vivo animal investigation of progeroid BAF, using CRISPR editing to introduce the NGPS mutation into the endogenous Drosophila baf gene. Progeroid BAF adults are born at expected frequencies, demonstrating that this BAF variant retains some function. However, tissue homeostasis is affected, supported by studies of the ovary, a tissue that depends upon BAF for stem cell survival and continuous oocyte production. We find that progeroid BAF causes defects in germline stem cell mitosis that delay anaphase progression and compromise chromosome segregation. We link these defects to decreased recruitment of centromeric proteins of the kinetochore, indicating dysfunction of cenBAF, a localized pool of dephosphorylated BAF produced by Protein Phosphatase PP4. We show that DNA damage increases in progenitor germ cells, which causes germ cell death due to activation of the DNA damage transducer kinase Chk2. Mitotic defects appear widespread, as aberrant chromosome segregation and increased apoptosis occur in another tissue. Together, these data highlight the importance of BAF in establishing centromeric structures critical for mitosis. Further, these studies link defects in cenBAF function to activation of a checkpoint that depletes progenitor reserves critical for tissue homeostasis, aligning with phenotypes of NGPS patients.

Molecular characterization of Vasa homolog in the pen shell Atrina pectinata: cDNA cloning and expression analysis during gonadal development

Vasa is an ATP-dependent RNA helicase of the DEAD (Asp-Glu-Ala-Asp) box family and a representative component of the germ plasm. In this study, we cloned the full-length vasa homolog in the bivalve Atrina pectinata (psvasa), and performed phylogenetic analysis, mRNA expression analysis for tissue-specific distributions, and immunostaining analysis to reveal its histological localization. The sequence of psvasa was 3587 bp in length and contained a 5' untranslated region of 150 bp, an open reading frame of 2214 bp, and a 3' untranslated region of 1223 bp. The deduced amino acid sequence of psvasa was 737 amino acids long and contained evolutionarily conserved sequences reported in other animals. The mRNA expression analysis showed the highest expression levels in the gonads. Expression was especially high in the ovaries, followed by the testes. The immunostaining analysis showed Vasa-positive cells in the developing gonads, suggesting the presence of putative germ stem cells contributing to the supply of germ cells. Furthermore, characteristic Vasa signals were observed in the basophilic nuclei of the oocytes, suggesting that psvasa plays an important role in the progression of meiosis in oocytes.

Vasa nucleates asymmetric translation along the mitotic spindle during unequal cell divisions

mRNA translation on the spindle is hypothesized to be an essential strategy for the localized production of cell regulators. This mechanism may be important particularly in early embryonic cells, which have a large diffusion volume and that undergo rapid cell divisions. Evidence to test such a hypothesis has been, however, limited. Here, we use an embryo with both symmetric and asymmetric cell divisions and manipulate Vasa protein, an RNA-helicase, on the spindle in live sea urchin embryos. We learned that the spindle serves as a major site of translation and that protein synthesis within a single spindle can be unequal and help drive asymmetric cell divisions during embryogenesis. Recruiting Vasa to the ectopic sub-cellular region induced a new site of translation, disturbed asymmetric translation on the spindle, and changed the cell fate. Based on these observations, we conclude that Vasa functions in localized translation, which provides a spatiotemporal control in protein synthesis and is essential for rapidly developing embryonic cells.

Association of mRNA translation with the mitotic spindle is thought to be involved in localized production of cell fate determinants. Here, the authors show Vasa facilitates asymmetric translation, which contributes to differential regulation during sea urchin embryogenesis.

Small regulatory noncoding RNAs in Drosophila melanogaster: biogenesis and biological functions

RNA interference (RNAi) is an important phenomenon that has diverse genetic regulatory functions at the pre- and posttranscriptional levels. The major trigger for the RNAi pathway is double-stranded RNA (dsRNA). dsRNA is processed to generate various types of major small noncoding RNAs (ncRNAs) that include microRNAs (miRNAs), small interfering RNAs (siRNAs) and PIWI-interacting RNAs (piRNAs) in Drosophila melanogaster (D. melanogaster). Functionally, these small ncRNAs play critical roles in virtually all biological systems and developmental pathways. Identification and processing of dsRNAs and activation of RNAi machinery are the three major academic interests that surround RNAi research. Mechanistically, some of the important biological functions of RNAi are achieved through: (i) supporting genomic stability via degradation of foreign viral genomes; (ii) suppressing the movement of transposable elements and, most importantly, (iii) post-transcriptional regulation of gene expression by miRNAs that contribute to regulation of epigenetic modifications such as heterochromatin formation and genome imprinting. Here, we review various routes of small ncRNA biogenesis, as well as different RNAi-mediated pathways in D. melanogaster with a particular focus on signaling pathways. In addition, a critical discussion of the most relevant and latest findings that concern the significant contribution of small ncRNAs to the regulation of D. melanogaster physiology and pathophysiology is presented.

Molecular approaches underlying the oogenic cycle of the scleractinian coral, Acropora tenuis

This study aimed to elucidate the physiological processes of oogenesis in Acropora tenuis. Genes/proteins related to oogenesis were investigated: Vasa, a germ cell marker, vitellogenin (VG), a major yolk protein precursor, and its receptor (LDLR). Coral branches were collected monthly from coral reefs around Sesoko Island (Okinawa, Japan) for histological observation by in situ hybridisation (ISH) of the Vasa (AtVasa) and Low Density Lipoprotein Receptor (AtLDLR) genes and immunohistochemistry (IHC) of AtVasa and AtVG. AtVasa immunoreactivity was detected in germline cells and ooplasm, whereas AtVG immunoreactivity was detected in ooplasm and putative ovarian tissues. AtVasa was localised in germline cells located in the retractor muscles of the mesentery, whereas AtLDLR was localised in the putative ovarian and mesentery tissues. AtLDLR was detected in coral tissues during the vitellogenic phase, whereas AtVG immunoreactivity was found in primary oocytes. Germline cells expressing AtVasa are present throughout the year. In conclusion, Vasa has physiological and molecular roles throughout the oogenic cycle, as it determines gonadal germline cells and ensures normal oocyte development, whereas the roles of VG and LDLR are limited to the vitellogenic stages because they act in coordination with lipoprotein transport, vitellogenin synthesis, and yolk incorporation into oocytes.

A kinesin Klp10A mediates cell cycle-dependent shuttling of Piwi between nucleus and nuage

The piRNA pathway protects germline genomes from selfish genetic elements (e.g. transposons) through their transcript cleavage in the cytoplasm and/or their transcriptional silencing in the nucleus. Here, we describe a mechanism by which the nuclear and cytoplasmic arms of the piRNA pathway are linked. We find that during mitosis of Drosophila spermatogonia, nuclear Piwi interacts with nuage, the compartment that mediates the cytoplasmic arm of the piRNA pathway. At the end of mitosis, Piwi leaves nuage to return to the nucleus. Dissociation of Piwi from nuage occurs at the depolymerizing microtubules of the central spindle, mediated by a microtubule-depolymerizing kinesin, Klp10A. Depletion of klp10A delays the return of Piwi to the nucleus and affects piRNA production, suggesting the role of nuclear-cytoplasmic communication in piRNA biogenesis. We propose that cell cycle-dependent communication between the nuclear and cytoplasmic arms of the piRNA pathway may play a previously unappreciated role in piRNA regulation.

The piRNA pathway that defends germline from selfish elements operates in two subpathways, one mediated by Piwi in Drosophila to silence transcription of targets in the nucleus and the other mediated by Aub and Ago3 to cleave transcripts of targets in the cytoplasm. How these two subpathways might coordinate with each other, particularly at the cell biological level, remains elusive. This study shows that Piwi interacts with Aub/Ago3 specifically in mitosis in nuage, the organelle that serves as the platform for piRNA cytoplasmic subpathway. Piwi returns to the nucleus at the end of mitosis, and our study suggests that dissociation of Piwi from nuage is facilitated by microtubule depolymerization by a kinesin Klp10A at the central spindle. We propose that cell-cycle-dependent interaction of two piRNA subpathways may play an important role in piRNA production.

Apoptosis-mediated vasa down-regulation controls developmental transformation in Japanese Copidosoma floridanum female soldiers

Copidosoma floridanum is a polyembryonic, caste-forming, wasp species. The ratio of investment in different castes changes with environmental stressors (e.g. multi-parasitism with competitors). The vasa gene was first identified in Drosophila melanogaster as a germ-cell-determining factor, and C. floridanum vasa (Cf-vas) gene positive cells have been known to develop into reproductive larvae. Cf-vas seems to control the ratio of investment in C. floridanum larval castes. In this study, we identified environmental factors that control Cf-vas mRNA expression in Japanese C. floridanum by examining Cf-vas mRNA expression under competitor (Meteorus pulchricornis) venom stress; we treated the male and female morulae with M. pulchricornis venom. We also assessed the effects of multi-parasitism of Japanese C. floridanum with M. pulchricornis and found an increasing number of female soldier larvae. The results showed that several amino acid sequences differ between the Japanese and US Cf-vas. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) showed that Japanese Cf-vas mRNA is expressed in both male and female larvae and pupae, but mRNA expression decreases in adults. Cf-vas mRNA expression significantly decreased, while C. floridanum dronc (Cf-dronc) mRNA expression increased, in female morulae after M. pulchricornis venom treatment at 20 h and 0 h of the culture period, respectively. Females and males showed different Cf-vas or Cf-dronc mRNA expression after M. pulchricornis venom treatment. Therefore, M. pulchricornis venom could affect the ratio of investment in different female castes of Japanese C. floridanum by decreasing Cf-vas mRNA expression via apoptosis.

Germ Cell Lineage Homeostasis in Drosophila Requires the Vasa RNA Helicase

The conserved RNA helicase Vasa is required for germ cell development in many organisms. In Drosophila melanogaster loss of PIWI-interacting RNA pathway components, including Vasa, causes Chk2-dependent oogenesis arrest. However, whether the arrest is due to Chk2 signaling at a specific stage and whether continuous Chk2 signaling is required for the arrest is unknown. Here, we show that absence of Vasa during the germarial stages causes Chk2-dependent oogenesis arrest. Additionally, we report the age-dependent decline of the ovariole number both in flies lacking Vasa expression only in the germarium and in loss-of-function vasa mutant flies. We show that Chk2 activation exclusively in the germarium is sufficient to interrupt oogenesis and to reduce ovariole number in aging flies. Once induced in the germarium, Chk2-mediated arrest of germ cell development cannot be overcome by restoration of Vasa or by downregulation of Chk2 in the arrested egg chambers. These findings, together with the identity of Vasa-associated proteins identified in this study, demonstrate an essential role of the helicase in the germ cell lineage maintenance and indicate a function of Vasa in germline stem cell homeostasis.

Germ plasm-related structures in marine medaka gametogenesis; novel sites of Vasa localization and the unique mechanism of germ plasm granule arising

Germ plasm, a cytoplasmic factor of germline cell differentiation, is suggested to be a perspective tool for in vitro meiotic differentiation. To discriminate between the: (1) germ plasm-related structures (GPRS) involved in meiosis triggering; and (2) GPRS involved in the germ plasm storage phase, we investigated gametogenesis in the marine medaka Oryzias melastigma. The GPRS of the mitosis-to-meiosis period are similar in males and females. In both sexes, five events typically occur: (1) turning of the primary Vasa-positive germ plasm granules into the Vasa-positive intermitochondrial cement (IMC); (2) aggregation of some mitochondria by IMC followed by arising of mitochondrial clusters; (3) intramitochondrial localization of IMC-originated Vasa; followed by (4) mitochondrial cluster degradation; and (5) intranuclear localization of Vasa followed by this protein entering the nuclei (gonial cells) and synaptonemal complexes (zygotene-pachytene meiotic cells). In post-zygotene/pachytene gametogenesis, the GPRS are sex specific; the Vasa-positive chromatoid bodies are found during spermatogenesis, but oogenesis is characterized by secondary arising of Vasa-positive germ plasm granules followed by secondary formation and degradation of mitochondrial clusters. A complex type of germ plasm generation, 'the follicle cell assigned germ plasm formation', was found in late oogenesis. The mechanisms discovered are recommended to be taken into account for possible reconstruction of those under in vitro conditions.

Germ plasm provides clues on meiosis: the concerted action of germ plasm granules and mitochondria in gametogenesis of the clam Ruditapes philippinarum

SummaryGerm plasm-related structures (GPRS) are known to accompany meiotic cell differentiation but their dynamics are still poorly understood. In this study, we analyzed the ultrastructural mechanisms of GPRS transformation during oogenesis and spermatogenesis of the bivalve mollusc Ruditapes philippinarum (Manila clam), exploring patterns of GPRS activity occurring at meiosis onset, sex-specific difference/similarity of such patterns, and the involvement of mitochondria during GPRS-assigned events. In the two sexes, the zygotene-pachytene stage of meiosis is anticipated by three shared steps. First, the dispersion of germ plasm granules containing the germ line determinant VASA occurs. Second, the VASA protein deriving from germ plasm granules enters neighbouring mitochondria and appears to induce mitochondrial matter release, as supported by cytochrome B localization outside the mitochondria. Third, intranuclear VASA entrance occurs and the protein appears involved in chromatin reorganization, as supported by VASA localization in synaptonemal complexes. In spermatogenesis, these three steps are sufficient for the normal course of meiosis. In oogenesis, these are followed by the action of 'germ plasm granule formation complex', a novel type of structure that appears alternative to the Balbiani body. The possibility of germ plasm involvement in reproductive technologies is also suggested.

A sisRNA/miRNA Axis Prevents Loss of Germline Stem Cells during Starvation in Drosophila

Animal reproduction responds to nutritional status. During starvation, Drosophila and Caenorhabditis elegans enter a period of reproductive diapause with increase apoptosis, while maintaining a stable pool of germline stem cells (GSCs). How GSCs are protected is not understood. Here, we show that a sisRNA/miRNA axis maintains ovarian GSCs during starvation in Drosophila. Starvation induces the expression of an ovary-enriched sisRNA sisR-2, which negatively regulates GSC maintenance via a fatty acid metabolism gene dFAR1. sisR-2 promotes the expression of bantam, which in turn inhibits the activity of sisR-2, forming a negative feedback loop. Therefore, bantam acts as a buffer to counteract sisR-2 activity to prevent GSC loss during starvation. We propose that the sisR-2/bantam axis confers robustness to GSCs in Drosophila.

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sisR-2 regulates the number of GSCs

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sisR-2 regulates GSC maintenance by repressing dFAR1

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bantam regulates GSC maintenance by repressing sisR-2 activity

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sisR-2/bantam axis protects GSCs from starvation

Germline Stem Cell Heterogeneity Supports Homeostasis in Drosophila

Adult and embryonic stem cells exhibit fluctuating gene expression; however, the biological significance of stem cell heterogeneity is not well understood. We show that, in Drosophila, female germline stem cells (GSCs) exhibit heterogeneous expression of a GSC differentiation-promoting factor Regena (Rga). The Drosophila homolog of human SON, dsn, is required to maintain GSC heterogeneity by suppressing sustained high levels of Rga. Reducing the expression of Rga in dsn mutants restores GSC heterogeneity and self-renewal. Thus, GSC heterogeneity is linked to GSC homeostasis.

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Female germline stem cells have heterogeneous Rga expression

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dsn suppresses rga transcription and maintains heterogeneity

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dsn maintains germline stem cells

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Lowering rga restores heterogeneity and suppresses dsn phenotypes

Do Gametes Woo? Evidence for Their Nonrandom Union at Fertilization

A fundamental tenet of inheritance in sexually reproducing organisms such as humans and laboratory mice is that gametes combine randomly at fertilization, thereby ensuring a balanced and statistically predictable representation of inherited variants in each generation. This principle is encapsulated in Mendel's First Law. But exceptions are known. With transmission ratio distortion, particular alleles are preferentially transmitted to offspring. Preferential transmission usually occurs in one sex but not both, and is not known to require interactions between gametes at fertilization. A reanalysis of our published work in mice and of data in other published reports revealed instances where any of 12 mutant genes biases fertilization, with either too many or too few heterozygotes and homozygotes, depending on the mutant gene and on dietary conditions. Although such deviations are usually attributed to embryonic lethality of the underrepresented genotypes, the evidence is more consistent with genetically-determined preferences for specific combinations of egg and sperm at fertilization that result in genotype bias without embryo loss. This unexpected discovery of genetically-biased fertilization could yield insights about the molecular and cellular interactions between sperm and egg at fertilization, with implications for our understanding of inheritance, reproduction, population genetics, and medical genetics.

Chl1 DNA helicase and Scc2 function in chromosome condensation through cohesin deposition

Chl1 DNA helicase promotes sister chromatid cohesion and associates with both the cohesion establishment acetyltransferase Eco1/Ctf7 and the DNA polymerase processivity factor PCNA that supports Eco1/Ctf7 function. Mutation in CHL1 results in precocious sister chromatid separation and cell aneuploidy, defects that arise through reduced levels of chromatin-bound cohesins which normally tether together sister chromatids (trans tethering). Mutation of Chl1 family members (BACH1/BRIP/FANCJ and DDX11/ChlR1) also exhibit genotoxic sensitivities, consistent with a role for Chl1 in trans tethering which is required for efficient DNA repair. Chl1 promotes the recruitment of Scc2 to DNA which is required for cohesin deposition onto DNA. There is limited evidence, however, that Scc2 also directs the deposition onto DNA of condensins which promote tethering in cis (intramolecular DNA links). Here, we test the ability of Chl1 to promote cis tethering and the role of both Chl1 and Scc2 to promote condensin recruitment to DNA. The results reveal that chl1 mutant cells exhibit significant condensation defects both within the rDNA locus and genome-wide. Importantly, chl1 mutant cell condensation defects do not result from reduced chromatin binding of condensin, but instead through reduced chromatin binding of cohesin. We tested scc2-4 mutant cells and similarly found no evidence of reduced condensin recruitment to chromatin. Consistent with a role for Scc2 specifically in cohesin deposition, scc2-4 mutant cell condensation defects are irreversible. We thus term Chl1 a novel regulator of both chromatin condensation and sister chromatid cohesion through cohesin-based mechanisms. These results reveal an exciting interface between DNA structure and the highly conserved cohesin complex.

DIP1 modulates stem cell homeostasis in Drosophila through regulation of sisR-1

Stable intronic sequence RNAs (sisRNAs) are by-products of splicing and regulate gene expression. How sisRNAs are regulated is unclear. Here we report that a double-stranded RNA binding protein, Disco-interacting protein 1 (DIP1) regulates sisRNAs in Drosophila. DIP1 negatively regulates the abundance of sisR-1 and INE-1 sisRNAs. Fine-tuning of sisR-1 by DIP1 is important to maintain female germline stem cell homeostasis by modulating germline stem cell differentiation and niche adhesion. Drosophila DIP1 localizes to a nuclear body (satellite body) and associates with the fourth chromosome, which contains a very high density of INE-1 transposable element sequences that are processed into sisRNAs. DIP1 presumably acts outside the satellite bodies to regulate sisR-1, which is not on the fourth chromosome. Thus, our study identifies DIP1 as a sisRNA regulatory protein that controls germline stem cell self-renewal in Drosophila.

Stable intronic sequence RNAs (sisRNAs) are by-products of splicing from introns with roles in embryonic development in Drosophila. Here, the authors show that the RNA binding protein DIP1 regulates sisRNAs in Drosophila, which is necessary for germline stem cell homeostasis.

Early gametogenesis in the Pacific oyster: new insights using stem cell and mitotic markers

While our knowledge of bivalve gametogenesis has progressed in recent times, more molecular markers are needed in order to develop tissue imaging. Here, we identified stem cell and mitotic markers to further characterize oyster early gametogenesis, mainly through immunofluorescence microscopy. Intense alkaline phosphatase activity, a non-specific marker for stem cells, was detected on the outer edge of the gonad ducts at the post-spawning stage, suggesting an abundance of undifferentiated cells very early during the sexual cycle. This observation was confirmed using an antibody against Sox2, a transcription factor specific for stem or germline cells, which labeled cells in the gonad duct inner mass and ciliated epithelium early during the initial oyster sexual cycle. Moreover, Vasa, a cytoplasmic marker for germline cells, was also detected in the gonad acini and duct cells, thus confirming that germline cells were abundant early on. In addition, the binding of the minichromosome maintenance MCM6 protein to chromatin indicated the gonad acini and duct cells were engaged in the cell cycle. DNA replication was indeed confirmed by an abundant in vivo incorporation of BrdU into the duct cell chromatin. Finally, proliferation of acini and duct cells was demonstrated by the chromatin-bound Ser10-phosphorylated histone H3, a mitotic marker. The markers for the cell cycle and mitosis used here thus indicate that acini and duct cells were already actively dividing early during the oyster sexual cycle. In addition, together with the stem cell markers, these data reveal that the epithelium delimiting the duct outer edge contains a dynamic population of undifferentiated cells.

Multiple Functions of the DEAD-Box Helicase Vasa in Drosophila Oogenesis

The DEAD-box helicase Vasa (Vas) has been most extensively studied in the fruit fly, Drosophila melanogaster, and numerous roles for it in germline development have been discovered. Here, we summarize the present state of knowledge about processes during oogenesis that involve Vas, as well as functions of Vas as a maternal determinant of embryonic spatial patterning and germ cell specification. We review literature that implicates Vas in Piwi-interacting RNA (piRNA) biogenesis in germline cells and in regulating mitosis in germline stem cells (GSCs). We describe the functions of Vas in translational activation of two mRNAs, gurken (grk) and mei-P26, which encode proteins that are important regulators of developmental processes, as Grk specifies both the dorsal-ventral and the anterior-posterior axis of the embryo and Mei-P26 promotes GSC differentiation. The role of Vas in assembly of polar granules, ribonucleoprotein particles that accumulate in the posterior pole plasm of the oocyte and are essential for germ cell specification and posterior embryonic patterning, is also described.

Regeneration in bipinnaria larvae of the bat star Patiria miniata induces rapid and broad new gene expression

BACKGROUND

Some metazoa have the capacity to regenerate lost body parts. This phenomenon in adults has been classically described in echinoderms, especially in sea stars (Asteroidea). Sea star bipinnaria larvae can also rapidly and effectively regenerate a complete larva after surgical bisection. Understanding the capacity to reverse cell fates in the larva is important from both a developmental and biomedical perspective; yet, the mechanisms underlying regeneration in echinoderms are poorly understood.

RESULTS

Here, we describe the process of bipinnaria regeneration after bisection in the bat star Patiria miniata. We tested transcriptional, translational, and cell proliferation activity after bisection in anterior and posterior bipinnaria halves as well as expression of SRAP, reported as a sea star regeneration associated protease (Vickery et al., 2001b). Moreover, we found several genes whose transcripts increased in abundance following bisection, including: Vasa, dysferlin, vitellogenin 1 and vitellogenin 2.

CONCLUSION

These results show a transformation following bisection, especially in the anterior halves, of cell fate reassignment in all three germ layers, with clear and predictable changes. These results define molecular events that accompany the cell fate changes coincident to the regenerative response in echinoderm larvae.

C-terminal residues specific to Vasa among DEAD-box helicases are required for its functions in piRNA biogenesis and embryonic patterning

The DEAD-box RNA helicase Vasa (Vas, also known as DDX4) is required for germ cell development. In Drosophila, analysis of hypomorphic mutations has implicated maternally expressed Vas in germ cell formation and posterior embryonic patterning. vas-null females, which rarely complete oogenesis, exhibit defects in mitotic progression of germline stem cells, Piwi-interacting RNA (piRNA)-mediated transposon silencing, and translation of Gurken (Grk), an EGFR ligand. The carboxy-terminal region of Vas orthologs throughout the animal kingdom consists of several acidic residues as well as an invariant tryptophan in the penultimate or ultimate position (Trp660 in Drosophila melanogaster). Using CRISPR/Cas9 gene editing, we made a substitution mutant in this residue. Replacing Trp660 by Glu (W660E) abolishes the ability of Vas to support germ cell formation and embryonic patterning and greatly reduces Vas activity in piRNA biogenesis, as measured by transposon silencing, and in activating Grk translation. A conservative substitution (W660F) has much milder phenotypic consequences. In addition, females expressing only a form of Vas in which the seven C-terminal amino acids were replaced with the corresponding residues from Belle (Bel, also known as DDX3) show defects in perinuclear nuage assembly and transposon silencing. Oogenesis in females expressing only the chimeric Vas arrests early; however, in a vas 1 background, in which early expression of endogenous Vas supports oogenesis, the chimeric protein supports posterior patterning and germ cell specification. These results indicate that the unique C-terminus of Vas is essential for its function in piRNA biogenesis and that the conserved Trp660 residue has an important functional role.

An unregulated regulator: Vasa expression in the development of somatic cells and in tumorigenesis

Growing evidence in diverse organisms shows that genes originally thought to function uniquely in the germ line may also function in somatic cells, and in some cases even contribute to tumorigenesis. Here we review the somatic functions of Vasa, one of the most conserved "germ line" factors among metazoans. Vasa expression in somatic cells is tightly regulated and often transient during normal development, and appears to play essential roles in regulation of embryonic cells and regenerative tissues. Its dysregulation, however, is believed to be an important element of tumorigenic cell regulation. In this perspectives paper, we propose how some conserved functions of Vasa may be selected for somatic cell regulation, including its potential impact on efficient and localized translational activities and in some cases on cellular malfunctioning and tumorigenesis.

Novel mutants of the aubergine gene

Aubergine is an RNA-binding protein of the Piwi clade, functioning in germline in the piRNA pathway that silences transposons and repetitive sequences. Several mutations of this gene exist, but they mostly result in truncated proteins or correspond to mutations that also affect neighboring genes. We have generated complete aubergine knock-out mutants that do not disrupt the neighboring genes. These novel mutants are characterized by PCR and sequencing. Their nature is confirmed by female sterility and by the presence of crystals in testes, common to the aubergine loss of function mutations. These mutants provide novel and more appropriate tools for the study of the piRNA pathway that controls genome stability.

A piece of the pi(e): The diverse roles of animal piRNAs and their PIWI partners

Small non-coding RNAs are indispensable to many biological processes. A class of endogenous small RNAs, termed PIWI-interacting RNAs (piRNAs) because of their association with PIWI proteins, has known roles in safeguarding the genome against inordinate transposon mobilization, embryonic development, and stem cell regulation, among others. This review discusses the biogenesis of animal piRNAs and their diverse functions together with their PIWI protein partners, both in the germline and in somatic cells, and highlights the evolutionarily conserved aspects of these molecular players in animal biology.

Loss of the Drosophila melanogaster DEAD box protein Ddx1 leads to reduced size and aberrant gametogenesis

Mammalian DDX1 has been implicated in RNA trafficking, DNA double-strand break repair and RNA processing; however, little is known about its role during animal development. Here, we report phenotypes associated with a null Ddx1 (Ddx1^AX) mutation generated in Drosophila melanogaster. Ddx1 null flies are viable but significantly smaller than control and Ddx1 heterozygous flies. Female Ddx1 null flies have reduced fertility with egg chambers undergoing autophagy, whereas males are sterile due to disrupted spermatogenesis. Comparative RNA sequencing of control and Ddx1 null third instars identified several transcripts affected by Ddx1 inactivation. One of these, Sirup mRNA, was previously shown to be overexpressed under starvation conditions and implicated in mitochondrial function. We demonstrate that Sirup is a direct binding target of Ddx1 and that Sirup mRNA is differentially spliced in the presence or absence of Ddx1. Combining Ddx1 null mutation with Sirup dsRNA-mediated knock-down causes epistatic lethality not observed in either single mutant. Our data suggest a role for Drosophila Ddx1 in stress-induced regulation of splicing.

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We describe a new Ddx1 null Drosophila line.

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Ddx1 null flies are smaller in size and display aberrant gametogenesis.

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Sirup splicing is altered in Ddx1 null flies.

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We show both a physical and a genetic interaction between Ddx1 and Sirup.

Essential elements for translation: the germline factor Vasa functions broadly in somatic cells

Vasa is a conserved RNA-helicase found in the germ lines of all metazoans tested. Whereas Vasa presence is often indicated as a metric for germline determination in animals, it is also expressed in stem cells of diverse origin. Recent research suggests, however, that Vasa has a much broader function, including a significant role in cell cycle regulation. Results herein indicate that Vasa is utilized widely, and often induced transiently, during development in diverse somatic cells and adult precursor tissues. We identified that Vasa in the sea urchin is essential for: (1) general mRNA translation during embryogenesis, (2) developmental re-programming upon manipulations to the embryo and (3) larval wound healing. We also learned that Vasa interacted with mRNAs in the perinuclear area and at the spindle in an Importin-dependent manner during cell cycle progression. These results suggest that, when present, Vasa functions are essential to contributing to developmental regulation.

In vivo mapping of the functional regions of the DEAD-box helicase Vasa

The maternally expressed Drosophila melanogaster DEAD-box helicase Vasa (Vas) is necessary for many cellular and developmental processes, including specification of primordial germ cells (pole cells), posterior patterning of the embryo, piRNA-mediated repression of transposon-encoded mRNAs, translational activation of gurken (grk) mRNA, and completion of oogenesis itself. Vas protein accumulates in the perinuclear nuage in nurse cells soon after their specification, and then at stage 10 Vas translocates to the posterior pole plasm of the oocyte. We produced a series of transgenic constructs encoding eGFP-Vas proteins carrying mutations affecting different regions of the protein, and analyzed in vivo which Vas functions each could support. We identified novel domains in the N- and C-terminal regions of the protein that are essential for localization, transposon repression, posterior patterning, and pole cell specification. One such functional region, the most C-terminal seven amino acids, is specific to Vas orthologues and is thus critical to distinguishing Vas from other closely related DEAD-box helicases. Surprisingly, we also found that many eGFP-Vas proteins carrying mutations that would be expected to abrogate DEAD-box helicase function localized to the nuage and posterior pole, and retained the capacity to support oogenesis, although they did not function in embryonic patterning, pole cell specification, grk activation, or transposon repression. We conclude from these experiments that Vas, a multifunctional protein, uses different domains and different molecular associations to carry out its various cellular and developmental roles.

Zebrafish vasa is required for germ-cell differentiation and maintenance

Vasa is a universal marker of the germ line in animals, yet mutations disrupting vasa cause sexually dimorphic infertility, with impaired development of the ovary in some animals and the testis in others. The basis for this sexually dimorphic requirement for Vasa is not clear; in most animals examined, both the male and female gonad express vasa throughout the life of the germ line. Here we characterized a loss-of-function mutation disrupting zebrafish vasa. We show that maternally provided Vasa is stable through the first ten days of development in zebrafish, and thus likely fulfills any early roles for Vasa during germ-line specification, migration, survival, and maintenance. Although zygotic Vasa is not essential for the development of juvenile gonads, vasa mutants develop exclusively as sterile males. Furthermore, phenotypes of vasa;p53 compound mutants are indistinguishable from those of vasa mutants, therefore the failure of vasa mutants to differentiate as females and to support germ-cell development in the testis is not due to p53-mediated apoptosis. Instead, we found that failure to progress beyond the pachytene stage of meiosis causes the loss of germ-line stem cells, leaving empty somatic tubules. Our studies provide insight into the function of zebrafish vasa during female meiosis, differentiation, and maintenance of germ-line stem cells.

vasa and piwi are required for mitotic integrity in early embryogenesis in the spider Parasteatoda tepidariorum

Studies in vertebrate and invertebrate model organisms on the molecular basis of primordial germ cell (PGC) specification have revealed that metazoans can specify their germ line either early in development by maternally transmitted cytoplasmic factors (inheritance), or later in development by signaling factors from neighboring tissues (induction). Regardless of the mode of PGC specification, once animal germ cells are specified, they invariably express a number of highly conserved genes. These include vasa and piwi, which can play essential roles in any or all of PGC specification, development, or gametogenesis. Although the arthropods are the most speciose animal phylum, to date there have been no functional studies of conserved germ line genes in species of the most basally branching arthropod clade, the chelicerates (which includes spiders, scorpions, and horseshoe crabs). Here we present the first such study by using molecular and functional tools to examine germ line development and the roles of vasa and piwi orthologues in the common house spider Parasteatoda (formerly Achaearanea) tepidariorum. We use transcript and protein expression patterns of Pt-vasa and Pt-piwi to show that primordial germ cells (PGCs) in the spider arise during late embryogenesis. Neither Pt-vasa nor Pt-piwi gene products are localized asymmetrically to any embryonic region before PGCs emerge as paired segmental clusters in opisthosomal segments 2-6 at late germ band stages. RNA interference studies reveal that both genes are required maternally for egg laying, mitotic progression in early embryos, and embryonic survival. Our results add to the growing body of evidence that vasa and piwi can play important roles in somatic development, and provide evidence for a previously hypothesized conserved role for vasa in cell cycle progression.

Balbiani body, nuage and sponge bodies--term plasm pathway players

In many animal species, germ cells are specified by maternally provided, often asymmetrically localized germ cell determinant, termed the germ plasm. It has been shown that in model organisms such as Xenopus laevis, Danio rerio and Drosophila melanogaster germ plasm components (various proteins, mRNAs and mitochondria) are delivered to the proper position within the egg cell by germline specific organelles, i.e. Balbiani bodies, nuage accumulations and/or sponge bodies. In the present article, we review the current knowledge on morphology, molecular composition and functioning of these organelles in main lineages of arthropods and different ovary types on the backdrop of data derived from the studies of the model vertebrate species.

CPF-associated phosphatase activity opposes condensin-mediated chromosome condensation

Functional links connecting gene transcription and condensin-mediated chromosome condensation have been established in species ranging from prokaryotes to vertebrates. However, the exact nature of these links remains misunderstood. Here we show in fission yeast that the 3′ end RNA processing factor Swd2.2, a component of the Cleavage and Polyadenylation Factor (CPF), is a negative regulator of condensin-mediated chromosome condensation. Lack of Swd2.2 does not affect the assembly of the CPF but reduces its association with chromatin. This causes only limited, context-dependent effects on gene expression and transcription termination. However, CPF-associated Swd2.2 is required for the association of Protein Phosphatase 1 PP1^Dis2 with chromatin, through an interaction with Ppn1, a protein that we identify as the fission yeast homologue of vertebrate PNUTS. We demonstrate that Swd2.2, Ppn1 and PP1^Dis2 form an independent module within the CPF, which provides an essential function in the absence of the CPF-associated Ssu72 phosphatase. We show that Ppn1 and Ssu72, like Swd2.2, are also negative regulators of condensin-mediated chromosome condensation. We conclude that Swd2.2 opposes condensin-mediated chromosome condensation by facilitating the function of the two CPF-associated phosphatases PP1 and Ssu72.

Failure to properly condense chromosomes prior to their segregation in mitosis can lead to genome instability. The evolutionary-conserved condensin complex is key to the condensation process but the molecular mechanisms underlying its localization pattern on chromosomes remain unclear. Previous observations showed that the localization of condensin is intimately linked to regions of high transcription, although, somewhat paradoxically, its association with chromatin is disrupted by a processive polymerase activity. Here we identify several RNA processing factors as negative regulators of condensin in fission yeast. Two of these factors associate with PP1 phosphatase as an independent entity within the Cleavage and Polyadenylation Factor (CPF), a complex key for 3′ end RNA processing. Lack of this module induces only minor and context-dependent effects on gene expression. Our data suggest that this module helps maintaining the proper level of phosphatase activity within the CPF and thereby opposes the function of condensin in mitotic chromosome condensation.

The DEAD Box RNA helicase VBH-1 is a new player in the stress response in C. elegans

For several years, DEAD box RNA helicase Vasa (DDX4) has been used as a bona fide germline marker in different organisms. C. elegans VBH-1 is a close homolog of the Vasa protein, which plays an important role in gametogenesis, germ cell survival and embryonic development. Here, we show that VBH-1 protects nematodes from heat shock and oxidative stress. Using the germline-defective mutant glp-4(bn2) we found that a potential somatic expression of vbh-1 might be important for stress survival. We also show that the VBH-1 paralog LAF-1 is important for stress survival, although this protein is not redundant with its counterpart. Furthermore, we observed that the mRNAs of the heat shock proteins hsp-1 and sip-1 are downregulated when vbh-1 or laf-1 are silenced. Previously, we reported that in C. elegans, VBH-1 was primarily expressed in P granules of germ cells and in the cytoplasm of all blastomeres. Here we show that during stress, VBH-1 co-localizes with CGH-1 in large aggregates in the gonad core and oocytes; however, VBH-1 aggregates do not overlap with CGH-1 foci in early embryos under the same conditions. These data demonstrate that, in addition to the previously described role for this protein in the germline, VBH-1 plays an important role during the stress response in C. elegans through the potential direct or indirect regulation of stress response mRNAs.

Multifunctionality of PIWI proteins in control of germline stem cell fate

PIWI proteins interacting with specific type of small RNAs (piRNAs) repress transposable elements in animals. Besides, they have been shown to participate in various cellular processes: in the regulation of heterochromatin formation including telomere structures, in the control of translation and the cell cycle, and in DNA rearrangements. PIWI proteins were first identified by their roles in the self-renewal of germline stem cells. PIWI protein functions are not limited to gonadogenesis, but the role in determining the fate of stem cells is their specific feature conserved throughout the evolution of animals. Molecular mechanisms underlying these processes are far from being understood. This review focuses on the role of PIWI proteins in the control of maintenance and proliferation of germinal stem cells and its relation to the known function of PIWI in transposon repression.

Analysis of Hydra PIWI proteins and piRNAs uncover early evolutionary origins of the piRNA pathway

To preserve genome integrity, an evolutionarily conserved small RNA-based silencing mechanism involving PIWI proteins and PIWI-interacting RNAs (piRNAs) represses potentially deleterious transposons in animals. Although there has been extensive research into PIWI proteins in bilaterians, these proteins remain to be examined in ancient phyla. Here, we investigated the PIWI proteins Hywi and Hyli in the cnidarian Hydra, and found that both PIWI proteins are enriched in multipotent stem cells, germline stem cells, and in the female germline. Hywi and Hyli localize to the nuage, a perinuclear organelle that has been implicated in piRNA-mediated transposon silencing, together with other conserved nuage and piRNA pathway components. Our findings provide the first report of nuage protein localization patterns in a non-bilaterian. Hydra PIWI proteins possess symmetrical dimethylarginines: modified residues that are known to aid in PIWI protein localization to the nuage and proper piRNA loading. piRNA profiling suggests that transposons are the major targets of the piRNA pathway in Hydra. Our data suggest that piRNA biogenesis through the ping-pong amplification cycle occurs in Hydra and that Hywi and Hyli are likely to preferentially bind primary and secondary piRNAs, respectively. Presumptive piRNA clusters are unidirectionally transcribed and primarily give rise to piRNAs that are antisense to transposons. These results indicate that various conserved features of PIWI proteins, the piRNA pathway, and their associations with the nuage were likely established before the evolution of bilaterians.

piRNA pathway and the potential processing site, the nuage, in the Drosophila germline

The accurate transfer of genetic material in germline cells during the formation of gametes is important for the continuity of the species. However, animal germline cells face challenges from transposons, which seek to spread themselves in the genome. This review focuses on studies in Drosophila melanogaster on how the genome protects itself from such a mutational burden via a class of gonad-specific small interfering RNAs, known as piRNAs (Piwi-interacting RNAs). In addition to silencing transposons, piRNAs also regulate other processes, such as chromosome segregation, mRNA degradation and germline differentiation. Recent studies revealed two modes of piRNA processing – primary processing and secondary processing (also known as ping-pong amplification). The primary processing pathway functions in both germline and somatic cells in the Drosophila ovaries by processing precursor piRNAs into 23–29 nt piRNAs. In contrast, the secondary processing pathway functions only in the germline cells where piRNAs are amplified in a feed-forward loop and require the Piwi-family proteins Aubergine and Argonaute3. Aubergine and Argonaute3 localize to a unique structure found in animal germline cells, the nuage, which has been proposed to function as a compartmentalized site for the ping-pong cycle. The nuage and the localized proteins are well-conserved, implying the importance of the piRNA amplification loop in animal germline cells. Nuage components include various types of proteins that are known to interact both physically and genetically, and therefore appear to be assembled in a sequential order to exert their function, resulting in a macromolecular RNA-protein complex dedicated to the silencing of transposons.

Spatio-temporal requirements for transposable element piRNA-mediated silencing during Drosophila oogenesis

During Drosophila oogenesis, transposable element (TE) repression involves the Piwi-interacting RNA (piRNA) pathway which ensures genome integrity for the next generation. We developed a transgenic model to study repression of the Idefix retrotransposon in the germline. Using a candidate gene KD-approach, we identified differences in the spatio-temporal requirements of the piRNA pathway components for piRNA-mediated silencing. Some of them (Aub, Vasa, Spn-E) are necessary in very early stages of oogenesis within the germarium and appear to be less important for efficient TE silencing thereafter. Others (Piwi, Ago3, Mael) are required at all stages of oogenesis. Moreover, during early oogenesis, in the dividing cysts within the germarium, Idefix anti-sense transgenes escape host control, and this is associated with very low piwi expression. Silencing of P-element-based transgenes is also strongly weakened in these cysts. This region, termed the 'Piwiless pocket' or Pilp, may ensure that new TE insertions occur and are transmitted to the next generation, thereby contributing to genome dynamics. In contrast, piRNA-mediated silencing is strong in germline stem cells in which TE mobilization is tightly repressed ensuring the continued production of viable germline cysts.

PIWI proteins are essential for early Drosophila embryogenesis

PIWI proteins, a subfamily of the ARGONAUTE/PIWI protein family, have been implicated in transcriptional and posttranscriptional gene regulation and transposon silencing mediated by small non-coding RNAs, especially piRNAs. Although these proteins are known to be required for germline development, their somatic function remains elusive. Here, we examine the maternal function of all three PIWI proteins in Drosophila; Piwi, Aubergine (Aub) and Argonaute3 (Ago3) during early embryogenesis. In syncytial embryos, Piwi displays an embryonic stage-dependent localization pattern. Piwi is localized in the cytoplasm during mitotic cycles 1-10. Between cycles 11 and 14, Piwi remains in the cytoplasm during mitosis but moves into the somatic nucleus during interphase. Beyond cycle 14, it stays in the nucleus. Aub and Ago3 are diffusely cytoplasmic from cycle 1 to 14. Embryos maternally depleted of any one of the three PIWI proteins display severe mitotic defects, including abnormal chromosome and nuclear morphology, cell cycle arrest, asynchronous nuclear division and aberrant nuclear migration. Furthermore, all three PIWI proteins are required for the assembly of mitotic machinery and progression through mitosis. Embryos depleted of maternal PIWI proteins also exhibit chromatin organization abnormalities. These observations indicate that maternal Piwi, Aub and Ago3 play a critical role in the maintenance of chromatin structure and cell cycle progression during early embryogenesis, with compromised chromatin integrity as a possible cause of the observed mitotic defects. Our study demonstrates the essential function of PIWI proteins in the first phase of somatic development.

The DEAD-box helicase Vasa: evidence for a multiplicity of functions in RNA processes and developmental biology

DEAD-box helicases related to the Drosophila protein Vasa (also known as Ddx4) are found throughout the animal kingdom. They have been linked to numerous processes in gametogenesis, germ cell specification, and stem cell biology, and alterations in Vasa expression are associated with malignancy of tumor cells and with some human male infertility syndromes. Experimental results indicating how Vasa contributes to all these different cellular and developmental processes are discussed, using examples from planarians, Caenorhabditis elegans, Drosophila, sea urchin, zebrafish, Xenopus, mouse, and human. Molecular, cellular, and developmental functions of Vasa and its orthologs are reviewed in this article. Evidence linking Vasa to translational regulation, to biogenesis of small RNAs, and to chromosome condensation is examined. Finally, potential overlapping functions between Vasa and related DEAD-box helicases (Belle, or Ddx3, and DEADSouth, or Ddx25) are explored. This article is part of a Special Issue entitled: The biology of RNA helicases - Modulation for life.

RNA interference in the nucleus: roles for small RNAs in transcription, epigenetics and beyond

A growing number of functions are emerging for RNA interference (RNAi) in the nucleus, in addition to well-characterized roles in post-transcriptional gene silencing in the cytoplasm. Epigenetic modifications directed by small RNAs have been shown to cause transcriptional repression in plants, fungi and animals. Additionally, increasing evidence indicates that RNAi regulates transcription through interaction with transcriptional machinery. Nuclear small RNAs include small interfering RNAs (siRNAs) and PIWI-interacting RNAs (piRNAs) and are implicated in nuclear processes such as transposon regulation, heterochromatin formation, developmental gene regulation and genome stability.

Lessons for inductive germline determination

Formation of the germline in an embryo marks a fresh round of reproductive potential, yet the developmental stage and location within the embryo where the primordial germ cells (PGCs) form differs wildly among species. In most animals, the germline is formed either by an inherited mechanism, in which maternal provisions within the oocyte drive localized germ-cell fate once acquired in the embryo, or an inductive mechanism that involves signaling between cells that directs germ-cell fate. The inherited mechanism has been widely studied in model organisms such as Drosophila melanogaster, Caenorhabditis elegans, Xenopus laevis, and Danio rerio. Given the rapid generation time and the effective adaptation for laboratory research of these organisms, it is not coincidental that research on these organisms has led the field in elucidating mechanisms for germline specification. The inductive mechanism, however, is less well understood and is studied primarily in the mouse (Mus musculus). In this review, we compare and contrast these two fundamental mechanisms for germline determination, beginning with the key molecular determinants that play a role in the formation of germ cells across all animal taxa. We next explore the current understanding of the inductive mechanism of germ-cell determination in mice, and evaluate the hypotheses for selective pressures on these contrasting mechanisms. We then discuss the hypothesis that the transition between these determination mechanisms, which has happened many times in phylogeny, is more of a continuum than a binary change. Finally, we propose an analogy between germline determination and sex determination in vertebrates-two of the milestones of reproduction and development-in which animals use contrasting strategies to activate similar pathways.

Polo-mediated phosphorylation of Maelstrom regulates oocyte determination during oogenesis in Drosophila

In Drosophila, Maelstrom is a conserved component of the perinuclear nuage, a germline-unique structure that appears to serve as a site for Piwi-interacting RNA (piRNA) production to repress deleterious transposons. Maelstrom also functions in the nucleus as a transcriptional regulator to repress the expression of microRNA-7, a process that is essential for the proper differentiation of germline stem cells. In this paper, we report another function of Maelstrom in regulating oocyte determination independently of its transposon silencing and germline stem cell differentiation activities. In Drosophila, the conserved serine 138 residue in Maelstrom is required for its phosphorylation, an event that promotes oocyte determination. Phosphorylation of Maelstrom is required for the repression of the pachytene checkpoint protein Sir2, but not for transposon silencing or for germline stem cell differentiation. We identify Polo as a kinase that mediates the phosphorylation of Maelstrom. Our results suggest that the Polo-mediated phosphorylation of Maelstrom may be a mechanism that controls oocyte determination by inactivating the pachytene checkpoint via the repression of Sir2 in Drosophila ovaries.

DEADSouth protein localizes to germ plasm and is required for the development of primordial germ cells in Xenopus laevis

DEADSouth mRNA is a component of germ plasm in Xenopus laevis and encodes a DDX25 DEAD-box RNA helicase. To determine the intracellular localization of DEADSouth protein, we injected mRNA encoding DEADSouth tagged with mCherry fluorescent protein into fertilized eggs from transgenic Xenopus expressing EGFP fused with a mitochondrial targeting signal. The DEADSouth-mCherry fusion protein was localized to the germ plasm, a mitochondria-rich region in primordial germ cells (PGCs). DEADSouth overexpression resulted in a reduction of PGC numbers after stage 20. Conversely, DEADSouth knockdown using an antisense locked nucleic acid gapmer inhibited movement of the germ plasm from the cortex to the perinuclear region, resulting in inhibition of PGC division at stage 12 and a decrease in PGC numbers at later stages. The knockdown phenotype was rescued by intact DEADSouth mRNA, but not mutant mRNA encoding inactive DEADSouth helicase. Surprisingly, it was also rescued by mouse vasa homolog and Xenopus vasa-like gene 1 mRNAs that encode DDX4 RNA helicases. The rescue was dependent on the 3' untranslated region (3'UTR) of DEADSouth mRNA, which was used for PGC-specific expression. The 3'UTR contributed to localization of the injected mRNA to the germ plasm, resulting in effective localization of DEADSouth protein. These results demonstrate that localization of DEADSouth helicase to the germ plasm is required for proper PGC development in Xenopus laevis.

PIWI Expression and Function in Cancer

PIWI proteins, a subclade of the Argonaute family proteins, are expressed predominantly in the germline and bind to PIWI-interacting RNAs (piRNAs), which are 25–31 nucleotides in length. The PIWI/piRNA pathway plays critical roles in germline development by regulating transposons and other targets to maintain genome integrity. While the functions of PIWI in the germline have been extensively investigated, recent studies have accumulated evidence that the human PIWI proteins, HIWI and HILI, are aberrantly expressed in a variety of cancers. This review summarizes our knowledge of PIWI expression in cancer and discusses its possible role in tumorigenesis.

Germline stem cells: origin and destiny

Germline stem cells are key to genome transmission to future generations. Over recent years, there have been numerous insights into the regulatory mechanisms that govern both germ cell specification and the maintenance of the germline in adults. Complex regulatory interactions with both the niche and the environment modulate germline stem cell function. This perspective highlights some examples of this regulation to illustrate the diversity and complexity of the mechanisms involved.

mRNA localization and translational control in Drosophila oogenesis

Localization of an mRNA species to a particular subcellular region can complement translational control mechanisms to produce a restricted spatial distribution of the protein it encodes. mRNA localization has been studied most in asymmetric cells such as budding yeast, early embryos, and neurons, but the process is likely to be more widespread. This article reviews the current state of knowledge about the mechanisms of mRNA localization and its functions in early embryonic development, focusing on Drosophila where the relevant knowledge is most advanced. Links between mRNA localization and translational control mechanisms also are examined.

vasa is expressed in somatic cells of the embryonic gonad in a sex-specific manner in Drosophila melanogaster

Vasa is a DEAD box helicase expressed in the Drosophila germline at all stages of development. vasa homologs are found widely in animals and vasa has become the gene of choice in identifying germ cells. I now show that Drosophila vasa expression is not restricted to the germline but is also expressed in a somatic lineage, the embryonic somatic gonadal precursor cells. This expression is sexually dimorphic, being maintained specifically in males, and is regulated post-transcriptionally. Although somatic Vasa expression is not required for gonad coalescence, these data support the notion that Vasa is not solely a germline factor.