The Xenopus localized messenger RNA An3 may encode an ATP-dependent RNA helicase

Cell repertoire and proliferation of germinative cells of the model cestode Mesocestoides corti

The phylum Platyhelminthes shares a unique population of undifferentiated cells responsible for the proliferation capacity needed for cell renewal, growth, tissue repair and regeneration. These cells have been extensively studied in free-living flatworms, whereas in cestodes the presence of a set of undifferentiated cells, known as germinative cells, has been demonstrated in classical morphology studies, but poorly characterized with molecular biology approaches. Furthermore, several genes have been identified as neoblast markers in free-living flatworms that deserve study in cestode models. Here, different cell types of the model cestode Mesocestoides corti were characterized, identifying differentiated and germinative cells. Muscle cells, tegumental cells, calcareous corpuscle precursor cells and excretory system cells were identified, all of which are non-proliferative, differentiated cell types. Besides those, germinative cells were identified as a population of small cells with proliferative capacity in vivo. Primary cell culture experiments in Dulbecco's Modified Eagle Medium (DMEM), Echinococcus hydatid fluid and hepatocyte conditioned media in non-reductive or reductive conditions confirmed that the germinative cells were the only ones with proliferative capacity. Since several genes have been identified as markers of undifferentiated neoblast cells in free-living flatworms, the expression of pumilio and pL10 genes was analysed by qPCR and in situ hybridization, showing that the expression of these genes was stronger in germinative cells but not restricted to this cell type. This study provides the first tools to analyse and further characterise undifferentiated cells in a model cestode.

Cloning and expression analysis of the DEAD-box/RNA helicase Oslaf-1 in Ovomermis sinensis

Ovomermis sinensis is a potentially-valuable nematode for controlling insect pests. The parasitic stage of the nematode absorbs nutrients in its host’s hemolymph to maintain its growth development and then kills the host when it emerges. At present, little known about its reproductive development, particularly the responsible molecular mechanism. More detailed research on the genes of reproductive development will not only help us understand the mechanisms underlying sex differentiation in the nematode, but would also be valuable for successfully cultivating them in vitro and using them for biocontrol. In this study, we used the homology cloning method to clone the full-length cDNA of a DEAD-box family gene (Oslaf-1) from O. sinensis. Then, using qRT-PCR technology to detect the expression pattern of the Oslaf-1 gene at different development stages and tissues, the gene was found to be highly expressed in the post-parasitic stage (P < 0.01) and ovarian (P < 0.05) of O. sinensis. Western blot analysis showed the same result that the gene is associated with gonadal development and function, but is not gonad-specific. In situ hybridization further demonstrated that the gene is widely expressed in early embryos and is mainly distributed in the gonadal area. However, the signal was mainly concentrated in the reproductive primordia in pre-parasitic juveniles. RNA interference (RNAi) studies revealed that the sex ratio of O. sinensis soaked in dsRNA of Oslaf-1 was not statistically different than the gfp dsRNA treated groups. Our results suggest that Oslaf-1 may play a vital role in the reproductive systems of the nematode. In addition, we speculate that the Oslaf-1 gene plays an important role during embryonic development and that it occurs and develops in the gonads of pre-parasitic juveniles of O. sinensis.

PL10 DEAD-Box Protein is Expressed during Germ Cell Differentiation in the Reptile Podarcis sicula (Family Lacertidae)

Among genes involved in the regulation of germ cell differentiation, those of DDX4/Vasa and the Ded1/DDX3 subfamilies encode for DEAD-box ATP-dependent RNA helicases, proteins involved in many mechanisms related to RNA processing. For the first time in reptiles, using specific antibodies at confocal microscopy, we analysed the localization pattern of a Ded1/DDX3 subfamily member in testis and ovary of Podarcis sicula (Ps-PL10) during the reproductive cycle. In testis, Ps-PL10 is expressed in the cytoplasm of spermatocytes and it is not detected in spermatogonia. Differently from Ps-VASA, in round spermatids, Ps-PL10 is not segregated in the chromatoid body but it accumulates in the cytoplasm of residual bodies, and mature spermatozoa are unstained. These observations suggest that in males, Ps-PL10 (1) is involved in spermatogenesis and (2) is then eliminated with residual bodies. In the ovary, Ps-PL10 is present with granules in the cytoplasm of early meiotic cells of the germinal bed (GB), while it is not present in oogonia and somatic cells of the GB stroma. In follicular cells of ovarian follicles, Ps-PL10 expression starts after their fusion with the oocyte. Numerous Ps-PL10 spots are visible in pyriform (nurse-like) cells concomitantly with the protein accumulation in the cytoplasm of differentiating oocyte. In pyriform cells, Ps-PL10 spots are present in the cytoplasm and nuclei, as observed for Ps-VASA, and in the nucleoli, suggesting for Ps-PL10 a role in rRNA processing and in the transport of molecules from the nucleus to cytoplasm and from nurse cells to the oocyte.

Global analysis of asymmetric RNA enrichment in oocytes reveals low conservation between closely related Xenopus species

Subcellular localization of mRNAs contributes to the generation of cellular asymmetries and cell fate determination. A comparative global analysis is given of animally and vegetally enriched RNAs in oocytes from two closely related Xenopus species.

RNAs that localize to the vegetal cortex during Xenopus laevis oogenesis have been reported to function in germ layer patterning, axis determination, and development of the primordial germ cells. Here we report on the genome-wide, comparative analysis of differentially localizing RNAs in Xenopus laevis and Xenopus tropicalis oocytes, revealing a surprisingly weak degree of conservation in respect to the identity of animally as well as vegetally enriched transcripts in these closely related species. Heterologous RNA injections and protein binding studies indicate that the different RNA localization patterns in these two species are due to gain/loss of cis-acting localization signals rather than to differences in the RNA-localizing machinery.

The Ded1/DDX3 subfamily of DEAD-box RNA helicases

In eukaryotic organisms, the orthologs of the DEAD-box RNA helicase Ded1p from yeast and DDX3 from human form a well-defined subfamily that is characterized by high sequence conservation in their helicase core and their N- and C- termini. Individual members of this Ded1/DDX3 subfamily perform multiple functions in RNA metabolism in both nucleus and cytoplasm. Ded1/DDX3 subfamily members have also been implicated in cellular signaling pathways and are targeted by diverse viruses. In this review, we discuss the considerable body of work on the biochemistry and biology of these proteins, including the recently discovered link of human DDX3 to tumorigenesis.

Putting RNAs in the right place at the right time: RNA localization in the frog oocyte

Localization of maternal mRNAs in many developing organisms provides the basis for both initial polarity during oogenesis and patterning during embryogenesis. Prominent examples of this phenomenon are found in Xenopus laevis, where localized maternal mRNAs generate developmental polarity along the animal/vegetal axis. Targeting of mRNA molecules to specific subcellular regions is a fundamental mechanism for spatial regulation of gene expression, and considerable progress has been made in defining the underlying molecular pathways.

Expression of DDX3 is directly modulated by hypoxia inducible factor-1 alpha in breast epithelial cells

DEAD box protein, DDX3, is aberrantly expressed in breast cancer cells ranging from weakly invasive to aggressive phenotypes and functions as an important regulator of cancer cell growth and survival. Here, we demonstrate that hypoxia inducible factor-1α is a transcriptional activator of DDX3 in breast cancer cells. Within the promoter region of the human DDX3 gene, we identified three putative hypoxia inducible factor-1 responsive elements. By luciferase reporter assays in combination with mutated hypoxia inducible factor-1 responsive elements, we determined that the hypoxia inducible factor-1 responsive element at position -153 relative to the translation start site is essential for transcriptional activation of DDX3 under hypoxic conditions. We also demonstrated that hypoxia inducible factor-1 binds to the DDX3 promoter and that the binding is specific, as revealed by siRNA against hypoxia inducible factor-1 and chromatin immunoprecipitation assays. Thus, the activation of DDX3 expression during hypoxia is due to the direct binding of hypoxia inducible factor-1 to hypoxia responsive elements in the DDX3 promoter. In addition, we observed a significant overlap in the protein expression pattern of hypoxia inducible factor-1α and DDX3 in MDA-MB-231 xenograft tumors. Taken together, our results demonstrate, for the first time, the role of DDX3 as a hypoxia-inducible gene that exhibits enhanced expression through the interaction of hypoxia inducible factor-1 with hypoxia inducible factor-1 responsive elements in its promoter region.

Requirement of cellular DDX3 for hepatitis C virus replication is unrelated to its interaction with the viral core protein

The cellular DEAD-box protein DDX3 was recently shown to be essential for hepatitis C virus (HCV) replication. Prior to that, we had reported that HCV core binds to DDX3 in yeast-two hybrid and transient transfection assays. Here, we confirm by co-immunoprecipitation that this interaction occurs in cells replicating the JFH1 virus. Consistent with this result, immunofluorescence staining of infected cells revealed a dramatic redistribution of cytoplasmic DDX3 by core protein to the virus assembly sites around lipid droplets. Given this close association of DDX3 with core and lipid droplets, and its involvement in virus replication, we investigated the importance of this host factor in the virus life cycle. Mutagenesis studies located a single amino acid in the N-terminal domain of JFH1 core that when changed to alanine significantly abrogated this interaction. Surprisingly, this mutation did not alter infectious virus production and RNA replication, indicating that the core–DDX3 interaction is dispensable in the HCV life cycle. Consistent with previous studies, siRNA-led knockdown of DDX3 lowered virus production and RNA replication levels of both WT JFH1 and the mutant virus unable to bind DDX3. Thus, our study shows for the first time that the requirement of DDX3 for HCV replication is unrelated to its interaction with the viral core protein.

Entamoeba histolytica EhDEAD1 is a conserved DEAD-box RNA helicase with ATPase and ATP-dependent RNA unwinding activities

RNA helicases are widely conserved key enzymes that perform multiple functions in RNA metabolism. Here, we present the cloning, expression and functional characterization of the EhDEAD1 RNA helicase in the protozoan parasite Entamoeba histolytica. According to its primary structure, EhDEAD1 is evolutionary related to yeast DED1 and human DDX3X RNA helicases, both involved in translation and cell cycle regulation. The EhDEAD1 predicted amino acid sequence exhibits the nine conserved motifs described for the DEAD-box SFII superfamily members reported in other organisms and it is evolutionary close to protozoan homologues. Purified recombinant EhDEAD1 protein presented ATPase activity and it was able to bind and unwind RNA in an ATPase-dependent manner in vitro. RT-PCR assays showed that EhDead1 gene is overtranscribed in the cell cycle S phase. Moreover, inhibition of EhDead1 gene expression by antisense RNA seemed to facilitate transition from S to G2/M phase. Intriguingly, our results showed that EhDEAD1 was unable to rescue two yeast Ded1 RNA helicase mutants affected in translation, in spite of the high sequence homology with yeast DED1.

Chordin affects pronephros development in Xenopus embryos by anteriorizing presomitic mesoderm

Spemann's organizer emits signals that pattern the mesodermal germ layer during Xenopus embryogenesis. In a previous study, we demonstrated that FGFR1 activity within the organizer is required for the production of both the somitic muscle- and pronephros-patterning signals by the organizer and the expression of chordin, an organizer-specific secreted protein (Mitchell and Sheets [2001] Dev. Biol. 237:295-305). Studies from others in both chicken and Xenopus embryos provide compelling evidence that pronephros forms by means of secondary induction signals emitted from anterior somites (Seufert et al. [1999] Dev. Biol. 215:233-242; Mauch et al. [2000] Dev. Biol. 220:62-75). Here we provide several lines of evidence in support of the hypothesis that chordin influences pronephros development by directing the formation of anterior somites. Chordin mRNA was absent in ultraviolet (UV) -irradiated embryos lacking pronepheros (average DAI<2) but was always found in UV-irradiated embryos that retain pronepheros (average DAI>2). Furthermore, ectopic expression of chordin in embryos and in tissue explants leads to the formation of anterior somites and pronephros. In these experiments, pronephros was only observed in association with muscle. Chordin diverted somatic muscle cells to more anterior positions within the somite file in chordin-induced secondary trunks and induced the expression of the anterior myogenic gene myf5. Finally, depletion of chordin mRNA with DEED antisense oligonucleotides substantially reduced somitic muscle and pronephric tubule and duct formation in whole embryos. These data and previous studies on ectoderm and endoderm (Sasai et al. [1995] Nature 377:757) support the idea that chordin functions as an anteriorizing signal in patterning the germ layers during vertebrate embryogenesis. Our data support the hypothesis that chordin directs the formation of anterior somites that in turn are necessary for pronephros development.

A PL10 vasa-like gene in the kuruma shrimp, Marsupenaeus japonicus, expressed during development and in adult gonad

A PL10 vasa-like gene was isolated from the Kuruma shrimp Marsupenaeus japonicus and therefore called Mjpl10. It is differentially expressed during embryonic, larval, and postlarval development, and in female and male gonads. Using absolute real-time reverse transcriptase-polymerase chain reaction (RT-PCR), we demonstrate that Mjpl10 transcripts are present in the two-cell embryo, suggesting it is maternally expressed, and continually at low levels throughout embryogenesis. Mjpl10 expression increases significantly in the first 25 h after hatching (nauplii IV) and then decreases in a linear fashion by 316-fold over the next 52-day period. Its continued expression throughout embryonic and larval development is compatible with a conserved role in early germ cell specification. Transcript levels of Mjpl10 are also detected in the ovary and testes of mature adults.

Differential expression of two vasa/PL10-related genes during gametogenesis in the special model system Rana

Germline cell fate decisions are primarily controlled at the post-transcriptional level with DEAD-box RNA helicases playing a crucial role in germline development. In this study, we report the identification of two DEAD-box vasa/PL10 orthologues (RlVlg and RlPL10) in a species complex of the genus Rana, characterized by hybridogenetic reproduction, an enigmatic process that involves the exclusion of an individual genome, and endoreduplication events. Both genes were expressed during the early stages of gametogenesis of R. ridibunda, R. lessonae, and their natural hybrid R. esculenta. RlVlg expression was germline specific. On the other hand, RlPL10 was also expressed in somatic tissues, although only at low levels. The two genes were expressed in different phases of mitotic and meiotic spermatogenetic divisions as demonstrated by immunostaining with an anti-H3 phosphohistone antibody. The data indicate that RlVlg and RlPL10 may represent useful markers for dissecting the molecular aspects of genome exclusion and endoreduplication of the hybridogenetic gametogenesis.

Vasa expression in a colonial ascidian, Botrylloides violaceus

Evolution of solitary or colonial life histories in tunicates is accompanied by dramatic developmental changes that affect morphology and reproduction. We compared vasa expression in a solitary ascidian and a closely related colonial ascidian, in an effort to uncover developmental mechanisms important during the evolution of these contrasting life histories, including the ability to reproduce by budding. In this study, we explored the origin of germ cells in new buds developing by asexual reproduction in a colonial ascidian, Botrylloides violaceus and compared it to the source of germ cells in a solitary ascidian Boltenia villosa. We studied expression by in situ hybridization of vasa, a DEAD box RNA helicase gene found in germ cells across the metazoans. In B. villosa, bv-vasa mRNA was expressed in putative germ cells and oocytes of adult gonads, and was sequestered into a posterior lineage during embryogenesis. In mature colonies of the ascidian B. violaceus, bot-vasa mRNA was expressed in putative spermatogonia, in oocytes of zooids, and in some circulating cells in the zooids and differentiating buds. We propose that expression of vasa in cells other than gonadal germ cells of zooids in a colonial ascidian may serve as a source of germ-line stem cells in the colony.

A Novel Ded1-like RNA Helicase Interacts with the Y-box Protein ctYB-1 in Nuclear mRNP Particles and in Polysomes

We have characterized a novel mRNA-binding protein, designated hrp84, in the dipteran Chironomus tentans and identified it as a DEAD-box RNA helicase. The protein contains the typical helicase core domain, a glycine-rich C-terminal part and a putative nuclear export signal in the N terminus. The protein belongs to the Ded1 subgroup of DEAD-box helicases, which is highly conserved from yeast (Ded1p) to mammals (DDX3). In tissue culture cells, hrp84 is present both in the nucleus and cytoplasm and, as shown by in vivo UV cross-linking, is bound to mRNA in both compartments. Immunoprecipitation experiments revealed that hpr84 is associated with the C. tentans homologue (ctYB-1) of the vertebrate Y-box protein YB-1 both in the nucleus and cytoplasm, and the two proteins also appear together in polysomes. The interaction is likely to be direct as shown by in vitro binding of purified components. We conclude that the mRNA-bound hrp84.ctYB-1 complex is formed in the nucleus and is translocated with mRNA into the cytoplasm and further into polysomes. As both Ded1 and YB-1 are known to regulate the initiation of translation, we propose that the RNA helicase-Y-box protein complex affects the efficiency of mRNA translation, presumably by modulating the conformation of the mRNP template.

vasa mRNA accumulates at the posterior pole during blastoderm formation in the flour beetle Tribolium castaneum

The correct specification of germ cells during embryogenesis is a fundamental step in life that ensures the existence of the next generation. Although different species display various cellular modes of generating germ cells, the product of the vasa gene proves to be a reliable marker of primordial germ cells in metazoans [Extavour and Akam (2003) 130:5869-5884; Raz (2000) 1:1017]. Here, I report the isolation of the vasa ortholog from the red flour beetle Tribolium castaneum, named Tc-vasa, and describe its sequence, its genomic organisation and its expression pattern during early embryogenesis. Unlike in Drosophila where vasa messenger RNA (mRNA) is ubiquitously distributed in the egg, Tc-vasa mRNA gradually accumulates at the posterior egg pole during blastoderm formation. Shortly before gastrulation, Tc-vasa mRNA marks a group of intra-blastodermal cells at the posterior pole. In the germ rudiment, a ball-like group of vasa-positive cells adheres to the growth zone at the posterior end of the embryo. These vasa-positive cells likely represent the primordial germ cells that have not been described in Tribolium prior to gonad formation. At the beginning of germ growth, a small band of vasa-positive cells starts to migrate along the dorsal side of the growth zone. vasa transcription ceases during further germ band extension. In contrast to Drosophila, Tc-vasa transcripts cannot be detected in the germ cells within the gonadal anlage after segmentation is completed.

vasa and nanos expression patterns in a sea anemone and the evolution of bilaterian germ cell specification mechanisms

Most bilaterians specify primordial germ cells (PGCs) during early embryogenesis using either inherited cytoplasmic germ line determinants (preformation) or induction of germ cell fate through signaling pathways (epigenesis). However, data from nonbilaterian animals suggest that ancestral metazoans may have specified germ cells very differently from most extant bilaterians. Cnidarians and sponges have been reported to generate germ cells continuously throughout reproductive life, but previous studies on members of these basal phyla have not examined embryonic germ cell origin. To try to define the embryonic origin of PGCs in the sea anemone Nematostella vectensis, we examined the expression of members of the vasa and nanos gene families, which are critical genes in bilaterian germ cell specification and development. We found that vasa and nanos family genes are expressed not only in presumptive PGCs late in embryonic development, but also in multiple somatic cell types during early embryogenesis. These results suggest one way in which preformation in germ cell development might have evolved from the ancestral epigenetic mechanism that was probably used by a metazoan ancestor.

Belle is a Drosophila DEAD-box protein required for viability and in the germ line

DEAD-box proteins are ATP-dependent RNA helicases that function in various stages of RNA processing and in RNP remodeling. Here, we report identification and characterization of the Drosophila protein Belle (Bel), which belongs to a highly conserved subfamily of DEAD-box proteins including yeast Ded1p, Xenopus An3, mouse PL10, human DDX3/DBX, and human DBY. Mutations in DBY are a frequent cause of male infertility in humans. Bel can substitute in vivo for Ded1p, an essential yeast translation factor, suggesting a requirement for Bel in translation initiation. Consistent with an essential cellular function, strong loss of function mutations in bel are recessive lethal with a larval growth defect phenotype. Hypomorphic bel mutants are male-sterile. Bel is also closely related to the Drosophila DEAD-box protein Vasa (Vas), a germ line-specific translational regulator. We find that Bel and Vas colocalize in nuage and at the oocyte posterior during oogenesis, and that bel function is required for female fertility. However, unlike Vas, Bel is not specifically enriched in embryonic pole cells. We conclude that the DEAD-box protein Bel has evolutionarily conserved roles in fertility and development.

Germ line development in the grasshopper Schistocerca gregaria: vasa as a marker

Vasa is a widely conserved germline marker, both in vertebrates and invertebrates. We identify a vasa orthologue, Sgvasa, and use it to study germline development in the grasshopper Schistocerca gregaria, a species in which no germ plasm has been identified. In adults, Sgvasa is specifically expressed in the ovary and testis. It is expressed at high levels during early oogenesis, but no detectable vasa RNA and little Vasa protein are present in mature unlaid eggs. None appears to be localized to any defined region of the egg cortex, suggesting that germline specification may not depend on maternal germ plasm expressing vasa. Vasa protein is expressed in most cleavage energids as they reach the egg surface and persists at high levels in most cells aggregating to form the embryonic primordium. However, after gastrulation, Vasa protein persists only in extraembryonic membranes and in cells at the outer margin of the late heart-stage embryo. In the embryo, it then become restricted to cells at the dorsal margin of the forming abdomen. In older embryos, these Vasa-positive cells move toward the midline; Vasa protein accumulates asymmetrically in their cytoplasm, a pattern closely resembling that of germ cells in late embryonic gonads. Thus, we suggest that the Vasa-stained cells in the abdominal margin are germ cells, as proposed by Nelson (1934), and not cardioblasts, as has been proposed by others.

Assembly and transport of a premessenger RNP particle

Salivary gland cells in the larvae of the dipteran Chironomus tentans offer unique possibilities to visualize the assembly and nucleocytoplasmic transport of a specific transcription product. Each nucleus harbors four giant polytene chromosomes, whose transcription sites are expanded, or puffed. On chromosome IV, there are two puffs of exceptional size, Balbiani ring (BR) 1 and BR 2. A BR gene is 35-40 kb, contains four short introns, and encodes a 1-MDa salivary polypeptide. The BR transcript is packed with proteins into a ribonucleoprotein (RNP) fibril that is folded into a compact ring-like structure. The completed RNP particle is released into the nucleoplasm and transported to the nuclear pore, where the RNP fibril is gradually unfolded and passes through the pore. On the cytoplasmic side, the exiting extended RNP fibril becomes engaged in protein synthesis and the ensuing polysome is anchored to the endoplasmic reticulum. Several of the BR particle proteins have been characterized, and their fate during the assembly and transport of the BR particle has been elucidated. The proteins studied are all added cotranscriptionally to the pre-mRNA molecule. The various proteins behave differently during RNA transport, and the flow pattern of each protein is related to the particular function of the protein. Because the cotranscriptional assembly of the pre-mRNP particle involves proteins functioning in the nucleus as well as proteins functioning in the cytoplasm, it is concluded that the fate of the mRNA molecule is determined to a considerable extent already at the gene level.

Cadherins and catenins, Wnts and SOXs: embryonic patterning in Xenopus

Wnt signaling plays a critical role in a wide range of developmental and oncogenic processes. Altered gene regulation by the canonical Wnt signaling pathway involves the cytoplasmic stabilization of beta-catenin, a protein critical to the assembly of cadherin-based cell-cell adherence junctions. In addition to binding to cadherins, beta-catenin also interacts with transcription factors of the TCF-subfamily of HMG box proteins and regulates their activity. The Xenopus embryo has proven to be a particularly powerful experimental system in which to study the role of Wnt signaling components in development and differentiation. We review this literature, focusing on the role of Wnt signaling and interacting components in establishing patterns within the early embryo.

RNA localization and germ cell determination in Xenopus

In many organisms the proper development of the embryo depends on the asymmetrical distribution of maternal RNAs and proteins in the egg. Although the Xenopus oocyte is radially symmetrical it contains distinct populations of maternal RNAs that are localized either in the animal or vegetal pole. The process of localization of RNAs in Xenopus oocytes occurs during the long period of oocyte differentiation and growth that is accompanied by the elaboration of oocyte polarity. Some of the vegetally localized RNAs, such as Vg1, VegT, and Xwnt11, are involved in axial patterning and germ layer specification. Others, such as Xdazl and Xcat2, which are located in the germ plasm, are likely to play a role in the specification of germ cell fate. We will discuss the different aspects of RNA localization in Xenopus in the context of the differentiation of the germ cells and the development of the oocyte polarity.

Regulation of steroidogenic enzymes and a novel testicular RNA helicase

Luteinizing hormone (LH) supports steroidogenesis and maintains testicular and ovarian function. Mediators of LH action exert homologous regulation of membrane receptors, steroidogenic enzymes and other regulatable genes of the Leydig cell (LC). Androgen and estrogen induced by LH could act through its cognate receptors in the LC to regulate gene expression. Although androgens are unquestionable essential for spermatogenesis and presumably exert their heterologous action through androgen receptors present in the Sertoli its regulatory mechanism in germinal cell maturation is far from clear. In contrast to physiological concentrations of gonadotropins which maintain the steroidogenic functions and LH and prolactin receptors in the gonads, high concentrations of gonadotropin (hCG) cause receptor down-regulation and desensitization of steroidogenic enzymes of the LCs in vivo (3beta-hydroxysteroid dehydrogenase types I and II, 17alpha-hydroxylase/17,20 lyase, and 17beta-hydroxysteroid dehydrogenase type III [17beta-HSD]). In addition, 17beta-HSD is regulated by compartmentalized endogenous glucose/ATP. The attenuation of steroidogenesis which results from receptor mediated activation by cognate hormone, but is independent of the subsequent phase of receptor down-regulation, is due to changes at the transcriptional level. Among the candidates affecting this regulation are active steroid metabolites (direct or indirect of steroids and other mediator(s) i.e. cAMP, putative transcription factors induced by LH action). Differential display assay revealed another gene which is transcriptionally regulated by gonadotropin termed GRTH (Gonadotropin Regulated Testicular Helicase). GRTH is a novel member of the DEAD-box family of RNA helicases, and is specifically expressed in LCs and meiotic LC of the testis. It is markedly up-regulated by hCG via cAMP-induced androgen formation in LCs at doses that cause down-regulation of receptors and steroidogenic enzymes. GRTH functions as a translational activator. Androgen produced by gonadotropin stimulation exerts intracrine/autocrine actions on GRTH, and also could influence transcription within the seminiferous tubule. GRTH may contribute to the control of steroidogenesis, including the restoration of down regulated cellular functions, and in the paracrine regulation of androgen dependent gene(s) involved in the meiotic process, and could thus have a crucial role in spermatogenesis.

RNA-binding proteins in mouse oocytes and embryos: expression of genes encoding Y box, DEAD box RNA helicase, and polyA binding proteins

Growth and differentiation of early embryos depends almost entirely on information which is maternally inherited in the form of macromolecules accumulated by the female gamete during its growth phase. Most of the maternal mRNAs synthesized by growing oocytes are not immediately recruited onto polysomes but are stored as translationally dormant messenger ribonucleoprotein (mRNP) particles. mRNA binding proteins which have been associated with masked mRNP complexes in Xenopus oocytes fall into two main categories, those having affinity for a variety of RNA sequences (members of the Y box and DEAD box RNA helicase families) and those which interact more specifically with 3' polyA tails (the polyA binding proteins or PABPs). The objective of this study was to determine whether mouse oocytes and embryos express sequences encoding a Y box protein, (MSY1); on RNA helicase, (RCK/p54); and a universally expressed PABP and testis specific isoform (PABP1 and PABPt, respectively). RNAs were amplified by RT/PCR and the identities of targeted cDNAs were confirmed by restriction analysis and/or direct sequencing. Relative steady state levels and time courses of accumulation/decay were compared by Northern hybridization. All of the sequences are transcribed as maternal mRNAs. MSY1 transcripts accumulated during the growth phase appear to be degraded in parallel with the bulk of maternal mRNAs by the mid-late two-cell stage. RCK/p54 mRNAs are most abundant in growing oocytes; steady state levels decline in primary and secondary oocytes, and degradation appears to be complete by the mid-late two-cell stage. Zygotic transcription of MSY1 and RCK/p54 is evident in four-cell stage embryos. Most of the PABP1 message accumulated by growing oocytes decays during meiotic maturation with transcription resuming in two-cell embryos. PABPt is expressed at very low levels in oocytes and embryos. Based on the temporal patterns of expression and the reported activities of homologous sequences in other systems, we suggest that these RNA binding proteins may participate in the post-transcriptional regulation of gene expression during the period of maternal control of development in the mouse.

Nuclear export of the DEAD box An3 protein by CRM1 is coupled to An3 helicase activity

We have recently identified the Xenopus laevis An3 protein as a bona fide substrate for the nuclear export receptor CRM1 (Exportin 1). An3 binds directly to CRM1 with high affinity via a leucine-rich nuclear export signal located in the extreme N terminus. An3 is a member of the DEAD box family of RNA helicases, which unwind RNA duplexes. RNA unwinding is coupled to hydrolysis of nucleoside triphosphates by the helicase, and the ATPase activity of several helicases is greatly stimulated by various polynucleotides. Here we report that dATP hydrolysis by An3 is stimulated approximately 6-fold by total RNA from X. laevis oocytes, whereas poly(U) RNA fails to enhance hydrolysis, suggesting the existence of a specific RNA activator for An3. Kinetic analysis reveals that a mutation within the conserved DEAD box motif reduces the rate of dATP hydrolysis by approximately 6-fold. In accordance with this, the DEAD box mutant is unable to unwind double-stranded RNA. Microinjection of the An3 DEAD box mutant into X. laevis oocytes nuclei reveals a significantly lower export rate as compared with wild-type An3 protein. This is not because the mutant has lower affinity toward CRM1, nor is it due to altered RNA binding capacity. This suggests that nuclear export of An3 protein by CRM1 is coupled to An3 helicase activity.

Cloning and characterization of a vasa-like gene in rainbow trout and its expression in the germ cell lineage

The origin of germ cells and the molecular mechanisms of primordial germ cell (PGC) determination in teleosts are unclear. Vasa is a member of the DEAD protein family and plays an indispensable role in germ cell determination in Drosophila and Xenopus species. In this study, we isolated and characterized a rainbow trout vasa cDNA as a first step towards understanding the molecular mechanisms of PGC determination and development and to develop a molecular marker to identify the PGCs in rainbow trout. Cloning of vasa cDNA was performed by degenerate- and RACE-PCR. The predicted amino acid sequence of rainbow trout Vasa contained eight consensus sequences for the DEAD protein family and five arginine-glycine-glycine repeats, a common character of known Vasa homologues. Overall amino acid similarity to the Vasa of Drosophila was 79.2%. Whole-mount in situ hybridization of eyed stage embryos (eighty somite stage) revealed that signals were localized to the putative PGCs. In adult rainbow trout tissues, both ovaries and testes contained large amounts of vasa gene transcripts. A reverse transcription-polymerase chain reaction analysis of unfertilized eggs proved that trout vasa is a maternal factor. Although we have not determined whether rainbow trout vasa functions as a germ cell determinant, its limited expression in the germ cell lineage proved that rainbow trout vasa can be used as a marker molecule for PGCs. This marker will make it possible to identify the PGCs or presumptive PGCs in early trout embryos whose germ cells can not be distinguished by morphological characteristics.

A novel gene family with a developmentally regulated expression in Xenopus laevis

We have isolated a new maternal gene called 4G2. 4G2 cDNA encodes a predicted protein of 501 amino acids, and its apparent molecular mass of 61 kDa was determined by SDS-PAGE of 4G2 recombinant protein expressed in E. coli or in vitro translated in rabbit reticulocyte lysate. Amino acid analysis of 4G2 revealed the RGD and LDV motif with a potential cell attachment activity. The open reading frames (ORF) also contained a consensus bipartite nuclear localization signal (NLS). There were number of expressed tag sequences (ESTs) from Drosophila, zebrafish, chicken, mouse, and human origin that encode a high degree of identity to the predicted 4G2 protein, thereby suggesting that 4G2 may constitute a novel gene family whose function has not been elucidated. We also present evidence that 4G2 transcript is maternally synthesized in stage IV oocyte, localized to animal hemisphere of egg, and zygotically reactivated in mid-neurula stage.

A novel gonadotropin-regulated testicular RNA helicase. A new member of the dead-box family

A gonadotropin-regulated testicular RNA helicase (GRTH) was identified and characterized. GRTH cloned from rat Leydig cell, mouse testis, and human testis cDNA libraries is a novel member of the DEAD-box protein family. GRTH is transcriptionally up-regulated by chorionic gonadotropin via cyclic AMP-induced androgen formation in the Leydig cell. It has ATPase and RNA helicase activities and increases translation in vitro. This helicase is highly expressed in rat, mouse, and human testes and weakly expressed in the pituitary and hypothalamus. GRTH is produced in both somatic (Leydig cells) and germinal (meiotic spermatocytes and round haploid spermatids) cells and is developmentally regulated. GRTH predominantly localized in the cytoplasm may function as a translational activator. This novel helicase could be relevant to the control of steroidogenesis and the paracrine regulation of androgen-dependent spermatogenesis in the testis.

fatvg encodes a new localized RNA that uses a 25-nucleotide element (FVLE1) to localize to the vegetal cortex of Xenopus oocytes

Vegetally localized transcripts have been implicated in a number of important biological functions, including cell fate determination and embryonic patterning. We have isolated a cDNA, fatvg, which encodes a localized maternal transcript that exhibits a localization pattern reminiscent of Vg1 mRNA. fatvg is the homologue of a mammalian gene expressed in adipose tissues. The fatvg transcript, unlike Vg1 which localizes strictly through the Late pathway, also associates with the mitochondrial cloud that is characteristic of the METRO or Early pathway. This suggests that fatvg mRNA may utilize both the METRO and Late pathways to localize to the vegetal cortex during oogenesis. We have dissected the cis-acting localization elements of fatvg mRNA and compared these elements with Vg1 mRNA. Our results indicate that, like most localized RNAs, in a variety of systems, transcripts of fatvg contain localization elements in the 3′UTR. The 3′UTR of fatvg mRNA contains multiple elements that are able to function independently; however, it functions most efficiently when all of the elements are present. We have defined a short 25-nucleotide element that can direct vegetal localization as a single copy. This element differs in sequence from previously described Vg1 localization elements, suggesting that different localization elements are involved in the localization of RNAs through the Late pathway.

RanGTP-regulated interactions of CRM1 with nucleoporins and a shuttling DEAD-box helicase

CRM1 is an export receptor mediating rapid nuclear exit of proteins and RNAs to the cytoplasm. CRM1 export cargoes include proteins with a leucine-rich nuclear export signal (NES) that bind directly to CRM1 in a trimeric complex with RanGTP. Using a quantitative CRM1-NES cargo binding assay, significant differences in affinity for CRM1 among natural NESs are demonstrated, suggesting that the steady-state nucleocytoplasmic distribution of shuttling proteins could be determined by the relative strengths of their NESs. We also show that a trimeric CRM1-NES-RanGTP complex is disassembled by RanBP1 in the presence of RanGAP, even though RanBP1 itself contains a leucine-rich NES. Selection of CRM1-binding proteins from Xenopus egg extract leads to the identification of an NES-containing DEAD-box helicase, An3, that continuously shuttles between the nucleus and the cytoplasm. In addition, we identify the Xenopus homologue of the nucleoporin CAN/Nup214 as a RanGTP- and NES cargo-specific binding site for CRM1, suggesting that this nucleoporin plays a role in export complex disassembly and/or CRM1 recycling.

Isolation of a novel gene, moc2, encoding a putative RNA helicase as a suppressor of sterile strains in Schizosaccharomyces pombe

A novel gene designated moc2, which encodes a putative RNA helicase, was isolated from Schizosaccharomyces pombe on the basis of its suppression of the sterility of two different mutant strains, one of which had elevated levels of cAMP and the other deregulated Ras functioning as a result of an ectopic expression of dominant negative RAS2. Moc2 is highly homologous to the RNA helicase DED1 of Saccharomyces cerevisiae (58% identity) and PL10 of mouse (50% identity). Disruption of the moc2 gene indicated that moc2 is essential for cell growth. The moc2 gene seems to have roles in both sexual differentiation and cell growth.

Hepatitis C virus core protein interacts with a human DEAD box protein DDX3

Several studies have implicated hepatitis C virus (HCV) core in influencing the expression of host genes. To identify cellular factors with a possible role in HCV replication and pathogenesis, we looked for cellular proteins that interact with the viral core protein. A human liver cDNA library was screened in a yeast two-hybrid assay to identify cellular proteins that bind to core. Several positive clones were isolated, one of which encoded the C-terminal 253 amino acids of a putative RNA helicase, a DEAD box protein designated DDX3. Bacterially expressed glutathione-S-transferase-DDX3 fusion protein specifically pulled down in vitro translated and radiolabeled HCV core, confirming a direct interaction. Immunofluorescent staining of HeLa cells with a polyclonal antiserum showed that DDX3 is located predominantly in nuclear speckles and at low levels throughout the cytoplasm. In cells infected with a recombinant vaccinia virus expressing HCV structural proteins (core, E1, and E2), DDX3 and core colocalized in distinct spots in the perinuclear region of the cytoplasm. The regions of the proteins involved in binding were found by deletion analysis to be the N-terminal 59 amino acid residues of core and a C-terminal RS-like domain of DDX3. The human DDX3 is a putative RNA helicase and a member of a highly conserved DEAD box subclass that includes murine PL10, Xenopus An3, and yeast Ded1 proteins. Their role in RNA metabolism or gene expression is unknown. The significance of core-helicase interaction in HCV replication and pathogenesis is discussed.

Hepatitis C virus core protein interacts with cellular putative RNA helicase

The nucleocapsid core protein of hepatitis C virus (HCV) has been shown to trans-act on several viral or cellular promoters. To get insight into the trans-action mechanism of HCV core protein, a yeast two-hybrid cloning system was used for identification of core protein-interacting cellular protein. One such cDNA clone encoding the DEAD box family of putative RNA helicase was obtained. This cellular putative RNA helicase, designated CAP-Rf, exhibits more than 95% amino acid sequence identity to other known RNA helicases including human DBX and DBY, mouse mDEAD3, and PL10, a family of proteins generally involved in translation, splicing, development, or cell growth. In vitro binding or in vivo coimmunoprecipitation studies demonstrated the direct interaction of the full-length/matured form and C-terminally truncated variants of HCV core protein with this targeted protein. Additionally, the protein's interaction domains were delineated at the N-terminal 40-amino-acid segment of the HCV core protein and the C-terminal tail of CAP-Rf, which encompassed its RNA-binding and ATP hydrolysis domains. Immunoblotting or indirect immunofluorescence analysis revealed that the endogenous CAP-Rf was mainly localized in the nucleus and to a lesser extent in the cytoplasm, and when fused with FLAG tag, it colocalized with the HCV core protein either in the cytoplasm or in the nucleus. Similar to other RNA helicases, this cellular RNA helicase has nucleoside triphosphatase-deoxynucleoside triphosphatase activity, but this activity is inhibited by various forms of homopolynucleotides and enhanced by the HCV core protein. Moreover, transient expression of HCV core protein in human hepatoma HuH-7 cells significantly potentiated the trans-activation effect of FLAG-tagged CAP-Rf or untagged CAP-Rf on the luciferase reporter plasmid activity. All together, our results indicate that CAP-Rf is involved in regulation of gene expression and that HCV core protein promotes the trans-activation ability of CAP-Rf, likely via the complex formation and the modulation of the ATPase-dATPase activity of CAP-Rf. These findings provide evidence that HCV may have evolved a distinct mechanism in alteration of host cellular gene expression regulation via the interaction of its nucleocapsid core protein and cellular putative RNA helicase known to participate in all aspects of cellular processes involving RNA metabolism. This feature of core protein may impart pleiotropic effects on host cells, which may partially account for its role in HCV pathogenesis.

Expression of vasa(vas)-related genes in germline cells and totipotent somatic stem cells of planarians

Planarians are known for their strong regenerative ability. This ability has been considered to reside in the totipotent somatic stem cell called the "neoblast." Neoblasts contain a unique cytoplasmic structure called the "chromatoid body," which has similar characteristics to the germline granules of germline cells of other animals. The chromatoid bodies decrease in number and size during cytodifferentiation and disappear in completely differentiated cells during regeneration. However, germ cells maintain the chromatoid body during their differentiation from neoblasts. These observations suggest that the chromatoid body is concerned with the totipotency of cells. To understand the molecular nature of the chromatoid body in the neoblast, we focused on vasa (vas)-related genes, since VAS and VAS-related proteins are known to be components of the germline granules in Drosophila and Caenorhabditis elegans. By PCR, two vas-related genes (Dugesia japonica vasa-like gene, DjvlgA and DjvlgB) were isolated, and they were shown to be expressed in germ cells. Interestingly, DjvlgA was also expressed in a number of somatic cells in the mesenchymal space. In regenerating planarians, accumulation of DjvlgA-expressing cells was observed in both the blastema and the blastema-proximal region. In X-ray-irradiated planarians, which had lost regenerative capacity, the number of DjvlgA-expressing cells decreased drastically. These results suggest that the product of DjvlgA may be a component of the chromatoid body and may be involved in the totipotency of the neoblast.

The protein family of RNA helicases

RNA helicases represent a large family of proteins that have been detected in almost all biological systems where RNA plays a central role. They are ubiquitously distributed over a wide range of organisms and are involved in nuclear and mitochondrial splicing processes, RNA editing, rRNA processing, translation initiation, nuclear mRNA export, and mRNA degradation. RNA helicases are described as essential factors in cell development and differentiation, and some of them play a role in transcription and replication of viral single-stranded RNA genomes. Comparisons of the conserved sequences reveal a close relationship between them and suggest that these proteins might be derived from a common ancestor. Biochemical studies have revealed a strong dependence of the unwinding activity on ATP hydrolysis. Although RNA helicase activity has only been demonstrated for a few examples yet, it is generally believed that all members of the largest subgroups, the DEAD and DEAH box proteins, exhibit this activity.

A Drosophila RNA helicase gene, pitchoune, is required for cell growth and proliferation and is a potential target of d-Myc

This article describes the characterization of a new Drosophila gene that we have called pitchoune (pit) (meaning small in Provence) because mutations in this gene produce larvae that cannot grow beyond the first instar larval stage although they can live as long as 7–10 days. All the tissues are equally affected and the perfectly shaped larvae are indistinguishable from first instar wild-type animals. Analysis of mutant somatic clones suggests a function in cell growth and proliferation, which is supported by the fact that cell proliferation is promoted by pit overexpression. Tagged-Pit, when transfected in S2 cells, localizes mainly to the nucleolus, pointing towards a possible role in ribosome biogenesis and, consequently, in protein biosynthesis. pit encodes a DEAD-box RNA helicase, a family of proteins involved in the control of RNA structure in many cellular processes and its closest homologue is a human DEAD-box RNA helicase, MrDb, whose corresponding gene transcription is directly activated by Myc-Max heterodimers (Grandori, C., Mac, J., Siebelt, F., Ayer, D. E. and Eisenman, R. N. (1996) EMBO J. 15, 4344–4357). The patterns of expression of d-myc and pit are superimposable. Ectopic expression of myc in the nervous system drives an ectopic expression of pit in this tissue indicating that in Drosophila as well, pit is a potential target of d-Myc. These results suggest that myc might promote cell proliferation by activating genes that are required in protein biosynthesis, thus linking cell growth and cell proliferation.

Expression of the 'dead box' RNA helicase p68 is developmentally and growth regulated and correlates with organ differentiation/maturation in the fetus

The human DEAD box protein p68 is an established RNA-dependent ATPase and RNA helicase, p68 has been highly conserved in evolution and appears to be essential for normal growth, suggesting that this protein plays an important role in the cell. Although the biochemical activities of p68 are fairly well characterized, little is known about its biological function. This report shows that p68 is detectable in quiescent cell lines, but its expression is induced by serum, suggesting that this protein may play a role in cell growth. It is also shown that both p68 mRNA and protein are differentially expressed in adult tissues; in this case, however, the levels do not always correlate with proliferation status, suggesting that the regulation of expression in the animal may be different from that in cell lines. Finally, it is shown that p68 expression is developmentally regulated and appears to correlate with organ differentiation/maturation. These findings suggest that p68 expression may not simply reflect proliferation/differentiation status and that it appears to be regulated in a more complex way.

Two copies of a subelement from the Vg1 RNA localization sequence are sufficient to direct vegetal localization in Xenopus oocytes

Localization of mRNA has emerged as a fundamental mechanism for generating polarity during development. In vertebrates, one example of this phenomenon is Vg1 RNA, which is localized to the vegetal cortex of Xenopus oocytes. Vegetal localization of Vg1 RNA is directed by a 340-nt sequence element contained within its 3′ untranslated region. To investigate how such cis-acting elements function in the localization process, we have undertaken a detailed analysis of the precise sequence requirements for vegetal localization within the 340-nt localization element. We present evidence for considerable redundancy within the localization element and demonstrate that critical sequences lie at the ends of the element. Importantly, we show that a subelement from the 5′ end of the Vg1 localization element is, when duplicated, sufficient to direct vegetal localization. We suggest that the Vg1 localization element is composed of smaller, redundant sequence motifs and identify one such 6-nt motif as essential for localization. These results allow insight into what constitutes an RNA localization signal and how RNA sequence elements may act in the localization process.

A vasa-like gene in zebrafish identifies putative primordial germ cells

The vasa gene is essential for germline formation in Drosophila. Vasa-related genes have been isolated from several organisms including nematode, frog and mammals. In order to gain insight into the early events in vertebrate germline development, zebrafish was chosen as a model. Two zebrafish vasa-related genes were isolated, pl10a and vlg. The pl10a gene was shown to be widely expressed during embryogenesis. The vlg gene and vasa belong to the same subfamily of RNA helicase encoding genes. Putative maternal vlg transcripts were detected shortly after fertilization and from the blastula stage onwards, expression was restricted to migratory cells most likely to be primordial germ cells.

Domain structure of human nuclear DNA helicase II (RNA helicase A)

Full-length human nuclear DNA helicase II (NDH II) was cloned and overexpressed in a baculovirus-derived expression system. Recombinant NDH II unwound both DNA and RNA. Limited tryptic digestion produced active helicases with molecular masses of 130 and 100 kDa. The 130-kDa helicase missed a glycine-rich domain (RGG-box) at the carboxyl terminus, while the 100-kDa form missed both its double-stranded RNA binding domains (dsRBDs) at the amino terminus and its RGG-box. Hence, the dsRBDs and the RGG-box were dispensable for unwinding. On the other hand, the isolated DEXH core alone could neither hydrolyze ATP nor unwind nucleic acids. These enzymatic activities were not regained by fusing a complete COOH or NH2 terminus to the helicase core. Hence, an active helicase required part of the NH2 terminus, the DEXH core, and a C-terminal extension of the core. Both dsRBDs and the RGG-box were bacterially expressed as glutathione S-transferase fusion proteins. The two dsRBDs had a strong affinity to double-stranded RNA and cooperated upon RNA binding, while the RGG-box bound preferentially to single-stranded DNA. A model is suggested in which the flanking domains influence and regulate the unwinding properties of NDH II.

PRH75, a new nucleus-localized member of the DEAD-box protein family from higher plants

The putative RNA helicases of the DEAD-box protein family are involved in pre-mRNA splicing, rRNA maturation, ribosome assembly, and translation. Members of this protein family have been identified in organisms from Escherichia coli to humans, but except for the translation initiation factor 4A, there have been no reports on the characterization of other DEAD-box proteins from plants. Here we report on a novel member of the DEAD-box protein family, the plant RNA helicase 75 (PRH75). PRH75 is localized in the nucleus and contains two domains for RNA binding. One is located at the C terminus and is similar to RGG RNA-binding domains of nucleus-localized RNA-binding proteins. The other one is located between amino acids 308 and 622, a region containing the conserved motif VI characteristic of DEAD-box proteins and known as the RNA-binding site of eIF-4A. The N-terminal 81 amino acids are sufficient for nuclear targeting of the protein. Northern and Western blot analyses show that PRH75 is mainly expressed in young and rapidly developing tissues. The purified recombinant PRH75 has a weak ATPase activity which is barely stimulated by RNA ligands. The fractionation of spinach whole-cell extracts by glycerol gradient centrifugation and gel filtration on a Superdex 200 column shows that the protein exists in a complex of about 500 kDa. Possible biological functions of PRH75 as well as structure-function relationships in the context of its modular primary structure are discussed.

An3 protein encoded by a localized maternal mRNA in Xenopus laevis is an ATPase with substrate-specific RNA helicase activity

ATP-dependent RNA helicases from the DEAD box family of proteins are involved in a number of RNA processing and utilization events. An3 protein from Xenopus laevis is an RNA helicase of the DEAD box family of proteins. An3 is synthesized by a mRNA that is localized to one end of Xenopus laevis oocytes. An3 protein is found in the nucleus of ooctes, and more specifically, during the middle stages of oocyte development, with extra nucleoli that contain amplified copies of rRNA genes in the nucleolus. By expressing glutathione-S-transferase:An3 fusion proteins in E. coli, sufficient amounts of An3 protein were isolated to examine its enzymatic activities. ATPase activity, NTP substrate range and RNA helicase activity were tested. An3 protein ATPase activity was evident but not stimulated by any of a variety of RNA tested. An3 protein was able to resolve the duplex formed by an in vitro substrate, in the presence of ATP or dATP. An3 required both 3' and 5' single-stranded regions of RNA flanking the RNA duplex it resolves.

Changes in nuclear localization of An3, a RNA helicase, during oogenesis and embryogenesis in Xenopus laevis

The immunolocalization of An3 protein, an ATP-dependent RNA helicase and a member of the DEAD box family, was compared with the localization of fibrillarin, a protein essential for rRNA processing, and snRNPs, which are involved in mRNA splicing reactions, during oogenesis and embryogenesis in Xenopus laevis. Although An3 protein was detected in the cytoplasm of all stages of oocytes, in most stages An3 protein was also present in the nucleus. Prior to stage I An3 protein was uniformly dispersed throughout the entire germinal vesicle; from stages I to V it was in nucleoli. By stage VI nucleolar labeling with anti-An3 disappeared and the protein was no longer present within nuclei. An3 reactivity was also present throughout the nuclei of follicle cells surrounding prestage I to stage VI oocytes. Both cytoplasmic and nuclear An3 staining were present in cells of stages 8 to 35 embryos; however, nuclear staining was punctate and uniformly distributed throughout the nucleoplasm. Fibrillarin was diffusely distributed throughout the entire germinal vesicle prior to stage I, localized exclusively to nucleoli of oocytes between stages I and VI and in nucleoli of stages 12 and 35 embryonic cells. Reactivity for snRNPs (anti-Sm) in germinal vesicles of prestage I oocytes was diffuse, and similar to the distribution of An3 and fibrillarin; in later stage oocytes anti-Sm staining was restricted to a population of granules, much fewer in number and more heterogeneous in size than nucleoli. Anti-Sm activity was apparent in nuclei of embryonic cells of stages 8 to 35 embryos. Although colocalization of the Sm epitope and An3 was not observed in developing oocytes and in embryonic cells, Sm reactive material was frequently found in close association with An3-positive nucleoli (oocytes) and nuclear deposits (embryonic cells). In stage IV and V oocytes treated with actinomycin D (4 micrograms/ml) to inhibit rRNA synthesis, nucleoli, which continued to possess fibrillarin, lacked An3; staining of follicle cell nuclei for An3 was unchanged. Treatment with 200 micrograms/ml actinomycin D to block mRNA synthesis, inhibited An3 but not fibrillarin staining in nuclei of prestage I oocytes and follicle cells. The changing patterns of An3 reactivity and the differential effects of actinomycin D on such localizations observed here are consistent with a role for An3 in the processing/production of RNA.

The mRNA encoding a beta subunit of heterotrimeric GTP-binding proteins is localized to the animal pole of Xenopus laevis oocyte and embryos

In order to provide evidence for a potential role of heterotrimeric GTP-binding proteins in the transduction of developmental signals, we prepared cDNAs from Xenopus laevis embryos and looked for fragments amplified between primers located in conserved sequences of the different subtypes of beta subunit. Using the amplified fragment as a probe, we cloned a member of the beta subunit family. The deduced protein sequence of the amphibian cDNA is highly homologous to the beta 1 subtype and, accordingly, we have named the Xenopus gene XG beta 1. In situ hybridization and RNase protection assay revealed that XG beta 1 mRNA is confined to the animal hemisphere of the mature oocyte. This localization of XG beta 1 mRNA is established at stage V during oogenesis. Following fertilization, the maternal mRNAs cosegregate with animal cells during cleavage stages. At gastrulation, transcripts are expressed in the dorsal ectoderm layer that will give rise to the central nervous system. Thus, XG beta 1 mRNA belongs to the small family of localized maternal mRNAs; as a transducing protein, its restriction to a subset of embryonic cells could mediate the distinct responsiveness which contributes to the patterning of the embryo.

Identification of new localized RNAs in the Xenopus oocyte by differential display PCR

We have identified localized transcripts in full-grown Xenopus oocytes by differential display PCR. One clone, An4a, has two transcripts, which localize to the animal half of the stage VI oocyte. The transcripts are expressed throughout early development, with embryonic expression primarily in anterior neural tissues. An4a has a high degree of sequence identity to a human cDNA clone of unknown function. Another clone, the previously identified beta-transducin repeat containing protein (beta-TrCP), has three transcripts with a unique pattern of localization, one localized to the animal half and two localized primarily to the vegetal cortex. This cDNA has previously been shown to rescue a yeast cell division cycle mutant, raising the possibility that the different Xenopus transcripts are involved in animal and vegetal cell cycles. Embryonic expression is primarily in the cement gland. These new localized transcripts contribute to the general observation that the vegetal cortex, but not the animal cortex, is a specific site for RNA localization.

Co-amplification and concomitant high levels of expression of a DEAD box gene with MYCN in human neuroblastoma

MYCN gene amplification is strongly correlated with poor prognosis in neuroblastoma (NB), the second most common solid pediatric tumor. However, increased MYCN expression seen in tumors that lack MYCN amplification does not correlate with aggressive clinical behavior. Whereas the MYCN gene spans only 7 kb, the MYCN amplicon has been shown to range in size from 350 kb to more than 1 Mb. Given the large size of the amplicon, it is possible that additional genes are co-amplified in NBs whose expression may contribute to the aggressive phenotype associated with MYCN-amplified tumors. We isolated a cDNA clone from a human NB library that is identical to DDXI, a gene recently reported to be preferentially expressed in two retinoblastoma cell lines that also express high levels of MYCN. DDXI belongs to a family of genes that encode DEAD (Asp-Glu-Ala-Asp) box proteins, putative ATP-dependent RNA helicases implicated in a number of cellular processes involving alterations of RNA secondary structure. We examined the frequency of DDXI amplification in 15 NB cell lines, 1 neuroepithelioma cell line, and 122 NB tumors by Southern blot analyses, and we found that 7 of 10 MYCN-amplified cell lines and 27 of 40 (68%) MYCN-amplified tumors also harbored multiple copies of the DDXI gene. Amplification of DDXI was associated with high levels of DDXI mRNA expression in the NB cell lines and tumors as examined by Northern analysis. Neither DDXI gene amplification nor enhanced expression was observed in tumors or cell lines that lacked MYCN amplification. Because RNA helicases play important roles in both post-transcriptional and translational gene regulation, high levels of DDXI expression consequent to genomic amplification may contribute to the malignant phenotype of a subset of NBs.