Apparent continuity between the messenger transport organizer and late RNA localization pathways during oogenesis in Xenopus

Xenopus laevis (Daudin, 1802) as a Model Organism for Bioscience: A Historic Review and Perspective

In vitro systems have been mainly promoted by authorities to sustain research by following the 3Rs principle, but continuously increasing amounts of evidence point out that in vivo experimentation is also of extreme relevance. Xenopus laevis, an anuran amphibian, is a significant model organism in the study of evolutionary developmental biology, toxicology, ethology, neurobiology, endocrinology, immunology and tumor biology; thanks to the recent development of genome editing, it has also acquired a relevant position in the field of genetics. For these reasons, X. laevis appears to be a powerful and alternative model to the zebrafish for environmental and biomedical studies. Its life cycle, as well as the possibility to obtain gametes from adults during the whole year and embryos by in vitro fertilization, allows experimental studies of several biological endpoints, such as gametogenesis, embryogenesis, larval growth, metamorphosis and, of course, the young and adult stages. Moreover, with respect to alternative invertebrate and even vertebrate animal models, the X. laevis genome displays a higher degree of similarity with that of mammals. Here, we have reviewed the main available literature on the use of X. laevis in the biosciences and, inspired by Feymann's revised view, "Plenty of room for biology at the bottom", suggest that X. laevis is a very useful model for all possible studies.

Comparison of RNA localization during oogenesis within Acipenser ruthenus and Xenopus laevis

The oocyte is a unique cell, from which develops a complex organism comprising of germ layers, tissues and organs. In some vertebrate species it is known that the asymmetrical localization of biomolecules within the oocyte is what drives the spatial differentiation of the daughter cells required for embryogenesis. This asymmetry is first established to produce an animal-vegetal (A-V) axis which reflects the future specification of the ectoderm, mesoderm, and endoderm layers. Several pathways for localization of vegetal maternal transcripts have already been described using a few animal models. However, there is limited information about transcripts that are localized to the animal pole, even though there is accumulating evidence indicating its active establishment. Here, we performed comparative TOMO-Seq analysis on two holoblastic cleavage models: Xenopus laevis and Acipenser ruthenus oocytes during oogenesis. We found that there were many transcripts that have a temporal preference for the establishment of localization. In both models, we observed vegetal transcript gradients that were established during either the early or late oogenesis stages and transcripts that started their localization during the early stages but became more pronounced during the later stages. We found that some animal gradients were already established during the early stages, however the majority were formed during the later stages of oogenesis. Some of these temporally localized transcripts were conserved between the models, while others were species specific. Additionally, temporal de novo transcription and also degradation of transcripts within the oocyte were observed, pointing to an active remodeling of the maternal RNA pool.

Specific binding of VegT mRNA localization signal to membranes in Xenopus oocytes

We have studied the interaction of a VegT mRNA localization signal sequence with the membranes of the mitochondrial cloud in Xenopus oocytes, and the binding of the VegT mRNA signal sequence to the lipid raft regions of the vesicles bounded by ordered and disordered phospholipid bilayers. RNA preference for the membranes of the mitochondrial cloud was confirmed using microscopy of a fluorescence resonance energy transfer from RNA molecules to membranes. Our studies show that VegT mRNA has a higher affinity for ordered regions of lipid bilayers. This conclusion is supported by the dissociation constant measurements for RNA-liposome complex and the visualization of the FRET signal between giant vesicles and RNA. Our data indicate that these affinities are sensitive and distinct to the location of the localization elements within the VegT mRNA localization signal structure. Therefore, specific binding of VegT mRNA localization signal sequence to membranes can be responsible for polarized distribution of VegT mRNA in Xenopus oocytes. We suggest that the mechanism of this binding can involve the interaction of the localization elements within the VegT mRNA signal sequence with lipid raft regions of the mitochondrial cloud membranes, thereby utilizing localization elements as novel lipid raft-binding RNA motifs.

Ciphers and Executioners: How 3'-Untranslated Regions Determine the Fate of Messenger RNAs

The sequences and structures of 3'-untranslated regions (3'UTRs) of messenger RNAs govern their stability, localization, and expression. 3'UTR regulatory elements are recognized by a wide variety of trans-acting factors that include microRNAs (miRNAs), their associated machinery, and RNA-binding proteins (RBPs). In turn, these factors instigate common mechanistic strategies to execute the regulatory programs encoded by 3'UTRs. Here, we review classes of factors that recognize 3'UTR regulatory elements and the effector machineries they guide toward mRNAs to dictate their expression and fate. We outline illustrative examples of competitive, cooperative, and coordinated interplay such as mRNA localization and localized translation. We further review the recent advances in the study of mRNP granules and phase transition, and their possible significance for the functions of 3'UTRs. Finally, we highlight some of the most recent strategies aimed at deciphering the complexity of the regulatory codes of 3'UTRs, and identify some of the important remaining challenges.

Xenopus laevis oocyte as a model for the study of the cytoskeleton

At the beginning of diplotene, the oocyte of Xenopus laevis is a cell of about 10-20 microns destined to increase 10,000-fold its size when the oocyte becomes filled with yolk platelets and has accumulated a great number of pigment granules in a half of its periphery. Its internal architecture is gradually accomplished during growth because of several factors, especially because of cytoskeletal changes. In the fully-grown oocyte, the cytoskeleton appears to sustain the eccentrically located germinal vesicle through arms radiating from the cortex to the germinal vesicle, a unique organization not to be found in other Amphibians. In this report, we summarized and analysed steps of cytoskeletal proteins and related mRNAs organization and function throughout diplotene stage, highlighting our studies in this animal model. The cytoskeletal proteins appear to exploit their activity with respect to ribosomal 60S subunit maturation and during translation. Most importantly, the polarity of the oocyte is achieved through a sophisticated and highly organized localization of mRNAs and cytoskeletal proteins in one side of the cell. This asymmetry will start the construction of the oocyte polarity that is instrumental for determining the characteristic of this cell, which will become an embryo. Moreover, in the same time membrane composition, conditioned by the underlying cytoskeletal organization, will acquire the prerequisites for sperm binding and fusion.

Mitochondria during sea urchin oogenesis

Sea urchin represents an ideal model for studies on fertilization and early development, but the achievement of egg competence and mitochondrial behaviour during oogenesis remain to be enlightened. Oocytes of echinoid, such as sea urchin, unlike other echinoderms and other systems, complete meiotic maturation before fertilization. Mitochondria, the powerhouse of eukaryotic cells, contain a multi-copy of the maternally inherited genome, and are involved directly at several levels in the reproductive processes, as their functional status influences the quality of oocytes and contributes to fertilization and embryogenesis. In the present paper, we report our latest data on mitochondrial distribution, content and activity during Paracentrotus lividus oogenesis. The analyses were carried out using confocal microscopy, in vivo incubating oocytes at different maturation stages with specific probes for mitochondria and mtDNA, and by immunodetection of Hsp56, a well known mitochondrial marker. Results show a parallel rise of mitochondrial mass and activity, and, especially in the larger oocytes, close to germinal vesicle (GV) breakdown, a considerable increase in organelle activity around the GV, undoubtedly for an energetic aim. In the mature eggs, mitochondrial activity decreases, in agreement with their basal metabolism. Further and significant information was achieved by studying the mitochondrial chaperonin Hsp56 and mtDNA. Results show a high increase of both Hsp56 and mtDNA. Taken together these results demonstrate that during oogenesis a parallel rise of different mitochondrial parameters, such as mass, activity, Hsp56 and mtDNA occurs, highlighting important tools in the establishment of developmental competence.

Mechanisms of Vertebrate Germ Cell Determination

Two unique characteristics of the germ line are the ability to persist from generation to generation and to retain full developmental potential while differentiating into gametes. How the germ line is specified that allows it to retain these characteristics within the context of a developing embryo remains unknown and is one focus of current research. Germ cell specification proceeds through one of two basic mechanisms: cell autonomous or inductive. Here, we discuss how germ plasm driven germ cell specification (cell autonomous) occurs in both zebrafish and the frog Xenopus. We describe the segregation of germ cells during embryonic development of solitary and colonial ascidians to provide an evolutionary context to both mechanisms. We conclude with a discussion of the inductive mechanism as exemplified by both the mouse and axolotl model systems. Regardless of mechanism, several general themes can be recognized including the essential role of repression and posttranscriptional regulation of gene expression.

Cytoskeletal proteins associate with components of the ribosomal maturation and translation apparatus in Xenopus stage I oocytes

Actin-based cytoskeleton (CSK) and microtubules may bind to RNAs and related molecules implicated in translation. However, many questions remain to be answered regarding the role of cytoskeletal components in supporting the proteins involved in steps in the maturation and translation processes. Here, we performed co-immunoprecipitation and immunofluorescence to examine the association between spectrins, keratins and tubulin and proteins involved in 60S ribosomal maturation and translation in Xenopus stage I oocytes, including ribosomal rpl10, eukaryotic initiation factor 6 (Eif6), thesaurins A/B, homologs of the eEF1α elongation factor, and P0, the ribosomal stalk protein. We found that rpl10 and eif6 cross-reacted with the actin-based CSK and with tubulin. rpl10 co-localizes with spectrin, particularly in the perinuclear region. eif6 is similarly localized. Given that upon ribosomal maturation, the insertion of rpl10 into the 60S subunit occurs simultaneously with the release of eif6, one can hypothesise that actin-based CSK and microtubules provide the necessary scaffold for the insertion/release of these two molecules and, subsequently, for eif6 transport and binding to the mature 60S subunit. P0 and thesaurins cross-reacted with only spectrin and cytokeratins. Thesaurins aggregated at the oocyte periphery, rendering this a territory favourable site for protein synthesis; the CSK may support the interaction between thesaurins and sites of the translating ribosome. Moreover, given that the assembly of the ribosome stalk, where P0 is located, to the 60S subunit is essential for the release of eif6, it can be hypothesised that the CSK can facilitate the binding of the stalk to the 60S.

Directional transport is mediated by a Dynein-dependent step in an RNA localization pathway

In vivo imaging of subcellular RNA localization in Xenopus oocytes reveals domains of transport directionality mediated by distinct molecular motors, with dynein providing a directional cue for polarized transport.

Cytoplasmic RNA localization is a key biological strategy for establishing polarity in a variety of organisms and cell types. However, the mechanisms that control directionality during asymmetric RNA transport are not yet clear. To gain insight into this crucial process, we have analyzed the molecular machinery directing polarized transport of RNA to the vegetal cortex in Xenopus oocytes. Using a novel approach to measure directionality of mRNA transport in live oocytes, we observe discrete domains of unidirectional and bidirectional transport that are required for vegetal RNA transport. While kinesin-1 appears to promote bidirectional transport along a microtubule array with mixed polarity, dynein acts first to direct unidirectional transport of RNA towards the vegetal cortex. Thus, vegetal RNA transport occurs through a multistep pathway with a dynein-dependent directional cue. This provides a new framework for understanding the mechanistic basis of cell and developmental polarity.

Like traffic on highways, molecular cargos are transported within cells on tracks that are collectively referred to as cytoskeletal networks. RNA molecules are one such cargo, and in many species, the localization of RNAs in egg cells or oocytes is essential for establishing the first asymmetries that are necessary for proper embryo development. RNAs can be actively transported by molecular motors that move cargos along the cytoskeletal tracks, but how such motors are capable of directing cargos to specific destinations within the cell is not yet known. Here we show that two motors, dynein and kinesin—known to carry out transport in opposite directions—are both directly involved in RNA localization in frog oocytes. To understand how these motors can promote directional cargo transport, we developed a system to monitor RNA transport in live oocytes. We find that the motor acting first in the pathway, dynein, is responsible for unidirectional transport. Bidirectional transport, mediated by kinesin, occurs subsequently on cytoskeletal tracks of opposing polarity near the RNA's final destination. Our results suggest a new model for directional transport comprising an initial directional cue that dominates over a later nondirectional step, acting to refine the ultimate cargo distribution.

Analysis of localization and reorganization of germ plasm in Xenopus transgenic line with fluorescence-labeled mitochondria

Germ plasm is found in germ-line cells of Xenopus and thought to include the determinant of primordial germ cells (PGCs). As mitochondria is abundant in germ plasm, vital staining of mitochondria was used to analyze the movement and function of germ plasm; however, its application was limited in early cleavage embryos. We made transgenic Xenopus, harboring enhanced green fluorescent protein (EGFP) fused to the mitochondria transport signal (Dria-line). Germ plasm with EGFP-labeled mitochondria was clearly distinguishable from the other cytoplasm, and retained mostly during one generation of germ-line cells in Dria-line females. Using the Dria-line, we show that germ plasm is reorganized from near the cell membrane to the perinuclear space at St. 9, dependent on the microtubule system.

DNA and RNA pattern of staining during oogenesis in zebrafish (Danio rerio): a confocal microscopy study

Oogenesis involves a sequence of cellular divisions and developmental changes leading to the formation of oocytes, whose role in development is to transfer genomic information to the next generation. During this process, the gene expression pattern changes considerably concomitant with genome remodeling, while genomic information is maintained. The development of the gonad in zebrafish is unique in that it goes through an initial ovarian phase and subsequently into either ovarian or testicular phases. How the germ cells choose to commit to an oogenic fate and enter meiosis or alternatively not to enter meiosis and commit to a spermatogenetic fate remains a key question in development. Lack of suitable markers has hampered the understanding of the principles controlling sex differentiation in zebrafish. The current study was aimed at finding substantive cytochemical markers to identify specific oocyte stages primarily focusing on the DNA and RNA component of cells, using fluorescent dyes: acridine orange and propidium iodide. The pattern of synthesis and appearance of nucleoli was stage specific and may be used to identify stages of oogenesis. A distinguishing and possibly diagnostic feature of the staining pattern observed was the low level of chromatin staining compared to other cellular structures. This may be related to the more diffuse state of chromatin that occurs prior to thickening of chromosomes from the pachytene stage onwards. Although the fluorescent dyes may be useful in determining the localization of nucleic acids in tissue sections, it was not possible to quantify the relative contribution of the DNA and RNA components of specific stages of oocyte growth.

Take the (RN)A-train: localization of mRNA to the endoplasmic reticulum

Protein translocation into the endoplasmic reticulum (ER) generally requires targeting of mRNAs encoding secreted or membrane proteins to the ER membrane. The prevalent view is that these mRNAs are delivered co-translationally, using the signal recognition particle (SRP) pathway. Here, SRP delivers signal sequence-containing proteins together with associated ribosomes and mRNA to the SRP receptor present on the ER surface. Recent studies demonstrate the presence of alternative pathways to recruit mRNAs to ER or to specific subdomains of the ER independent of SRP or translation. Such targeting of specific mRNAs to the ER subdomains allows the cell to sort proteins before translocation or to ensure co-localization of ER and mRNAs at specific locations. Translation-independent association of mRNAs involves ER-linked RNA-binding proteins and represents an alternative pathway of mRNA delivery to the ER. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Regulation of cell polarity and RNA localization in vertebrate oocytes

It has long been appreciated that the inheritance of maternal cytoplasmic determinants from different regions of the egg can lead to differential specification of blastomeres during cleavage. Localized RNAs are important determinants of cell fate in eggs and embryos but are also recognized as fundamental regulators of cell structure and function. This chapter summarizes recent molecular and genetic experiments regarding: (1) mechanisms that regulate polarity during different stages of vertebrate oogenesis, (2) pathways that localize presumptive protein and RNA determinants within the polarized oocyte and egg, and (3) how these determinants act in the embryo to determine the ultimate cell fates. Emphasis is placed on studies done in Xenopus, where extensive work has been done in these areas, and comparisons are drawn with fish and mammals. The prospects for future work using in vivo genome manipulation and other postgenomic approaches are also discussed.

Spectrin labeling during oogenesis in zebrafish (Danio rerio)

Progression through mitosis and meiosis during early zebrafish ovarian development is accompanied by highly regulated series of transformations in the architecture of oocytes. These cytoskeletal-dependent membrane events may be assumed to be brought about by deployment of proteins. While the cytoskeleton and its associated proteins play a pivotal role in each of these developmental transitions, it remains unclear how specific cytoskeletal proteins participate in regulating diverse processes of oocyte development in zebrafish. Results from this study show that a pool of spectrin accumulates during oogenesis and parallels an increase in volume of oocytes at pre-vitellogenic stages of development. Spectrin labeling is restricted to the surface of oogonia, the cortex of post-pachytene oocytes and later accumulates on the cytoplasm of pre-vitellogenic and vitellogenic oocytes. Results here suggest a correlation between spectrin labeling, increased cytoplasm volume of oocytes, an increase in the number of nucleoli and accumulation of cytoplasmic organelles. Overall, these results suggest that synthesis and storage of spectrin during pre-vitellogenic stages of oogenesis primes the egg with a pre-established pool of membrane-cytoskeletal precursors for use during embryogenesis, and that the presence of spectrin at the oocyte sub-cortex is essential for maintaining oocyte structure.

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.

The role of membranes and membrane trafficking in RNA localization

Eukaryotic cells possess highly sophisticated membrane trafficking pathways that define specific membrane domains and provide a means for moving vesicles between them (Mostov, Su, and ter Beest, 2003, Nat. Cell Biol. 5, 287-293). Here, I review recent data that indicate a role for membrane trafficking in mRNA localization. Specifically, I review evidence that some localized mRNAs are anchored to specific membrane domains and/or transported on membranous organelles or vesicles to specific subcellular sites. This review is not intended as a discussion on indirect influences of membrane trafficking on mRNA localization. I will not, for example, discuss the role of membrane trafficking in the regulation of extracellular signalling events that could indirectly influence mRNA localization through polarization of the actin or microtubule cytoskeleton (for examples, see reviews by Drubin and Nelson, 1996, Cell 84, 335-344; Shulman and St Johnston, 1999, Trends Cell Biol. 9, M60-M64).

Polarity and asymmetry during mouse oogenesis and oocyte maturation

Cell polarity and asymmetry play a fundamental role in embryo development. The unequal segregation of determinants, cues, and activities is the major event in the differentiation of cell fate and function in all multicellular organisms. In oocytes, polarity and asymmetry in the distribution of different molecules are prerequisites for the progression and proper outcome of embryonic development. The mouse oocyte, like the oocytes of other mammals, seems to apply a less stringent strategy of polarization than other vertebrates. The mouse embryo undergoes a regulative type of development, which permits the full rectification of development even if the embryo loses up to half of its cells or its size is experimentally doubled during the early stages of embryogenesis. Such pliability is strongly related to the proper oocyte polarization before fertilization. Thus, the molecular mechanisms leading to the development and maintenance of oocyte polarity must be included in any fundamental understanding of the principles of embryo development. In this chapter, we provide an overview of current knowledge regarding the development and maintenance of polarity and asymmetry in the distribution of organelles and molecules in the mouse oocyte. Curiously, the mouse oocyte becomes polarized at least twice during ontogenesis; the question of how this phenomenon is achieved and what role it might play is addressed in this chapter.

New insights into the regulation of RNP granule assembly in oocytes

In a variety of cell types in plants, animals, and fungi, ribonucleoprotein (RNP) complexes play critical roles in regulating RNA metabolism. These RNP granules include processing bodies and stress granules that are found broadly across cell types, as well as RNP granules unique to the germline, such as P granules, polar granules, sponge bodies, and germinal granules. This review focuses on RNP granules localized in oocytes of the major model systems, Caenorhabditis elegans, Drosophila, Xenopus, mouse, and zebrafish. The signature families of proteins within oocyte RNPs include Vasa and other RNA-binding proteins, decapping activators and enzymes, Argonaute family proteins, and translation initiation complex proteins. This review describes the many recent insights into the dynamics and functions of RNP granules, including their roles in mRNA degradation, mRNA localization, translational regulation, and fertility. The roles of the cytoskeleton and cell organelles in regulating RNP granule assembly are also discussed.

Polarizing animal cells via mRNA localization in oogenesis and early development

The localization of mRNAs in developing animal cells is essential for establishing cellular polarity and setting up the body plan for subsequent development. Cellular and molecular mechanisms by which maternal mRNAs are localized during oogenesis have been extensively studied in Drosophila and Xenopus. In contrast, evidence for mechanisms used in the localization of mRNAs encoded by developmentally important genes has also been accumulating in several other organisms. This offers the opportunity to unravel the fundamental mechanisms of mRNA localization shared among many species, as well as unique mechanisms specifically acquired or retained by animals based on their developmental needs. In addition to maternal mRNAs, the localization of zygotically expressed mRNAs in the cells of cleaving embryos is also important for early development. In this review, mRNA localization dynamics in the oocytes/eggs of Drosophila and Xenopus are first summarized, and evidence for localized mRNAs in the oocytes/eggs and cleaving embryos of other organisms is then presented.

Expression of XNOA 36 in the mitochondrial cloud of Xenopus laevis oocytes

In Xenopus laevis oocytes a mitochondrial cloud (MC) is found between the nucleus and the plasma membrane at stages I–II of oogenesis. The MC contains RNAs that are transported to the future vegetal pole at stage II of oogenesis. In particular, germinal plasm mRNAs are found in the Message Transport Organiser (METRO) region, the MC region opposite to the nucleus. At stages II–III, a second pathway transports Vg1 and VegT mRNAs to the area where the MC content merges with the vegetal cortex. Microtubules become polarized at the sites of migration of Vg1 and VegT mRNAs through an unknown signalling mechanism. In early meiotic stages, the centrioles are almost completely lost with their remnants being dispersed into the cytoplasm and the MC, which may contain a MTOC to be used in the later localization pathway of the mRNAs. In mammals, XNOA 36 encodes a member of a highly conserved protein family and localises to the nucleolus or in the centromeres. In the Xenopus late stage I oocyte, XNOA 36 mRNA is transiently segregated in one half of the oocyte, anchored by a cytoskeletal network that contains spectrin. Here we found that XNOA 36 transcript also localises to the nucleoli and in the METRO region. XNOA 36 protein immunolocalization, using an antibody employed for the library immunoscreening that depicted XNOA 36 expression colonies, labels the migrating MC, the cytoplasm of stage I oocytes and in particular the vegetal cortex facing the MC. The possible role of XNOA 36 in mRNA anchoring to the vegetal cortex or in participating in early microtubule reorganization is discussed.

Nuage morphogenesis becomes more complex: two translocation pathways and two forms of nuage coexist in Drosophila germline syncytia

We have developed a simple and reliable method of preserving antigen immunoreactivity with concomitant excellent retention of the cell ultrastructure. Using this method, we have been able to follow the origin and developmental stages of nuage accumulations within the nurse cell/oocyte syncytium in the ovary of the fruit fly, Drosophila melanogaster, at the ultrastructural level. We have found two morphologically and biochemically distinct forms of nuage material in the nurse cell cytoplasm: translocating accumulations of nuage containing the Vasa protein, termed sponge bodies and stationary polymorphic accumulations of nuage enriched in Argonaute and Survival of motor neuron proteins. Immunogold labeling combined with confocal fluorescent and ultrastructural analyses have revealed that the Vasa-containing nuage accumulations remain closely associated with the cisternae of the endoplasmic reticulum throughout their lifetimes. The migration mechanism of the Vasa-positive nuage appears distinct from the microtubule-dependent translocation of oskar ribonucleoprotein complexes. We postulate that these two distinct nuage translocation pathways converge in the formation of the polar granules within the polar/germ plasm of the oocyte posterior pole. We also provide morphological and immunocytochemical evidence that these polymorphic nuage accumulations correspond to the recently described cytoplasmic domains termed U body-P body complexes.

Transmission of mitochondrial DNA diseases and ways to prevent them

Recent reports of strong selection of mitochondrial DNA (mtDNA) during transmission in animal models of mtDNA disease, and of nuclear transfer in both animal models and humans, have important scientific implications. These are directly applicable to the genetic management of mtDNA disease. The risk that a mitochondrial disorder will be transmitted is difficult to estimate due to heteroplasmy—the existence of normal and mutant mtDNA in the same individual, tissue, or cell. In addition, the mtDNA bottleneck during oogenesis frequently results in dramatic and unpredictable inter-generational fluctuations in the proportions of mutant and wild-type mtDNA. Pre-implantation genetic diagnosis (PGD) for mtDNA disease enables embryos produced by in vitro fertilization (IVF) to be screened for mtDNA mutations. Embryos determined to be at low risk (i.e., those having low mutant mtDNA load) can be preferentially transferred to the uterus with the aim of initiating unaffected pregnancies. New evidence that some types of deleterious mtDNA mutations are eliminated within a few generations suggests that women undergoing PGD have a reasonable chance of generating embryos with a lower mutant load than their own. While nuclear transfer may become an alternative approach in future, there might be more difficulties, ethical as well as technical. This Review outlines the implications of recent advances for genetic management of these potentially devastating disorders.

A transient asymmetric distribution of XNOA 36 mRNA and the associated spectrin network bisects Xenopus laevis stage I oocytes along the future A/V axis

In Xenopus oogenesis, the mechanisms governing the localisation of molecules crucial for primary axis determination have been uncovered in recent years. In stage I oocytes, the mitochondrial cloud (MC) entraps RNAs implicated in germ line specification and other RNAs, such as Xwnt-11 and Xlsirts, that are later delivered to the vegetal pole. Microfilaments and microtubules gradually develop in the cytoplasm, sustaining organelles as well as the MC. At stage III, other mRNAs migrate to the vegetal hemisphere through a microtubule-dependent mechanism. We report here the isolation of a cDNA encoding XNOA 36, a highly conserved protein, whose function is to date not fully understood. The XNOA 36 transcript is abundantly accumulated in stage I oocytes where it decorates a filamentous network. At the end of stage I the transcript gradually segregates in a sector of the oocyte surrounding the MC and opposite the ovarian hylum. Here, XNOA 36 mRNA distributes in a gradient-like pattern extending from a peripheral network towards the interior of the oocyte. This distribution is similar to that of alpha-spectrin mRNA. Both mRNAs are segregated in one half of the 250 microm oocytes, with the MC located between the XNOA 36/alpha-spectrin mRNA-labelled and unlabelled regions. XNOA 36 mRNA localisation was uncoupled from that of alpha-spectrin mRNA by cytochalasin B or ice-nocodazole treatments, suggesting that the two transcripts rely on different mechanisms for their localisation. However, immunolocalisation experiments coupled with in situ hybridisation revealed that the XNOA 36 transcript co-localises with the protein spectrin. This observation, together with the finding that XNOA 36 mRNA co-precipitates with spectrin, indicates that these two molecules interact physically. In conclusion, our data suggest that XNOA 36 mRNA is localized and/or anchored in the oocyte through a cytoskeletal network containing spectrin. The putative implications of this finding are discussed.

Is the mitochondrial cloud the selection machinery for preferentially transmitting wild-type mtDNA between generations? Rewinding Müller's ratchet efficiently

In animal mitochondrial DNA inheritance, it remains largely unclear where the mitochondrial genetic bottleneck localizes and how it works in rewinding Müller's ratchet. In a variety of different animals germ plasm mRNAs typically aggregate along with numerous mitochondria to form the mitochondrial cloud (MC) during oogenesis. The MC has been found to serve as messenger transport organizer for germ plasm mRNAs. Germ plasm RNAs in MC will specifically distribute to the primordial germ cells of the future embryo. It has been proposed that the MC might be the site where selected mitochondria accumulate for specific transmission to grandchildren but this idea received relatively little attention and the criterion by which mitochondria are selected remains unknown. Our recent results in zebrafish provided further evidence for selective mitochondria accumulation in the MC by showing that mitochondria with high-inner membrane potential tend to be recruited preferentially into the MC, and these mitochondria are transported along with germ plasm to the cortex of the vegetal pole. By analyzing the composition, behavior and functions of the MC, and in reviewing related literature, we found strong support for the proposition that the MC corresponds to the position and function of the mitochondrial genetic bottleneck.

Detection of protein-RNA complexes in Xenopus oocytes

There is a remarkable variety of mechanisms for controlling post-transcriptional gene expression that is achieved through the formation of ribonucleoprotein (RNP) complexes on specific cis-acting regions of mRNA. These complexes regulate splicing, nuclear and cytoplasmic polyadenylation, stability, localization, and translation. Thus, it is important to be able to detect the association of specific proteins with specific RNAs within the context of these RNP complexes. We describe a method to test for protein-RNA complexes in Xenopus oocytes. The procedure combines immunoprecipitation with reverse transcription-PCR (RT-PCR) and does not entail chemical or photo crosslinking. Microinjected mRNA is efficiently translated in Xenopus oocytes; thus, in cases where primary antibody is not available, an epitope-tagged version of the protein can be expressed for utilization in this procedure. The inclusion of control mRNAs has provided no evidence of nonspecific protein reassociation to RNA during or subsequent to cell lysis. The method has been used to document the association of certain trans-acting factors specifically with localized mRNAs in Xenopus oocytes.

Multiple-Particle-Tracking to investigate viscoelastic properties in living cells

Cell mechanical properties play an important role in determining many cellular activities. Passive microrheology techniques, such as Multiple-Particle-Tracking (MPT) give an insight into the structural rearrangements and viscoelastic response of a wide range of materials, in particular soft materials and complex fluids like cell cytoplasm in living cells. The technique finds an important field of application in large cells such as oocytes where, during their growth, several organelles and molecules are displaced in specific territories of the cell instrumental for later embryonic development. To measure cell mechanics, cells are usually deformed by many techniques that are slow and often invasive. To overcome these limits, the MPT technique is applied. Probe particles are embedded in the viscoelastic sample and their properties are extracted from the thermal fluctuation spectra measured using digital video-microscopy. The Brownian motion of a probe particle immersed in a network is directly related to the network's mechanical properties. Particles exhibit larger motions when their local environments are less rigid or less viscous. The mean-square-displacement (MSD) of the particle's trajectory is used to quantify its amplitude of motions over different time scales.

Interactions of 40LoVe within the ribonucleoprotein complex that forms on the localization element of Xenopus Vg1 mRNA

Proline rich RNA-binding protein (Prrp), which associates with mRNAs that employ the late pathway for localization in Xenopus oocytes, was used as bait in a yeast two-hybrid screen of an expression library. Several independent clones were recovered that correspond to a paralog of 40LoVe, a factor required for proper localization of Vg1 mRNA to the vegetal cortex. 40LoVe is present in at least three alternatively spliced isoforms; however, only one, corresponding to the variant identified in the two-hybrid screen, can be crosslinked to Vg1 mRNA. In vitro binding assays revealed that 40LoVe has high affinity for RNA, but exhibits little binding specificity on its own. Nonetheless, it was only found associated with localized mRNAs in oocytes. 40LoVe also interacts directly with VgRBP71 and VgRBP60/hnRNP I; it is the latter factor that likely determines the binding specificity of 40LoVe. Initially, 40LoVe binds to Vg1 mRNA in the nucleus and remains with the RNA in the cytoplasm. Immunohistochemical staining of oocytes shows that the protein is distributed between the nucleus and cytoplasm, consistent with nucleocytoplasmic shuttling activity. 40LoVe is excluded from the mitochondrial cloud, which is used by RNAs that localize through the early (METRO) pathway in stage I oocytes; nonetheless, it is associated with at least some early pathway RNAs during later stages of oogenesis. A phylogenetic analysis of 2xRBD hnRNP proteins combined with other experimental evidence suggests that 40LoVe is a distant homolog of Drosophila Squid.

RNA localization to the Balbiani body in Xenopus oocytes is regulated by the energy state of the cell and is facilitated by kinesin II

Xenopus oocytes provide an excellent model system for understanding the cis-elements and protein factors that carry out mRNA localization in vertebrate cells. More than 20 mRNAs have been identified that localize to the vegetal cortex during stages II-IV of oogenesis. The earliest localizing RNAs are presorted to a subcellular structure, the Balbiani body (also called the mitochondrial cloud in Xenopus), of stage I oocytes prior to entering the vegetal cortex. While some evidence has suggested that diffusion drives RNA localization to the Balbiani body, a role for temperature and metabolic energy in this process has not been explored. To address this issue, we developed a quantitative assay to monitor RNA localization in stage I oocytes. Here we show that the rate of RNA accumulation to the Balbiani body is highly dependent on temperature and the intracellular concentration of ATP. In fact, while ATP depletion severely impairs RNA localization, increasing the intracellular concentration of ATP by a factor of two doubles the localization rate, indicating that ATP is limiting under normal conditions. We also show that RNA localization in stage I oocytes is reduced by inhibition of kinesin II, and that the Xcat-2 RNA localization element recruits kinesin II to the Balbiani body. We conclude from these studies that the energy state of the cell regulates the rate of RNA localization to the Balbiani body and that this process, at least to some extent, involves kinesin II.

Bucky ball functions in Balbiani body assembly and animal-vegetal polarity in the oocyte and follicle cell layer in zebrafish

The Balbiani body is an evolutionarily conserved asymmetric aggregate of organelles that is present in early oocytes of all animals examined, including humans. Although first identified more than 150 years ago, genes acting in the assembly of the Balbiani body have not been identified in a vertebrate. Here we show that the bucky ball gene in the zebrafish is required to assemble this universal aggregate of organelles. In the absence of bucky ball the Balbiani body fails to form, and vegetal mRNAs are not localized in oocytes. In contrast, animal pole localized oocyte markers are expanded into vegetal regions in bucky ball mutants, but patterning within the expanded animal pole remains intact. Interestingly, in bucky ball mutants an excessive number of cells within the somatic follicle cell layer surrounding the oocyte develop as micropylar cells, an animal pole specific cell fate. The single micropyle permits sperm to fertilize the egg in zebrafish. In bucky ball mutants, excess micropyles cause polyspermy. Thus bucky ball provides the first genetic access to Balbiani body formation in a vertebrate. We demonstrate that bucky ball functions during early oogenesis to regulate polarity of the oocyte, future egg and embryo. Finally, the expansion of animal identity in oocytes and somatic follicle cells suggests that somatic cell fate and oocyte polarity are interdependent.

Mitochondrial behavior during oogenesis in zebrafish: a confocal microscopy analysis

The behavior of mitochondria during early oogenesis remains largely unknown in zebrafish. We used three mitochondrial probes (Mito Tracker Red CMXRos, Mito Tracker Green FM, and JC-1) to stain early zebrafish oocyte mitochondria, and confocal microscopy to analyze mitochondrial aggregation and distribution. By using fluorescence recovery after photobleaching (FRAP), we traced mitochondrial movement. The microtubule assembly inhibitor nocodazole and microfilament inhibitor cytochalasin B (CB) were used to analyze the role of microtubules and microfilaments on mitochondrial movement. By using the dual emission probe, JC-1, and oxidative phosphorylation uncoupler, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP), we determined the distribution of active and inactive (low-active) mitochondria. Green/red fluorescence ratios of different sublocations in different oocyte groups stained by JC-1 were detected in merged (green and red) images. Our results showed that mitochondria exhibited a unique distribution pattern in early zebrafish oocytes. They tended to aggregate into large clusters in early stage I oocytes, but in a threadlike state in latter stage I oocytes. We detected a lower density mitochondrial area and a higher density mitochondrial area on opposite sides of the germinal vesicle. The green/red fluorescence ratios in different sublocations in normal oocytes were about 1:1. This implies that active mitochondria were distributed in all sublocations. FCCP treatment caused significant increases in the ratios. CB and nocodazole treatment caused an increase of the ratios in clusters and mitochondrial cloud, but not in dispersed areas. Mitochondria in different sublocations underwent fast dynamic movement. Inhibition or disruption of microtubules or microfilaments resulted in even faster mitochondrial free movement.

Pix1 and Pix2 are novel WD40 microtubule-associated proteins that colocalize with mitochondria in Xenopus germ plasm and centrosomes in human cells

In many animals, the germ line develops from a distinct mitochondria-rich region of embryonic cytoplasm called the germ plasm. However, the protein composition of germ plasm and its formation remain poorly understood, except in Drosophila. Here, we show that Xpat, a recently identified protein component of Xenopus germ plasm, interacts via its C-terminal domain with a novel protein, xPix1. Xpat and xPix1 are co-expressed in ovaries, eggs and early embryos and colocalize to the mitochondrial cloud and germ plasm in stage I and stage VI oocytes, respectively. Although Xpat appears unique to Xenopus, Pix proteins, which contain an N-terminal WD40 domain and C-terminal coiled-coil, are widely conserved. In humans, two proteins, Pix1 and Pix2, are expressed at varying levels in different cancer cell lines. Importantly, as well as localizing to mitochondria, human Pix proteins localize to centrosomes and associate with microtubules in vitro and in vivo. Although, Pix proteins are stably expressed through the cell cycle, Pix2 concentrates on microtubule structures in mitosis and microinjection of Pix antibodies interferes with cell division. Based on these data, we propose that Pix1 and Pix2 are microtubule-associated adaptor proteins that likely contribute to a range of developmental and cell division processes.

The efficiency of Xenopus primordial germ cell migration depends on the germplasm mRNA encoding the PDZ domain protein Grip2

A microarray analysis of vegetal pole sequences in the egg and early Xenopus laevis embryo identified Unigene Xl.14891 as a vegetally localized RNA. Analysis of the Xenopus tropicalis genome showed this Unigene to be localized near the 3' end of the Grip2 (glutamate receptor interacting protein 2) transcription unit. RACE showed that the Unigene represented the 3' UTR of Grip2 mRNA. Grip2 mRNA is present in the mitochondrial cloud of late pre-vitellogenic oocytes and then in the germplasm through oogenesis and early development until tailbud tadpole stages. Interference with Grip2 mRNA translation using two antisense morpholino oligos (MOs) impairs primordial germ cell (PGC) migration to the germinal ridges. Both MOs also inhibit swimming movements of the tailbud tadpole, known to involve glutamate receptors. We conclude that Grip2 has several functions in the embryo, including enabling efficient PGC migration.

The Balbiani body in the oocytes of a common cellar spider, Pholcus phalangioides (Araneae: Pholcidae)

Previtellogenic oocytes of a common cellar spider, Pholcus phalangioides, contain a single aggregation of organelles referred here to as the Balbiani body. It is a well defined ooplasmic structure predominantly composed of fine granular nuage, RNA rich material but comprising also mitochondria, vesicles of endoplasmic reticulum and stacks of Golgi cysternae. The Balbiani body originates early during previtellogenesis in the form of a cap-shaped mass in juxtaposition to one pole of the oocyte nucleus. During later stages of previtellogenic growth the Balbiani body translocates as a single body towards the ooplasm periphery. The results presented indicate that Balbiani body translocation is cytoskeleton independent. Balbiani body repositioning does not result in the localization of its components to any distinct, asymmetrically situated region of the ooplasm but, instead, ends up with their even dispersion in the oocyte cortex. The Balbiani body in Pholcus does not seem to be implicated either in germ cell determination or organelle inheritance. Its homology with similar organelle accumulations in the oocytes of other species is discussed.

Ribonucleoprotein remodeling during RNA localization

Cytoplasmic RNA localization is a means to create polarity by restricting protein expression to a discrete subcellular location. RNA localization is a multistep process that begins with the recognition of cis-acting sequences within the RNA by specific trans-factors, and RNAs are localized in ribonucleoprotein (RNP) complexes that contain both the RNA and numerous protein components. Components of the localization machinery transport the RNP complex, usually in a translationally repressed state, to a distinct subcellular region, resulting in spatially restricted gene expression. Recent efforts to identify both the cis- and trans-factors required for RNA localization have elucidated RNA-protein interactions that are remodeled during localization.

Balbiani cytoplasm in oocytes of a primitive fish, the sturgeon Acipenser gueldenstaedtii, and its potential homology to the Balbiani body (mitochondrial cloud) of Xenopus laevis oocytes

The oocytes of many organisms, including frogs and fish, contain a distinct cytoplasmic organelle called the Balbiani body. Because of the scarcity of published information and the tremendous variability in the appearance, ultrastructure, and composition of Balbiani bodies between species, the function of the Balbiani body and its inter-species homology remain a mystery. In Xenopus laevis, the Balbiani body is known to play a role in transporting germ cell determinants and localized RNAs to the oocyte vegetal cortex. In fish, however, the molecular composition of the Balbiani body has not been studied to date, and its function remains completely unknown. We have studied the ultrastructure and molecular composition of previtellogenic oocytes of the sturgeon, Acipenser gueldenstaedtii, by using electron microscopy, in situ hybridization, and immunostaining. We have found that sturgeon oocytes contain two distinct zones of cytoplasm: homogeneous (organelle-free) and granular (organelle-rich). We have also found that the granular ooplasm, which we term the Balbiani cytoplasm, shares important homologies, in both ultrastructure and molecular composition, with Xenopus Balbiani bodies.

40LoVe interacts with Vg1RBP/Vera and hnRNP I in binding the Vg1-localization element

Localizing mRNAs within the cytoplasm gives cells the ability to spatially restrict protein production, a powerful means to regulate gene expression. Localized mRNA is often visible in microscopically observable particles or granules, and the association of mRNA localization with these structures is an indication that particles or granules may be essential to the localization process. Understanding how such structures form will therefore be important for understanding the function of localization RNPs (L-RNPs). We previously identified a novel component of an L-RNP from the Vg1 mRNA from Xenopus oocytes called 40LoVe. 40LoVe interaction with the Vg1-localization element (Vg1LE) was previously shown to be dependent on the VM1 and E2 sequence motifs within the Vg1LE that cross-link to hnRNP I and Vg1RBP/Vera, respectively. We report interaction of these motif-binding proteins with 40LoVe and identify a 40LoVe-Xenopus hnRNP D/AUF1 interaction. We further demonstrate that titration of VM1 and E2 motif binding activity in vivo surprisingly suggests that the motif binding proteins have differing roles during Vg1LE-dependent mRNA localization.

Xenopus Dead end mRNA is a localized maternal determinant that serves a conserved function in germ cell development

Germ plasm formation is considered to define the first step in germ cell development. Xenopus Dead end represents a germ plasm specific transcript that is homologous to the previously characterized zebrafish dead end, which is required for germ cell migration and survival. XDead end mRNA localizes to the vegetal pole of Xenopus oocytes; in contrast to all other known germ plasm associated transcripts in Xenopus, XDead end is transported via the late transport pathway, suggesting a different mode of germ plasm restriction. Vegetal localization in the oocyte is achieved via a localization element mapping to a 251 nucleotide element in the 3'-UTR. This RNA sequence binds to a set of proteins characteristic for the late localization pathway and to one additional protein of 38 kDa. Inhibition of XDead end translation in Xenopus embryos results in a loss of primordial germ cells at tadpole stages of development. Early specification events do not seem to be affected, but the primordial germ cells fail to migrate dorsally and eventually disappear. This phenotype is very similar to what has been observed in the zebrafish, indicating that the role of XDead end in germ cell development has been conserved in evolution.

Xenopus Xpat protein is a major component of germ plasm and may function in its organisation and positioning

In many animals, including Drosophila, C. elegans, zebrafish and Xenopus, the germ line is specified by maternal determinants localised in a distinct cytoplasmic structure called the germ plasm. This is consists of dense granules, mitochondria, and specific localised RNAs. We have characterised the expression and properties of the protein encoded by Xpat, an RNA localised to the germ plasm of Xenopus. Immunofluorescence and immunoblotting showed that this novel protein is itself a major constituent of germ plasm throughout oogenesis and early development, although it is also present in other regions of oocytes and embryos, including their nuclei. We found that an Xpat-GFP fusion protein can localise correctly in cultured oocytes, in early oocytes to the 'mitochondrial cloud', from which germ plasm originates, and in later oocytes to the vegetal cortex. The localisation process was microtubule-dependent, while cortical anchoring required microfilaments. Xpat-GFP expressed in late stage oocytes assembled into circular fields of multi-particulate structures resembling endogenous fields of germ plasm islands. Furthermore these structures could be induced to form at ectopic sites by manipulation of culture conditions. Ectopic Xpat-GFP islands were able to recruit mitochondria, a major germ plasm component. These data suggest that Xpat protein has an important role in Xenopus germ plasm formation, positioning and maintenance.

Endoplasmic reticulum: one continuous network compartmentalized by extrinsic cues

The endoplasmic reticulum (ER) is an extensive three-dimensional network that stretches from the inner nuclear envelope to the cell cortex with a single, continuous membrane and a single, continuous lumen. Yet the ER contains specialized regions that carry out unique functions. The question that immediately arises is how the ER can be compartmentalized if it is continuous, and the answer to this is that cellular landmarks with unique sub-cellular distributions impose non-uniformity on the ER from outside, creating structural and functional sub-domains of the ER.

Identification of 40LoVe, a Xenopus hnRNP D family protein involved in localizing a TGF-beta-related mRNA during oogenesis

Asymmetric distribution of cellular components underlies many biological processes, and the localization of mRNAs within domains of the cytoplasm is one important mechanism of establishing and maintaining cellular asymmetry. mRNA localization often involves assembly of large ribonucleoproteins (RNPs) in the cytoplasm. Using an RNA affinity chromatography approach, we investigated localization RNP formation on the vegetal localization element (VLE) of the mRNA encoding Vg1, a Xenopus TGF-beta family member. We identified 40LoVe, an hnRNP D family protein, as a specific VLE binding protein from Xenopus oocytes. Interaction of 40LoVe with the VLE strictly correlates with the ability of the RNA to localize, and antibodies against 40LoVe inhibit vegetal localization in vivo in oocytes. Our results associate an hnRNP D protein with mRNA localization and have implications for several functions mediated by this important protein family.

Evidence for common machinery utilized by the early and late RNA localization pathways in Xenopus oocytes

In Xenopus, an early and a late pathway exist for the selective localization of RNAs to the vegetal cortex during oogenesis. Previous work has suggested that distinct cellular mechanisms mediate localization during these pathways. Here, we provide several independent lines of evidence supporting the existence of common machinery for RNA localization during the early and late pathways. Data from RNA microinjection assays show that early and late pathway RNAs compete for common localization factors in vivo, and that the same short RNA sequence motifs are required for localization during both pathways. In addition, quantitative filter binding assays demonstrate that the late localization factor Vg RBP/Vera binds specifically to several early pathway RNA localization elements. Finally, confocal imaging shows that early pathway RNAs associate with a perinuclear microtubule network that connects to the mitochondrial cloud of stage I oocytes suggesting that motor driven transport plays a role during the early pathway as it does during the late pathway. Taken together, our data indicate that common machinery functions during the early and late pathways. Thus, RNA localization to the vegetal cortex may be a regulated process such that differential interactions with basal factors determine when distinct RNAs are localized during oogenesis.

RNA localization mechanisms in oocytes

In many animals, normal development depends on the asymmetric distribution of maternal determinants, including various coding and noncoding RNAs, within the oocyte. The temporal and spatial distribution of localized RNAs is determined by intricate mechanisms that regulate their movement and anchoring. These mechanisms involve cis-acting sequences within the RNA molecules and a multitude of trans-acting factors, as well as a polarized cytoskeleton, molecular motors and specific transporting organelles. The latest studies show that the fates of localized RNAs within the oocyte cytoplasm are predetermined in the nucleus and that nuclear proteins, some of them deposited on RNAs during splicing, together with the components of the RNA-silencing pathway, dictate the proper movement, targeting, anchoring and translatability of localized RNAs.

Two distinct Staufen isoforms in Xenopus are vegetally localized during oogenesis

Localization of mRNA is an important way of generating early asymmetries in the developing embryo. In Drosophila, Staufen is intimately involved in the localization of maternally inherited mRNAs critical for cell fate determination in the embryo. We show that double-stranded RNA-binding Staufen proteins are present in the oocytes of a vertebrate, Xenopus, and are localized to the vegetal cytoplasm, a region where important mRNAs including VegT and Vg1 mRNA become localized. We identified two Staufen isoforms named XStau1 and XStau2, where XStau1 was found to be the principal Staufen protein in oocytes, eggs, and embryos, the levels of both proteins peaking during mid-oogenesis. In adults, Xenopus Staufens are principally expressed in ovary and testis. XStau1 was detectable throughout the oocyte cytoplasm by immunofluorescence and was concentrated in the vegetal cortical region from stage II onward. It showed partial codistribution with subcortical endoplasmic reticulum (ER), raising the possibility that Staufen may anchor mRNAs to specific ER-rich domains. We further showed that XStau proteins are transiently phosphorylated by the MAPK pathway during meiotic maturation, a period during which RNAs such as Vg1 RNA are released from their tight localization at the vegetal cortex. These findings provide evidence that Staufen proteins are involved in targeting and/or anchoring of maternal determinants to the vegetal cortex of the oocyte in Xenopus. The Xenopus oocyte should thus provide a valuable system to dissect the role of Staufen proteins in RNA localization and vertebrate development.

Localization of RNAs to the mitochondrial cloud in Xenopus oocytes through entrapment and association with endoplasmic reticulum

The germ cell lineage in Xenopus is specified by the inheritance of germ plasm, which originates within a distinct "mitochondrial cloud" (MC) in previtellogenic oocytes. Germ plasm contains localized RNAs implicated in germ cell development, including Xcat2 and Xdazl. To understand the mechanism of the early pathway through which RNAs localize to the MC, we applied live confocal imaging and photobleaching analysis to oocytes microinjected with fluorescent Xcat2 and Xdazl RNA constructs. These RNAs dispersed evenly throughout the cytoplasm through diffusion and then became progressively immobilized and formed aggregates in the MC. Entrapment in the MC was not prevented by microtubule disruption and did not require localization to germinal granules. Immobilized RNA constructs codistributed and showed coordinated movement with densely packed endoplasmic reticulum (ER) concentrated in the MC, as revealed with Dil16(3) labeling and immunofluorescence analysis. Vg1RBP/Vera protein, which has been implicated in linking late pathway RNAs to vegetal ER, was shown to bind specifically both wild-type Xcat2 3' untranslated region and localization-defective constructs. We found endogenous Vg1RBP/Vera and Vg1RBP/Vera-green fluorescent protein to be largely excluded from the MC but subsequently to codistribute with Xcat2 and ER at the vegetal cortex. We conclude that germ line RNAs localize into the MC through a diffusion/entrapment mechanism involving Vg1RBP/Vera-independent association with ER.

Hermes is a localized factor regulating cleavage of vegetal blastomeres in Xenopus laevis

We have identified the RNA-binding protein Hermes in a screen for vegetally localized RNAs in Xenopus oocytes. The RNA localizes to the vegetal cortex through both the message transport organizer (METRO) and late pathways. Hermes mRNA and protein are both detected at the vegetal cortex of the oocyte; however, the protein is degraded within a several hour period during oocyte maturation. Injection of antisense morpholino oligonucleotides (HE-MO) against Hermes caused a precocious reduction in Hermes protein present during maturation and resulted in a phenotype characterized by cleavage defects in vegetal blastomeres. The phenotype can be partially rescued by injecting Hermes mRNA. These results demonstrate that the localized RNA-binding protein Hermes functions during oocyte maturation to regulate the cleavage of specific vegetally derived cell lineages. Hermes most likely performs its function by regulating the translation or processing of one or more target RNAs. This is an important mechanism by which the embryo can generate unique cell lineages. The regulation of region-specific cell division is a novel function for a localized mRNA.