Xenopus poly (A) binding protein maternal RNA is localized during oogenesis and associated with large complexes in blastula

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.

Temporally coordinated signals progressively pattern the anteroposterior and dorsoventral body axes

The vertebrate body plan is established through the precise spatiotemporal coordination of morphogen signaling pathways that pattern the anteroposterior (AP) and dorsoventral (DV) axes. Patterning along the AP axis is directed by posteriorizing signals Wnt, fibroblast growth factor (FGF), Nodal, and retinoic acid (RA), while patterning along the DV axis is directed by bone morphogenetic proteins (BMP) ventralizing signals. This review addresses the current understanding of how Wnt, FGF, RA and BMP pattern distinct AP and DV cell fates during early development and how their signaling mechanisms are coordinated to concomitantly pattern AP and DV tissues.

Novel animal pole-enriched maternal mRNAs are preferentially expressed in neural ectoderm

BACKGROUND

Many animals utilize maternal mRNAs to pre-pattern the embryo before the onset of zygotic transcription. In Xenopus laevis, vegetal factors specify the germ line, endoderm, and dorsal axis, but there are few studies demonstrating roles for animal-enriched maternal mRNAs. Therefore, we carried out a microarray analysis to identify novel maternal transcripts enriched in 8-cell-stage animal blastomeres.

RESULTS

We identified 39 mRNAs isolated from 8-cell animal blastomeres that are >4-fold enriched compared to vegetal pole mRNAs. We characterized 14 of these that are of unknown function. We validated the microarray results for 8/14 genes by qRT-PCR and for 14/14 genes by in situ hybridization assays. Because no developmental functions are reported yet, we provide the expression patterns for each of the 14 genes. Each is expressed in the animal hemisphere of unfertilized eggs, 8-cell animal blastomeres, and diffusely in blastula animal cap ectoderm, gastrula ectoderm and neural ectoderm, neural crest (and derivatives) and cranial placodes (and derivatives). They have varying later expression in some mesodermal and endodermal tissues in tail bud through larval stages.

CONCLUSIONS

Novel animal-enriched maternal mRNAs are preferentially expressed in ectodermal derivatives, particularly neural ectoderm. However, they are later expressed in derivatives of other germ layers.

Calpain2 protease: A new member of the Wnt/Ca(2+) pathway modulating convergent extension movements in Xenopus

Calpains are a family of calcium-dependent intracellular cysteine proteases that regulate several physiological processes by limited cleavage of different substrates. The role of Calpain2 in embryogenesis is not clear with conflicting evidence from a number of mouse knockouts. Here we report the temporal and spatial expression of Calpain2 in Xenopus laevis embryos and address its role in Xenopus development. We show that Calpain2 is expressed maternally with elevated expression in neural tissues and that Calpain2 activity is spatially and temporally regulated. Using a Calpain inhibitor, a dominant negative and a morpholino oligonoucleotide we demonstrate that impaired Calpain2 activity results in defective convergent extension both in mesodermal and neural tissues. Specifically, Calpain2 downregulation results in loss of tissue polarity and blockage of mediolateral intercalation in Keller explants without affecting adherens junction turnover. We further show that Calpain2 is activated in response to Wnt5a and that the inhibitory effect of Wnt5a expression on animal cap elongation can be rescued by blocking Calpain2 function. This suggests that Calpain2 activity needs to be tightly regulated during convergent extension. Finally we show that expression of Xdd1 blocks the membrane translocation of Calpain2 suggesting that Calpain2 activation is downstream of Dishevelled. Overall our data show that Calpain2 activation through the Wnt/Ca(2+) pathway and Dishevelled can modulate convergent extension movements.

Asymmetric Localization of CK2α During Xenopus Oogenesis

The establishment of the dorso-ventral axis is a fundamental process that occurs after fertilization. Dorsal axis specification in frogs starts immediately after fertilization, and depends upon activation of Wnt/β-catenin signaling. The protein kinase CK2α can modulate Wnt/β-catenin signaling and is necessary for dorsal axis specification in Xenopus laevis. Our previous experiments show that CK2α transcripts and protein are animally localized in embryos, overlapping the region where Wnt/β-catenin signaling is activated. Here we determined whether the animal localization of CK2α in the embryo is preceded by its localization in the oocyte. We found that CK2α transcripts were detected from stage I, their levels increased during oogenesis, and were animally localized as early as stage III. CK2α transcripts were translated during oogenesis and CK2α protein was localized to the animal hemisphere of stage VI oocytes. We cloned the CK2α 3’UTR and showed that the 2.8 kb CK2α transcript containing the 3’UTR was enriched during oogenesis. By injecting ectopic mRNAs, we demonstrated that both the coding and 3’UTR regions were necessary for proper CK2α transcript localization. This is the first report showing the involvement of coding and 3’UTR regions in animal transcript localization. Our findings demonstrate the pre-localization of CK2α transcript and thus, CK2α protein, in the oocyte. This may help restrict CK2α expression in preparation for dorsal axis specification.

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 shroom family proteins play broad roles in the morphogenesis of thickened epithelial sheets

Thickened epithelial sheets are found in a wide variety of organ systems and the mechanisms governing their morphogenesis remain poorly defined. We show here, through expression patterns and functional studies, that Shroom family proteins are broadly involved in generating thickened epithelial sheets. Through in situ hybridization, we report the temporal and spatial expression patterns of the four Shroom family members during early Xenopus development, from oocytes to tadpole stage embryos. Further, we show that Shroom1 and 2 mRNAs are maternally expressed, while Shroom3 and Shroom4 are zygotic transcripts. In addition, maternal Shroom1 and 2 mRNAs localize in the animal hemisphere of the Xenopus egg and early blastula. During later development, all four Shroom family proteins are broadly expressed in developing epithelial organs, and the epithelial cells that express Shrooms are elongated. Moreover, we show that ectopic expression of Shroom2, like Shroom3, is able to increase cell height and that loss of Shroom2 function results in a failure of cell elongation in the neural epithelium. Together, these data suggest that Shroom family proteins play an important role in the morphogenesis of several different epithelial tissues during development. Developmental Dynamics 238:1480-1491, 2009. (c) 2009 Wiley-Liss, Inc.

Cytoskeleton-associated large RNP complexes in tobacco male gametophyte (EPPs) are associated with ribosomes and are involved in protein synthesis, processing, and localization

The progamic phase of male gametophyte development involves activation of synthetic and catabolic processes required for the rapid growth of the pollen tube. It is well-established that both transcription and translation play an important role in global and specific gene expression patterns during pollen maturation. On the contrary, germination of many pollen species has been shown to be largely independent of transcription but vitally dependent on translation of stored mRNAs. Here, we report the first structural and proteomic data about large ribonucleoprotein particles (EPPs) in tobacco male gametophyte. These complexes are formed in immature pollen where they contain translationally silent mRNAs. Although massively activated at the early progamic phase, they also serve as a long-term storage of mRNA transported along with the translational machinery to the tip region. Moreover, EPPs were shown to contain ribosomal subunits, rRNAs and a set of mRNAs. Presented results extend our view of EPP complexes from mere RNA storage and transport compartment in particular stages of pollen development to the complex and well-organized machinery devoted to mRNA storage, transport and subsequent controlled activation resulting in protein synthesis, processing and precise localization. Such an organization is extremely useful in fast tip-growing pollen tube. There, massive and orchestrated protein synthesis, processing, and transport must take place in accurately localized regions. Moreover, presented complex role of EPPs in tobacco cytoplasmic mRNA and protein metabolism makes them likely to be active in another plant species too. Expression of vast majority of the closest orthologues of EPP proteins also in Arabidopsis male gametophyte further extends this concept from tobacco to Arabidopsis, the model species with advanced tricellular pollen.

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.

Xvelo1 uses a novel 75-nucleotide signal sequence that drives vegetal localization along the late pathway in Xenopus oocytes

Vegetally localized RNAs in Xenopus laevis oocytes are involved in the patterning of the early embryo as well as in cell fate specification. Here we report on the isolation and characterization of a novel, vegetally localized RNA in Xenopus oocytes termed Xvelo1. It encodes a protein of unknown biological function and it represents an antisense RNA for XPc1 over a length of more than 1.8 kb. Xvelo1 exhibits a localization pattern reminiscent of the late pathway RNAs Vg1 and VegT; it contains RNA localization elements (LE) which do not match with the consensus structural features as deduced from Vg1 and VegT LEs. Nevertheless, the protein binding pattern as observed for Xvelo1-LE in UV cross-linking experiments and coimmunoprecipitation assays is largely overlapping with the one obtained for Vg1-LE. These observations suggest that the structural features recognized by the protein machinery that drives localization of maternal mRNAs along the late pathway in Xenopus oocytes must be redefined.

A homolog of FBP2/KSRP binds to localized mRNAs in Xenopus oocytes

A Xenopus oocyte expression library was screened for proteins that bind to the 340-nucleotide localization element of Vg1 mRNA. Four different isolates encoded a Xenopus homolog of the human transcription factor, FUSE-binding protein 2 (FBP2). This protein has been independently identified as the splicing regulatory factor KSRP. The only significant difference between the Xenopus protein, designated VgRBP71, and KSRP is the absence of a 58 amino acid segment near the N-terminal of the former. In vivo binding assays show that VgRBP71 is associated with mRNAs localized to either the vegetal or animal hemispheres, but was not found with control mRNAs. Unlike other factors that bind to the localization element of Vg1 mRNA, VgRBP71 does not accumulate at the vegetal cortex with the mRNA; rather, it is present in the nucleus and throughout the cytoplasm at all stages of oogenesis. Cytoplasmic VgRBP71 appears to be most concentrated at the cell cortex. VgRBP71 interacts with Prrp, another protein that binds to the Vg1 localization element; this association does not require the presence of Vg1 mRNA.

Biochemical characterization of a cellular structure retaining vegetally localized RNAs in Xenopus late stage oocytes

Two pathways operate during Xenopus oogenesis to localize a small number of RNAs to the vegetal cortex. Correct localization of these RNAs is essential to normal development as the proteins they encode are involved in specifying cell type and in patterning the early embryo. Binding these RNAs to the vegetal cortex and thus preserving their localized condition is a critical step, although little is known about how this is achieved. In this study, we have used a biochemical approach to examine the anchoring step. Xlsirts, an abundant localized RNA (locRNA), was selectively enriched in a detergent-insoluble fraction (DIF) prepared from oocytes that had completed the RNA localization process. These putative RNA-anchoring complexes were analyzed by density gradient centrifugation and in RNA-protein binding assays. Cortical Xlsirts and other localized RNAs are specifically found in the heavy region of sucrose gradients and in the pellet, quite different from other cellular RNPs. Four proteins were identified by UV-crosslinking that bound the Xlsirts localization signal in the cortex, but not in the soluble fraction. These are likely members of the anchoring complex and appear to include vera, a characterized Vg1 RNA binding protein. Vera was found to co-sediment with other locRNAs found in the vegetal cortex, suggesting that it is a common component of locRNPs. Finally, we found that locRNPs extracted into the soluble fraction had the same buoyant density as typical ooplasmic RNPs. We propose that locRNAs are organized and anchored in the cortex as typical RNPs.

A proline-rich protein binds to the localization element of Xenopus Vg1 mRNA and to ligands involved in actin polymerization

A 340 nucleotide element within the 3' untranslated region of Vg1 mRNA determines its localization to the vegetal cortex of Xenopus oocytes. To identify protein factors that bind to this region, we screened a cDNA expression library with an RNA probe containing this sequence. Five independent isolates encoded a protein (designated Prrp for proline-rich RNA binding protein) having two RNP domains followed by multiple polyproline segments. Prrp and Vg1 mRNAs are co-localized to the vegetal cortex of stage IV oocytes, substantiating an interaction between the two in vivo. Prrp also associates with VegT mRNA, which like Vg1 mRNA uses the late localization pathway, but not with Xcat-2 or Xwnt-11 mRNAs, which use the early pathway. The proline-rich domain of Prrp interacts with profilin, a protein that promotes actin polymerization. Prrp can also associate with the EVH1 domain of Mena, another microfilament-associated protein. Since the anchoring of Vg1 mRNA to the vegetal cortex is actin dependent, one function of Prrp may be to facilitate local actin polymerization, representing a novel function for an RNA binding protein.

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.

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.

Spatially regulated translation in embryos: asymmetric expression of maternal Wnt-11 along the dorsal-ventral axis in Xenopus

Transition from symmetry to asymmetry is a central theme in cell and developmental biology. In Xenopus embryos, dorsal-ventral asymmetry is initiated by a microtubule-dependent cytoplasmic rotation during the first cell cycle after fertilization. Here we show that the cytoplasmic rotation initiates differential cytoplasmic polyadenylation of maternal Xwnt-11 RNA, encoding a member of the Wnt family of cell-cell signaling factors. Translational regulation of Xwnt-11 mRNA along the dorsal-ventral axis results in asymmetric accumulation of Xwnt-11 protein. These results demonstrate spatially regulated translation of a maternal cell-signaling factor along the vertebrate dorsal-ventral axis and represent a novel mechanism for Wnt gene regulation. Spatial regulation of maternal RNA translation, which has been established in invertebrates, appears to be an evolutionarily conserved mechanism in the generation of intracellular asymmetry and the consequential formation of the multicellular body pattern.

Polarizing genetic information in the egg: RNA localization in the frog oocyte

RNA localization is a powerful strategy used by cells to localize proteins to subcellular domains and to control protein synthesis regionally. In germ cells, RNA targeting has profound implications for development, setting up polarities in genetic information that drive cell fate during embryogenesis. The frog oocyte offers a useful system for studying the mechanism of RNA localization. Here, we discuss critically the process of RNA localization during frog oogenesis. Three major pathways have been identified that are temporally and spatially separated in oogenesis. Each pathway uses a different mechanism to effect RNA localization. In some cases, localization elements within the 3' untranslated region have been identified and have provided unique insights into the localization process. This important field is still in its infancy, however, and much remains to be learned.

Xcat2 RNA is a translationally sequestered germ plasm component in Xenopus

In Xenopus, the inheritance of germ plasm by a small subset of blastomeres during early development is thought to direct these cells into the germ cell lineage. We show that Xcat2 RNA, related to Drosophila nanos, is a germ plasm component that is translationally repressed during oogenesis. Xcat2 protein was not detected in oocytes at times prior to, or after its RNA was localized in germ plasm, suggesting Xcat2 RNA is functionally sequestered soon after transcription. Indeed, Xcat2 RNA is found in a dense non-polysomal compartment in oocytes. Repression of translation was not relieved by substituting the Xcat2 3'UTR with that of beta-globin. Immunodetection of Xcat2 protein during blastula and gastrula stages coincides with the time of symmetric segregation of the germ plasm and a net increase in the number of primordial germ cells. Xcat2 is capable of binding RNA in vitro and we propose that it may function to translationally regulate other RNAs specific to primordial germ cells.

RNA sorting in Xenopus oocytes and embryos

Cytoplasmic localization of mRNA molecules has emerged as a powerful mechanism for generating spatially restricted gene expression. This process is an important contributor to cell polarity in both somatic cells and oocytes, and can provide the basis for patterning during embryonic development. In vertebrates, this phenomenon is perhaps best documented in the frog, Xenopus laevis, where polarity along the animal-vegetal axis coincides with the localization of numerous mRNA molecules. Research over the last several years has made exciting progress toward understanding the molecular mechanisms underlying cytoplasmic mRNA localization.