Fertilization of cultured Xenopus oocytes and use in studies of maternally inherited molecules

A fractionation-based protocol to investigate RNA solubility phase transition during Xenopus oocyte maturation

While RNAs are soluble in vitro, their solubility may be altered when incorporated into some protein complexes inside the cell. The solubility phase transition of RNAs is thus indicative of changes in the function and activity of RNAs. Here, we present a protocol for the assessment of RNA solubility phase transition during Xenopus oocyte maturation. We describe steps for sample preparation, cell fractionation, RNA extraction, real-time PCR, and analysis of the obtained results. For complete details on the use and execution of this protocol, please refer to Hwang et al. (2023).1.

Gene Therapy for Genetic Syndromes: Understanding the Current State to Guide Future Care

Gene therapy holds promise as a life-changing option for individuals with genetic variants that give rise to disease. FDA-approved gene therapies for Spinal Muscular Atrophy (SMA), cerebral adrenoleukodystrophy, β-Thalassemia, hemophilia A/B, retinal dystrophy, and Duchenne Muscular Dystrophy have generated buzz around the ability to change the course of genetic syndromes. However, this excitement risks over-expansion into areas of genetic disease that may not fit the current state of gene therapy. While in situ (targeted to an area) and ex vivo (removal of cells, delivery, and administration of cells) approaches show promise, they have a limited target ability. Broader in vivo gene therapy trials have shown various continued challenges, including immune response, use of immune suppressants correlating to secondary infections, unknown outcomes of overexpression, and challenges in driving tissue-specific corrections. Viral delivery systems can be associated with adverse outcomes such as hepatotoxicity and lethality if uncontrolled. In some cases, these risks are far outweighed by the potentially lethal syndromes for which these systems are being developed. Therefore, it is critical to evaluate the field of genetic diseases to perform cost-benefit analyses for gene therapy. In this work, we present the current state while setting forth tools and resources to guide informed directions to avoid foreseeable issues in gene therapy that could prevent the field from continued success.

Solubility phase transition of maternal RNAs during vertebrate oocyte-to-embryo transition

The oocyte-to-embryo transition (OET) is regulated by maternal products stored in the oocyte cytoplasm, independent of transcription. How maternal products are precisely remodeled to dictate the OET remains largely unclear. In this work, we discover the dynamic solubility phase transition of maternal RNAs during Xenopus OET. We have identified 863 maternal transcripts that transition from a soluble state to a detergent-insoluble one after oocyte maturation. These RNAs are enriched in the animal hemisphere, and many of them encode key cell cycle regulators. In contrast, 165 transcripts, including nearly all Xenopus germline RNAs and some vegetally localized somatic RNAs, undergo an insoluble-to-soluble phase transition. This phenomenon is conserved in zebrafish. Our results demonstrate that the phase transition of germline RNAs influences their susceptibility to RNA degradation machinery and is mediated by the remodeling of germ plasm. This work thus identifies important remodeling mechanisms that act on RNAs to control vertebrate OET.

TMEM16A activation for the fast block to polyspermy in the African clawed frog does not require conventional activation of egg PLCs

Fertilization of an egg by more than one sperm, a condition known as polyspermy, leads to gross chromosomal abnormalities and is embryonic lethal for most animals. Consequently, eggs have evolved multiple processes to stop supernumerary sperm from entering the nascent zygote. For external fertilizers, such as frogs and sea urchins, fertilization signals a depolarization of the egg membrane, which serves as the fast block to polyspermy. Sperm can bind to, but will not enter, depolarized eggs. In eggs from the African clawed frog, Xenopus laevis, the fast block depolarization is mediated by the Ca2+-activated Cl- channel TMEM16A. To do so, fertilization activates phospholipase C, which generates IP3 to signal a Ca2+ release from the ER. Currently, the signaling pathway by which fertilization activates PLC during the fast block remains unknown. Here, we sought to uncover this pathway by targeting the canonical activation of the PLC isoforms present in the X. laevis egg: PLCγ and PLCβ. We observed no changes to the fast block in X. laevis eggs inseminated in inhibitors of tyrosine phosphorylation, used to stop activation of PLCγ, or inhibitors of Gαq/11 pathways, used to stop activation of PLCβ. These data suggest that the PLC that signals the fast block depolarization in X. laevis is activated by a novel mechanism.

A review of the reproductive system in anuran amphibians

Reproductive biology is an important topic that is well explored in many vertebrates, but information about frogs' reproductive mechanisms could be improved. Therefore, this review aims to provide organized and specific information on frog reproduction. First, we developed schemes that illustrate the general information regarding reproductive biological mechanisms in frogs in a specific way. Then, we described the physiological, histological, and morphological mechanisms of each organ of the reproductive system of male and female frogs. Finally, this manuscript may contribute to a broader understanding of anuran reproductive biology. Since, understanding frogs' reproductive system permits one to make a comparison with reproduction with other anurans.

A CRISPR/Cas-Based Method for Precise DNA Integration in Xenopus laevis Oocytes Followed by Intracytoplasmic Sperm Injection (ICSI) Fertilization

Xenopus has long had a reputation for being a powerful model organism for use in developmental cell and biochemistry research. With the advent of gene-editing technologies, and the full genome sequencing of Xenopus genomes revealing the extent of the genetic conservation between Xenopus and humans, Xenopus has the potential to become an ideal model for human genetic disease. However, the inability to produce non-mosaic, precise DNA insertions through homology directed repair has limited the strength of Xenopus this field. Furthermore, it has prevented researchers from taking full advantage of fusion tagging, a method for directly tagging genes with either epitope or fluorescent tags, allowing the visualization, quantification, and tracking of proteins without the use of protein-specific antibodies. Here, we describe a method for precise DNA insertion into oocytes using CRISPR/Cas9, followed by in vitro maturation and fertilization by intracytoplasmic sperm injection (ICSI), culminating in the production of embryos carrying a non-mosaic, heterozygous insertion.

Regulation of RNA localization during oocyte maturation by dynamic RNA-ER association and remodeling of the ER

Asymmetric localization of mRNAs is crucial for cell polarity and cell fate determination. By performing fractionation RNA-seq, we report here that a large number of maternal RNAs are associated with the ER in Xenopus oocytes but are released into the cytosol after oocyte maturation. We provide evidence that the majority of ER-associated RNA-binding proteins (RBPs) remain associated with the ER after oocyte maturation. However, all ER-associated RBPs analyzed exhibit reduced binding to some of their target RNAs after oocyte maturation. Our results further show that the ER is remodeled massively during oocyte maturation, leading to the formation of a widespread tubular ER network in the animal hemisphere that is required for the asymmetric localization of mRNAs in mature eggs. Thus, our findings demonstrate that dynamic regulation of RNA-ER association and remodeling of the ER are important for the asymmetric localization of RNAs during development.

Maternal Wnt11b regulates cortical rotation during Xenopus axis formation: analysis of maternal-effect wnt11b mutants

Asymmetric signalling centres in the early embryo are essential for axis formation in vertebrates. These regions (e.g. amphibian dorsal morula, mammalian anterior visceral endoderm) require stabilised nuclear β-catenin, but the role of localised Wnt ligand signalling activity in their establishment remains unclear. In Xenopus, dorsal β-catenin is initiated by vegetal microtubule-mediated symmetry breaking in the fertilised egg, known as 'cortical rotation'. Localised wnt11b mRNA and ligand-independent activators of β-catenin have been implicated in dorsal β-catenin activation, but the extent to which each contributes to axis formation in this paradigm remains unclear. Here, we describe a CRISPR-mediated maternal-effect mutation in Xenopus laevis wnt11b.L. We find that wnt11b is maternally required for robust dorsal axis formation and for timely gastrulation, and zygotically for left-right asymmetry. Importantly, we show that vegetal microtubule assembly and cortical rotation are reduced in wnt11b mutant eggs. In addition, we show that activated Wnt coreceptor Lrp6 and Dishevelled lack behaviour consistent with roles in early β-catenin stabilisation, and that neither is regulated by Wnt11b. This work thus implicates Wnt11b in the distribution of putative dorsal determinants rather than in comprising the determinants themselves. This article has an associated 'The people behind the papers' interview.

Optimized design of antisense oligomers for targeted rRNA depletion

RNA sequencing (RNA-seq) is extensively used to quantify gene expression transcriptome-wide. Although often paired with polyadenylate (poly(A)) selection to enrich for messenger RNA (mRNA), many applications require alternate approaches to counteract the high proportion of ribosomal RNA (rRNA) in total RNA. Recently, digestion using RNaseH and antisense DNA oligomers tiling target rRNAs has emerged as an alternative to commercial rRNA depletion kits. Here, we present a streamlined, more economical RNaseH-mediated rRNA depletion with substantially lower up-front costs, using shorter antisense oligos only sparsely tiled along the target RNA in a 5-min digestion reaction. We introduce a novel Web tool, Oligo-ASST, that simplifies oligo design to target regions with optimal thermodynamic properties, and additionally can generate compact, common oligo pools that simultaneously target divergent RNAs, e.g. across different species. We demonstrate the efficacy of these strategies by generating rRNA-depletion oligos for Xenopus laevis and for zebrafish, which expresses two distinct versions of rRNAs during embryogenesis. The resulting RNA-seq libraries reduce rRNA to <5% of aligned reads, on par with poly(A) selection, and also reveal expression of many non-adenylated RNA species. Oligo-ASST is freely available at https://mtleelab.pitt.edu/oligo to design antisense oligos for any taxon or to target any abundant RNA for depletion.

Dissection of the Ovulatory Process Using ex vivo Approaches

Ovulation is a unique physiological phenomenon that is essential for sexual reproduction. It refers to the entire process of ovarian follicle responses to hormonal stimulation resulting in the release of mature fertilization-competent oocytes from the follicles and ovaries. Remarkably, ovulation in different species can be reproduced out-of-body with high fidelity. Moreover, most of the molecular mechanisms and signaling pathways engaged in this process have been delineated using in vitro ovulation models. Here, we provide an overview of the major molecular and cytological events of ovulation observed in frogs, primarily in the African clawed frog Xenopus laevis, using mainly ex vivo approaches, with the focus on meiotic oocyte maturation and follicle rupture. For the purpose of comparison and generalization, we also refer extensively to ovulation in other biological species, most notoriously, in mammals.

Zinc protection of fertilized eggs is an ancient feature of sexual reproduction in animals

One of the earliest and most prevalent barriers to successful reproduction is polyspermy, or fertilization of an egg by multiple sperm. To prevent these supernumerary fertilizations, eggs have evolved multiple mechanisms. It has recently been proposed that zinc released by mammalian eggs at fertilization may block additional sperm from entering. Here, we demonstrate that eggs from amphibia and teleost fish also release zinc. Using Xenopus laevis as a model, we document that zinc reversibly blocks fertilization. Finally, we demonstrate that extracellular zinc similarly disrupts early embryonic development in eggs from diverse phyla, including Cnidaria, Echinodermata, and Chordata. Our study reveals that a fundamental strategy protecting human eggs from fertilization by multiple sperm may have evolved more than 650 million years ago.

X-linked α-thalassemia with mental retardation is downstream of protein kinase A in the meiotic cell cycle signaling cascade in Xenopus oocytes and is dynamically regulated in response to DNA damage†

X-linked α-thalassemia with mental retardation (ATRX) is a chromatin remodeling protein that belongs to the SWItch/sucrose non-fermentable (SWI2/SNF2) family of helicase/ATPases. During meiosis, ATRX is necessary for heterochromatin formation and maintenance of chromosome stability in order to ensure proper assembly of the metaphase II spindle. Previously, we established ATRX as a novel progesterone regulated protein during bovine meiotic maturation, in addition to being dynamically regulated in response to DNA damage in oocytes. In the present study, we utilize the Xenopus laevis model system to further elucidate the signaling pathways regulating ATRX expression within the oocyte. Here, we present an analysis of endogenous ATRX protein expression during oogenesis, oocyte meiotic maturation, and early embryonic development. ATRX expression is dynamically regulated as evidenced by loss of the protein in metaphase II of meiosis. The downstream activation of meiosis via protein kinase A inhibition resulted in a similar decrease in ATRX protein expression. We demonstrate that the ATRX protein is detected in ubiquitin immuno-precipitates from germinal vesicle oocyte extracts and experimentally demonstrate that proteosomal degradation is responsible for the decreased expression of ATRX during meiosis. ATRX expression is significantly increased in response to gamma-irradiation induced DNA damage in oocytes and embryos. This increased expression is independent of p53 protein expression in apoptotic embryos, as determined by the expression of active caspase-3. Thus, regulation of ATRX protein expression impacts on G2-M progression and ultimately has consequences for cell survival.

A single KH domain in Bicaudal-C links mRNA binding and translational repression functions to maternal development

Bicaudal-C (Bicc1) is a conserved RNA-binding protein that represses the translation of selected mRNAs to control development. In Xenopus embryos, Bicc1 binds and represses specific maternal mRNAs to control anterior-posterior cell fates. However, it is not known how Bicc1 binds its RNA targets or how binding affects Bicc1-dependent embryogenesis. Focusing on the KH domains, we analyzed Bicc1 mutants for their ability to bind RNA substrates in vivo and in vitro Analyses of these Bicc1 mutants demonstrated that a single KH domain, KH2, was crucial for RNA binding in vivo and in vitro, while the KH1 and KH3 domains contributed minimally. The Bicc1 mutants were also assayed for their ability to repress translation, and results mirrored the RNA-binding data, with KH2 being the only domain essential for repression. Finally, maternal knockdown and rescue experiments indicated that the KH domains were essential for the regulation of embryogenesis by Bicc1. These data advance our understanding of how Bicc1 selects target mRNAs and provide the first direct evidence that the RNA binding functions of Bicc1 are essential for both Bicc1-dependent translational repression and maternal vertebrate development.

Oocyte Host-Transfer and Maternal mRNA Depletion Experiments in Xenopus

This protocol details the oocyte host-transfer method in Xenopus, using transplantation by intraperitoneal injection. This approach is suitable for the overexpression of mRNAs and for the use of antisense oligonucleotides to deplete maternal mRNAs, which are not replaced until zygotic genome activation in the mid-blastula transition. Xenopus oocyte host-transfer can also be used for highly efficient mutagenesis in the F₀ generation by prefertilization injection of genome editing reagents.

The TMEM16A channel mediates the fast polyspermy block in Xenopus laevis

In their preceding paper, Wozniak et al. show that fertilization increases intracellular Ca²⁺ in Xenopus laevis eggs by activating an IP₃ signaling cascade. Here, they reveal that Ca²⁺ subsequently opens the Cl⁻ channel TMEM16A to allow Cl⁻ efflux, cell depolarization, and fast block to polyspermy.

In externally fertilizing animals, such as sea urchins and frogs, prolonged depolarization of the egg immediately after fertilization inhibits the entry of additional sperm—a phenomenon known as the fast block to polyspermy. In the African clawed frog Xenopus laevis, this depolarization is driven by Ca²⁺-activated Cl⁻ efflux. Although the prominent Ca²⁺-activated Cl⁻ currents generated in immature X. laevis oocytes are mediated by X. laevis transmembrane protein 16a (xTMEM16A) channels, little is known about the channels that contribute to the fast block in mature eggs. Moreover, the gamete undergoes a gross transformation as it develops from an immature oocyte into a fertilization-competent egg. Here, we report the results of our approach to identify the Ca²⁺-activated Cl⁻ channel that triggers the fast block. By querying published proteomic and RNA-sequencing data, we identify two Ca²⁺-activated Cl⁻ channels expressed in fertilization-competent X. laevis eggs: xTMEM16A and X. laevis bestrophin 2A (xBEST2A). By exogenously expressing xTMEM16A and xBEST2A in axolotl cells lacking endogenous Ca²⁺-activated currents, we characterize the effect of inhibitors on currents mediated by these channels. None of the inhibitors tested block xBEST2A currents specifically. However, 2-(4-chloro-2-methylphenoxy)-N-[(2-methoxyphenyl)methylideneamino]-acetamide (Ani9) and N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid (MONNA) each reduce xTMEM16A currents by more than 70% while only nominally inhibiting those generated by xBEST2A. Using whole-cell recordings during fertilization, we find that Ani9 and MONNA effectively diminish fertilization-evoked depolarizations. Additionally, these inhibitors lead to increased polyspermy in X. laevis embryos. These results indicate that fertilization activates TMEM16A channels in X. laevis eggs and induces the earliest known event triggered by fertilization: the fast block to polyspermy.

PLC and IP3-evoked Ca2+ release initiate the fast block to polyspermy in Xenopus laevis eggs

The fast block to polyspermy is achieved in Xenopus laevis eggs by fertilization-induced depolarization. Wozniak et al. show that fertilization activates a signaling cascade involving phospholipase C, IP₃, and intracellular Ca²⁺ release, which induces depolarization via Ca²⁺-activated Cl⁻ efflux.

The prevention of polyspermy is essential for the successful progression of normal embryonic development in most sexually reproducing species. In external fertilizers, the process of fertilization induces a depolarization of the egg’s membrane within seconds, which inhibits supernumerary sperm from entering an already-fertilized egg. This fast block requires an increase of intracellular Ca²⁺ in the African clawed frog, Xenopus laevis, which in turn activates an efflux of Cl⁻ that depolarizes the cell. Here we seek to identify the source of this intracellular Ca²⁺. Using electrophysiology, pharmacology, bioinformatics, and developmental biology, we explore the requirement for both Ca²⁺ entry into the egg from the extracellular milieu and Ca²⁺ release from an internal store, to mediate fertilization-induced depolarization. We report that although eggs express Ca²⁺-permeant ion channels, blockade of these channels does not alter the fast block. In contrast, insemination of eggs in the presence of Xestospongin C—a potent inhibitor of inositol 1,4,5-trisphosphate (IP₃)-induced Ca²⁺ release from the endoplasmic reticulum (ER)—completely inhibits fertilization-evoked depolarization and increases the incidence of polyspermy. Inhibition of the IP₃-generating enzyme phospholipase C (PLC) with U73122 similarly prevents fertilization-induced depolarization and increases polyspermy. Together, these results demonstrate that fast polyspermy block after fertilization in X. laevis eggs is mediated by activation of PLC, which increases IP₃ and evokes Ca²⁺ release from the ER. This ER-derived Ca²⁺ then activates a Cl⁻ channel to induce the fast polyspermy block. The PLC-induced cascade of events represents one of the earliest known signaling pathways initiated by fertilization.

Primordial Germ Cell Transplantation for CRISPR/Cas9-based Leapfrogging in Xenopus

The creation of mutant lines by genome editing is accelerating genetic analysis in many organisms. CRISPR/Cas9 methods have been adapted for use in the African clawed frog, Xenopus, a longstanding model organism for biomedical research. Traditional breeding schemes for creating homozygous mutant lines with CRISPR/Cas9-targeted mutagenesis have several time-consuming and laborious steps. To facilitate the creation of mutant embryos, particularly to overcome the obstacles associated with knocking out genes that are essential for embryogenesis, a new method called leapfrogging was developed. This technique leverages the robustness of Xenopus embryos to "cut and paste" embryological methods. Leapfrogging utilizes the transfer of primordial germ cells (PGCs) from efficiently-mutagenized donor embryos into PGC-ablated wildtype siblings. This method allows for the efficient mutation of essential genes by creating chimeric animals with wildtype somatic cells that carry a mutant germline. When two F0 animals carrying "leapfrog transplants" (i.e., mutant germ cells) are intercrossed, they produce homozygous, or compound heterozygous, null F1 embryos, thus saving a full generation time to obtain phenotypic data. Leapfrogging also provides a new approach for analyzing maternal effect genes, which are refractory to F0 phenotypic analysis following CRISPR/Cas9 mutagenesis. This manuscript details the method of leapfrogging, with special emphasis on how to successfully perform PGC transplantation.

Role of maternal Xenopus syntabulin in germ plasm aggregation and primordial germ cell specification

The localization and organization of mitochondria- and ribonucleoprotein granule-rich germ plasm is essential for many aspects of germ cell development. In Xenopus, germ plasm is maternally inherited and is required for the specification of primordial germ cells (PGCs). Germ plasm is aggregated into larger patches during egg activation and cleavage and is ultimately translocated perinuclearly during gastrulation. Although microtubule dynamics and a kinesin (Kif4a) have been implicated in Xenopus germ plasm localization, little is known about how germ plasm distribution is regulated. Here, we identify a role for maternal Xenopus Syntabulin in the aggregation of germ plasm following fertilization. We show that depletion of sybu mRNA using antisense oligonucleotides injected into oocytes results in defects in the aggregation and perinuclear transport of germ plasm and subsequently in reduced PGC numbers. Using live imaging analysis, we also characterize a novel role for Sybu in the collection of germ plasm in vegetal cleavage furrows by surface contraction waves. Additionally, we show that a localized kinesin-like protein, Kif3b, is also required for germ plasm aggregation and that Sybu functionally interacts with Kif3b and Kif4a in germ plasm aggregation. Overall, these data suggest multiple coordinate roles for kinesins and adaptor proteins in controlling the localization and distribution of a cytoplasmic determinant in early development.

Controlling the Messenger: Regulated Translation of Maternal mRNAs in Xenopus laevis Development

The selective translation of maternal mRNAs encoding cell-fate determinants drives the earliest decisions of embryogenesis that establish the vertebrate body plan. This chapter will discuss studies in Xenopus laevis that provide insights into mechanisms underlying this translational control. Xenopus has been a powerful model organism for many discoveries relevant to the translational control of maternal mRNAs because of the large size of its oocytes and eggs that allow for microinjection of molecules and the relative ease of manipulating the oocyte to egg transition (maturation) and fertilization in culture. Consequently, many key studies have focused on the expression of maternal mRNAs during the oocyte to egg transition (the meiotic cell cycle) and the rapid cell divisions immediately following fertilization. This research has made seminal contributions to our understanding of translational regulatory mechanisms, but while some of the mRNAs under consideration at these stages encode cell-fate determinants, many encode cell cycle regulatory proteins that drive these early cell cycles. In contrast, while maternal mRNAs encoding key developmental (i.e., cell-fate) regulators that function after the first cleavage stages may exploit aspects of these foundational mechanisms, studies reveal that these mRNAs must also rely on distinct and, as of yet, incompletely understood mechanisms. These findings are logical because the functions of such developmental regulatory proteins have requirements distinct from cell cycle regulators, including becoming relevant only after fertilization and then only in specific cells of the embryo. Indeed, key maternal cell-fate determinants must be made available in exquisitely precise amounts (usually low), only at specific times and in specific cells during embryogenesis. To provide an appreciation for the regulation of maternal cell-fate determinant expression, an overview of the maternal phase of Xenopus embryogenesis will be presented. This section will be followed by a review of translational mechanisms operating in oocytes, eggs, and early cleavage-stage embryos and conclude with a discussion of how the regulation of key maternal cell-fate determinants at the level of translation functions in Xenopus embryogenesis. A key theme is that the molecular asymmetries critical for forming the body axes are established and further elaborated upon by the selective temporal and spatial regulation of maternal mRNA translation.

High-efficiency non-mosaic CRISPR-mediated knock-in and indel mutation in F0 Xenopus

The revolution in CRISPR-mediated genome editing has enabled the mutation and insertion of virtually any DNA sequence, particularly in cell culture where selection can be used to recover relatively rare homologous recombination events. The efficient use of this technology in animal models still presents a number of challenges, including the time to establish mutant lines, mosaic gene editing in founder animals, and low homologous recombination rates. Here we report a method for CRISPR-mediated genome editing in Xenopus oocytes with homology-directed repair (HDR) that provides efficient non-mosaic targeted insertion of small DNA fragments (40-50 nucleotides) in 4.4-25.7% of F0 tadpoles, with germline transmission. For both CRISPR/Cas9-mediated HDR gene editing and indel mutation, the gene-edited F0 embryos are uniformly heterozygous, consistent with a mutation in only the maternal genome. In addition to efficient tagging of proteins in vivo, this HDR methodology will allow researchers to create patient-specific mutations for human disease modeling in Xenopus.

Generation of a Xenopus laevis F1 albino J strain by genome editing and oocyte host-transfer

Completion of the Xenopus laevis genome sequence from inbred J strain animals has facilitated the generation of germline mutant X. laevis using targeted genome editing. In the last few years, numerous reports have demonstrated that TALENs are able to induce mutations in F0 Xenopus embryos, but none has demonstrated germline transmission of such mutations in X. laevis. In this report we used the oocyte host-transfer method to generate mutations in both tyrosinase homeologs and found highly-penetrant germline mutations; in contrast, embryonic injections yielded few germline mutations. We also compared the distribution of mutations in several F0 somatic tissues and germ cells and found that the majority of mutations in each tissue were different. These results establish that X. laevis J strain animals are very useful for generating germline mutations and that the oocyte host-transfer method is an efficient technique for generating mutations in both homeologs.

Extracellular Ca2+ Is Required for Fertilization in the African Clawed Frog, Xenopus laevis

Background

The necessity of extracellular Ca²⁺ for fertilization and early embryonic development in the African clawed frog, Xenopus laevis, is controversial. Ca²⁺ entry into X. laevis sperm is reportedly required for the acrosome reaction, yet fertilization and embryonic development have been documented to occur in high concentrations of the Ca²⁺ chelator BAPTA. Here we sought to resolve this controversy.

Methodology/principal finding

Using the appearance of cleavage furrows as an indicator of embryonic development, we found that X. laevis eggs inseminated in a solution lacking added divalent cations developed normally. By contrast, eggs inseminated in millimolar concentrations of BAPTA or EGTA failed to develop. Transferring embryos to varying solutions after sperm addition, we found that extracellular Ca²⁺ is specifically required for events occurring within the first 30 minutes after sperm addition, but not after. We found that the fluorescently stained sperm were not able to penetrate the envelope of eggs inseminated in high BAPTA, whereas several had penetrated the vitelline envelope of eggs inseminated without a Ca²⁺ chelator, or with BAPTA and saturating CaCl₂. Together these results indicate that fertilization does not occur in high concentrations of Ca²⁺ chelators. Finally, we found that the jelly coat includes >5 mM of readily diffusible Ca²⁺.

Conclusions/Significance

Taken together, these data are consistent with requirement of extracellular Ca²⁺ for fertilization. Based on our findings, we hypothesize that the jelly coat surrounding the egg acts as a reserve of readily available Ca²⁺ ions to foster fertilization in changing extracellular milieu.

Leapfrogging: primordial germ cell transplantation permits recovery of CRISPR/Cas9-induced mutations in essential genes

CRISPR/Cas9 genome editing is revolutionizing genetic loss-of-function analysis but technical limitations remain that slow progress when creating mutant lines. First, in conventional genetic breeding schemes, mosaic founder animals carrying mutant alleles are outcrossed to produce F1 heterozygotes. Phenotypic analysis occurs in the F2 generation following F1 intercrosses. Thus, mutant analyses will require multi-generational studies. Second, when targeting essential genes, efficient mutagenesis of founders is often lethal, preventing the acquisition of mature animals. Reducing mutagenesis levels may improve founder survival, but results in lower, more variable rates of germline transmission. Therefore, an efficient approach to study lethal mutations would be useful. To overcome these shortfalls, we introduce 'leapfrogging', a method combining efficient CRISPR mutagenesis with transplantation of mutated primordial germ cells into a wild-type host. Tested using Xenopus tropicalis, we show that founders containing transplants transmit mutant alleles with high efficiency. F1 offspring from intercrosses between F0 animals that carry embryonic lethal alleles recapitulate loss-of-function phenotypes, circumventing an entire generation of breeding. We anticipate that leapfrogging will be transferable to other species.

A gradient of maternal Bicaudal-C controls vertebrate embryogenesis via translational repression of mRNAs encoding cell fate regulators

Vertebrate Bicaudal-C (Bicc1) has important biological roles in the formation and homeostasis of multiple organs, but direct experiments to address the role of maternal Bicc1 in early vertebrate embryogenesis have not been reported. Here, we use antisense phosphorothioate-modified oligonucleotides and the host-transfer technique to eliminate specifically maternal stores of both bicc1 mRNA and Bicc1 protein from Xenopus laevis eggs. Fertilization of these Bicc1-depleted eggs produced embryos with an excess of dorsal-anterior structures and overexpressed organizer-specific genes, indicating that maternal Bicc1 is crucial for normal embryonic patterning of the vertebrate embryo. Bicc1 is an RNA-binding protein with robust translational repression function. Here, we show that the maternal mRNA encoding the cell-fate regulatory protein Wnt11b is a direct target of Bicc1-mediated repression. It is well established that the Wnt signaling pathway is crucial to vertebrate embryogenesis. Thus, the work presented here links the molecular function of Bicc1 in mRNA target-specific translation repression to its biological role in the maternally controlled stages of vertebrate embryogenesis.

Small C-terminal Domain Phosphatase 3 Dephosphorylates the Linker Sites of Receptor-regulated Smads (R-Smads) to Ensure Transforming Growth Factor β (TGFβ)-mediated Germ Layer Induction in Xenopus Embryos

Germ layer induction is one of the earliest events shortly after fertilization that initiates body formation of vertebrate embryos. In Xenopus, the maternally deposited transcriptional factor VegT promotes the expression of zygotic Nodal/Activin ligands that further form a morphogen gradient along the vegetal-animal axis and trigger the induction of the three germ layers. Here we found that SCP3 (small C-terminal domain phosphatase 3) is maternally expressed and vegetally enriched in Xenopus embryos and is essential for the timely induction of germ layers. SCP3 is required for the full activation of Nodal/Activin and bone morphogenetic protein signals and functions via dephosphorylation in the linker regions of receptor-regulated Smads. Consistently, the linker regions of receptor-regulated Smads are heavily phosphorylated in fertilized eggs, and this phosphorylation is gradually removed when embryos approach the midblastula transition. Knockdown of maternal SCP3 attenuates these dephosphorylation events and the activation of Nodal/Activin and bone morphogenetic protein signals after midblastula transition. This study thus suggested that the maternal SCP3 serves as a vegetally enriched, intrinsic factor to ensure a prepared status of Smads for their activation by the upcoming ligands during germ layer induction of Xenopus embryos.

Manipulation and in vitro maturation of Xenopus laevis oocytes, followed by intracytoplasmic sperm injection, to study embryonic development

Amphibian eggs have been widely used to study embryonic development. Early embryonic development is driven by maternally stored factors accumulated during oogenesis. In order to study roles of such maternal factors in early embryonic development, it is desirable to manipulate their functions from the very beginning of embryonic development. Conventional ways of gene interference are achieved by injection of antisense oligonucleotides (oligos) or mRNA into fertilized eggs, enabling under- or over-expression of specific proteins, respectively. However, these methods normally require more than several hours until protein expression is affected, and, hence, the interference of gene functions is not effective during early embryonic stages. Here, we introduce an experimental system in which expression levels of maternal proteins can be altered before fertilization. Xenopus laevis oocytes obtained from ovaries are defolliculated by incubating with enzymes. Antisense oligos or mRNAs are injected into defolliculated oocytes at the germinal vesicle (GV) stage. These oocytes are in vitro matured to eggs at the metaphase II (MII) stage, followed by intracytoplasmic sperm injection (ICSI). By this way, up to 10% of ICSI embryos can reach the swimming tadpole stage, thus allowing functional tests of specific gene knockdown or overexpression. This approach can be a useful way to study roles of maternally stored factors in early embryonic development.

Maternal syntabulin is required for dorsal axis formation and is a germ plasm component in Xenopus

In amphibians and teleosts, early embryonic axial development is driven by maternally deposited mRNAs and proteins, called dorsal determinants, which migrate to the presumptive dorsal side of the embryo in a microtubule-dependent manner after fertilization. Syntabulin is an adapter protein that binds to kinesin KIF5B and to the transmembrane protein Syntaxin1. In zebrafish, a mutation in Syntabulin causes complete embryo ventralization. It is unknown whether Syntabulin plays an analogous role during early development of other species, a question addressed here in Xenopus laevis. in situ hybridization of syntabulin mRNA was carried out at different stages of Xenopus development. In oocytes, syntabulin transcripts were localized to the vegetal cortex of large oocytes and the mitochondrial cloud of very young oocytes. We extended the zebrafish data by finding that during cleavage Xenopus syntabulin mRNA localized to the germ plasm and was later expressed in primordial germ cells (PGCs). This new finding suggested a role for Syntabulin during germ cell differentiation. The functional role of maternal syntabulin mRNA was investigated by knock-down with phosphorothioate DNA antisense oligos followed by oocyte transfer. The results showed that syntabulin mRNA depletion caused the complete loss of dorso-anterior axis formation in frog embryos. Consistent with the ventralized phenotype, syntabulin-depleted embryos displayed severe reduction of dorsal markers and ubiquitous transcription of the ventral marker sizzled. Syntabulin was required for the maternal Wnt/β-Catenin signal, since ventralization could be completely rescued by injection of β-catenin (or syntabulin) mRNA. The data suggest an evolutionarily conserved role for Syntabulin, a protein that bridges microtubule motors and membrane vesicles, during dorso-ventral axis formation in the vertebrates.

Aven is dynamically regulated during Xenopus oocyte maturation and is required for oocyte survival

We have analyzed the expression and function of the cell death and cell cycle regulator Aven in Xenopus. Analysis of Xenopus Aven expression in oocytes and embryos revealed a band close to the predicted molecular weight of the protein (36 kDa) in addition to two bands of higher molecular weight (46 and 49 kDa), one of which was determined to be due to phosphorylation of the protein. The protein is primarily detected in the cytoplasm of oocytes and is tightly regulated during meiotic and mitotic cell cycles. Progesterone stimulation of oocytes resulted in a rapid loss of Aven expression with the protein levels recovering before germinal vesicle breakdown (GVBD). This loss of Aven is required for the G2–M1 cell cycle transition. Aven morpholino knockdown experiments revealed that early depletion of the protein increases progesterone sensitivity and facilitates GVBD, but prolonged depletion of Aven results in caspase-3 activation and oocyte death by apoptosis. Phosphorylated Aven (46 kDa) was found to bind Bcl-x_L in oocytes, but this interaction was lost in apoptotic oocytes. Thus, Aven alters progesterone sensitivity in oocytes and is critical for oocyte survival.

Coco regulates dorsoventral specification of germ layers via inhibition of TGFβ signalling

One of the earliest steps in embryonic development is the specification of the germ layers, the subdivision of the blastula embryo into endoderm, mesoderm and ectoderm. Maternally expressed members of the Transforming Growth Factor β (TGFβ) family influence all three germ layers; the ligands are required to induce endoderm and mesoderm, whereas inhibitors are required for formation of the ectoderm. Here, we demonstrate a vital role for maternal Coco, a secreted antagonist of TGFβ signalling, in this process. We show that Coco is required to prevent Activin and Nodal signals in the dorsal marginal side of the embryo from invading the prospective ectoderm, thereby restricting endoderm- and mesoderm-inducing signals to the vegetal and marginal zones of the pre-gastrula Xenopus laevis embryo.

Maternal Dead-End1 is required for vegetal cortical microtubule assembly during Xenopus axis specification

Vertebrate axis specification is an evolutionarily conserved developmental process that relies on asymmetric activation of Wnt signaling and subsequent organizer formation on the future dorsal side of the embryo. Although roles of Wnt signaling during organizer formation have been studied extensively, it is unclear how the Wnt pathway is asymmetrically activated. In Xenopus and zebrafish, the Wnt pathway is triggered by dorsal determinants, which are translocated from the vegetal pole to the future dorsal side of the embryo shortly after fertilization. The transport of dorsal determinants requires a unique microtubule network formed in the vegetal cortex shortly after fertilization. However, molecular mechanisms governing the formation of vegetal cortical microtubule arrays are not fully understood. Here we report that Dead-End 1 (Dnd1), an RNA-binding protein required for primordial germ cell development during later stages of embryogenesis, is essential for Xenopus axis specification. We show that knockdown of maternal Dnd1 specifically interferes with the formation of vegetal cortical microtubules. This, in turn, impairs translocation of dorsal determinants, the initiation of Wnt signaling, organizer formation, and ultimately results in ventralized embryos. Furthermore, we found that Dnd1 binds to a uridine-rich sequence in the 3'-UTR of trim36, a vegetally localized maternal RNA essential for vegetal cortical microtubule assembly. Dnd1 anchors trim36 to the vegetal cortex in the egg, promoting high concentrations of Trim36 protein there. Our work thus demonstrates a novel and surprising function for Dnd1 during early development and provides an important link between Dnd1, mRNA localization, the microtubule cytoskeleton and axis specification.

Maternal mRNA knock-down studies: antisense experiments using the host-transfer technique in Xenopus laevis and Xenopus tropicalis

The ability to inhibit the activity of maternally stored gene products in Xenopus has led to numerous insights into early developmental mechanisms. Oocytes can be cultured and manipulated in vitro and then implanted into the body cavity of a host female to make them competent for fertilization. Here, we summarize the methods for obtaining, culturing, and fertilizing Xenopus oocytes, with the goal of inhibiting maternal gene function through antisense oligonucleotide-mediated mRNA knock-down. We describe a simplified technique for implanting donor oocytes into host females using intraperitoneal injection. Also, we present optimized methods for performing the host-transfer procedure with Xenopus tropicalis oocytes.

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

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

Activity of long-chain acyl-CoA synthetase is required for maintaining meiotic arrest in Xenopus laevis

In most vertebrates, fully grown oocytes are arrested in meiotic prophase I and only resume the cell cycle upon external stimuli, such as hormones. The proper arrest and resumption of the meiotic cycle is critical for reproduction. A Galpha(S) signaling pathway essential for the arrest is conserved in organisms from Xenopus to mouse and human. A previous gene association study implicated that mutations of human ACSL6 may be related to premature ovarian failure. However, functional roles of ACSL6 in human infertility have yet to be reported. In the present study, we found that triacsin C, a potent and specific inhibitor for ACSL, triggers maturation in Xenopus and mouse oocytes in the absence of hormone, suggesting ACSL activity is required for the oocyte arrest. In Xenopus, acsl1b may fulfill a major role in the process, because inhibition of acsl1b by knocking down its RNA results in abnormal acceleration of oocyte maturation. Such abnormally matured eggs cannot support early embryonic development. Moreover, direct inhibition of protein palmitoylation, which lies downstream of ACSLs, also causes oocyte maturation. Furthermore, palmitoylation of Galpha(s), which is essential for its function, is inhibited when the ACSL activity is blocked by triacsin C in Xenopus. Thus, disruption of ACSL activity causes inhibition of the Galpha(s) signaling pathway in the oocytes, which may result in premature ovarian failure in human.

Translation of incenp during oocyte maturation is required for embryonic development in Xenopus laevis

The chromosome passenger complex (CPC) consists of Aurora-B kinase and several other subunits. One of these, incenp, binds Aurora-B and regulates its kinase activity. During Xenopus oocyte maturation, incenp accumulates through translation, contributing to aurora-b activation. A previous study has demonstrated that inhibition of incenp translation during oocyte maturation diminishes aurora-b activation but does not interfere with oocyte maturation, characterized by normal maturation-specific cyclin-b phosphorylation, degradation, and resynthesis. Here we have extended these findings, showing that inhibition of incenp translation during oocyte maturation did not interfere with meiosis I or II, as indicated by the normal emission of the first polar body and metaphase II arrest, followed by the successful emission of the second polar body upon parthenogenetic egg activation. Most importantly, however, when transferred to host frogs and subsequently ovulated, the incenp-deficient eggs were fertilized but failed to undergo mitotic cleavage. Thus, translation of incenp during oocyte maturation appears to be part of oocyte cytoplasmic maturation, preparing the egg for the rapid mitosis following fertilization.

Xenopus as a model to study alternative splicing in vivo

An increasing number of genes are being identified for which the corresponding mRNAs contain different combinations of the encoded exons. This highly regulated exon choice, or alternative splicing, is often tissue-specific and potentially could differentially affect cellular functions. Alternative splicing is therefore not only a means to increase the coding capacity of the genome, but also to regulate gene expression during differentiation or development. To both evaluate the importance for cellular functions and define the regulatory pathways of alternative splicing, it is necessary to progress from the in vitro or ex vivo experimental models actually used towards in vivo whole-animal studies. We present here the amphibian, Xenopus, as an experimental model highly amenable for such studies. The various experimental approaches that can be used with Xenopus oocytes and embryos to characterize regulatory sequence elements and factors are presented and the advantages and drawbacks of these approaches are discussed. Finally, the real possibilities for large-scale identification of mRNAs containing alternatively spliced exons, the tissue-specific patterns of exon usage and the way in which these patterns are modified by perturbing the relative amount of splicing factors are discussed.

Maternal Oct1/2 is required for Nodal and Vg1/Univin expression during dorsal-ventral axis specification in the sea urchin embryo

The TGFβ family member Nodal is expressed early in the presumptive ventral ectoderm of the early sea urchin embryo and its activity is crucial for dorsal-ventral (D/V) axis specification. Analysis of the nodal promoter identified a number of critical binding sites for transcription factors of different families including Sox, Oct, TCF and bZIP, but in most cases the specific factors that regulate nodal expression are not known. In this study, we report that the maternal factor Oct1/2 functions as a positive regulator of nodal and that its activity is essential for the initiation of nodal expression. Inhibition of Oct1/2 mRNA translation produced embryos with severe axial defects similar to those observed following inhibition of Nodal function. We show that perturbing Oct1/2 function specifically disrupted specification of the ventral and dorsal ectodermal regions and that these effects were caused by the failure of nodal to be expressed early in development. Furthermore, we identified the key gene vg1/univin, which is also necessary for nodal expression, as an additional factor that was completely dependent on Oct1/2 for its zygotic expression. These data demonstrate that the maternal Oct1/2 protein plays an early and essential role in D/V axis specification by initiating the expression of nodal and vg1/univin, two genes that act at the top of the D/V ectoderm gene regulatory network.

A functional analysis of MELK in cell division reveals a transition in the mode of cytokinesis during Xenopus development

MELK is a serine/threonine kinase involved in several cell processes, including the cell cycle, proliferation, apoptosis and mRNA processing. However, its function remains elusive. Here, we explored its role in the Xenopus early embryo and show by knockdown that xMELK (Xenopus MELK) is necessary for completion of cell division. Consistent with a role in cell division, endogenous xMELK accumulates at the equatorial cortex of anaphase blastomeres. Its relocalization is highly dynamic and correlates with a conformational rearrangement in xMELK. Overexpression of xMELK leads to failure of cytokinesis and impairs accumulation at the division furrow of activated RhoA - a pivotal regulator of cytokinesis. Furthermore, endogenous xMELK associates and colocalizes with the cytokinesis organizer anillin. Unexpectedly, our study reveals a transition in the mode of cytokinesis correlated to cell size and that implicates xMELK. Collectively, our findings disclose the importance of xMELK in cytokinesis during early development and show that the mechanism of cytokinesis changes during Xenopus early development.

Defolliculation of Xenopus oocytes

It is possible to microinject Xenopus oocytes that are still contained within their ovarian follicles, but most researchers find it more convenient to work with defolliculated oocytes. Defolliculation can be carried out enzymatically by treatment with collagenase, or it can be performed manually. Enzymatic treatment is recommended for preparation of large numbers of oocytes (more than 1000); however, this treatment causes complications because it often damages the quality of the oocytes. The manual procedure, which requires some practice, is recommended for experiments requiring only a few hundred oocytes. This protocol details both the enzymatic and manual procedures for defolliculation of Xenopus oocytes.

Fertilization of Xenopus oocytes using the host transfer method

Studying the contribution of maternally inherited molecules to vertebrate early development is often hampered by the time and expense necessary to generate maternal-effect mutant animals. Additionally, many of the techniques to overexpress or inhibit gene function in organisms such as Xenopus and zebrafish fail to sufficiently target critical maternal signaling pathways, such as Wnt signaling. In Xenopus, manipulating gene function in cultured oocytes and subsequently fertilizing them can ameliorate these problems to some extent. Oocytes are manually defolliculated from donor ovary tissue, injected or treated in culture as desired, and then stimulated with progesterone to induce maturation. Next, the oocytes are introduced into the body cavity of an ovulating host female frog, whereupon they will be translocated through the host's oviduct and acquire modifications and jelly coats necessary for fertilization. The resulting embryos can then be raised to the desired stage and analyzed for the effects of any experimental perturbations. This host-transfer method has been highly effective in uncovering basic mechanisms of early development and allows a wide range of experimental possibilities not available in any other vertebrate model organism.

Identification of germ plasm-associated transcripts by microarray analysis of Xenopus vegetal cortex RNA

RNA localization is a common mechanism for regulating cell structure and function. Localized RNAs in Xenopus oocytes are critical for early development, including germline specification by the germ plasm. Despite the importance of these localized RNAs, only approximately 25 have been identified and fewer are functionally characterized. Using microarrays, we identified a large set of localized RNAs from the vegetal cortex. Overall, our results indicate a minimum of 275 localized RNAs in oocytes, or 2-3% of maternal transcripts, which are in general agreement with previous findings. We further validated vegetal localization for 24 candidates and further characterized three genes expressed in the germ plasm. We identified novel germ plasm expression for reticulon 3.1, exd2 (a novel exonuclease-domain encoding gene), and a putative noncoding RNA. Further analysis of these and other localized RNAs will likely identify new functions of germ plasm and facilitate the identification of cis-acting RNA localization elements.

Dicalcin inhibits fertilization through its binding to a glycoprotein in the egg envelope in Xenopus laevis

Fertilization comprises oligosaccharide-mediated sperm-egg interactions, including sperm binding to an extracellular egg envelope, sperm penetration through the envelope, and fusion with an egg plasma membrane. We show that Xenopus dicalcin, an S100-like Ca(2+)-binding protein, present in the extracellular egg envelope (vitelline envelope (VE)), is a suppressive mediator of sperm-egg interaction. Preincubation with specific antibody greatly increased the efficiency of in vitro fertilization, whereas prior application of exogenous dicalcin substantially inhibited fertilization as well as sperm binding to an egg and in vitro sperm penetration through the VE protein layer. Dicalcin showed binding to protein cores of gp41 and gp37, constituents of VE, in a Ca(2+)-dependent manner and increased in vivo reactivity of VE with a lectin, Ricinus communis agglutinin I, which was accounted for by increased binding ability of gp41 to the lectin and greater exposure of gp41 to an external environment. Our findings strongly suggest that dicalcin regulates the distribution of oligosaccharides within the VE through its binding to the protein core of gp41, probably by modulating configuration of oligosaccharides on gp41 and the three-dimensional structure of VE framework, and thereby plays a pivotal role in sperm-egg interactions during fertilization.

The use of antisense oligonucleotides in Xenopus oocytes

The ability to manipulate gene expression in Xenopus oocytes and then generate fertilized embryos by transfer into host females has made it possible to rapidly characterize maternal signaling pathways in vertebrate development. Maternal mRNAs in particular can be efficiently depleted using antisense deoxyoligonucleotides (oligos), mediated by endogenous RNase-H activity. Since the microinjection of antisense reagents or mRNAs into eggs after fertilization often fails to affect maternal signaling pathways, mRNA depletion in the Xenopus oocyte is uniquely suited to assessing maternal functions. In this review, we highlight the advantages of using antisense in Xenopus oocytes and describe basic methods for designing and choosing effective oligos. We also summarize the procedures for fertilizing cultured oocytes by host-transfer and interpreting the specificity of antisense effects. Although these methods can be technically demanding, the use of antisense in oocytes can be used to address biological questions that are intractable in other experimental settings.

Vegetally localized Xenopus trim36 regulates cortical rotation and dorsal axis formation

Specification of the dorsoventral axis in Xenopus depends on rearrangements of the egg vegetal cortex following fertilization, concomitant with activation of Wnt/beta-catenin signaling. How these processes are tied together is not clear, but RNAs localized to the vegetal cortex during oogenesis are known to be essential. Despite their importance, few vegetally localized RNAs have been examined in detail. In this study, we describe the identification of a novel localized mRNA, trim36, and characterize its function through maternal loss-of-function experiments. We find that trim36 is expressed in the germ plasm and encodes a ubiquitin ligase of the Tripartite motif-containing (Trim) family. Depletion of maternal trim36 using antisense oligonucleotides results in ventralized embryos and reduced organizer gene expression. We show that injection of wnt11 mRNA rescues this effect, suggesting that Trim36 functions upstream of Wnt/beta-catenin activation. We further find that vegetal microtubule polymerization and cortical rotation are disrupted in trim36-depleted embryos, in a manner dependent on Trim36 ubiquitin ligase activity. Additionally, these embryos can be rescued by tipping the eggs 90 degrees relative to the animal-vegetal axis. Taken together, our results suggest a role for Trim36 in controlling the stability of proteins regulating microtubule polymerization during cortical rotation, and subsequently axis formation.

Antiapoptotic role for ornithine decarboxylase during oocyte maturation

Ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, is a nonredundant and essential gene in all eukaryotes. During the mitotic cell cycle, ODC exhibits two activity peaks: one at the G(1)/S transition and one during the G(2)/M transition. The physiological role of this cell cycle-dependent ODC activity dynamic is not clear. Previous studies have reported a significant elevation of ODC activity during Xenopus oocyte maturation, which resembles mitotic G(2)/M transition. In order to study the roles of ODC activity in the oocytes, we utilized antisense morpholino (xODC mo) oligonucleotides to inhibit ODC translation. We report here that xODC mo abolished ODC activity increase during oocyte maturation. xODC mo-injected oocytes underwent germinal vesicle breakdown, emitted the first polar body, and reached metaphase II, thus completing nuclear maturation. However, the metaphase II oocytes exhibited high levels of reactive oxygen species and became apoptotic. When transferred to host frogs and subsequently ovulated, these eggs were fertilized but exhibited embryo fragmentation. Translation of ODC is therefore integral to cytoplasmic maturation, protecting metaphase II oocytes from reactive oxygen species-induced apoptosis.

Maternal Tgif1 regulates nodal gene expression in Xenopus

In Xenopus, the maternal transcription factor VegT is necessary and sufficient to initiate the expression of nodal-related genes, which are central to many aspects of early development. However, little is known about regulation of VegT activity. Using maternal loss-of-function experiments, we show that the maternal homeoprotein, Tgif1, antagonizes VegT and plays a central role in anteroposterior patterning by negatively regulating a subset of nodal-related genes. Depletion of Tgif1 causes the anteriorization of embryos and the up-regulation of nodal paralogues nr5 and nr6. Furthermore, Tgif1 inhibits activation of nr5 by VegT in a manner that requires a C-terminal Sin3 corepressor-interacting domain. Tgif1 has been implicated in the transcriptional corepression of transforming growth factor-beta (TGFbeta) and retinoid signaling. However, we show that Tgif1 does not inhibit these pathways in early development. These results identify an essential role for Tgif1 in the control of nodal expression and provide insight into Tgif1 function and mechanisms controlling VegT activity.

Maternal Interferon Regulatory Factor 6 is required for the differentiation of primary superficial epithelia in Danio and Xenopus embryos

Early in the development of animal embryos, superficial cells of the blastula form a distinct lineage and adopt an epithelial morphology. In different animals, the fate of these primary superficial epithelial (PSE) cells varies, and it is unclear whether pathways governing segregation of blastomeres into the PSE lineage are conserved. Mutations in the gene encoding Interferon Regulatory Factor 6 (IRF6) are associated with syndromic and non-syndromic forms of cleft lip and palate, consistent with a role for Irf6 in development of oral epithelia, and mouse Irf6 targeted null mutant embryos display abnormal differentiation of oral epithelia and skin. In Danio rerio (zebrafish) and Xenopus laevis (African clawed frog) embryos, zygotic irf6 transcripts are present in many epithelial tissues including the presumptive PSE cells and maternal irf6 transcripts are present throughout all cells at the blastula stage. Injection of antisense oligonucleotides with ability to disrupt translation of irf6 transcripts caused little or no effect on development. By contrast, injection of RNA encoding a putative dominant negative Irf6 caused epiboly arrest, loss of gene expression characteristic of the EVL, and rupture of the embryo at late gastrula stage. The dominant negative Irf6 disrupted EVL gene expression in a cell autonomous fashion. These results suggest that Irf6 translated in the oocyte or unfertilized egg suffices for early development. Supporting the importance of maternal Irf6, we show that depletion of maternal irf6 transcripts in X. laevis embryos leads to gastrulation defects and rupture of the superficial epithelium. These experiments reveal a conserved role for maternally-encoded Irf6 in differentiation of a simple epithelium in X. laevis and D. rerio. This epithelium constitutes a novel model tissue in which to explore the Irf6 regulatory pathway.

Inhibition of apoptotic potency by ligand stimulated thyroid hormone receptors located in mitochondria

We recently reported that shortened thyroid hormone receptor isoforms (TRs) can target mitochondria and acutely modulate inositol 1,4,5 trisphosphate (IP3)-mediated Ca2+ signaling when activated by thyroid hormone 3,5,3'-tri-iodothyronine (T3). Stimulation occurs via an increase in mitochondrial metabolism that is independent of transcriptional activity. Here, we present evidence that T3-bound xTRbetaA1s inhibit apoptotic activity mediated by cytochrome c release. An assay for apoptotic potency was modified to measure the ability of Xenopus oocyte extracts to induce morphological changes in isolated liver nuclei. Apoptotic potency was significantly decreased when oocyte extract was prepared from xTRbetaA1 expressing oocytes and treated with T3. The ability of T3 treatment to inhibit apoptosis was dependent on the expression of xTRbetaA1s in the mitochondrial fraction, not in the cytosolic fraction. T3 treatment also increased the membrane potential of isolated mitochondria prepared from oocytes expressing xTRbetaA1s but not from wildtype controls. We conclude that T3 acutely regulates cytochrome c release in a potential dependent manner by activating TRs located within mitochondria.

Long- and short-range signals control the dynamic expression of an animal hemisphere-specific gene in Xenopus

Little is known of the control of gene expression in the animal hemisphere of the Xenopus embryo. Here we show that expression of FoxI1e, a gene essential for normal ectoderm formation, is expressed regionally within the animal hemisphere, in a highly dynamic fashion. In situ hybridization shows that FoxI1e is expressed in a wave-like fashion that is initiated on the dorsal side of the animal hemisphere, extends across to the ventral side by the mid-gastrula stage, and is then turned off in the dorsal ectoderm, the neural plate, at the neurula stage. It is confined to the inner layers of cells in the animal cap, and is expressed in a mosaic fashion throughout. We show that this dynamic pattern of expression is controlled by both short- and long-range signals. Notch signaling controls both the mosaic, and dorsal/ventral changes in expression, and is controlled, in turn, by Vg1 signaling from the vegetal mass. FoxI1e expression is also regulated by nodal signaling downstream of VegT. Canonical Wnt signaling contributes only to late changes in the FoxI1e expression pattern. These results provide new insights into the roles of vegetally localized mRNAs in controlling zygotic genes expressed in the animal hemisphere by long-range signaling. They also provide novel insights into the role of Notch signaling at the earliest stages of vertebrate development.

G-protein-coupled signals control cortical actin assembly by controlling cadherin expression in the early Xenopus embryo

During embryonic development, each cell of a multicellular organ rudiment polymerizes its cytoskeletal elements in an amount and pattern that gives the whole cellular population its characteristic shape and mechanical properties. How does each cell know how to do this? We have used the Xenopus blastula as a model system to study this problem. Previous work has shown that the cortical actin network is required to maintain shape and rigidity of the whole embryo, and its assembly is coordinated throughout the embryo by signaling through G-protein-coupled receptors. In this paper, we show that the cortical actin network colocalizes with foci of cadherin expressed on the cell surface. We then show that cell-surface cadherin expression is both necessary and sufficient for cortical actin assembly and requires the associated catenin p120 for this function. Finally, we show that the previously identified G-protein-coupled receptors control cortical actin assembly by controlling the amount of cadherin expressed on the cell surface. This identifies a novel mechanism for control of cortical actin assembly during development that might be shared by many multicellular arrays.