The effect of cytochalasin D on protein synthesis in Xenopus laevis oocytes

The voltage sensing phosphatase (VSP) localizes to the apical membrane of kidney tubule epithelial cells

Voltage-sensing phosphatases (VSPs) are transmembrane proteins that couple changes in membrane potential to hydrolysis of inositol signaling lipids. VSPs catalyze the dephosphorylation of phosphatidylinositol phosphates (PIPs) that regulate diverse aspects of cell membrane physiology including cell division, growth and migration. VSPs are highly conserved among chordates, and their RNA transcripts have been detected in the adult and embryonic stages of frogs, fish, chickens, mice and humans. However, the subcellular localization and biological function of VSP remains unknown. Using reverse transcriptase-PCR (RT-PCR), we show that both Xenopus laevis VSPs (Xl-VSP1 and Xl-VSP2) mRNAs are expressed in early embryos, suggesting that both Xl-VSPs are involved in early tadpole development. To understand which embryonic tissues express Xl-VSP mRNA, we used in situ hybridization (ISH) and found Xl-VSP mRNA in both the brain and kidney of NF stage 32-36 embryos. By Western blot analysis with a VSP antibody, we show increasing levels of Xl-VSP protein in the developing embryo, and by immunohistochemistry (IHC), we demonstrate that Xl-VSP protein is specifically localized to the apical membrane of both embryonic and adult kidney tubules. We further characterized the catalytic activity of both Xl-VSP homologs and found that while Xl-VSP1 catalyzes 3- and 5-phosphate removal, Xl-VSP2 is a less efficient 3-phosphatase with different substrate specificity. Our results suggest that Xl-VSP1 and Xl-VSP2 serve different functional roles and that VSPs are an integral component of voltage-dependent PIP signaling pathways during vertebrate kidney tubule development and function.

Evolutionary origin and phylogenetic analysis of the novel oocyte-specific eukaryotic translation initiation factor 4E in Tetrapoda

The transcriptionally active, growing oocyte accumulates mRNAs essential for early stages of development, the oocyte-to-embryo transition, in a stable, dormant form. Translational repression of mRNAs in eggs of various species is conferred by interactions, either direct or via intermediate proteins, of repressive factors bound to the 3'-untranslated regions with the proteins of the eukaryotic translation initiation factor 4E (eIF4E) family bound to the 5'-cap of the transcripts. Recently, a novel oocyte-specific eIF4E encoded by the Eif41b gene in mammals has been identified by our group. To further investigate this gene, the available cDNA libraries, as well as genome assemblies of nonmammalian vertebrates, were surveyed. This analysis revealed that the Eif4e1b gene arose in Tetrapoda as a result of the ancestral Eif4e locus duplication. Unlike other known proteins of three subfamilies comprising eIF4E family (eIF4E1, eIF4E2, and eIF4E3), cDNA library evidence suggests that Eif41b locus has an oocyte-restricted expression across all classes of Tetrapoda. To further understand the role of eIF4E1B during oocyte maturation, injections of antisense morpholino nucleotides in the X. tropicalis fully-grown stage VI oocytes were performed. The resulted ablation of eIF4E1B protein led to significant acceleration of oocyte maturation after progesterone induction; morpholino-injected oocytes formed the metaphase plate 30 min faster than the control groups. These results suggest that eIF4E1B protein acts as a repressor in translational regulation of maternal mRNAs activated during, and required for, oocyte maturation.

Xenopus tropicalis oocytes: more than just a beautiful genome

For more than 30 yr, Xenopus laevis has been the animal of choice for studying the biochemical regulation of the meiotic and early mitotic vertebrate cell cycles. Attracted by its diploid genome, several laboratories have begun using the similar, although evolutionarily distinct, frog Xenopus tropicalis for studies of vertebrate development. Comparisons between the two species indicate that their development is similar in most respects. Both frogs share many advantages, including their amenability to manipulation and their ability to produce large numbers of high-quality oocytes and eggs year round. In addition, X. tropicalis possesses several advantages that, when combined with its potential for genetic studies, makes it an attractive, complementary model for vertebrate developmental biology. In this chapter, we note some of these advantages and describe in detail techniques we have adapted for the study of meiosis in X. tropicalis.

Regulation of the G2/M transition in oocytes of xenopus tropicalis

The molecular events regulating hormone-induced oocyte activation and meiotic maturation are probably best understood in Xenopus laevis. In X. laevis, progesterone activates the G2-arrested oocyte, induces entry into M phase of meiosis I (MI) and resumption of the meiotic cell cycles, and leads to the formation of a mature, fertilizable egg. Oocytes of Xenopus tropicalis offer several practical advantages over those of X. laevis, including faster and more synchronous meiotic cell cycle progression, less seasonal variability, and the availability of transgenic approaches. Previous work found several similarities in the pathways regulating oocyte maturation in the two species. Here, we report several additional ones that are conserved in X. tropicalis. (1). Injection of Mos mRNA into G2-arrested oocytes activates the MAP kinase cascade and induces the G2/MI transition. (2). Injection of the beta subunit of the kinase CK2 (a negative regulator of Mos and oocyte activation) delays the G2/MI transition. (3). Elevating PKA activity blocks progesterone-induced maturation; repressing PKA activity induces entry into MI in the absence of progesterone. (4). LF (anthrax lethal factor), which cleaves certain MAP kinase kinases, strongly reduces both the rate and extent of entry into MI. In contrast to the one previously reported major difference between oocytes of the two species, we find that injection of egg cytoplasm ("MPF activity") into G2-arrested X. tropicalis oocytes induces entry into meiosis I even when protein synthesis is blocked, just as it does in oocytes of X. laevis. These results indicate that much of what we have learned from studies of X. laevis oocytes holds for those of X. tropicalis, and suggest that X. tropicalis oocytes offer a good experimental system for investigating certain questions that require a rapid, synchronous progression through the G2/meiosis I transition.

Characterization of MPF and MAPK activities during meiotic maturation of Xenopus tropicalis oocytes

Resumption of meiosis in oocytes of Xenopus tropicalis required translation but not transcription, and was marked by the appearance of a white spot and a dark ring, coincident with entry into metaphase I and the onset of anaphase I, respectively. Cyclin B(2)/p34(cdc2) activity increased prior to the first meiotic division, declined at the onset of anaphase I, and subsequently increased again. The capacity of egg cytoplasm to induce germinal vesicle breakdown (GVBD) was inhibited by cycloheximide, despite the fact that these oocytes contained cyclin B(2)/p34(cdc2) complexes. However, cycloheximide-treated oocytes underwent GVBD following injection of constitutively active mitogen-activated protein kinase (MAPK) kinase 2 (MEK2), p33(Ringo), or Delta 90 cyclin B. MAPK activity increased just prior to the first meiotic division and remained stable thereafter. Although injection of constitutively active MEK2 induced GVBD, treatment with the MEK inhibitors U0126 or anthrax lethal factor delayed GVBD and prevented spindle formation. Interestingly, the ability of egg cytoplasm to induce GVBD was unaffected by the inhibition of MEK activity. Our results indicate that the synthesis of a novel or short-lived protein(s) which acts in a MEK-independent fashion is required in order for egg cytoplasm to induce GVBD in X. tropicalis oocytes.

Changes in protein association with intracellular membranes of Xenopus laevis oocytes during maturation and activation

Intracellular membranes isolated from fully grown immature oocytes, mature oocytes (eggs) and activated eggs of Xenopus laevis were fractionated through a discontinuous sucrose density gradient into light, intermediate and heavy fractions. Electron microscopy showed that the light and intermediate fractions consisted mainly of smooth membranes, while the heavy fraction consisted mainly of rough membranes and mitochondria. Variations in the proteins associate with samples taken at different stages were observed by SDS-PAGE. The following differences were consistently observed: a 200 kDa protein was present only in the intermediate fraction of activated eggs, 29 and 44 kDa proteins were present only in the intermediate fractions of immature oocytes and activated eggs, and 120 and 145 kDa proteins were present only in the heavy fractions of mature oocytes and activated eggs. Examination of Western blots showed that cyclins A and B2 did not associate with membrane fractions at any stage of meiosis. Instead, cyclin A was present in the cytosols of mature oocytes and cyclin B2 was present in the cytosols of immature and mature oocytes. c-mos protein was detected in the cytosols and occasionally in the light fractions of mature oocytes and activated eggs. While alpha- and beta-tubulins were detected in the light and intermediate fractions at all the stages of meiosis examined, only beta-tubulin was present in the heavy fraction. beta-tubulin present in the heavy fraction was detected only at interphase, i.e. in immature oocytes and activated eggs, and not in mature oocytes. Using immunogold labelling we confirmed these results and found evidence to suggest that beta-tubulin associates with the rough endoplasmic reticulum of interphase cells by a linking protein.

Cytochalasin B inhibits phosphoinositide hydrolysis in rat hippocampal slices

The effect of cytochalasin B on phosphoinositide (PI) hydrolysis was examined in rat hippocampal slices. Pretreatment of the slices with cytochalasin B caused a significant decrease in PI hydrolysis elicited by carbachol, norepinephrine, or by high K+. This effect was cytochalasin B dose- and time-dependent and was not mimicked by cytochalasin D, vinblastine, colchicine, or phloretin. In contrast, in [3H]inositol-prelabeled hippocampal membranes, cytochalasin B did not affect PI hydrolysis elicited by GTP gamma S and GTP gamma S plus carbachol. Similar result was obtained using the membranes prepared from the slices pretreated with cytochalasin B. The inhibitory effect of cytochalasin B on the carbachol-response was observed in SK-N-SH human neuroblastoma cells, but not in cultured rat astrocytes. These results indicate that cytochalasin B inhibits PI hydrolysis in neuron-specific manner and that its action may be an indirect cellular mechanism other than interaction with cytoskeleton elements.