Growth of anuran oocytes in serum-supplemented medium

Amphibian (Euphlyctis cyanophlyctis) in vitro ovarian culture system to assess impact of aquatic agrochemical contaminants on female reproduction

The present study is an attempt to screen impacts of aquatic agrochemical contaminants (acephate, atrazine and cypermethrin) on development and growth of follicles, in in vitro-cultured ovarian fragments of frog (Euphlyctis cyanophlyctis). Ovarian lobes removed surgically from gravid females were cut into small pieces and cultured in vitro in presence of graded (0.01 or 0.1 μg/ml of culture medium) concentrations of test chemicals or estradiol-17β (positive controls) or culture medium alone (controls) in quadruplicate sets at 23 ± 1 °C temperature for 20 days in a humidified sterile chamber. On 21st day, they were fixed in Bouin's fluid and used for differential follicle counting (n = 3 sets) and histology (n = 1 set). In vitro exposure of ovarian fragments to test chemicals caused a decline in previtellogenic follicles, maintenance of large yolky follicles, incorporation of brown granules into early vitellogenic follicles and decrease in follicular atresia compared to corresponding controls. These results suggest that ovarian follicles are greatly sensitive to chemical exposure during their transition from previtellogenic to vitellogenic growth phase and in vitro ovarian culture system may be used as a tool to assess the effects of aquatic agrochemical contaminants on ovarian function.

Xepac protein and IP3/Ca2+ pathway implication during Xenopus laevis vitellogenesis

The objective of this study was to elucidate the signalling pathways initiated by cAMP once inside the Xenopus laevis oocyte, where it triggers and maintains vitellogenin endocytic uptake. Our results showed the presence of Xepac transcripts at all stages of oogenesis and we demonstrated that a cAMP analogue that exclusively activates Xepac, 8-CPT, was able to rescue the endocytic activity in oocytes with uncoupled gap junctions. Inhibition experiments for the IP3/Ca2+ signalling pathway showed either a complete inhibition or a significant reduction of the vitellogenic process. These results were confirmed with the rescue capability of the A-23187 ionophore in those oocyte batches in which the IP3/Ca2+ pathway was inhibited. Taking our findings into account, we propose that the cAMP molecule binds Xepac protein enabling it to activate the IP3/Ca2+ pathway, which is necessary to start and maintain X. laevis vitellogenin uptake.

Involvement of cAMP and calmodulin in endocytic yolk uptake during Xenopus laevis oogenesis

The aim of the present study was to show the participation and physiological role of calmodulin (CaM) and cAMP during vitellogenin endocytic uptake in the amphibian Xenopus laevis. The results showed a differential distribution of CaM in the ovary follicles during oogenesis. The CaM intracellular localization was not affected by gap junction's downregulation and CaM inhibition did not completely abolished the endocytic activity of oocytes. We showed that cAMP was able to completely rescue the endocytic competence in follicles in which gap junctional communication had been disrupted by octanol. Moreover cAMP was capable of restoring oocyte endocytic capability in the presence of octanol and stelazine, a CaM inhibitor. We propose that, in Vtg uptake regulation, cAMP is upstream of CaM during the endocytic signalling pathway.

Visualization of mRNA localization in Xenopus oocytes

Visualization of in vivo mRNA localization provides a tool for understanding steps in the mechanism of transport. Here we detail a method of fluorescently labeling mRNA transcripts and microinjecting them into Xenopus laevis oocytes followed with imaging by confocal microscopy. This technique overcomes a significant hurdle of imaging RNA in the frog oocyte while providing a rapid method of visualizing mRNA localization in high resolution.

Full-grown oocytes from Xenopus laevis resume growth when placed in culture

When most full-grown, follicle cell-invested oocytes from Xenopus laevis are placed in an appropriate culture medium, they resume growth and remain physiologically healthy for at least 2-3 weeks. Rates of growth by full-grown oocytes in vitro generally approximate and can even exceed the most rapid growth rate achieved by vitellogenic oocytes in vivo. Resumption of oocyte growth can be correlated with the loss of investing follicle cells, which under normal conditions appear to interfere with vitellogenin and nutrient access to the oocyte. The final size reached by the oocyte within the ovary is thus not an intrinsic property of the oocyte but is extrinsically imposed by the somatic environment.

Localization of mRNA and axis formation during Xenopus embryogenesis

This paper summarizes our recent work concerned with the developmental polarity of the frog egg and the patterning of the embryonic body plan. In two separate projects, we are studying genes involved in setting up basic embryonic axes. One of these genes, Vg1, codes for a maternal mRNA that is localized in the frog egg. The Vg1 gene is used in studies on the induction of mesoderm and as a probe to understand how the polarity of an egg is established. A second gene, Xhox3, contains a homeodomain and is differentially expressed in the axial mesoderm. Our studies suggest that this homeobox gene is critically involved in setting up different positional values along the anteroposterior axis.

Implication of gap junction coupling in amphibian vitellogenin uptake

The aim of the present study was to investigate the physiological role and the expression pattern of heterologous gap junctions during Xenopus laevis vitellogenesis. Dye transfer experiments showed that there are functional gap junctions at the oocyte/follicle cell interface during the vitellogenic process and that octanol uncouples this intercellular communication. The incubation of vitellogenic oocytes in the presence of biotinylated bovine serum albumin (b-BSA) or fluorescein dextran (FDX), showed that oocytes develop stratum of newly formed yolk platelets. In octanol-treated follicles no sign of nascent yolk sphere formation was observed. Thus, experiments in which gap junctions were downregulated with octanol showed that coupled gap junctions are required for endocytic activity. RT-PCR analysis showed that the expression of connexin 43 (Cx43) was first evident at stage II of oogenesis and increased during the subsequent vitellogenic stages (III, IV and V), which would indicate that this Cx is related to the process that regulates yolk uptake. No expression changes were detected for Cx31 and Cx38 during vitellogenesis. Based on our results, we propose that direct gap junctional communication is a requirement for endocytic activity, as without the appropriate signal from surrounding epithelial cells X. laevis oocytes were unable to endocytose VTG.

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

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

Evidence for overlapping, but not identical, protein machineries operating in vegetal RNA localization along early and late pathways in Xenopus oocytes

RNAs that localize to the vegetal cortex of Xenopus oocytes are involved in early embryonic patterning and cell fate specification. Two mechanistically distinct pathways lead to RNA enrichment at the vegetal cortex: the early and the late. While several candidate proteins that seem to operate in the late localization pathway have been identified, proteins involved in the early pathway remain to be identified. In this study, we report on the isolation of a novel vegetally localized RNA in Xenopus oocytes that makes use of the early pathway and encodes a protein with a conserved but functionally uncharacterized NIF-motif. The localization signal of XNIF was mapped to a 300-nucleotide region in the 5'-UTR, which is able to mediate both accumulation to the mitochondrial cloud in stage I oocytes, as well as vegetal transport in later stage oocytes. The XNIF-LE contains 16 copies of the previously defined CAC-containing signal motifs for RNA localization. A critical number of such repeats seems to be required for accumulation in the mitochondrial cloud along the early pathway, but additional repeats seem to be required for localization along the late pathway. Cross-linking experiments identify two novel proteins of 62 and 64 kDa that interact with the XNIF-LE but not with the Vg1-LE that operates in the late pathway. Conversely, at least two of the previously identified VgRBPs, Vg1RBP1 and Prrp, also bind to the XNIF-LE. Thus, overlapping, but not identical, protein machineries mediate vegetal RNA localization along early and late pathways in Xenopus oocytes.

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 consensus RNA signal that directs germ layer determinants to the vegetal cortex of Xenopus oocytes

RNA localization is an important mechanism for generating cellular diversity and polarity in the early embryo. In Xenopus, the correct localization of the RNA encoding the T-box transcription factor VegT is essential for the correct spatial organization and identity of endoderm and mesoderm. Although localization signals in the 3' UTR have been identified for many localized RNAs, insight into what constitutes an RNA localization signal remains elusive. To investigate possible common features between signals that direct different RNAs to the same subcellular region, we carried out a detailed analysis of the uncharacterized VegT RNA localization signal and compared it with the well-studied Vg1 localization signal. Both RNAs localize to the vegetal cortex during the same period of oogenesis. Our results suggest a common RNA localization signal at the level of clustered redundant protein-binding motifs and trans-acting factors. We propose that what characterizes RNA localization signals in general is not the nucleotide sequence or secondary structure per se, but the critical clustering of specific redundant protein-binding motifs.

The developing Xenopus oocyte specifies the type of gonadotropin-stimulated steroidogenesis performed by its associated follicle cells

As a response to gonadotropin, amphibian ovarian follicles primarily synthesize and secrete estradiol-17 beta (E2) during vitellogenesis and progesterone (P) when fully grown. Stage IV (vitellogenic) and stage VI (full-grown) ovarian follicles from Xenopus laevis, as well as intermediate sizes, were used to explore this change in steroidogenesis. Optimum steroidogenesis occurred in both stage IV and stage VI follicle exposed for 6 h to 20 IU human chorionic gonadotropin/mL. Although the total amounts of steroid fund were about the same, the E2/P ratios ranged from 26 to 35 for intact stage IV follicles, but only 0.02-0.03 for intact stage VI follicles. Steroid-producing follicle cells were isolated from stage IV and stage VI follicles by non-enzymatic procedures, were washed and were tested for steroidogenic activity in the absence of oocytes. In both cases, P was the predominant steroid produced (E2/P = 0.004-0.04), so the presence of stage IV, but not stage VI, oocytes appears to be necessary for E2 production as a response to gonadotropin. Octanol had no significant effect on the E2/P ratio of intact stage IV follicle. Dissected oocyte/follicle cell preparations from stage IV follicles were also periodically challenged with gonadotropin over 72 h, during which time most follicle cells detached from the oocyte and formed a monolayer over the bottom of the culture dish. The relatively high E2/P ratios for such preparations showed no significant change when stimulated with gonadotropin at various times over the 72 h, as long as the medium was not replaced. We conclude that the estrogenic effect of stage IV oocytes is most likely mediated by a secretory product rather than by gap junctions or by cell contact. Because the X. laevis oocyte has been shown to be a self-differentiating cell, the steroidogenic shift that occurs in developing ovarian follicles appears to be fundamentally regulated by the growing oocyte as if undergoes a physiological change rather than by different gonadotropins.

Localization of Xcat-2 RNA, a putative germ plasm component, to the mitochondrial cloud in Xenopus stage I oocytes

The mitochondrial cloud is a unique cell structure found in stage I Xenopus oocytes that plays a role in mitochondriogenesis and in the distribution of germ plasm to the vegetal pole. Xcat-2 RNA specifically localizes to the mitochondrial cloud and moves with it to the vegetal subcortex in stage II oocytes. Later, in the 4-cell embryo, it is found in a pattern identical to the germ plasm. Following microinjection into stage I oocytes, synthetic Xcat-2 RNAs localize to the mitochondrial cloud within 22 hours. Transcripts are stable over this time period with very little evidence of degradation. The Xcat-2 3′untranslated region was found to be both required and sufficient for mitochondrial cloud localization. Further deletion analysis narrowed this localization signal to a 250 nucleotide region at the proximal end of the 3′untranslated region. This region is different from, but overlaps with, a domain previously shown to be sufficient to direct Xcat-2 to the vegetal cortex in stage IV oocytes. Examination of early stage I oocytes reveals a time when Xcat-2 is uniformly distributed, arguing against vectorial nuclear export into the mitochondrial cloud. Analysis of localization at different time points does not suggest active transport to the mitochondrial cloud. We postulate that localization occurs by selective entrapment of Xcat-2 within the cloud by localized binding sites.

Yolk platelets in Xenopus oocytes maintain an acidic internal pH which may be essential for sodium accumulation

Yolk platelets constitute an embryonic endocytic compartment that stores maternally synthesized nutrients. The pH of Xenopus yolk platelets, measured by photometry on whole oocytes which had endocytosed FITC-vitellogenin, was found to be acidic (around pH 5.6). Experiments on digitonin-permeabilized oocytes showed that acidification was due to the activity of an NEM- and bafilomycin A1-sensitive vacuolar proton-ATPase. Proton pumping required chloride, but was not influenced by potassium or sodium. Passive proton leakage was slow, probably due to the buffer capacity of the yolk, and was dependent on the presence of cytoplasmic monovalent cations. In particular, sodium could drive proton efflux through an amiloride-sensitive Na+/H+ exchanger. 8-Bromo-cyclic-AMP was found to increase acidification, suggesting that pH can be regulated by intracellular second messengers. The moderately acidic pH does not promote degradation of the yolk platelets, which in oocytes are stable for weeks, but it is likely to be required to maintain the integrity of these organelles. Furthermore, the pH gradient created by the proton pump, when coupled with the Na+/H+ exchanger, is probably responsible for the accumulation and storage of sodium into the yolk platelets during oogenesis.

Functional reconstitutional of the human epidermal growth factor receptor system in Xenopus oocytes

We have expressed the human EGF receptor (hEGF-R) in Xenopus oocytes by injecting mRNA synthesized in vitro using SP6 vectors containing receptor cDNAs. Each oocyte could express over 1 x 10(10) receptors of a single affinity class and these were able to bind and rapidly internalize EGF. Occupancy resulted in receptor tyrosine autophosphorylation, downregulation, and release of intracellular calcium. Occupied receptors also rapidly induced meiotic maturation in stage VI oocytes. Receptors lacking tyrosine kinase activity bound EGF normally, but did not downregulate or induce any biological responses. The rate of oocyte maturation was proportional to hEGF-R occupancy and was significantly faster than progesterone-induced maturation at nanomolar EGF concentrations. Mutant hEGF-R truncated at residue 973 displayed identical phenotypes in both mammalian cells and oocytes in that they were defective in their ability to release intracellular calcium, undergo ligand induced internalization and receptor downregulation. However, these receptors were fully capable of inducing oocyte maturation. The remarkable retention of specific biological activities of different hEGF-R in the context of oocytes suggests that this receptor system interacts with generally available cellular components that have been conserved during evolution. In addition, it suggests that cell surface tyrosine kinase activity may play an important role in regulating resumption of the cell cycle.

Oogenesis in Fundulus heteroclitus. VI. Establishment and verification of conditions for vitellogenin incorporation by oocytes in vitro

A procedure was developed for studying vitellogenin (VTG) incorporation by vitellogenic oocytes of Fundulus heteroclitus in vitro. Since homologous VTG can be obtained from this animal only with great difficulty, the use of [32P]VTG from Xenopus laevis was explored as an alternative. Vitellogenic as well as maturational-stage oocytes were found to sequester X. laevis [32P]VTG from the medium, and incorporation was found to be linear with time for at least up to 12 hr. Once incorporated into the oocyte, [32P]VTG did not appear to undergo turnover. The effect of different [32P]VTG concentrations on incorporation indicated that the uptake mechanism was saturable. Unlabeled F. heteroclitus VTG and X. laevis VTG were also found to compete effectively with X. laevis [32P]VTG, whereas bovine serum albumin did not. These results represent the first documentation of a successful culture system for receptor-mediated VTG incorporation by teleost oocytes.

The material mRNA Vg1 is correctly localized following injection into Xenopus oocytes

The animal and vegetal ends of Xenopus oocytes have distinctly different developmental fates. At the molecular level, several maternal mRNAs have been isolated that are localized to either the animal or vegetal hemisphere. One of these mRNAs, Vg1, is distributed homogeneously throughout the cytoplasm of early-stage oocytes and gets localized during oogenesis to a tight shell at the vegetal cortex of middle and late-stage oocytes. We have used an in vitro culture system to demonstrate that exogeneous Vg1 mRNA injected into middle-stage, but not late-stage, oocytes gets localized in a similar fashion to the endogenous message. Furthermore, translation of Vg1 mRNA is not required for the localization of the message itself. These results show that the information necessary to interpret the animal-vegetal polarity in oocytes is present in the naked mRNA transcript.

Differentiation of the animal-vegetal axis in Xenopus laevis oocytes. I. Polarized intracellular translocation of platelets establishes the yolk gradient

The animal-vegetal axis of the oocyte of Xenopus laevis is recognizable not only by the pattern of surface pigmentation, but also by the distribution of yolk platelets, with the largest platelets (congruent to 14 microns in diameter) and 70% of the total yolk protein localized in the vegetal hemisphere. We have used fluorescent and radioactive vitellogenins (yolk protein precursors) to study the spatial and temporal patterns of yolk deposition along this axis. We find that the rate of uptake of vitellogenin is nearly uniform over the surface of vitellogenic oocytes of all sizes. Once formed, yolk platelets in the animal hemisphere move inward, around the germinal vesicle, and into the central region of the vegetal hemisphere. Yolk platelets of the vegetal hemisphere do not actively move but are slowly displaced from the surface by successive layers of younger platelets arising and enlarging near the surface. The oldest yolk platelets, which arise circumcortically at the beginning of vitellogenesis in stage II and III oocytes, eventually come to reside in the vegetal hemisphere of stage VI oocytes, in the upper portion of the cup-shaped region of largest platelets. The vegetal hemisphere thus gains the majority of yolk protein by directed intracellular transport from the animal hemisphere adding to the amount directly sequestered by the vegetal hemisphere.

An enzymatic method for radiolabeling vertebrate vitellogenin

Phosphoprotein kinases from Xenopus and chicken liver have been purified and these enzymes have been used to label Xenopus vitellogenin, a phosphoprotein, to high specific activity with [gamma-32P]ATP. The enzymes were isolated by (NH4)2SO4 fractionation followed by chromatography on DE-52 cellulose and phosphocellulose. This procedure resulted in greater than 20,000-fold enrichment for the enzymes. Both enzyme preparations were used to selectively label vitellogenin in the serum of estrogen-treated animals. Thus, isolation of the vitellogenin prior to radiolabeling was not necessary. The [32P]vitellogenin labeled in situ was incorporated by oocytes at a rate similar to [32P]vitellogenin labeled in vivo, was translocated to the yolk platelets, and was correctly processed into the yolk proteins.

Differential postendocytotic compartmentation in Xenopus oocytes is mediated by a specifically bound ligand

Xenopus laevis oocytes were used as a model system to study the intracellular fate of proteins incorporated by endocytosis. We found that the intracellular stability and compartmentation of proteins incorporated by receptor-mediated endocytosis differed substantially from that of proteins incorporated by nonspecific endocytosis. After its uptake, the specifically sequestered yolk precursor protein vitellogenin was converted to the yolk proteins lipovitellin and phosvitin which were stable with time (up to 13 days in culture). In contrast, nonspecifically incorporated albumin (125I-labeled or 3H-labeled bovine serum albumin) was rapidly degraded. To determine whether the differential stability of these proteins was related to their entry into different postendocytotic compartments, we examined the intracellular transfer pathways taken by these proteins. The transfer of vitellogenin from coated vesicles to the yolk platelets was found to involve a secondary compartment formed by the fusion of the incoming endosomes. This compartment, termed transitional yolk bodies (TYB), underwent a progressive condensation until it attained its terminal density (1.21 g/cm3) after approximately 1.5 hr. The fusion of the TYB with the yolk platelets then occurred coincidentally with the time at which vitellogenin was proteolytically processed into the yolk proteins within the TYB. When the proteolytic cleavage of vitellogenin was blocked there was no fusion of the two compartments. In contrast, we found that albumin incorporated in the absence of vitellogenin was directly transferred from endosomes to yolk platelets without the formation of, or fusion with a secondary compartment. However, when oocytes were exposed simultaneously to albumin and vitellogenin both proteins followed identical routes of compartmentation (that of vitellogenin) with no evidence of direct transfer of either protein to the yolk platelets. These results suggest that the incorporation of a specifically bound ligand can result in the formation of a unique intracellular compartment. Moreover, since yolk platelets were able to fuse only with vesicles lacking occupied receptors (in the case of albumin alone) or with a compartment in which the specific ligand had been proteolytically cleaved and presumably released from its receptor (in the case of VTG), we suggest that occupied receptors can act as a transmembrane signal which directs the postendocytotic compartmentation of proteins.