Mouse oocyte maturation is affected by lithium via the polyphosphoinositide metabolism and the microtubule network

Metabolite abundance in bovine preovulatory follicular fluid is influenced by follicle developmental progression post estrous onset in cattle

Introduction: Preovulatory follicle response to the luteinizing hormone (LH) surge leads to metabolic, molecular, and functional changes in the oocyte and somatic follicular cells from the onset of estrus to ovulation. Follicular fluid contains metabolites, miRNAs, proteins, and hormones that are byproducts of follicular metabolism and support cellular processes of oocyte, cumulus, and granulosa constituents. Numerous studies have highlighted the importance of follicular fluid composition to support fertility, but critical gaps exist toward understanding dynamic modifications in the follicular fluid metabolome from estrous onset to ovulation. The hypothesis was that abundance of follicular fluid metabolites is dependent on follicle progression post LH surge and variability in follicular fluid metabolome profiles indicate key processes required for preparation of the follicle and oocyte for optimal fertility. The objective was to generate preovulatory follicular fluid metabolome profiles and discern differences in the metabolome of preovulatory follicular fluid samples collected at onset of estrus, 11 h post estrous onset, and 18 h post estrous onset. Methods: Estrus was synchronized in non-lactating Jersey cows (n=40) and follicular fluid was collected immediately after the first observed standing mount (hr 0) or at approximately h 11 or 18 after the first standing mount. Ultra-High-Performance Liquid Chromatography-High Resolution Mass Spectrometry was performed on preovulatory follicular fluid samples (n = 9 collected at hr 0, 9 at h 11, and 10 at h 18) and a multiple linear model was performed to determine if time post estrous onset impacted metabolite abundance. Results: Metabolites influenced by time post estrous onset were tested for enrichment in KEGG pathways. Ninety metabolites were identified in follicular fluid samples. Twenty metabolites differed in abundance among timepoints post estrous onset (p ≤ 0.05). Pathways corresponding to amino acid and energy metabolism were enriched with metabolites impacted by time post estrous onset (FDR ≤ 0.10). Discussion: Results from the current study indicate early response to the LH surge to increase bioavailability of amino acids and metabolites used by the cumulus and granulosa cells for energy production and shuttled into the oocyte to support meiotic maturation. Such metabolites may later be used by the ovulatory follicle for protein production.

Toxicological and nutritional status of trace elements in hair of women with in vitro fertilization (IVF) pregnancy and their 9-month-old children

The objective of the present study was to assess toxic and nutritional trace element and mineral status in hair of women with IVF pregnancy and their children. Inductively-coupled plasma mass-spectrometry was used to assess hair trace element levels of 50 women with IVF pregnancy and 158 controls with spontaneous pregnancy and their children. Women with IVF pregnancy were characterized by significantly elevated hair As, Hg, Li, K, Na, and reduced Fe, Si, and Zn contents. Children from IVF pregnancy had significantly lower values of hair Cr, Fe, Mg, Sr, and Al content when compared to the control values, whereas hair Hg and Mo levels were higher. Hair trace element levels were associated with pregnancy complications and infertility, but not newborn characteristics. The results suggest the need for preconceptional monitoring and correction of the levels of toxic and essential elements in women in order to improve the course pregnancy and child development.

How to Achieve High-Quality Oocytes? The Key Role of Myo-Inositol and Melatonin

Assisted reproductive technologies (ART) have experienced growing interest from infertile patients seeking to become pregnant. The quality of oocytes plays a pivotal role in determining ART outcomes. Although many authors have studied how supplementation therapy may affect this important parameter for both in vivo and in vitro models, data are not yet robust enough to support firm conclusions. Regarding this last point, in this review our objective has been to evaluate the state of the art regarding supplementation with melatonin and myo-inositol in order to improve oocyte quality during ART. On the one hand, the antioxidant effect of melatonin is well known as being useful during ovulation and oocyte incubation, two occasions with a high level of oxidative stress. On the other hand, myo-inositol is important in cellular structure and in cellular signaling pathways. Our analysis suggests that the use of these two molecules may significantly improve the quality of oocytes and the quality of embryos: melatonin seems to raise the fertilization rate, and myo-inositol improves the pregnancy rate, although all published studies do not fully agree with these conclusions. However, previous studies have demonstrated that cotreatment improves these results compared with melatonin alone or myo-inositol alone. We recommend that further studies be performed in order to confirm these positive outcomes in routine ART treatment.

Inositol Treatment and ART Outcomes in Women with PCOS

Polycystic ovary syndrome (PCOS) affects 5-10% of women in reproductive age and is characterized by oligo/amenorrhea, androgen excess, insulin resistance, and typical polycystic ovarian morphology. It is the most common cause of infertility secondary to ovulatory dysfunction. The underlying etiology is still unknown but is believed to be multifactorial. Insulin-sensitizing compounds such as inositol, a B-complex vitamin, and its stereoisomers (myo-inositol and D-chiro-inositol) have been studied as an effective treatment of PCOS. Administration of inositol in PCOS has been shown to improve not only the metabolic and hormonal parameters but also ovarian function and the response to assisted-reproductive technology (ART). Accumulating evidence suggests that it is also capable of improving folliculogenesis and embryo quality and increasing the mature oocyte yield following ovarian stimulation for ART in women with PCOS. In the current review, we collate the evidence and summarize our current knowledge on ovarian stimulation and ART outcomes following inositol treatment in women with PCOS undergoing in vitro fertilization (IVF) and/or intracytoplasmic sperm injection (ICSI).

Effects of myo-inositol in women with PCOS: a systematic review of randomized controlled trials

Polycystic ovary syndrome (PCOS) affects 5%-10% of women in reproductive age, and it is the most common cause of infertility due to ovarian dysfunction and menstrual irregularity. Several studies have reported that insulin resistance is common in PCOS women, regardless of the body mass index. The importance of insulin resistance in PCOS is also suggested by the fact that insulin-sensitizing compounds have been proposed as putative treatments to solve the hyperinsulinemia-induced dysfunction of ovarian response to endogenous gonadotropins. Rescuing the ovarian response to endogenous gonadotropins reduces hyperandrogenemia and re-establishes menstrual cyclicity and ovulation, increasing the chance of a spontaneous pregnancy. Among the insulin-sensitizing compounds, there is myo-inosiol (MYO). Previous studies have demonstrated that MYO is capable of restoring spontaneous ovarian activity, and consequently fertility, in most patients with PCOS. With the present review, we aim to provide an overview on the clinical outcomes of the MYO use as a treatment to improve ovarian function and metabolic and hormonal parameters in women with PCOS.

Protein kinases and protein phosphatases that regulate meiotic maturation in mouse oocytes

Oocytes arrest at prophase of meiosis I (MI) and in vivo do not resume meiosis until they receive ovulatory cues. Meiotic resumption entails two rounds of chromosome segregation without an intervening round of DNA replication and an arrest at metaphase of meiosis II (MII); fertilization triggers exit from MII and entry into interphase. During meiotic resumption, there is a burst of protein phosphorylation and dephosphorylation that dramatically changes during the course of oocyte meiotic maturation. Many of these phosphorylation and dephosphorylation events are key to regulating meiotic cell cycle arrest and/or progression, chromosome dynamics, and meiotic spindle assembly and disassembly. This review, which is subdivided into sections based upon meiotic cell cycle stages, focuses on the major protein kinases and phosphatases that have defined requirements during meiosis in mouse oocytes and, when possible, connects these regulatory pathways.

The production of mature oocytes from adult ovaries following primary follicle culture in a marsupial

A model marsupial culture system has been developed whereby individual primary follicles, obtained from adult ovaries, can be grown in vitro to the antral stage and oocytes retrieved from these follicles can achieve nuclear maturation (metaphase II) in the presence of LH. Primary follicles isolated from adult Sminthopsis macroura ovaries were cultured individually in one of four systems: microdrops under oil, upright, inverted, or roller culture. After 6 days of culture, cumulus–oocyte complexes (COCs) were excised from early antral follicles and incubated for an additional 24 h to assess meiotic competence and the effects of LH and lithium on oocyte maturation. Histology and transmission electron microscopy established normal in vivo standards and verified oocyte and follicular integrity following culture. On day 6 of culture, follicle viability was significantly greater in the inverted system (73%) than in the other three systems (10–46%). The inverted system was the most effective in supporting development with follicles demonstrating progressive growth during culture and showing antral signs by day 4. Meiotic resumption during COC culture was facilitated by LH, but hindered by lithium. The ability to resume meiosis and progress to metaphase II was equivalent in oocytes retrieved following follicle culture and those matured in vivo. This study highlights the importance of oxygen and nutrient availability during marsupial follicle culture, and demonstrates for the first time that primary follicles isolated from adult mammalian ovaries can undergo normal growth and development in vitro, to produce mature, meiotically competent oocytes.

Involvement of GABAA receptor in Bufo arenarum oocyte maturation

Amphibian oocytes meiotic arrest is released under the stimulus of progesterone; this hormone interacts with the oocyte surface and starts a cascade of events leading to the activation of a cytoplasmic maturation promoting factor (MPF) that induces germinal vesicle breakdown (GVBD), chromosome condensation and extrusion of the first polar body. The aim of this work was to determine whether the activation of a GABAA receptor is able to induce GVBD in fully grown denuded oocytes of Bufo arenarum and to analyse its possible participation in progesterone-induced maturation. We also evaluated the role of purines and phospholipids in the maturation process induced by a GABAA receptor agonist such as muscimol. Our results indicated that the activation of the GABAA receptor by muscimol induces maturation in a dose- and time-dependent manner and that this activation is a genuine maturation that enables oocytes to form pronuclei. Assays with a receptor antagonist, picrotoxine, showed that the maturation induced by muscimol was inhibited. Treatment with picrotoxine, however, shows that the participation of GABAA receptor in progesterone-induced maturation is not significant. In addition, our results indicate that high intracellular levels of purines obtained by the use of db-AMPc and theophylline or the inhibition of the phosphatidylinositol 4,5-bisphosphate (PIP2 hydrolysis by neomycin and PIP2 turn over by LiCl, respectively, inhibited the maturation induced by muscimol. Treatment with H-7 indicated, however, that PKC activation is not necessary for GVBD induced by the GABAA receptor agonist. Results suggest that the transduction pathway used by the GABAA receptor to induce maturation is different from those used by progesterone.

Insulin signaling in mouse oocytes

Continuous exposure of follicles/oocytes to elevated levels of insulin compromises embryonic developmental competence, although the underlying cellular mechanisms are unknown. The objectives of the present study were to determine whether mouse oocytes have insulin receptors and a functional insulin signaling cascade, and whether insulin exposure during oocyte growth or maturation influences meiotic progression and chromatin remodeling. Immunoblot and immunocytochemical analyses of germinal vesicle-intact (GVI) oocytes demonstrated the presence of insulin receptor-beta. Insulin receptor expression in oocytes was increased by gonadotropin stimulation, and remained elevated throughout meiotic maturation. Fully grown GVI oocytes contained 3-phosphoinositide-dependent protein kinase-1 (PDPK1), thymoma viral proto-oncogene 1 (AKT1), and glycogen synthase kinase 3 (GSK3). In vitro maturation of GVI oocytes in 5 microg/ml insulin had no influence on meiotic progression or the incidence of normal metaphase II (MII) chromosome condensation. Treatment of oocytes during maturation had no effect on GSK3A/B protein expression or phosphorylation of S21/9. However, the culturing of preantral follicles for 10 days with 5 microg/ml insulin increased the phosphorylation of oocyte GSK3B, indicating GSK3 inactivation. The rates of development to metaphase I (MI) were similar for oocytes obtained from insulin-treated follicles and controls, whereas the incidence of abnormal MI chromatin condensation was significantly higher in oocytes obtained from follicles cultured with insulin compared to those cultured without insulin. These results demonstrate that oocytes contain a functional insulin signaling pathway, and that insulin exposure during oocyte growth results in chromatin remodeling aberrations. These findings begin to elucidate the mechanisms by which chronic elevated insulin influences oocyte meiosis, chromatin remodeling, and embryonic developmental competence.

Spontaneous calcium oscillations and nuclear PLC-β1 in human GV oocytes

Our aim was to investigate if human oocytes, like mouse oocytes, exhibit spontaneous Ca(2+) oscillations and nuclear translocation of PLC-beta1 prior to germinal vesicle breakdown (GVBD), and to correlate these events with the evolution of chromatin configuration as a landmark for the meiosis resumption kinetics. Human germinal vesicle (GV) oocytes were either loaded with Fluo-3 probe to record Ca(2+) signals or fixed for subsequent fluorescent labeling of both chromatin and PLC-beta1, and immunogold labeling of PLC-beta1. Here for the first time, we show that human oocytes at the GV-stage exhibit spontaneous Ca(2+) oscillations. Interestingly, only oocytes with a large diameter and characterized by a compact chromatin surrounding the nucleolus of the GV could reveal these kind of oscillations. We also observed a translocation of PLC-beta1 from the cytoplasm towards the nucleus during in vitro maturation of human oocytes. Spontaneous calcium oscillations and nuclear translocation of PLC-beta1 may reflect some degree of oocyte maturity. The impact of our results may be very helpful to understand and resolve many enigmatic problems usually encountered during the in vitro meiotic maturation of human GV oocytes.

Interactions of the meiotic spindle with mitotic chromosomes in GV mouse oocytes

During mitosis, a spindle checkpoint detects chromosome misalignment and halts the cell cycle progression. In meiosis of female germ cells, however, it is debatable whether such a checkpoint is present. This research employed a unique model in the mouse, mitotic chromosomes transferred to meiotic cytoplasts to investigate whether a meiotic oocyte's microtubule apparatus can effectively separate mitotic metaphase chromosomes, and whether a spindle checkpoint exists during its division. The intact germinal vesicle (GV) oocytes, enucleated GV cytoplasts, and enucleated GV cytoplasts at 15 h in-vitro maturation were transferred with a metaphase fibroblast cell. When mitotic chromosomes were transferred into enucleated or intact mouse GV oocytes, the first bipolar meiotic spindles were established and the reconstructed oocytes were able to extrude polar bodies. However, none of the reconstructed oocytes showed complete and accurate alignment of chromosomes, except the enucleated GV cytoplasts reconstructed after maturation. The spindle formation and polar body extrusion suggest that the first meiotic spindle was functional, and the chromosome misalignment did not prevent the onset of anaphase. The data indicate that a spindle checkpoint, providing surveillance of misaligned chromosomes, was overridden or compromised by the incompatibility between somatic chromosomes and meiotic spindles during the first meiotic division.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

The thiol reagent, thimerosal, irreversibly inhibits meiosis reinitiation in mouse oocyte when applied during a very early and narrow temporal window: a pharmacological analysis

The effect of the sulfhydryl reagent, thimerosal (TMS) on meiosis resumption in germinal vesicle (GV)-stage denuded mouse oocytes was studied. It irreversibly inhibits both GV breakdown (GVBD) and the first polar body (pb1) extrusion in concentration- and time-dependent manners, the most striking result being the very early and narrow temporal window during which denuded primary oocytes released from their follicle are susceptible to a pulse of the drug. This inhibition is bypassed by dithiothreitol (DTT) with an efficiency declining with time, while thiosalicylic acid (TA), an analog of TMS devoid of the mercury atom, has no effect on meiosis reinitiation. These results strongly suggest that the inhibitory effect of TMS is a consequence of its sulfhydryl group oxidising activity. The molecular target(s) of this inhibitory oxidation should however be identified. In contrast to DTT, okadaic acid (OA), known to bypass the inhibitory effect of drugs interfering with protein kinase activities, only induces chromatin condensation and GVBD in TMS-pulsed oocytes with a delay of about 8 hr as compared to the control situation. This confirms that a very early thiol oxidation induced by TMS exerts a much more dramatic effect on resumption on meiosis than any pharmacological manipulation of protein kinase activities leading to activation of MPF.

Glycogen synthase kinase-3 regulates mouse oocyte homologue segregation

Intracellular regulation of oocyte meiosis is not completely understood. However, reversible phosphorylation, which involves serine/threonine protein kinases and phosphatases (PP), is an important mediator. Glycogen synthase kinase-3 (GSK-3) is a highly conserved serine/threonine protein kinase. Currently no reports exist on presence or function of GSK-3 in mammalian oocytes. The aim of this study was to determine GSK-3 presence/absence, transcript and protein expression, intracellular protein distribution, and to investigate the functional importance of GSK-3 in mouse oocyte meiosis. Germinal vesicle-intact (GVI) oocytes contained both GSK-3 transcript and protein. Although GSK-3 beta-isoform is the only transcript identifiable in GVI oocytes, both alpha- and beta-isoforms were recognized by Western blot analysis. In growing, meiotic-incompetent oocytes GSK-3 was present, diffusely located throughout the cytoplasm and absent in the nucleus, whereas in meiotic-competent oocytes this cytoplasmic GSK-3 displays a predominant peri-oolemma staining. Treatment of mouse GVI oocytes with lithium chloride (LiCl), which inhibits both inositol monophosphatase (IMPase) and GSK-3, had no significant influence on oocyte viability, morphology, or development to metaphase II (MII). However, LiCl caused abnormal spindle formation and significantly increased incidence of abnormal homologue segregation during the first meiotic division. L690,330, which is a specific IMPase inhibitor, had no significant effect on oocyte viability, morphology, MII development, or homologue segregation. This is the first report of GSK-3 in mammalian oocytes. LiCl inhibition of mouse oocyte GSK-3 modified organization of microtubules and/or function of meiotic spindles thus compromising segregation of condensed bivalent chromosomes.

beta-Catenin in early development of the lancelet embryo indicates specific determination of embryonic polarity

The lancelet (amphioxus) embryo develops from a miolecithal egg and starts gastrulation when it is approximately 400 cells in size, in a fashion similar to that of some non-chordate deuterostomes. Throughout this type of gastrulation, the embryo develops characteristics such as the notochord and hollow nerve cord that commonly appear in chordates. beta-Catenin is an important factor in initiating body patterning. The behavior and developmental pattern of this protein in early lancelet development was examined in this study. Cytoplasmic beta-catenin was localized to the animal pole after fertilization and then was incorporated asymmetrically into the blastomeres during the first cleavage. Asymmetric distribution was observed at least until the 32-cell stage. The first nuclear localization was at the 64-cell stage, and involved all of the cells. At the initial gastrula stage, however, concentrated beta-catenin was found on the dorsal side. LiCl treatment affected the asymmetric pattern of beta-catenin during the first cleavage. LiCl also changed distribution of nuclear beta-catenin at the initial gastrula stage: distribution extended to cells on the animal side. Apparently associated with this change, expression domains of goosecoid, lhx3 and otx also changed to a radially symmetric pattern centered at the animal pole. However, LiCl-treated embryos were able to establish embryonic polarity. The present study suggests that in the lancelet embryo, polarity determination is independent of dorsal morphogenesis.

Follicular fluid and serum concentrations of myo-inositol in patients undergoing IVF: relationship with oocyte quality

BACKGROUND

The follicular microenvironment is an important determinant of oocyte development. The aim of this study was to examine whether the myo-inositol (MI) content in human follicular fluid (FF) was associated with better oocyte quality.

METHODS

A total of 53 patients treated with IVF was recruited to a prospective observational study. FF and serum samples collected were divided into two groups: group A consisted of FF associated with matured and fertilized oocytes, whilst group B was from follicles with immature and unfertilized oocytes.

RESULTS

Patient's age, total ampoules of HMG used, days of stimulation, basal levels of FSH, estradiol (E(2)) levels on the day of HCG, and serum MI content were not significantly different between the two groups. FF volume and its MI content were significantly higher in group A compared with group B (P < 0.05). The levels of MI in FF were positively correlated with the amount of E(2) in their corresponding FF samples and also correlated with embryo quality.

CONCLUSIONS

We propose that higher concentrations of MI and E(2) in human FF appear to play a role in follicular maturity and provide a marker of good quality oocytes.

Chromosomal influence on meiotic spindle assembly: abnormal meiosis I in female Mlh1 mutant mice

In mouse oocytes, the first meiotic spindle is formed through the action of multiple microtubule organizing centers rather than a pair of centrosomes. Although the chromosomes are thought to play a major role in organizing the meiotic spindle, it remains unclear how a stable bipolar spindle is established. We have studied the formation of the first meiotic spindle in murine oocytes from mice homozygous for a targeted disruption of the DNA mismatch repair gene, Mlh1. In the absence of the MLH1 protein meiotic recombination is dramatically reduced and, as a result, the vast majority of chromosomes are present as unpaired univalents at the first meiotic division. The orientation of these univalent chromosomes at prometaphase suggests that they are unable to establish stable bipolar spindle attachments, presumably due to the inability to differentiate functional kinetochore domains on individual sister chromatids. In the presence of this aberrant chromosome behavior a stable first meiotic spindle is not formed, the spindle poles continue to elongate, and the vast majority of cells never initiate anaphase. These results suggest that, in female meiotic systems in which spindle formation is based on the action of multiple microtubule organizing centers, the chromosomes not only promote microtubule polymerization and organization but their attachment to opposite spindle poles acts to stabilize the forming spindle poles.

Inhibition of phosphoinositide metabolism or chelation of intracellular calcium blocks FSH-induced but not spontaneous meiotic resumption in mouse oocytes

Mammalian oocytes are arrested at the diplotene phase of the first meiotic division until ovulation. In the mouse, germinal vesicle breakdown (GVBD) and progression to metaphase II is thought to be triggered by a positive signal originating in the follicular cells following stimulation by the luteinizing hormone (LH) surge. Isolated, fully grown oocytes can also undergo spontaneous reinitiation of meiosis in vitro in the absence of gonadotrophin stimulation. To investigate the mechanism of meiotic resumption, inhibitors of phosphoinositide metabolism and an intracellular calcium chelator were used during maturation in vitro under different conditions. In a series of experiments, isolated cumulus cell-oocyte complexes (COCs) maintained in meiotic arrest by hypoxanthine were induced to resume meiosis by treatment with follicle-stimulating hormone (FSH). Under these conditions, both LiCl and neomycin, which inhibit phosphoinositide hydrolysis, produced a dose-dependent inhibitory effect on meiotic resumption. Similar results were obtained when FSH-induced meiotic resumption was observed in the presence of the acetoxymethyl ester form of 1, 2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA/AM), an intracellular calcium chelator. In hypoxanthine-arrested oocytes, GVBD induced by epidermal growth factor (EGF), which mimics FSH action in in vitro maturation, was also repressed by LiCl and neomycin. Conversely, meiotic resumption triggered by a pulse of 8-bromo-cyclic adenosine monophosphate (8-Br cAMP) was not affected by these two inhibitors. In experiments in which oocytes were cultured under conditions which permit spontaneous meiotic maturation, resumption of meiosis was not affected by either inhibition of phosphoinositide hydrolysis or chelation of intracellular calcium. Therefore, it appears that meiotic resumption induced by hormone stimulation requires activation of the phosphoinositide pathway and mobilization of intracellular calcium. In contrast, spontaneous maturation probably occurs through a different mechanism because it is not affected by inhibition of this signaling pathway.

Inhibition of GSK-3beta leading to the loss of phosphorylated MAP-1B is an early event in axonal remodelling induced by WNT-7a or lithium

WNT-7a induces axonal spreading and branching in developing cerebellar granule neurons. This effect is mediated through the inhibition of GSK-3beta, a serine/threonine kinase and a component of the WNT pathway. Lithium, an inhibitor of GSK-3beta, mimics WNT-7a in granule cells. Here we examined further the effect of GSK-3beta inhibition on cytoskeletal re-organisation. Lithium induces axonal spreading and increases growth cone area and perimeter. This effect is associated with the absence or reduction of stable microtubules in spread areas. Lithium induces the loss of a phosphorylated form of MAP-1B, a microtubule associated protein involved in axonal outgrowth. Down-regulation of the phosphorylated MAP-1B, MAP-1B-P, from axonal processes occurs before axonal remodelling is evident. In vitro phosphorylation assays show that MAP-1B-P is generated by direct phosphorylation of MAP-1B by GSK-3beta. WNT-7a, like lithium, also leads to loss of MAP-1B-P from spread axons and growth cones. Our data suggest that WNT-7a and lithium induce changes in microtubule dynamics by inhibiting GSK-3beta which in turn lead to changes in the phosphorylation of MAP-1B. These findings suggest a novel role for GSK-3beta and WNTs in axonal remodelling and identify MAP-1B as a new target for GSK-3beta and WNT.

Acquisition of meiotic competence is related to the functionality of the phosphoinositide/calcium signaling pathway in the mouse oocyte

The meiosis resumption process has been related to spontaneous cytoplasmic InsP3-dependent calcium oscillations in fully grown mouse oocytes. Our purpose was to determine whether the acquisition of meiotic competence during the growth phase of oogenesis was associated with that of Ca2+ oscillations and whether these oscillations were dependent on the phosphoinositide cycle. We used confocal laser scanning microscopy to image free calcium ions in fluo-3/AM-loaded oocytes recovered from 12- to 26-day-old mice for 15 min following follicular release. As expected, oocytes isolated from 12-day-old mice were totally incompetent to undergo GVB in vitro, whereas the GVB rate increased progressively with mouse age and oocyte diameter. The percentage of oocytes exhibiting spontaneous calcium oscillations and that of oocytes resuming meiosis were similarly correlated with the female age, with incompetent oocytes failing to exhibit spontaneous Ca2+ oscillations. It is noteworthy that regardless of the stage of growth, thapsigargin induced an ooplasmic calcium release from the InsP3-sensitive stores when it was added to the culture medium. However, intracytoplasmic microinjection of InsP3 induced a shorter sequence of Ca2+ oscillations in 12-day-old mouse oocytes than in 15-day-old mouse oocytes and, whereas InsP3 increased the GVB rate at 15 days, it was unable to induce GVB at 12 days. These data lead us to conclude that the acquisition of meiotic competence is related to the functionality of the InsP3 pathway and, correspondingly, to the oocyte's ability to generate spontaneous cytoplasmic InsP3-dependent calcium oscillations.