Requirement for glucose in ligand-stimulated meiotic maturation of cumulus cell-enclosed mouse oocytes.

In this study, the effect of different energy sources used in Eagle's minimum essential medium on the meiotic maturation of mouse oocytes in culture was examined. The effects of glucose (5.5 mmol 1(-1)), pyruvate (0.23 mmol 1(-1)) and glutamine (2 mmol 1(-1)) in different combinations were tested on the maturation of denuded oocytes in the presence or absence of 300 mumol dibutyryl cAMP 1(-1) during 17-18 h of culture. In the absence of cyclic nucleotide, only oocytes from those groups containing pyruvate resumed maturation at a high frequency (99-100% germinal vesicle breakdown); all other combinations resulted in < or = 54% germinal vesicle breakdown. When dibutyryl cAMP was introduced, all pyruvate-containing groups exhibited maturation frequencies of about 50%, whereas maturation in all other groups was negligible (< or = 10% GVB). Pyruvate was also important for the maintenance of viability in denuded oocytes (> or = 86% viability in pyruvate-containing medium; < or = 35% viability in pyruvate-free groups). When cumulus cell-enclosed oocytes were cultured in medium without inhibitor, all combinations of energy substrates supported high frequencies of maturation (> or = 89% germinal vesicle breakdown) and viability (> or = 91%). The addition of dibutyryl cAMP resulted in inhibition of meiotic maturation (5-33% germinal vesicle breakdown) in all cultures except the pyruvate-alone group (97% germinal vesicle breakdown). Viability in cumulus cell-enclosed oocytes was greatest when two or more energy substrates were present in the medium. Follicle-stimulating hormone (FSH) produced a stimulation of meiotic maturation in all cultures of meiotically arrested cumulus cell-enclosed oocytes, but maximal induction of germinal vesicle breakdown was dependent upon D-glucose. Concanavalin A (ConA)-induced meiotic maturation was also dependent upon D-glucose. Uptake and metabolism of D-glucose by the cumulus cells is important in mediating the stimulatory effects of these ligands on oocyte maturation because (1) both FSH and ConA stimulated uptake of D-glucose and 2-deoxyglucose but not 3-O-methylglucose; (2) phloretin prevented the stimulatory action of FSH and ConA on germinal vesicle breakdown at a concentration that suppressed ligand-induced uptake of D-glucose; (3) 2-deoxyglucose, a hexose that suppresses glycolysis, prevented the induction of meiotic maturation by FSH and ConA and (4) D-mannose, a glycolysable sugar, was as effective as D-glucose in supporting the ligand effects.(ABSTRACT TRUNCATED AT 400 WORDS)     

Maturation of the mouse oocyte-cumulus cell complex: stimulation by lectins.

We have used carbohydrate-binding proteins, or lectins, as tools to investigate the physiological phenomena associated with the preovulatory maturation of the oocyte-cumulus cell complex. Certain lectins are mitogens, and since other mitogenic agents such as growth factors are known to stimulate meiotic maturation and cumulus expansion, we tested the ability of lectins to provoke these physiological responses. Cumulus cell-enclosed oocytes (CEO) from primed mice were maintained in meiotic arrest in vitro with dibutyryl cyclic adenosine 3',5'-monophosphate (dbcAMP) and treated with one of eleven different lectins. With the exception of pokeweed mitogen (PWM), all of the mitogenic lectins tested were able to induce germinal vesicle breakdown (GVB) in meiotically arrested oocytes, and this action required the presence of the somatic cumulus cells; in fact, either there was no effect or maturation was suppressed when cumulus cell-free oocytes (denuded oocytes; DO) were treated with lectins. None of the nonmitogenic lectins stimulated meiotic maturation in either CEO or DO. The mitogenic lectin concanavalin A (Con A) also induced maturation in CEO when meiotic arrest was maintained with hypoxanthine, guanosine, or 3-isobutyl-1-methylxanthine. The kinetics of spontaneous oocyte maturation in inhibitor-free medium were not altered by Con A. Only the mitogenic lectins that induced meiotic maturation stimulated cumulus expansion, with Con A the most active lectin. The actions of Con A on the maturation of the oocyte-cumulus cell complex were inhibited by methyl-alpha-D-mannopyranoside as predicted by its sugar-binding specificity. These results demonstrate that (1) lectins can stimulate maturation of the mouse oocyte-cumulus cell complex; (2) mitogenicity is associated with the positive activity of the lectins; and (3) cumulus cells mediate the stimulatory action of lectins on oocyte maturation, while inhibition of GVB occurs at the oocyte level. These data support the idea that common signals mediate the mitogenic and maturation-inducing actions of lectins.     

Factors influencing resumption of meiotic maturation and cumulus expansion of porcine oocyte-cumulus cell complexes in vitro

The present study was undertaken to examine effects of various combinations of epidermal growth factor (EGF), transforming growth factor-b?₁ (TGF-b?₁), follicle-stimulating hormone (FSH), luteinizing hormone (LH), androstenedione (A₄), and estradiol-17b? (E₂) on meiotic maturation and cumulus expansion in the pig using an in vitro model system. Oocyte-cumulus cell complexes (OCC) were cultured in the media containing the abovementioned agents for 24 hr and were observed for germinal vesicle breakdown (GVBD), indicative of initiation of meiotic maturation, and for expansion of their cumulus cells. Treatment with EGF significantly increased (P < 0.05) incidence of GVBD, with maximal stimulation occurring at 1 ng/ml (55% vs. 12% in the control). Concentrations of EGF as low as 100 pg/ml significantly stimulated GVBD over control (37% vs. 12%). Addition of EGF (1 ng/ml) and FSH (1.5 μg/ml) together and LH (2 μg/ml) and FSH (1.5 μg/ml) together resulted in significantly higher (P < 0.01) GVBD levels than were observed in response to EGF, FSH, or LH alone. Addition of E₂ (1 μg/ml) had no effect by itself but significantly decreased the incidence of GVBD in the presence of FSH and of LH + FSH. Addition of A₄ (1 μg/ml) significantly reduced the percentage of oocytes undergoing GVBD when added alone or with FSH. Although both EGF and LH stimulated cumulus expansion, FSH was more effective in stimulating cumulus expansion than EGF or LH. TGF-b?₁ had no effect on GVBD or cumulus expansion. These studies indicate that these hormones may have differing roles in oocyte maturation and that their interactions may be part of an intricate system regulating the maturation of oocytes during follicular development in vivo. © 1993 Wiley-Liss, Inc.     

Localization and function of the Ska complex during mouse oocyte meiotic maturation

The Ska (spindle and kinetochore-associated) complex is composed of three proteins: Ska1, Ska2 and Ska3. It is required for stabilizing kinetochore-microtubule (KT-MT) interactions and silencing spindle checkpoint during mitosis. However, its roles in meiosis remain unclear. The present study was designed to investigate the localization and function of the Ska complex during mouse oocyte meiotic maturation. Our results showed that the localization and function of Ska complex in mouse oocyte meiosis differ in part from those in mitosis. Injection of low dose exogenous Myc-Ska mRNA showed that, instead of localizing to the kinetochores (KTs) and mediating KT-MT interactions from pro-metaphase to mid-anaphase stages as in mitosis, the members of the Ska complex were only localized on spindle microtubules from the Pro-MI to MII stages in mouse oocyte meiosis. Time-lapse live imaging analysis showed that knockdown of any member of the Ska complex by Morpholino injection into mouse oocytes resulted in spindle movement defects and enlarged polar bodies. Depletion of the whole Ska complex disrupted the stability of the anaphase spindle and influenced the extrusion of the first polar body. Taken together, these results show that the Ska complex plays an important role in meiotic spindle migration and anaphase spindle stability during mouse oocyte maturation.     

Oocyte-dependent activation of mitogen-activated protein kinase (ERK1/2) in cumulus cells is required for the maturation of the mouse oocyte-cumulus cell complex

Luteinizing hormone (LH) induces maturational processes in oocyte-cumulus cell complexes (OCC) of preovulatory follicles that include both resumption of meiosis in the oocyte and expansion (mucification) of the cumulus oophorus. Both processes require activation of mitogen-activated protein kinase (MAPK) in granulosa cells. Here, it is reported that inhibition of MAPK activation prevented gonadotropin-stimulated resumption of meiosis as well as the rise in expression of two genes whose products are necessary for normal cumulus expansion, Has2 and Ptgs2. However, inhibition of MAPK did not block gonadotropin-induced elevation of granulosa cell cAMP, indicating that the activation of MAPK required for inducing GVB and cumulus expansion is downstream of cAMP. Moreover, activation of MAPK in cumulus cells requires one or more paracrine factors from the oocyte to induce GVB and cumulus expansion; MAPK activation alone is not sufficient to initiate these maturational processes. This study demonstrates a remarkable interaction between the oocyte and cumulus cells that is essential for gonadotropin-induced maturational processes in OCC. By enabling gonadotropin-dependent MAPK activation in granulosa cells, oocytes promote the generation of a return signal from these cells that induces the resumption of meiosis. It also appears that an oocyte-dependent pathway downstream from oocyte-enabled activation of MAPK, and distinct from that promoting the resumption of meiosis, governs cumulus expansion.     

Gonadotropin-induced murine oocyte maturation in vivo is not associated with decreased cyclic adenosine monophosphate in the oocyte-cumulus cell complex

The cyclic adenosine monophosphate (cAMP) content of intact oocyte-cumulus cell complexes at various times after the induction of oocyte maturation in mice in vivo was correlated with the time of commitment by the oocytes to undergo germinal vesicle breakdown (GVB) and metabolic coupling between the oocyte and cumulus cells. Seventy-nine percent of the oocytes either underwent GVB or were committed to do so by 2 h after injection of human chorionic gonadotropin (hCG). This occurred without a decrease in the coupling between cumulus cells and the oocyte and with increasing cAMP levels in the oocyte-cumulus cell complex. Maintenance of threshold levels of cAMP within mammalian oocytes appears essential for the maintenance of meiotic arrest, but data presented here suggest that oocyte maturation in mice is induced by gonadotropins in nonatretic follicles in vivo by some mechanism other than one which decreases the cAMP content of the intact oocyte-cumulus cell complex.     

Decreases in heterologous metabolic and dye coupling, but not in electrical coupling, accompany meiotic resumption in hamster oocyte-cumulus complexes

The temporal relationship between resumption of meiosis and reduction in either heterologous intercellular coupling, or magnitude of oocyte or cumulus cell resting potential in hamster oocyte-cumulus complexes was investigated. Coupling was assessed qualitatively by lucifer yellow dye transfer and quantitatively by transfer of radiolabeled uridine metabolites or electrical current after culture of complexes in various systems previously characterized either to maintain meiotic arrest or to permit meiotic resumption. In each of the three systems which permitted meiotic resumption, cumulus to oocyte metabolic and dye coupling and oocyte to cumulus dye coupling decreased progressively with time after release from meiotic arrest. In contrast, no similar temporal changes in metabolic or dye coupling were observed in any complex after culture in either of the two systems which maintained meiotic arrest. Analysis of the extent of heterologous ionic coupling revealed that in neither direction was a decrease in ionic uncoupling consistently associated with reinitiation of meiosis. Furthermore, while the resting potential of both the oocyte and cumulus cell underwent changes characteristic of each system employed, the level of neither cell membrane potential was specific to meiotic status. These results support the hypothesis that meiotic maturation in hamster oocytes is accompanied by disruption of the integrity of intercellular, non-ionic coupling between the oocyte and its adherent cumulus cells. The data show, however, that no specific alteration either in the extent of ionic coupling or in the oocyte or cumulus cell resting potential is prerequisite for meiotic resumption in this species.     

Relationship between growth and meiotic maturation of the mouse oocyte

Oocytes of various sizes were isolated from trypsinized ovaries of juvenile mice, cultured in a chemically defined medium, and scored for the resumption and completion of meiotic maturation. Oocytes recovered from mice younger than 15 days remained in the germinal vesicle stage, whereas those from mice 15 days or older resumed meiosis at a frequency which increased with the age of the mice. The mean diameter of the oocytes recovered also increased with the age of the mice. Within individual litters, the mean diameter of oocytes which failed to mature (incompetent oocytes) was significantly less than that of oocytes which matured (competent oocytes). The frequency of premature metaphase I arrest decreased markedly as the age of the mice and oocyte volume increased. These results suggest that the ability to resume meiosis is acquired at a specific stage of oocyte growth in the juvenile mouse, and that the ability to complete meiotic maturation is acquired subsequently. These oocytes provide an in vitro system with which to study the control of meiosis in the mammal.     

Metabolism of radiolabeled glucose by mouse oocytes and oocyte-cumulus cell complexes.

This study was carried out to examine the metabolism of [1-14C]-, [6-14C]-, and [5-3H]glucose by oocyte-cumulus cell complexes (OCC) and denuded oocytes (DO) and to test the hypothesis that metabolism of glucose through the pentose phosphate pathway is associated with meiotic induction. OCC or DO were cultured in hanging drops suspended from the cap of a microfuge tube, with NaOH serving as a trap to collect released 3H2O or 14CO2. Preliminary experiments established that this culture system supports both spontaneous and ligand-induced meiotic maturation. An initial time course experiment (1.5-6 h) showed that hypoxanthine-treated OCC from eCG-primed animals metabolized glucose principally via glycolysis, with an increase to 2.7-fold in response to FSH. Though more [1-14C]glucose was oxidized than [6-14C]glucose, its metabolism was about two orders of magnitude less than that of [5-3H]glucose. Also, FSH significantly increased oxidation of [1-14C]glucose but not [6-14C]glucose, indicating a preferential activation of the pentose phosphate pathway. Pyrroline carboxylate, an activator of the pentose phosphate pathway, increased the activity of this pathway to over 2-fold but failed to affect glucose oxidation through the tricarboxylic acid cycle. Glycolytic metabolism was increased by 25%. The addition of pyruvate to pyruvate-free medium resulted in significant reduction in the metabolism of all three glucose analogues. In OCC retrieved from hCG-injected, primed mice and cultured under hormone-free conditions, metabolic responses were similar to those in FSH-treated complexes cultured in hypoxanthine. DO metabolized glucose, but at a much reduced rate when compared to OCC. Pyruvate reduced the consumption of all three glucose analogues by DO. Pyrroline carboxylate reduced [5-3H]glucose metabolism by DO but had little effect on [1-14C]- and [6-14C]glucose oxidation. These data demonstrate metabolism of glucose by both DO and OCC, but reveal that cumulus cells are more active than the oocyte in this regard. In addition, induction of maturation by FSH, hCG, or pyrroline carboxylate was accompanied by a significant increase in the oxidation of [1-14C]glucose but not [6-14C]glucose by OCC, supporting a proposed role for the pentose phosphate pathway in meiotic induction.     

Metabolic, fluorescent dye and electrical coupling between hamster oocytes and cumulus cells during meiotic maturation in vivo and in vitro

Heterologous intercellular communication was determined qualitatively by lucifer yellow dye transfer and quantitatively by transfer of radiolabeled uridine metabolites and electrical current in hamster oocyte-cumulus complexes during meiotic maturation in vitro and in vivo. In addition, changes in cell resting potentials during maturation were recorded. Significantly less time was required for germinal vesicle breakdown (GVBD) in oocytes matured in vitro than in oocytes stimulated in vivo (1.81 +/- 0.06 hr, N = 13 vs 2.46 +/- 0.07 hr, N = 18, respectively, P less than 0.001). Resting potentials of the oocyte (RP-o) and cumulus cells (RP-c) significantly increased contemporaneously with GVBD in vitro (RP-o: from -18.9 +/- 3.2 mV to -33.2 +/- 2.9 mV, P less than 0.001; RP-c: from -16.3 +/- 1.9 mV to -27.5 +/- 2.6 mV, P less than 0.001) and in vivo after hCG injection (RP-o: from -16.8 +/- 5.9 mV to -30.1 +/- 3.9 mV, P less than 0.001; RP-c: from -15.5 +/- 3.8 mV to -26.3 +/- 3.2 mV, P less than 0.001). RP-o and RP-c progressively increased with time of culture up to 7 hr (maximum time examined) while the values reached maxima in in vivo matured oocytes 4.5 hr post-hCG and subsequently declined concomitant with the onset of cumulus expansion. Cumulus to oocyte coupling decreased progressively with time after release from meiotic arrest both in vitro and in vivo, as assessed by a progressive reduction in transfer of either uridine marker or lucifer yellow from the cumulus cell to the oocyte. By 4.5 hr after hCG injection, cumulus expansion had begun in 100% of complexes examined. Expansion was extensive by 7 hr post-hCG and spread of lucifer yellow from a cumulus cell was limited to very few adjacent cumulus cells. Oocyte to cumulus cell metabolic coupling also decreased progressively with time in both treatment groups. Examination of the extent of heterologous ionic coupling revealed that ionic coupling exhibited biphasic and, bidirectionally parallel, increases during meiotic maturation. While these temporal changes were observed in both groups, the coupling ratios were much greater in those complexes matured in vitro than in vivo. These results show that dye, metabolic, and electrical coupling exist between the immature hamster oocyte and its surrounding cumulus cells but that during the early stages of meiosis, metabolic and dye coupling decrease, while electrical coupling increases biphasically.     

Inhibitory effect of purines in meiotic maturation of denuded mouse oocytes.

The potential action of purines, such as hypoxanthine and adenosine, in meiotic arrest was examined using denuded mouse oocytes. The spontaneous meiotic maturation of denuded oocytes was significantly inhibited by hypoxanthine and/or adenosine in a dose-dependent manner. Germinal vesicle breakdown (GVBD) was inhibited even at a low concentration (1 nM) of hypoxanthine, when hypoxanthine was microinjected into the cytoplasm of denuded oocytes. This inhibitory action was potentiated by co-injection with allopurinol, a metabolic blocker of hypoxanthine that can block a metabolic pathway to uric acid. By contrast, a microinjection of adenosine was no longer effective in inhibiting GVBD. Inhibitory action of purines in meiotic maturation was correlated with sustaining intracellular cAMP levels. GVBD was resumed by econazole, one of the nitroimidazole derivatives which act as inhibitors of catalytic subunit of adenylate cyclase. This compound was effective in counteracting the effect of adenosine, but not the action of 3-isobutyl-1-methylxanthine (IBMX) on GVBD, indicating that adenosine is probably exerted at the level of oocyte plasmalemma. These data suggest that the inhibitory action of hypoxanthine and adenosine in oocyte meiotic maturation may be involved in the regulation of cAMP metabolism in a differential manner.     

Stability of cyclin B protein during meiotic maturation and the first mitotic cell division in mouse oocytes

This report examines in detail the metabolism of the cyclin protein B1 during meiotic maturation and following the activation of mature mouse oocytes using immunoprecipitation of the radiolabelled protein. The net synthesis of cyclin B increases progressively during meiotic maturation, reaching its maximum levels at least 1 h before oocytes exit into metaphase of meiosis II (MII). This increase correlates with the rise in cdc2 kinase activity reported previously and suggests an association between the length of the first meiotic M phase (MI) and the net synthesis of cyclin B, that seems to regulate the time required for the cdc2 kinase to reach its maximum activity. Moreover, no marked degradation of cyclin B was observed before the MI to MII transition and that which occurs does so independently of the presence of microtubules, which are essential for cyclin degradation during metaphase II arrest and exit of oocytes into interphase of the first mitotic cell cycle. Cyclin B is degraded rapidly during the transitions MI to MII, MII to the first mitotic interphase and MII to an abortive third metaphase state (MIII). However, whilst its degradation was incomplete during the MI to MII transition, virtually no cyclin B protein was detected following both the MII to interphase and MII to MIII transitions. Thus, the decision of oocytes to exit into MIII, or interphase is not controlled at the level of cyclin B degradation. Lastly, in aging, non-activated oocytes, the net synthesis of cyclin B declines. Whereas, in activated eggs cultured in parallel although the rate of net synthesis declines initially, it is effectively ‘rescued’ being two-fold greater than in non-activated oocytes of an equivalent age. This gradual fall in the net synthesis of cyclin B observed in aging oocytes may contribute to the increasing ease with which they become activated, compared to recently ovulated oocytes.     

Physiological changes in oocyte-cumulus cell complexes from diabetic mice that potentially influence meiotic regulation

We have previously shown that the type I diabetic condition significantly alters meiotic regulation in mouse oocytes. In the present study, possible physiological deficiencies underlying such meiotic dysfunction were examined in oocyte-cumulus cell complexes (OCC) from type I diabetic mice. Whereas the diabetic condition did not affect glycolysis or the tricarboxylic acid cycle, the increased flux of glucose through the pentose phosphate pathway in response to FSH treatment was suppressed. De novo purine synthesis was also compromised, and ATP levels were reduced in freshly isolated OCC. Additionally, diabetes resulted in a reduction in FSH-mediated cAMP synthesis. The responsiveness of the oocyte to cAMP was also affected; fewer oocytes were induced to resume maturation after a stimulatory pulse with cAMP analogs. Meiotic induction triggered by FSH was significantly reduced, but that stimulated by phorbol ester or epidermal growth factor was affected to a much lesser extent. In addition to metabolic deficiencies, the cell-cell communication between the oocyte and the cumulus cells was reduced in diabetic mice as determined by coupling assays. Thus, numerous physiological parameters are affected by type I diabetes, and these changes may collectively contribute to altered meiotic regulation.     

Microtubule patterning during meiotic maturation in mouse oocytes is determined by cell cycle-specific sorting and redistribution of gamma-tubulin.

The topography of microtubule assembly events during meiotic maturation of animal oocytes demands tight spatial control and temporal precision. To better understand what regulates the timing and location of microtubule assembly, synchronously maturing mouse oocytes were evaluated with respect to gamma-tubulin, pericentrin, and total tubulin polymer fractions at specific stages of meiotic progression. gamma-Tubulin remained associated with cytoplasmic centrosomes through diakinesis of meiosis-1. Following chromatin condensation and perinuclear centrosome aggregation, gamma-tubulin relocated to a nuclear lamina-bounded compartment in which meiosis-1 spindle assembly occurred. gamma-Tubulin was stably associated with the meiotic spindle from prometaphase-1 through to anaphase-2, but also exhibited cell cycle-specific relocalization to cytoplasmic centrosomes. Specifically, anaphase onset of both meiosis-1 and -2 was characterized by the concomitant appearance of gamma-tubulin and microtubule nucleation in subcortical centrosomes. Brief pulses of taxol applied at specific cell cycle stages enhanced detection of gamma-tubulin compartmentalization, consistent with a gamma-tubulin localization-dependent spatial restriction of microtubule assembly during meiotic progression. In addition, a taxol pulse during meiotic resumption impaired subsequent gamma-tubulin sorting, resulting in monopolar spindle formation and cell cycle arrest in meiosis-1; despite cell cycle arrest, polar body extrusion occurred roughly on schedule. Therefore, sorting of gamma-tubulin is involved in both the timing of location of meiotic spindle assembly as well as the coordination of karyokinesis and cytokinesis in mouse oocytes.     

Steroidogenesis by the human oocyte-cumulus cell complex in vitro

The ability of the human oocyte-cumulus cell complex to synthesize progesterone, androgens and estrogens and to modify its endocrine environment in vitro was investigated. Germinal-vesicle stage oocytes with adhering layers of cumulus cells were recovered from human ovaries and maintained for 40–50 h in vitro in a culture medium with or without antral fluid.The results show that oocyte-cumulus (0-C) cell complexes were capable of synthesizing progesterone, androgens and estrogens. Oocytes with the capacity of resuming meiosis in vitro were part of an 0-C complex producing significantly more progesterone than those 0-C complexes containing oocytes incapable of resuming meiosis. Irrespective of the stage of oocyte maturation at the end of culture, testosterone and estrone were respectively the major androgen and estrogen produced.It is concluded that the oocyte-cumulus compartment of the antral follicle is a steroidogenically competent unit and that it has the capacity to modify the endocrine microenvironment of the follicle.     

Induction of meiotic maturation in mouse oocytes by adenosine analogs

In this study we have examined the meiosis-inducing influence of adenosine analogs in mouse oocytes. When a varied group of nucleosides and nucleotides were tested on overnight cultures of hypoxanthine-arrested, cumulus cell-enclosed oocytes (CEO), halogenated adenosine nucleosides, but not native adenosine, exhibited a significant meiosis-inducing capability. When tested under a variety of conditions, meiotic induction by 8-bromo-adenosine (8-Br-Ado) and a second adenosine analog, methylmercaptopurine riboside (MMPR), was especially potent in denuded oocytes (DO) compared to CEO and was not dependent on the type of inhibitor chosen to maintain meiotic arrest. Germinal vesicle breakdown (GVB) was stimulated with rapid kinetics and was preceded by an increase in AMP-activated protein kinase (AMPK) activity. Moreover, compound C, an inhibitor of AMPK, blocked the meiosis-inducing activities of both adenosine analogs. When tested for an effect on meiotic progression to metaphase II (MII) in spontaneously maturing CEO, 8-Br-Ado and the AMPK activator, 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR), increased the percentage of MII-stage oocytes, but MMPR decreased this number. Adenosine and inhibitors of de novo purine synthesis had no effect on the completion of maturation, while compound C suppressed this process. These results support the proposition that oocyte AMPK mediates the positive influence of AICAR and 8-Br-Ado on both the initiation and completion of meiotic maturation. The role of AMPK in MMPR action is less clear.     

Regulation of histone acetylation during meiotic maturation in mouse oocytes

Histone acetylation is an important epigenetic modification implicated in the regulation of chromatin structure and, subsequently, gene expression. Global histone deacetylation was reported in mouse oocytes during meiosis but not mitosis. The regulation of this meiosis-specific deacetylation has not been elucidated. Here, we demonstrate that p34(cdc2) kinase activity and protein synthesis are responsible for the activation of histone deacetylases and the inhibition of histone acetyltransferases (HATs), respectively, resulting in deacetylation of histone H4 at lysine-12 (H4K12) during mouse oocyte meiosis. Temporal changes in the acetylation state of H4K12 were examined immunocytochemically during meiotic maturation using an antibody specific for acetylated H4K12. H4K12 was deacetylated during the first meiosis, temporarily acetylated around the time of the first polar body (PB1) extrusion, and then deacetylated again during the second meiosis. Because these changes coincided with the known oscillation pattern of p34(cdc2) kinase activity, we investigated the involvement of the kinase in H4K12 deacetylation. Roscovitine, an inhibitor of cyclin-dependent kinase activity, prevented H4K12 deacetylation during both the first and second meiosis, suggesting that p34(cdc2) kinase activity is required for deacetylation during meiosis. In addition, cycloheximide, a protein synthesis inhibitor, also prevented deacetylation. After PB1 extrusion, at which time H4K12 had been deacetylated, H4K12 was re-acetylated in the condensed chromosomes by treatment with cycloheximide but not with roscovitine. These results demonstrate that HATs are present but inactivated by newly synthesized protein(s) that is (are) not involved in p34(cdc2) kinase activity. Our results suggest that p34(cdc2) kinase activity induces the deacetylation of H4K12 and that the deacetylated state is maintained by newly synthesized protein(s) that inhibits HAT activity during meiosis.     

Localization and function of mSpindly during mouse oocyte meiotic maturation

Spindly was first identified in Drosophila; its homologues are termed SPDL-1 in Caenorhabditis elegans and Hs Spindly/hSpindly in humans. In all species, Spindly and its homologues function by recruiting dynein to kinetochores and silencing SAC in mitosis of somatic cells. Depletion of Spindly causes an extensive metaphase arrest during somatic mitoses in Drosophila, C. elegans and humans. In Drosophila, Spindly is required for shedding of Rod and Mad2 from the kinetochores in metaphase; in C. elegans, SPDL-1 presides over the recruitment of dynein and MDF-1 to the kinetochores; in humans, Hs Spindly is required for recruiting both dynein and dynactin to kinetochores but it is dispensable for removal of checkpoint proteins from kinetochores. The present study was designed to investigate the localization and function of the Spindly homologue (mSpindly) during mouse oocyte meiotic maturation by immunofluorescent analysis, and by overexpression and knockdown of mSpindly. We found that mSpindly was typically localized to kinetochores when chromatin condensed into chromosomes after GVBD. In metaphase of both first meiosis and second meiosis, mSpindly was localized not only to kinetochores but also to the spindle poles. Overexpression of mSpindly did not affect meiotic progression, but its depletion resulted in an arrest of the pro-MI/MI stage, failure of anaphase entry and subsequent polar body emission, and in abnormal spindle morphology and misaligned chromosomes. Our data suggest that mSpindly participates in SAC silencing and in spindle formation as a recruiter and/or a transporter of kinetochore proteins in mouse oocytes, but that it needs to cooperate with other factors to fulfill its function.     

Changes in histone acetylation during oocyte meiotic maturation in the diabetic mouse

Although there is considerable evidence that diabetes can adversely affect meiosis in mammalian oocytes, acetylation status of oocytes in a diabetic environment remains unclear. The objective was to determine acetylation or deacetylation patterns (based on immunostaining) of H3K9, H3K14, H4K5, H4K8, H4K12, and H4K16 sites at various stages during meiosis in murine oocytes from control and diabetic mice. According to quantitative real time polymerase chain reaction (qPCR), mean ± SEM relative expression of Gcn5 (1.70 ± 0.14 at metaphase [M]I and 1.27 ± 0.01 at MII, respectively), Ep300 (1.74 ± 0.04 at MI and 1.80 ± 0.001 at MII), and Pcaf (2.01 ± 0.03 at MI and 1.41 ± 0.18 at MII) mRNA in oocytes from diabetic mice were higher than those from controls (P < 0.05), whereas there was no difference (P > 0.05) during the germinal vesicle (GV) stage between the two groups (1.23 ± 0.04 for Gcn5, 0.82 ± 0.06 for Ep300, and 0.80 ± 0.07 for Pcaf). Conversely, relative mRNA expression concentrations of Hdac1, Hdac2, Hdac3, Sirt1 and Sirt2 during the germinal vesicle stage were lower in oocytes of diabetic mice (0.24 ± 0.03 for Hdac1, 0.11 ± 0.001 for Hdac2, 0.31 ± 0.03 for Hdac3, 0.28 ± 0.02 for Sirt1, and 0.55 ± 0.02 for Sirt2; P < 0.05). Similarly, the expression concentrations of these genes at the MI stage were lower in oocytes from diabetic mice (0.79 ± 0.12 for Hdac1, 0.72 ± 0.001 for Hdac2, 0.02 ± 0.001 for Sirt1, and 0.84 ± 0.08 for Sirt2; P < 0.05). Their expression concentrations at the MII stage were also lower in oocytes from diabetic mice (0.46 ± 0.03 for Hdac1, 0.93 ± 0.01 for Hdac2, 0.56 ± 0.01 for Hdac3, 0.01 ± 0.002 for Sirt1, and 0.84 ± 0.04 for Sirt2; P < 0.05). At the MI stage, however, there was no difference in the expression of Hdac3 between the two groups of oocytes (0.96 ± 0.03; P > 0.05). Taken together, diabetes altered the intracellular histone modification system, which may have contributed to changes in histone acetylation, and may be involved in the compromised maturation rate of oocytes in diabetic humans.