Second meiotic spindle integrity requires MEK/MAP kinase activity in mouse eggs

Vitrification of bovine germinal vesicle oocytes significantly decreased the methylation level of their in vitro derived MII oocytes

CONTEXT

The vitrification of oocytes is important for the conservation of animals, and the effect of vitrification on methylation patterns of bovine oocytes remains unclear.

AIMS

This article aims to investigate the effect of vitrification on the DNA methylation patterns on vitrified GV oocytes and their in vitro derived MII oocytes.

METHODS

5-MeC staining and single-cell whole genome bisulphite sequencing (SC-WGBS) were utilised to analyse fresh GV oocytes (F_GV group), MII oocytes (F_MII group), vitrified GV oocytes (V_GV group) and their in vitro derived MII oocytes (V_MII group).

KEY RESULTS

Results of both 5-MeC staining and SC-WGBS showed that no significant difference was found between the F_GV group and the V_GV group, while the methylation level of the V_MII group was significantly lower than that of the F_MII group. Moreover, supplementation of 2μM resveratrol (Res) in IVM medium significantly improved maturation and development ability of vitrified GV oocytes by restoring their DNA methylation levels.

CONCLUSION

In conclusion, vitrification of bovine GV oocytes significantly decreased the DNA methylation level of their in vitro derived MII oocytes, and 2μM Res improved their development ability by restoring DNA methylation level.

IMPLICATIONS

Our results provide an efficient approach to improve the maturation and fertilisation ability of vitrified GV oocytes.

Melatonin improves the first cleavage of parthenogenetic embryos from vitrified-warmed mouse oocytes potentially by promoting cell cycle progression

Background

This study investigated the effect of melatonin (MT) on cell cycle (G1/S/G2/M) of parthenogenetic zygotes developed from vitrified-warmed mouse metaphase II (MII) oocytes and elucidated the potential mechanism of MT action in the first cleavage of embryos.

Results

After vitrification and warming, oocytes were parthenogenetically activated (PA) and in vitro cultured (IVC). Then the spindle morphology and chromosome segregation in oocytes, the maternal mRNA levels of genes including Miss, Doc1r, Setd2 and Ythdf2 in activated oocytes, pronuclear formation, the S phase duration in zygotes, mitochondrial function at G1 phase, reactive oxygen species (ROS) level at S phase, DNA damage at G2 phase, early apoptosis in 2-cell embryos, cleavage and blastocyst formation rates were evaluated. The results indicated that the vitrification/warming procedures led to following perturbations 1) spindle abnormalities and chromosome misalignment, alteration of maternal mRNAs and delay in pronucleus formation, 2) decreased mitochondrial membrane potential (MMP) and lower adenosine triphosphate (ATP) levels, increased ROS production and DNA damage, G1/S and S/G2 phase transition delay, and delayed first cleavage, and 3) increased early apoptosis and lower levels of cleavage and blastocyst formation. Our results further revealed that such negative impacts of oocyte cryopreservation could be alleviated by supplementation of warming, recovery, PA and IVC media with 10^− 9 mol/L MT before the embryos moved into the 2-cell stage of development.

Conclusions

MT might promote cell cycle progression via regulation of MMP, ATP, ROS and maternal mRNA levels, potentially increasing the first cleavage of parthenogenetic zygotes developed from vitrified–warmed mouse oocytes and their subsequent development.

Heat shock protein 90α couples with the MAPK-signaling pathway to determine meiotic maturation of porcine oocytes

Heat shock protein 90 (Hsp90) functions as a molecular chaperone in its interaction with clients to influence multiple cellular and physiological processes. However, our current understanding on Hsp90's relationship with mammalian oocyte maturation is still very limited. Here, we aimed to investigate Hsp90's effect on pig oocyte meiotic maturation. Endogenous Hsp90α was constantly expressed at both mRNA and protein levels in porcine maturing oocytes. Addition of 2 µM 17-allylamino-17-demethoxygeldanamycin (17-AAG), the Hsp90 inhibitor, to in vitro mature cumulus-oocyte complexes (COC) significantly decreased Hsp90α protein level (P < 0.05), delayed germinal vesicle breakdown (GVBD) (P < 0.05), and impeded the first polar body (PB1) extrusion (P < 0.01) of porcine oocytes. 2 µM 17-AAG treatment during in vitro maturation also decreased the subsequent development competence as indicated by the lower cleavage (P < 0.001) and higher fragmentation (P < 0.001) rates of parthenotes, whereas no effects on the percentage and average cell number of blastocysts were found. Immunodepletion of Hsp90α by antibody microinjection into porcine oocytes at germinal vesicle and metaphase II stages induced similar defects of meiotic maturation and parthenote development, to that resulted from 2 µM inhibitor 17-AAG. For oocytes treated by 2 µM 17-AAG, the cytoplasm and membrane actin levels were weakened (P < 0.01), and the spindle assembly was disturbed (P < 0.05), due to decreased p-ERK1/2 level (P < 0.05). However, the mitochondrial function and early apoptosis were not affected, as demonstrated by rhodamine 123 staining and Annexin V assays. Our findings indicate that Hsp90α can couple with mitogen-activated protein kinase to regulate cytoskeletal structure and orchestrate meiotic maturation of porcine oocytes.

ERK Plays a Role in Chromosome Alignment and Participates in M-Phase Progression

Cell division, a prerequisite for cell proliferation, is a process in which each daughter cell inherits one complete set of chromosomes. The mitotic spindle is a dedicated apparatus for the alignment and segregation of chromosomes. Extracellular signal-regulated kinase (ERK) 1/2 plays crucial roles in cell cycle progression, particularly during M-phase. Although, association with the mitotic spindle has been reported, the precise roles played by ERK in the dynamics of the mitotic spindle and in M-phase progression remain to be elucidated. In this study, we used MEK inhibitors U0126 and GSK1120212 to dissect the roles of ERK in M-phase progression and chromosome alignment. Fluorescence microscopy revealed that ERK is localized to the spindle microtubules in a manner independent of Src kinase, which is one of the kinases upstream of ERK at mitotic entry. ERK inhibition induces an increase in the number of prophase cells and a decrease in the number of anaphase cells. Time-lapse imaging revealed that ERK inhibition perturbs chromosome alignment, thereby preventing cells from entering anaphase. These results suggest that ERK plays a role in M-phase progression by regulating chromosome alignment and demonstrate one of the mechanisms by which the aberration of ERK signaling may produce cancer cells.

MAPK3/1 (ERK1/2) and Myosin Light Chain Kinase in Mammalian Eggs Affect Myosin-II Function and Regulate the Metaphase II State in a Calcium- and Zinc-Dependent Manner

Vertebrate eggs are arrested at metaphase of meiosis II, a state classically known as cytostatic factor arrest. Maintenance of this arrest until the time of fertilization and then fertilization-induced exit from metaphase II are crucial for reproductive success. Another key aspect of this meiotic arrest and exit is regulation of the metaphase II spindle, which must be appropriately localized adjacent to the egg cortex during metaphase II and then progress into successful asymmetric cytokinesis to produce the second polar body. This study examined the mitogen-activated protein kinases MAPK3 and MAPK1 (also known as ERK1/2) as regulators of these two related aspects of mammalian egg biology, specifically testing whether this MAPK pathway affected myosin-II function and whether myosin-II perturbation would produce some of the same effects as MAPK pathway perturbation. Inhibition of the MEK1/2-MAPK pathway with U0126 leads to reduced levels of phosphorylated myosin-regulatory light chain (pMRLC) and causes a reduction in cortical tension, effects that are mimicked by treatment with the myosin light chain kinase (MLCK) inhibitor ML-7. These data indicate that one mechanism by which the MAPK pathway acts in eggs is by affecting myosin-II function. We further show that MAPK or MLCK inhibition induces loss of normal cortical spindle localization or parthenogenetic egg activation. This parthenogenesis is dependent on cytosolic and extracellular calcium and can be rescued by hyperloading eggs with zinc, suggesting that these effects of inhibition of MLCK or the MAPK pathway are linked with dysregulation of ion homeostasis.

The dynamics of MAPK inactivation at fertilization in mouse eggs

Egg activation at fertilization in mammals is initiated by prolonged Ca(2+) oscillations that trigger the completion of meiosis and formation of pronuclei. A fall in mitogen-activated protein kinase (MAPK) activity is essential for pronuclear formation, but the precise timing and mechanism of decline are unknown. Here, we have measured the dynamics of MAPK pathway inactivation during fertilization of mouse eggs using novel chemiluminescent MAPK activity reporters. This reveals that the MAPK activity decrease begins during the Ca(2+) oscillations, but MAPK does not completely inactivate until after pronuclear formation. The MAPKs present in eggs are Mos, MAP2K1 and MAP2K2 (MEK1 and MEK2, respectively) and MAPK3 and MAPK1 (ERK1 and ERK2, respectively). Notably, the MAPK activity decline at fertilization is not explained by upstream destruction of Mos, because a decrease in the signal from a Mos-luciferase reporter is not associated with egg activation. Furthermore, Mos overexpression does not affect the timing of MAPK inactivation or pronuclear formation. However, the late decrease in MAPK could be rapidly reversed by the protein phosphatase inhibitor, okadaic acid. These data suggest that the completion of meiosis in mouse zygotes is driven by an increased phosphatase activity and not by a decline in Mos levels or MEK activity.

Mouse embryos stressed by physiological levels of osmolarity become arrested in the late 2-cell stage before entry into M phase

Preimplantation mouse embryos of many strains become arrested at the 2-cell stage if the osmolarity of culture medium that normally supports development to blastocysts is raised to approximately that of their normal physiological environment in the oviduct. Arrest can be prevented if molecules that serve as "organic osmolytes" are present in the medium, because organic osmolytes, principally glycine, are accumulated by embryos to provide intracellular osmotic support and regulate cell volume. Medium with an osmolarity of 310 mOsM induced arrest of approximately 80% of CF1 mouse embryos at the 2-cell stage, in contrast to the approximately 100% that progressed beyond the 2-cell stage at 250 or 301 mOsM with glycine. The nature of this arrest induced by physiological levels of osmolarity is unknown. Arrest was reversible by transfer to lower-osmolarity medium at any point during the 2-cell stage, but not after embryos would normally have progressed to the 4-cell stage. Cessation of development likely was not due to apoptosis, as shown by lack of external annexin V binding, detectable cytochrome c release from mitochondria, or nuclear DNA fragmentation. Two-cell embryos cultured at 310 mOsM progressed through the S phase, and zygotic genome activation markers were expressed. However, most embryos failed to initiate the M phase, as evidenced by intact nuclei with decondensed chromosomes, low M-phase promoting factor activity, and an inactive form of CDK1, although a few blastomeres were arrested in metaphase. Thus, embryos become arrested late in the G(2) stage of the second embryonic cell cycle when stressed by physiological osmolarity in the absence of organic osmolytes.

HCO3(-)/Cl(-) exchange inactivation and reactivation during mouse oocyte meiosis correlates with MEK/MAPK-regulated Ae2 plasma membrane localization

Background

Germinal Vesicle (GV) stage mouse oocytes in first meiotic prophase exhibit highly active HCO₃⁻/Cl⁻ exchange—a class of transport nearly ubiquitously involved in regulation of intracellular pH and cell volume. During meiosis, however, oocyte HCO₃⁻/Cl⁻ exchange becomes inactivated during first metaphase (MI), remains inactive in second metaphase (MII), and is reactivated only after egg activation. Previous work using pharmacological manipulations had indicated that activity of the MEK/MAPK signaling pathway was negatively correlated with HCO₃⁻/Cl⁻ exchange activity during meiosis. However, the mechanism by which the exchanger is inactivated during meiotic progression had not been determined, nor had the role of MEK/MAPK been directly established.

Methodology/Principal Findings

Expression of a constitutively active form of MEK (MAP kinase kinase), which prevented the normal downregulation of MAPK after egg activation, also prevented reactivation of HCO₃⁻/Cl⁻ exchange. Conversely, suppression of endogenous MAPK activity with dominant negative MEK activated the normally quiescent HCO₃⁻/Cl⁻ exchange in mature MII eggs. A GFP-tagged form of the HCO₃⁻/Cl⁻ exchanger isoform Ae2 (Slc4a2) was strongly expressed at the GV oocyte plasma membrane, but membrane localization decreased markedly during meiotic progression. A similar pattern for endogenous Ae2 was confirmed by immunocytochemistry. The loss of membrane-localized Ae2 appeared selective, since membrane localization of a GFP-tagged human dopamine D1 receptor did not change during meiotic maturation.

Conclusions

Direct manipulation of MAPK activity indicated that GFP-tagged Ae2 localization depended upon MAPK activity. Inactivation of HCO₃⁻/Cl⁻ exchange during the meiotic cell cycle may therefore reflect the loss of Ae2 from the oocyte plasma membrane, downstream of MEK/MAPK signaling. This identifies a novel role for MEK/MAPK-mediated cytostatic factor (CSF) activity during meiosis in membrane protein trafficking in mouse oocytes, and shows for the first time that selective retrieval of membrane proteins is a feature of meiosis in mammalian oocytes.

The extracellular calcium-sensing receptor is expressed in the cumulus-oocyte complex in mammals and modulates oocyte meiotic maturation

The extracellular calcium-sensing receptor (CASR) plays an important role in cells involved in calcium (Ca2+) homeostasis by directly sensing changes in the extracellular Ca2+ ion concentration. We previously reported the localization and quantitative expression of CASR protein in human oocytes. In this study, we examined the expression and the functional role of CASR during oocyte meiotic maturation in a large mammal animal model, the horse. As in humans, CASR protein was found to be expressed in equine oocytes and cumulus cells. Western-blot analysis revealed a single 130 kDa band in denuded oocytes and a doublet of 130-120 kDa in cumulus cells. CASR labeling was observed by confocal microscopy in cumulus cells and in oocytes on the plasma membrane and within the cytoplasm at all examined stages of meiosis. Functionally, the CASR allosteric effector NPS R-467, in the presence of 2.92 mM external Ca2+, increased oocyte maturation rate in a dose-dependent manner and its stimulatory effect was attenuated by pre-treatment with the CASR antagonist NPS 2390. NPS R-467 had no effect in suboptimal external Ca2+ (0.5 mM), indicating that it requires higher external Ca2+ to promote oocyte maturation. In oocytes treated with NPS R-467, CASR staining increased at the plasmalemma and was reduced in the cytosol. Moreover, NPS R-467 increased the activity of MAPK, also called ERK, in cumulus cells and oocytes. These results provide evidence of a novel signal transduction pathway modulating oocyte meiotic maturation in mammals in addition to the well-known systemic hormones.