Regulation of fusion of the nucleolar precursor bodies following activation of mouse oocytes: roles of the maturation-promoting factors and mitogen-activated protein kinases

NEDD4-like ubiquitin ligase 2 protein (NEDL2) in porcine spermatozoa, oocytes, and preimplantation embryos and its role in oocyte fertilization†

The ubiquitin-proteasome system plays diverse regulatory and homeostatic roles in mammalian reproduction. Ubiquitin ligases are the substrate-specific mediators of ubiquitin-binding to its substrate proteins. The NEDD4-like ubiquitin ligase 2 (aliases NEDL2, HECW2) is a HECT-type ubiquitin ligase that contains one N-terminal HECW ubiquitin ligase domain, one C-terminal HECT ubiquitin ligase domain, one C2 domain, and two WW protein-protein interaction modules. Beyond its predicted ubiquitin-ligase activity, its cellular functions are largely unknown. Current studies were designed to investigate the content and distribution of NEDL2 in porcine spermatozoa, oocytes, zygotes, and early preimplantation embryos, and in cumulus cells before and after in vitro maturation with oocytes, and fibroblast cells as positive control by western blot and immunocytochemistry, and to examine its roles during oocyte fertilization. Multiple isoforms of NEDL2 were identified by WB. One at approximately 52 kDa was detected only in the germinal vesicle (GV) stage and metaphase II oocytes, and in early preimplantation embryos. Other isoforms were high mass bands at 91, 136, and 155 kDa, which were only detected in somatic cells. Interestingly, ejaculated spermatozoa prominently displayed the same 52 kDa band as oocytes; they also had two minor bands of 74 and 129 kDa, which were not detected in somatic cells or oocytes. By immunofluorescence, NEDL2 showed a diffused cytoplasmic localization in all cell types and accumulated in distinct foci in the germinal vesicles (GVs) of immature oocytes, in maternal and paternal pronuclei of zygotes and nuclei of embryo blastomeres and somatic cells. In blastocysts, the labeling intensity of NEDL2 was stronger in the inner cell mass than in trophoblast, indicating higher NEDL2 content in the ICM cells than in trophectoderm. NEDL2 abundance was 10 times higher in post-maturation oocyte-surrounding cumulus cells than that of cumulus cells before in vitro maturation with hormones, indicating that NEDL2 may have a unique role in cumulus cells after ovulation. Microinjection of anti-NEDL2 antibody into oocyte before IVF did not affect the percentage of oocytes fertilized, percentage of oocytes cleaved, or blastocyst formation. However, the anti-NEDL2 antibody decreased the number of pronuclei, accelerated the formation of nuclear precursor bodies at 6 h postfertilization, inhibited sperm DNA decondensation, and resulted in more fertilized oocytes without male pronuclear formation. In summary, NEDL2 may play a key role during fertilization, especially during sperm DNA decondensation.

Single nucleolus precursor body formation in the pronucleus of mouse zygotes and SCNT embryos

Mammalian oocytes and zygotes have nucleoli that are transcriptionally inactive and structurally distinct from nucleoli in somatic cells. These nucleoli have been termed nucleolus precursor bodies (NPBs). Recent research has shown that NPBs are important for embryonic development, but they are only required during pronuclear formation. After fertilization, multiple small NPBs are transiently formed in male and female pronuclei and then fuse into a single large NPB in zygotes. In cloned embryos produced by somatic cell nuclear transfer (SCNT), multiple NPBs are formed and maintained in the pseudo-pronucleus, and this is considered an abnormality of the cloned embryos. Despite this difference between SCNT and normal embryos, it is unclear how the size and number of NPBs in pronuclei is determined. Here, we show that in mouse embryos, the volume of NPB materials plays a major role in the NPB scaling through a limiting component mechanism and determines whether a single or multiple NPBs will form in the pronucleus. Extra NPB- and extra MII spindle-injection experiments demonstrated that the total volume of NPBs was maintained regardless of the pronucleus number and the ratio of pronucleus/NPB is important for fusion into a single NPB. Based on these results, we examined whether extra-NPB injection rescued multiple NPB maintenance in SCNT embryos. When extra-NPBs were injected into enucleated-MII oocytes before SCNT, the number of NPBs in pseudo-pronuclei of SCNT embryos was reduced. These results indicate that multiple NPB maintenance in SCNT embryos is caused by insufficient volume of NPB.

SIRT1 induces resistance to apoptosis in human granulosa cells by activating the ERK pathway and inhibiting NF-κB signaling with anti-inflammatory functions

SIRT1, a member of the sirtuin family, has recently emerged as a vital molecule in controlling ovarian function. The aims of the present study were to investigate SIRT1 expression and analyze SIRT1-mediated apoptosis in human granulosa cells (GCs). Human ovarian tissues were subjected to immunohistochemistry for localization of SIRT1 expression. SIRT1 knockdown in a human ovarian GC tumor line (COV434) was achieved by small interfering RNA, and the relationship between apoptosis and SIRT1 was assessed by quantitative reverse transcription polymerase chain reaction and western blotting. We further detected SIRT1 expression in human luteinized GCs. Associations among SIRT1 knockdown, SIRT1 stimulation (resveratrol) and expression of ERK1/2 and apoptotic regulatory proteins were analyzed in cell lines and luteinized GCs. Resveratrol downregulated the levels of nuclear factor (NF)-κB/p65, but this inhibitory effect was attenuated by suppressing SIRT1 activity. The NF-κB/p65 inhibitor pyrrolidine dithiocarbamate achieved similar anti-apoptosis effects. These results suggest that SIRT1 might play an anti-apoptotic role in apoptosis processes in GCs, possibly by sensing and regulating the ERK1/2 pathway, which has important clinical implications. Thus, our study provides a mechanistic link, whereby activation of SIRT1 function might help to sustain human reproduction by maintaining GCs as well as oocytes, offering a novel approach for developing a new class of therapeutic anti-inflammatory agents.

Expression profiles and function analysis of microRNAs in postovulatory aging mouse oocytes

In this study, microRNA (miRNA) profiles in postovulatory aging mouse oocytes were analyzed by microarray screening and RT-qPCR. Hierarchical cluster analysis on the microarray data and KEGG pathway enrichment analysis on the mRNAs targeted by differentially expressed (DE) miRNAs between two adjacent egg-ages suggest that while only a mild alteration in miRNA expression occurred from 13 to 18 h, a great change took place from 18 to 24 h post hCG injection. Theoretical exploration on functions of the predicted target genes suggest that KEGG pathways enriched by 13-18 h DE miRNAs are correlated with early events of oocyte aging while pathways most enriched by 18-24 h or 24-30 h DE miRNAs are correlated with the late symptoms of aged oocytes. Experimental verification on functions of the key proteins predicted by the KEGG analysis and injection of miR-98 mimics or inhibitors further confirmed that miRNAs played stimulatory/inhibitory roles in postovulatory oocyte aging. In conclusion, marked changes in miRNA expression are associated with significant alterations in function and morphology of postovulatory aging oocytes.

DMBA acts on cumulus cells to desynchronize nuclear and cytoplasmic maturation of pig oocytes

As an environmental pollutant and carcinogen, 7,12-dimethylbenz[a]anthracene (DMBA) can destroy ovarian follicles at all developmental stages in rodents. However, the underlying molecular mechanism remains obscure. In the present study, we aim to address how DMBA affects the in vitro maturation and development of porcine oocytes. We discovered that for 20 μM DMBA-treated cumulus-oocyte complexes (COCs), the rate of oocyte germinal vesicle breakdown (GVBD) was significantly altered, and the extrusion rate of first polar body was increased. Moreover, oocytes from 20 μM DMBA-treated COCs had significant down-regulation of H3K9me3 and H3K27me3, up-regulation of H3K36me3, higher incidence of DNA double strand breaks (DSBs) and early apoptosis. In striking contrast, none of these changes happened to 20 μM DMBA-treated cumulus-denuded oocytes (CDOs). Furthermore, 20 μM DMBA treatment increased the reactive oxygen species (ROS) level, decreased mitochondrial membrane potential (Δ Ψm), and inhibited developmental competence for oocytes from both COC and CDO groups. Collectively, our data indicate DMBA could act on cumulus cells via the gap junction to disturb the synchronization of nuclear and ooplasmic maturation, and reduce the developmental competence of oocytes.

Localisation of RNAs and proteins in nucleolar precursor bodies of early mouse embryos

Early embryos of all mammalian species contain morphologically distinct but transcriptionally silent nucleoli called the nucleolar precursor bodies (NPBs), which, unlike normal nucleoli, have been poorly studied at the biochemical level. To bridge this gap, here we examined the occurrence of RNA and proteins in early mouse embryos with two fluorochromes - an RNA-binding dye pyronin Y (PY) and the protein-binding dye fluorescein-5'-isothiocyanate (FITC). The staining patterns of zygotic NPBs were then compared with those of nucleolus-like bodies (NLBs) in fully grown surrounded nucleolus (SN)-type oocytes, which are morphologically similar to NPBs. We show that both entities contain proteins, but unlike NLBs, NPBs are significantly impoverished for RNA. Detectable amounts of RNA appear on the NPB surface only after resumption of rDNA transcription and includes pre-rRNAs and 28S rRNA as evidenced by fluorescence in situ hybridisation with specific oligonucleotide probes. Immunocytochemical assays demonstrate that zygotic NPBs contain rRNA processing factors fibrillarin, nucleophosmin and nucleolin, while UBF (the RNA polymerase I transcription factor) and ribosomal proteins RPL26 and RPS10 are not detectable. Based on the results obtained and data in the contemporary literature, we suggest a scheme of NPB assembly and maturation to normal nucleoli that assumes utilisation of maternally derived nucleolar proteins but of nascent rRNAs.

Oocyte vitrification modifies nucleolar remodeling and zygote kinetics-a sibling study

Purpose

Oocyte vitrification does not affect embryo quality after oocyte warming, making this method effective in the preservation of female fertility. Morphokinetic parameters can be used to predict the competence of an embryo produced from fresh oocytes. Our aim was to study the effect of oocyte vitrification on zygote-embryo kinetics (pl).

Methods

The embryo-kinetics of fresh and sibling vitrified/warmed oocytes were compared to determine the consequences of oocyte preservation on the timing of embryo development. A 44-hours time-lapse analysis, from the time of ICSI (t0), of 179 fertilized fresh oocytes was compared to 168 fertilized sibling vitrified/warmed oocytes.

Results

Oocyte vitrification accelerated pronuclear disappearance, one-cell stage timing and modified nucleoli activity by increasing their number and decreasing their diameter at the zygote stage. In contrast, embryo kinetics during cleavage were similar to those observed for fresh sibling oocytes based on the parameters examined in this study.

Conclusions

At the zygote stage, oocyte vitrification induces changes in pronuclei stability, probably due to pronuclei envelop instability as well as modifications in nucleoli functionality. Therefore, the predictive morphokinetic parameters on embryo competence found from fresh oocytes must be revised when applied on embryos from vitrified/warmed oocytes.

Role of cytoskeleton in regulating fusion of nucleoli: a study using the activated mouse oocyte model

Although fusion of nucleoli was observed during pronuclear development of zygotes and the behavior of nucleoli in pronuclei has been suggested as an indicator of embryonic developmental potential, the mechanism for nucleolar fusion is unclear. Although both cytoskeleton and the nucleolus are important cellular entities, there are no special reports on the relationship between the two. Role of cytoskeleton in regulating fusion of nucleoli was studied using the activated mouse oocyte model. Mouse oocytes were cultured for 6 h in activating medium (Ca²⁺-free CZB medium containing 10 mM SrCl₂) supplemented with or without inhibitors for cytoskeleton or protein synthesis before pronuclear formation, nucleolar fusion, and the activity of maturation-promoting factor (MPF) were examined. Whereas treatment with microfilament inhibitor cytochalasin D or B or intermediate filament inhibitor acrylamide suppressed nucleolar fusion efficiently, treatment with microtubule inhibitor demecolcine or nocodazole or protein synthesis inhibitor cycloheximide had no effect. The cytochalasin D- or acrylamide-sensitive temporal window coincided well with the reported temporal window for nucleolar fusion in activated oocytes. Whereas a continuous incubation with demecolcine prevented pronuclear formation, pronuclei formed normally when demecolcine was excluded during the first hour of activation treatment when the MPF activity dropped dramatically. The results suggest that 1) microfilaments and intermediate filaments but not microtubules support nucleolar fusion, 2) proteins required for nucleolar fusion including microfilaments and intermediate filaments are not de novo synthesized, and 3) microtubule disruption prevents pronuclear formation by activating MPF.

Roles of MAPK and spindle assembly checkpoint in spontaneous activation and MIII arrest of rat oocytes

Rat oocytes are well known to undergo spontaneous activation (SA) after leaving the oviduct, but the SA is abortive with oocytes being arrested in metaphase III (MIII) instead of forming pronuclei. This study was designed to investigate the mechanism causing SA and MIII arrest. Whereas few oocytes collected from SD rats at 13 h after hCG injection that showed 100% of mitogen-activated protein kinase (MAPK) activities activated spontaneously, all oocytes recovered 19 h post hCG with MAPK decreased to below 75% underwent SA during in vitro culture. During SA, MAPK first declined to below 45% and then increased again to 80%; the maturation-promoting factor (MPF) activity fluctuated similarly but always began to change ahead of the MAPK activity. In SA oocytes with 75% of MAPK activities, microtubules were disturbed with irregularly pulled chromosomes dispersed over the spindle and the spindle assembly checkpoint (SAC) was activated. When MAPK decreased to 45%, the spindle disintegrated and chromosomes surrounded by microtubules were scattered in the ooplasm. SA oocytes entered MIII and formed several spindle-like structures by 6 h of culture when the MAPK activity re-increased to above 80%. While SA oocytes showed one Ca²⁺ rise, Sr²⁺-activated oocytes showed several. Together, the results suggested that SA stimuli triggered SA in rat oocytes by inducing a premature MAPK inactivation, which led to disturbance of spindle microtubules. The microtubule disturbance impaired pulling of chromosomes to the spindle poles, caused spindle disintegration and activated SAC. The increased SAC activity reactivated MPF and thus MAPK, leading to MIII arrest.