Timing of Plk1 and MPF activation during porcine oocyte maturation

Different p53 genotypes regulating different phosphorylation sites and subcellular location of CDC25C associated with the formation of polyploid giant cancer cells

Background

Our previous studies have confirmed that cobalt chloride (CoCl₂) can induce the formation of polyploid giant cancer cells (PGCCs), which is the key to the heterogeneity of solid tumors. PGCC formation is closely related to the abnormal expression of cell cycle-related proteins and cell fusion. In this study, we investigated the molecular mechanism of PGCCs formation by detecting the expression of cell cycle-related proteins in mutant and wild-type p53 cancer cell lines.

Methods

HEY, BT-549, SKOv3 and MDA-MB-231 cells were treated with CoCl₂ and the cell cycle was detected by flow cytometry. The expression and subcellular localization of cell cycle-related proteins, kinases, and P53 were compared before and after CoCl₂ treatment. Immunoprecipitation was used to analyze the interacting proteins of pCDC25C-Ser216 and pCDC25C-Ser198. The clinicopathologic significances of these cell cycle-related proteins and protein kinases expression were studied.

Results

CoCl₂ induced the formation of PGCCs and G2/M arrest. CDC25C, cyclin B1, and CDK1 expressions after CoCl₂ treatment were lower than that in control cells. Cytoplasmic CDC25C was degraded by ubiquitin-dependent proteasome. The expression of P53 and phosphokinases including CHK1, CHK2, PLK1, and Aurora A increased after CoCl₂ treatment. The expression of pCDC25C-Ser216 and pCDC25C-Ser198 depended upon the genotype of p53. The expressions of cell cycle-related proteins and kinases gradually increased with the development of ovarian cancer and breast cancer.

Conclusion

CHK1, CHK2–pCDC25C-Ser216–cyclin B1–CDK1, and Aurora A–PLK1–pCDC25C-Ser198–cyclin B1–CDK1 signaling pathways may participate in the formation of PGCCs and different phosphorylation sites of CDC25C may be associated with the genotype of p53.

Plk1 is essential for proper chromosome segregation during meiosis I/meiosis II transition in pig oocytes

BACKGROUND

Polo-like kinase 1 (Plk1), as a characteristic regulator in meiosis, organizes multiple biological events of cell division. Although Plk1 has been implicated in various functions in somatic cell mitotic processes, considerably less is known regarding its function during the transition from metaphase I (MI) to metaphase II (MII) stage in oocyte meiotic progression.

METHODS

In this study, the possible role of Plk1 during the MI-to-MII stage transition in pig oocytes was addressed. Initially, the spatiotemporal expression and subcellular localization pattern of Plk1 were revealed in pig oocytes from MI to MII stage using indirect immunofluorescence and confocal microscopy imaging techniques combined with western blot analyses. Moreover, a highly selective Plk1 inhibitor, GSK461364, was used to determine the potential role of Plk1 during this MI-to-MII transition progression.

RESULTS

Upon expression, Plk1 exhibited a specific dynamic intracellular localization, and co-localization of Plk1 with α-tubulin was revealed in the meiotic spindle of pig oocyte during the transition from MI to MII stage. GSK461364 treatment significantly blocked the first polar body (pbI) emission in a dose-dependent manner and resulted in a failure of meiotic maturation, with a larger percentage of the GSK461364-treated oocytes arresting in the anaphase-telophase I (ATI) stage. Further subcellular structure examination results showed that inhibition of Plk1 with GSK461364 had no visible effect on spindle assembly but caused a significantly higher proportion of the treated oocytes to have obvious defects in homologous chromosome segregation at ATI stage.

CONCLUSIONS

Thus, these results indicate that Plk1 plays an essential role during the meiosis I/meiosis II transition in porcine oocytes, and the regulation is associated with Plk1's effects on homologous chromosome segregation in the ATI stage.

Plk1 inhibition leads to a failure of mitotic division during the first mitotic division in pig embryos

PURPOSE

This study was conducted to examine the dynamic distribution of polo-like 1 kinase (Plk1) and the possible role it plays in first mitotic division during early porcine embryo development.

METHODS

Indirect immunofluorescence and confocal microscopy imaging techniques combined with western blot analyses were used to study the dynamic expression and subcellular localization of Plk1 protein in pig parthenogenetic embryos. Finally, a selective Plk1 inhibitor, GSK461364, was used to evaluate the potential role of Plk1 during this special stage.

RESULTS

The results showed that Plk1 upon expression exhibited specific dynamic intracellular localization, which closely correlated with the α-tubulin distribution during the first mitotic division. GSK461364 treatment resulted in cleavage failure, with majority of the GSK461364-treated embryos being arrested in prometaphase. Further results of the subcellular structure examination showed that GSK461364 treatment led to a significantly higher proportion of the treated embryos having abnormal spindles and misarranged chromosomes at the prometaphase stage.

CONCLUSIONS

Thus, these results indicated that Plk1 is essential for porcine embryos to complete the first mitotic division. Furthermore, Plk1 regulation was associated with effects on spindle assembly and chromosome arrangement.

The optimal period of Ca-EDTA treatment for parthenogenetic activation of porcine oocytes during maturation culture

The changes triggered by sperm-induced activation of oocytes, which are required for normal oocyte development, can be mediated by other agents, thereby inducing the parthenogenesis. In this study, we exposed porcine oocytes to 1 mM Ca-EDTA, a metal-ion chelator, at various intervals during 48 hr of in vitro maturation to determine the optimum period of Ca-EDTA treatment for parthenogenetic activation. When the oocytes were cultured with or without Ca-EDTA from 36 hr (post-12), 24 hr (post-24), 12 hr (post-36) and 0 hr (post-48) after the start of maturation culture, the blastocyst formation rates were significantly higher (P<0.05) in the post-24, post-36 and post-48 groups (3.3%, 4.0% and 2.6%, respectively) than those in the control group without treatment (0%). Furthermore, when the oocytes were cultured with Ca-EDTA for 0 hr (control), 12 hr (pre-12), 24 hr (pre-24), 36 hr (pre-36) and 48 hr (pre-48) from the start of maturation culture, the oocytes formed blastocysts only in the pre-36 and pre-48 groups (0.4% or 0.8%, respectively). Pronuclei (<66.7%) were observed only when the periods of Ca-EDTA treatment were more than 12 hr during maturation culture. In the control group, no pronuclei were detected. Our findings demonstrate that porcine immature oocytes can be parthenogenetically activated by Ca-EDTA treatment for at least 24 hr to 36 hr during maturation culture, leading to pronucleus formation followed by the formation of blastocysts.

Production of small RNAs by mammalian Dicer

MicroRNA (miRNA) and RNA interference (RNAi) pathways employ RNase III Dicer for the biogenesis of small RNAs guiding post-transcriptional repression. Requirements for Dicer activity differ in the two pathways. The biogenesis of miRNAs requires a single Dicer cleavage of a short hairpin precursor to produce a small RNA with a precisely defined sequence, while small RNAs in RNAi come from a processive cleavage of a long double-stranded RNA (dsRNA) into a pool of small RNAs with different sequences. While Dicer is generally conserved among eukaryotes, its substrate recognition, cleavage, and biological roles differ. In Metazoa, a single Dicer can function as a universal factor for RNAi and miRNA pathways or as a factor adapted specifically for one of the pathways. In this review, we focus on the structure, function, and evolution of mammalian Dicer. We discuss key structural features of Dicer and other factors defining Dicer substrate repertoire and biological functions in mammals in comparison with invertebrate models. The key for adaptation of Dicer for miRNA or RNAi pathways is the N-terminal helicase, a dynamically evolving Dicer domain. Its functionality differs between mammals and invertebrates: the mammalian Dicer is well adapted to produce miRNAs while its ability to support RNAi is limited.

Sculpting the Transcriptome During the Oocyte-to-Embryo Transition in Mouse

In mouse, the oocyte-to-embryo transition entails converting a highly differentiated oocyte to totipotent blastomeres. This transition is driven by degradation of maternal mRNAs, which results in loss of oocyte identity, and reprogramming of gene expression during the course of zygotic gene activation, which occurs primarily during the two-cell stage and confers blastomere totipotency. Full-grown oocytes are transcriptionally quiescent and mRNAs are remarkably stable in oocytes due to the RNA-binding protein MSY2, which stabilizes mRNAs, and low activity of the 5' and 3' RNA degradation machinery. Oocyte maturation initiates a transition from mRNA stability to instability due to phosphorylation of MSY2, which makes mRNAs more susceptible to the RNA degradation machinery, and recruitment of dormant maternal mRNAs that encode for critical components of the 5' and 3' RNA degradation machinery. Small RNAs (miRNA, siRNA, and piRNA) play little, if any, role in mRNA degradation that occurs during maturation. Many mRNAs are totally degraded but a substantial fraction is only partially degraded, their degradation completed by the end of the two-cell stage. Genome activation initiates during the one-cell stage, is promiscuous, low level, and genome wide (and includes both inter- and intragenic regions) and produces transcripts that are inefficiently spliced and polyadenylated. The major wave of genome activation in two-cell embryos involves expression of thousands of new genes. This unique pattern of gene expression is the product of maternal mRNAs recruited during maturation that encode for transcription factors and chromatin remodelers, as well as dramatic changes in chromatin structure due to incorporation of histone variants and modified histones.

PLK4 is essential for meiotic resumption in mouse oocytes

Polo-like kinase (PLK) 4 is a unique member of the PLK family that plays vital roles in centriole biogenesis during mitosis. The localization of PLK4 on centrioles must be precisely regulated during mitosis to ensure correct centriole duplication. However, little is known about the function of PLK4 in mammalian oocyte meiosis. We addressed this question by examining the expression and localization of PLK4 in mouse oocytes and using RNA interference and protein overexpression to investigate its function in meiosis. PLK4 expression peaked at the germinal vesicle breakdown (GVBD) stage, and the protein was localized in the cytoplasm throughout meiotic maturation. Depletion of PLK4 caused meiotic arrest at the GV stage and suppressed CYCLINB1 and CDC2 activities. Moreover, PLK4 depletion prevented the de-phosphorylation of CDC2-Tyr15 in nucleus and induced a decrease in the level of the CDC25C protein. PLK1 overexpression failed to rescue GV-stage arrest in PLK4-depleted oocytes, whereas overexpressing PLK4 resulted in normal GVBD in oocytes in which PLK1 activity was inhibited. In addition, PLK4 overexpression did not cause abnormal spindle formation or affect extrusion of the first polar body. These results illustrate the fact that PLK4 is essential for meiotic resumption but may not influence spindle formation in mouse oocytes during meiotic maturation.

Multiple requirements of PLK1 during mouse oocyte maturation

Polo-like kinase 1 (PLK1) orchestrates multiple events of cell division. Although PLK1 function has been intensively studied in centriole-containing and rapidly cycling somatic cells, much less is known about its function in the meiotic divisions of mammalian oocytes, which arrest for a long period of time in prophase before meiotic resumption and lack centrioles for spindle assembly. Here, using specific small molecule inhibition combined with live mouse oocyte imaging, we comprehensively characterize meiotic PLK1's functions. We show that PLK1 becomes activated at meiotic resumption on microtubule organizing centers (MTOCs) and later at kinetochores. PLK1 is required for efficient meiotic resumption by promoting nuclear envelope breakdown. PLK1 is also needed to recruit centrosomal proteins to acentriolar MTOCs to promote normal spindle formation, as well as for stable kinetochore-microtubule attachment. Consequently, PLK1 inhibition leads to metaphase I arrest with misaligned chromosomes activating the spindle assembly checkpoint (SAC). Unlike in mitosis, the metaphase I arrest is not bypassed by the inactivation of the SAC. We show that PLK1 is required for the full activation of the anaphase promoting complex/cyclosome (APC/C) by promoting the degradation of the APC/C inhibitor EMI1 and is therefore essential for entry into anaphase I. Moreover, our data suggest that PLK1 is required for proper chromosome segregation and the maintenance of chromosome condensation during the meiosis I-II transition, independently of the APC/C. Thus, our results define the meiotic roles of PLK1 in oocytes and reveal interesting differential requirements of PLK1 between mitosis and oocyte meiosis in mammals.

Dual effects of hydrogen sulfide donor on meiosis and cumulus expansion of porcine cumulus-oocyte complexes

Hydrogen sulfide (H₂S) has been revealed to be a signal molecule with second messenger action in the somatic cells of many tissues, including the reproductive tract. The aim of this study was to address how exogenous H₂S acts on the meiotic maturation of porcine oocytes, including key maturation factors such as MPF and MAPK, and cumulus expansion intensity of cumulus-oocyte complexes. We observed that the H₂S donor, Na₂S, accelerated oocyte in vitro maturation in a dose-dependent manner, following an increase of MPF activity around germinal vesicle breakdown. Concurrently, the H₂S donor affected cumulus expansion, monitored by hyaluronic acid production. Our results suggest that the H₂S donor influences oocyte maturation and thus also participates in the regulation of cumulus expansion. The exogenous H₂S donor apparently affects key signal pathways of oocyte maturation and cumulus expansion, resulting in faster oocyte maturation with little need of cumulus expansion.

Aurora kinase B modulates chromosome alignment in mouse oocytes

The elevated incidence of aneuploidy in human oocytes warrants study of the molecular mechanisms regulating proper chromosome segregation. The Aurora kinases are a well-conserved family of serine/threonine kinases that are involved in proper chromosome segregation during mitosis and meiosis. Here we report the expression and localization of all three Aurora kinase homologs, AURKA, AURKB, and AURKC, during meiotic maturation of mouse oocytes. AURKA, the most abundantly expressed homolog, localizes to the spindle poles during meiosis I (MI) and meiosis II (MII), whereas AURKB is concentrated at kinetochores, specifically at metaphase of MI (Met I). The germ cell-specific homolog, AURKC, is found along the entire length of chromosomes during both meiotic divisions. Maturing oocytes in the presence of the small molecule pan-Aurora kinase inhibitor, ZM447439 results in defects in meiotic progression and chromosome alignment at both Met I and Met II. Over-expression of AURKB, but not AURKA or AURKC, rescues the chromosome alignment defect suggesting that AURKB is the primary Aurora kinase responsible for regulating chromosome dynamics during meiosis in mouse oocytes.

CDC2/SPDY transiently associates with endoplasmic reticulum exit sites during oocyte maturation

BACKGROUND

Mammalian oocytes acquire competence to be fertilized during meiotic maturation. The protein kinase CDC2 plays a pivotal role in several key maturation events, in part through controlled changes in CDC2 localization. Although CDC2 is involved in initiation of maturation, a detailed analysis of CDC2 localization at the onset of maturation is lacking. In this study, the subcellular distribution of CDC2 and its regulatory proteins cyclin B and SPDY in combination with several organelle markers at the onset of pig oocyte maturation has been investigated.

RESULTS

Our results demonstrate that CDC2 transiently associates with a single domain, identified as a cluster of endoplasmic reticulum (ER) exit sites (ERES) by the presence of SEC23, in the cortex of maturing porcine oocytes prior to germinal vesicle break down. Inhibition of meiosis resumption by forskolin treatment prevented translocation of CDC2 to this ERES cluster. Phosphorylated GM130 (P-GM130), which is a marker for fragmented Golgi, localized to ERES in almost all immature oocytes and was not affected by forskolin treatment. After removal of forskolin from the culture media, the transient translocation of CDC2 to ERES was accompanied by a transient dispersion of P-GM130 into the ER suggesting a role for CDC2 in redistributing Golgi components that have collapsed into ERES further into the ER during meiosis. Finally, we show that SPDY, rather than cyclin B, colocalizes with CDC2 at ERES, suggesting a role for the CDC2/SPDY complex in regulating the secretory pathway during oocyte maturation.

CONCLUSION

Our data demonstrate the presence of a novel structure in the cortex of porcine oocytes that comprises ERES and transiently accumulates CDC2 prior to germinal vesicle breakdown. In addition, we show that SPDY, but not cyclin B, localizes to this ERES cluster together with CDC2.

The cell cycle control protein cdc25C is present, and phosphorylated on serine 214 in the transition from germinal vesicle to metaphase II in human oocyte meiosis

Cdc25C is a dual specificity phosphatase essential for dephosphorylation and activation of cyclin-dependent kinase 1 (cdk1), a prerequisite step for mitosis in all eucaryotes. Cdc25C activation requires phosphorylation on at least six sites including serine 214 (S214) which is essential for metaphase/anaphase transit. Here, we have investigated S214 phosphorylation during human meiosis with the objectives of determining if this mitotic phosphatase cdc25C participates in final meiotic divisions in human oocytes. One hundred forty-eight human oocytes from controlled ovarian stimulation protocols were stained for immunofluorescence: 33 germinal vesicle (GV), 37 metaphase stage I (MI), and 78 unfertilized metaphase stage II (MII). Results were stage dependent, identical, independent of infertility type, or stimulation protocol. During GV stages, phospho-cdc25C is localized at the oocyte periphery. During early meiosis I (MI), phosphorylated cdc25C is no longer detected until onset of meiosis I. Here, phospho-cdc25C localizes on interstitial microtubules and at the cell periphery corresponding to the point of polar body expulsion. As the first polar body reaches the periphery, phosphorylated cdc25C is localized at the junction corresponding to the mid body position. On polar body expulsion, the interior signal for phospho-cdc25C is lost, but remains clearly visible in the extruded polar body. In atresic or damaged oocytes, the polar body no longer stains for phospho-cdc25C. Human cdc25C is both present and phosphorylated during meiosis I and localizes in a fashion similar to that seen during human mitotic divisions implying that the involvement of cdc25C is conserved and functional in meiotic cells.

Selective degradation of transcripts in mammalian oocytes and embryos

During the last decade several gene expression analysis studies have been carried out to investigate the transcriptional profile of bovine embryos in response to various culture and treatments conditions. Despite this fact, the function of a large number of genes in mammalian embryogenesis has not yet been investigated or is not known. The conventional gene-knockout experiments have been used extensively to study the function of genes in mammalian embryogenesis. However, these studies are relatively slow and cannot keep pace with the rapid accumulation of new sequence information produced by various genome projects. For this, the posttranscriptional gene silencing (PTGS) by double-stranded RNA (dsRNA), or RNA interference (RNAi), has emerged as a new tool for studying gene function in an increasing number of organisms. The present review will focus on recent developments in the use of RNAi for selective degradation of transcripts in mammalian embryos to elucidate their function in early development.

Cdc25 and Wee1: analogous opposites?

Movement through the cell cycle is controlled by the temporally and spatially ordered activation of cyclin-dependent kinases paired with their respective cyclin binding partners. Cell cycle events occur in a stepwise fashion and are monitored by molecular surveillance systems to ensure that each cell cycle process is appropriately completed before subsequent events are initiated. Cells prevent entry into mitosis while DNA replication is ongoing, or if DNA is damaged, via checkpoint mechanisms that inhibit the activators and activate the inhibitors of mitosis, Cdc25 and Wee1, respectively. Once DNA replication has been faithfully completed, Cdc2/Cyclin B is swiftly activated for a timely transition from interphase into mitosis. This sharp transition is propagated through both positive and negative feedback loops that impinge upon Cdc25 and Wee1 to ensure that Cdc2/Cyclin B is fully activated. Recent reports from a number of laboratories have revealed a remarkably complex network of kinases and phosphatases that coordinately control Cdc25 and Wee1, thereby precisely regulating the transition into mitosis. Although not all factors that inhibit Cdc25 have been shown to activate Wee1 and vice versa, a number of regulatory modules are clearly shared in common. Thus, studies on either the Cdc25 or Wee1-regulatory arm of the mitotic control pathway should continue to shed light on how both arms are coordinated to smoothly regulate mitotic entry.