NuMA distribution and microtubule configuration in rabbit oocytes and cloned embryos

Microtubule depolymerization attenuates WNT4/CaMKIIα signaling in mouse uterus and leads to implantation failure

Microtubule (MT) dynamics plays a crucial role in fertilization and early embryonic development; however its involvement in uterus during embryo implantation remains unclear. Herein, we report the effect of microtubule depolymerization during embryo implantation in BALB/c mice. Intrauterine treatment with depolymerizing agent nocodazole at pre-implantation phase (D4, 07:00 h) in mice resulted into mitigation in receptivity markers viz. LIF, HoxA10, Integrin-β3, IHH, WNT4 and led to pregnancy failure. MT depolymerization in endometrial epithelial cells (EECs) also inhibited the blastocyst attachment and the adhesion. The decreased expression of MT polymerization-related proteins TPPP and α/β-tubulin in luminal and glandular epithelial cells along with the alteration in morphology of pinopodes in the luminal epithelium was observed in nocodazole receiving uteri. Nocodazole treatment also led to increased intracellular Ca+2levels in EECs, which indicated that altered Ca+2homeostasis might be responsible for implantation failure. Microtubule depolymerization inhibited WNT4 and Fz-2 interaction, thereby suppressing the downstream WNT4/CaMKIIα signaling cascades calmodulin and calcineurin which led to attenuation of NF-κB transcriptional promoter activity in EECs. MT depolymerization or CaMKIIα knockdown inhibited the transcription factor NFAT and NF-κB expression along with reduced secretion of prostaglandins PGE2 and PGF2α in mouse EECs. Overall, MT depolymerization impaired the WNT4/CaMKIIα signaling and suppressed the secretion of PGE2 and PGF2α in EECs which may be responsible for implantation failure in mice.

Meiotic spindle assembly and chromosome segregation in oocytes

Centrosomes play a key role in organizing the microtubule spindle that separates chromosomes during mitosis. Bennabi et al. review how microtubule spindle formation and chromosomal segregation also occur in oocytes during cell division by meiosis despite the absence of centrosomes.

Oocytes accumulate maternal stores (proteins, mRNAs, metabolites, etc.) during their growth in the ovary to support development after fertilization. To preserve this cytoplasmic maternal inheritance, they accomplish the difficult task of partitioning their cytoplasm unequally while dividing their chromosomes equally. Added to this complexity, most oocytes, for reasons still speculative, lack the major microtubule organizing centers that most cells use to assemble and position their spindles, namely canonical centrosomes. In this review, we will address recent work on the mechanisms of meiotic spindle assembly and chromosome alignment/segregation in female gametes to try to understand the origin of errors of oocyte meiotic divisions. The challenge of oocyte divisions appears indeed not trivial because in both mice and humans oocyte meiotic divisions are prone to chromosome segregation errors, a leading cause of frequent miscarriages and congenital defects.

Nek9 regulates spindle organization and cell cycle progression during mouse oocyte meiosis and its location in early embryo mitosis

Nek9 (also known as Nercc1), a member of the NIMA (never in mitosis A) family of protein kinases, regulates spindle formation, chromosome alignment and segregation in mitosis. Here, we showed that Nek9 protein was expressed from germinal vesicle (GV) to metaphase II (MII) stages in mouse oocytes with no detectable changes. Confocal microscopy identified that Nek9 was localized to the spindle poles at the metaphase stages and associated with the midbody at anaphase or telophase stage in both meiotic oocytes and the first mitotic embyros. Depletion of Nek9 by specific morpholino injection resulted in severely defective spindles and misaligned chromosomes with significant pro-MI/MI arrest and failure of first polar body (PB1) extrusion. Knockdown of Nek9 also impaired the spindle-pole localization of γ-tubulin and resulted in retention of the spindle assembly checkpoint protein Bub3 at the kinetochores even after 10 h of culture. Live-cell imaging analysis also confirmed that knockdown of Nek9 resulted in oocyte arrest at the pro-MI/MI stage with abnormal spindles, misaligned chromosomes and failed polar body emission. Taken together, our results suggest that Nek9 may act as a MTOC-associated protein regulating microtubule nucleation, spindle organization and, thus, cell cycle progression during mouse oocyte meiotic maturation, fertilization and early embryo cleavage.

Effect of donor cell type on nuclear remodelling in rabbit somatic cell nuclear transfer embryos

Cloned rabbits have been produced for many years by somatic cell nuclear transfer (SCNT). The efficiency of cloning by SCNT, however, has remained extremely low. Most cloned embryos degenerate in utero, and the few that develop to term show a high incidence of post-natal death and abnormalities. The cell type used for donor nuclei is an important factor in nuclear transfer (NT). As reported previously, NT embryos reconstructed with fresh cumulus cells (CC-embryos) have better developmental potential than those reconstructed with foetal fibroblasts (FF-embryos) in vivo and in vitro. The reason for this disparity in developmental capacity is still unknown. In this study, we compared active demethylation levels and morphological changes between the nuclei of CC-embryos and FF-embryos shortly after activation. Anti-5-methylcytosine immunofluorescence of in vivo-fertilized and cloned rabbit embryos revealed that there was no detectable active demethylation in rabbit zygotes or NT-embryos derived from either fibroblasts or CC. In the process of nuclear remodelling, however, the proportion of nuclei with abnormal appearance in FF-embryos was significantly higher than that in CC-embryos during the first cell cycle. Our study demonstrates that the nuclear remodelling abnormality of cloned rabbit embryos may be one important factor for the disparity in developmental success between CC-embryos and FF-embryos.

Dynamic distribution of NuMA and microtubules in human fetal fibroblasts, developing oocytes and somatic cell nuclear transferred embryos

BACKGROUND

The nuclear mitotic apparatus (NuMA) plays a central role in the assembly and maintenance of spindle poles. Somatic cell nuclear transfer (SCNT) studies on non-human primates have shown that meiotic spindle removal during enucleation causes depletion of NuMA and the minus-end-directed motor protein (HSET) from the ooplasm, and this in turn leads to failure of embryo development. To determine whether NuMA from somatic cells could compensate for NuMA loss during enucleation, the distribution of NuMA and microtubule organization were investigated in human fibroblasts, developing oocytes and SCNT embryos.

METHODS

Human fetal fibroblasts, oocytes at various maturation stages and human embryos reconstructed by different SCNT methods were analyzed for NuMA and α-tubulin using immunofluorescent confocal microscopy.

RESULTS

NuMA was detected in interphase nuclei of fibroblasts and oocytes. During mitosis and meiosis, NuMA relocated to the domain surrounding the two spindle poles. During the enucleation process, NuMA was removed along with the meiotic spindle. At 2 h after injection into a donor cell, transitory bipolar spindles were organized and NuMA was detected in the reformed poles. NuMA could be detected spreading uniformly across the nucleoplasm of one pseudo-pronucleus in SCNT embryos but was excluded from the nucleolus. Regardless of the method used for SCNT (enucleation-injection or injection-pronuclei enucleation), NuMA aggregated and relocated to the reformed spindle poles at metaphase of the first mitotic event. At interphase, NuMA relocated throughout the nucleus in developmentally arrested SCNT embryos.

CONCLUSIONS

Our results show that donor cell nuclei contain NuMA, which might contribute to the maintenance of spindle morphology in SCNT embryos. Normal spindle and NuMA expression were found in human SCNT embryos at different developmental stages.

Trichostatin A (TSA) improves the development of rabbit-rabbit intraspecies cloned embryos, but not rabbit-human interspecies cloned embryos

The interspecies somatic cell nuclear transfer (iSCNT) technique for therapeutic cloning gives great promise for treatment of many human diseases. However, the incomplete nuclear reprogramming and the low blastocyst rate of iSCNT are still big problems. Herein, we observed the effect of TSA on the development of rabbit-rabbit intraspecies and rabbit-human interspecies cloned embryos. After treatment with TSA for 6 hr during activation, we found that the blastocyst rate of rabbit-rabbit cloned embryos was more than two times higher than that of untreated embryos; however, the blastocyst rate of TSA-treated rabbit-human interspecies cloned embryos decreased. We also found evident time-dependent histone deacetylation-reacetylation changes in rabbit-rabbit cloned embryos, but not in rabbit-human cloned embryos from fusion to 6 hr after activation. Our results suggest that TSA-treatment does not improve blastocyst development of rabbit-human iSCNT embryos and that abnormal histone deacetylation-reacetylation changes in iSCNT embryos may account for their poor blastocyst development.

Trichostatin A and nuclear reprogramming of cloned rabbit embryos

To investigate the influence of histone deacetylases on nuclear reprogramming after nuclear transfer, we treated the cloned embryos with a histone deacetylase inhibitor, Trichostatin A (TSA). In the present study, global changes in acetylation of histone H3-lysine 14, histone H4-lysine 12, and histone H4-lysine 5 were studied in rabbit in vivo fertilized embryos, somatic cell nuclear transfer (SCNT) embryos, and TSA-treated SCNT embryos. From the pronuclear to the morula stage, the deacetylation-reacetylation changes in acetylation of histone H3-lysine 14 and histone H4-lysine 12 occurred in both fertilized embryos and TSA-treated cloned embryos; however, the distribution pattern in untreated cloned embryos failed to display such changes. More interesting, the signal of acetylation of histone H4-lysine 12 in cloned embryos was detected in both the inner cell mass and the trophectoderm, whereas TSA-treated cloned embryos showed the same staining pattern as fertilized embryos and the staining was limited to the inner cell mass. The histone acetylation pattern of TSA-treated SCNT embryos appeared to be more similar to that of normal embryos, indicating that TSA could improve nuclear reprogramming after nuclear transfer.