Aurora B prevents aneuploidy via MAD2 during the first mitotic cleavage in oxidatively damaged embryos

Mechanism of chromosomal mosaicism in preimplantation embryos and its effect on embryo development

Aneuploidy is one of the main causes of miscarriage and in vitro fertilization failure. Mitotic abnormalities in preimplantation embryos are the main cause of mosaicism, which may be influenced by several endogenous factors such as relaxation of cell cycle control mechanisms, defects in chromosome cohesion, centrosome aberrations and abnormal spindle assembly, and DNA replication stress. In addition, incomplete trisomy rescue is a rare cause of mosaicism. However, there may be a self-correcting mechanism in mosaic embryos, which allows some mosaicisms to potentially develop into normal embryos. At present, it is difficult to accurately diagnose mosaicism using preimplantation genetic testing for aneuploidy. Therefore, in clinical practice, embryos diagnosed as mosaic should be considered comprehensively based on the specific situation of the patient.

Healthy Live Births after the Transfer of Mosaic Embryos: Self-Correction or PGT-A Overestimation?

The implementation of next generation sequencing (NGS) in preimplantation genetic testing for aneuploidy (PGT-A) has led to a higher prevalence of mosaic diagnosis within the trophectoderm (TE) sample. Regardless, mosaicism could potentially increase the rate of live-born children with chromosomic syndromes, though available data from the transfer of embryos with putative PGT-A mosaicism are scarce but reassuring. Even with lower implantation and higher miscarriage rates, mosaic embryos can develop into healthy live births. Therefore, this urges an explanation for the disappearance of aneuploid cells throughout development, to provide guidance in the management of mosaicism in clinical practice. Technical overestimation of mosaicism, together with some sort of "self-correction" mechanisms during the early post-implantation stages, emerged as potential explanations. Unlike the animal model, in which the elimination of genetically abnormal cells from the future fetal lineage has been demonstrated, in human embryos this capability remains unverified even though the germ layer displays an aneuploidy-induced cell death lineage preference with higher rates of apoptosis in the inner cell mass (ICM) than in the TE cells. Moreover, the reported differential dynamics of cell proliferation and apoptosis between euploid, mosaic, and aneuploid embryos, together with pro-apoptosis gene products (cfDNA and mRNA) and extracellular vesicles identified in the blastocoel fluid, may support the hypothesis of apoptosis as a mechanism to purge the preimplantation embryo of aneuploid cells. Alternative hypotheses, like correction of aneuploidy by extrusion of a trisomy chromosome or by monosomic chromosome duplication, are even, though they represent an extremely rare phenomenon. On the other hand, the technical limitations of PGT-A analysis may lead to inaccuracy in embryo diagnoses, identifying as "mosaic" those embryos that are uniformly euploid or aneuploid. NGS assumption of "intermediate copy number profiles" as evidence of a mixture of euploid and aneuploid cells in a single biopsy has been reported to be poorly predictive in cases of mosaicism diagnosis. Additionally, the concordance found between the TE and the ICM in cases of TE biopsies displaying mosaicism is lower than expected, and it correlates differently depending on the type (whole chromosome versus segmental) and the level of mosaicism reported. Thus, in cases of low-/medium-level mosaicism (<50%), aneuploid cells would rarely involve the ICM and other regions. However, in high-level mosaics (≥50%), abnormal cells in the ICM should display higher prevalence, revealing more uniform aneuploidy in most embryos, representing a technical variation in the uniform aneuploidy range, and therefore might impair the live birth rate.

PGT-A mosaicism based on NGS intermediate copy numbers: is it time to stop reporting them?

Mosaicism represents a genuine real phenomenon, but its high prevalence and undisclosed clinical significance, stress the burden on genetic counseling and the management of PGT-A results. Even though the assumption of mosaicism from NGS intermediate chromosome copy number profiles may represent a reasonable interpretation, other potential technical reasons, including amplification bias, contamination, biopsy technique, or the analysis algorithms, may constitute alternative explanations. Thresholds confining mosaicism ranges are established according to models employing mixtures of normal and abnormal cells with steady conditions of quantity and quality which are unable to reflect the full extent of variability present in a trophectoderm (TE) biopsy specimen. When the concordance of TE with the ICM is considered, mosaic TE biopsies poorly correlate with the chromosomal status of the remaining embryo, displaying mostly ICM aneuploidy in cases of TE high-range mosaics diagnosis and euploidy when mosaicism grade in TE is less than 50% (low-mid range mosaicism), which implies an evident overestimation of mosaicism results. Indeed, a binary classification of NGS profiles that excludes mosaic ranges, including only euploid and aneuploid diagnosis, provides higher specificity and accuracy in identifying abnormal embryos and discarding them. As intermediate copy number profiles do not represent strong evidence of mosaicism but only an inaccurate and misleading assumption, and considering that no increased risk has been reported in the offspring, until diagnosis specificity is improved and its clinical implications are determined, laboratories should consider limiting predictions to euploid and aneuploid and stop reporting mosaicism.

Effect of Aurora kinase B on polyploidy and decidualization in mouse uterus

RESEARCH QUESTION

Decidualization is critical to the establishment of mouse normal pregnancy. The fibroblast-like stromal cells in the process form polyploid multinucleated cells. Aurora kinase B (Aurora B) has previously been shown to regulate polyploidy in various cells. However, whether Aurora B regulates the formation of decidual cell polyploidization and its regulatory mechanisms remain poorly understood.

DESIGN

Establish decidualization model of mouse primary endometrial stromal cells in vitro. Construct pseudopregnancy mouse models and delayed-activation mouse models. Detect Aurora B and polyploidization related genes in mouse uteri treated by Aurora B specific inhibitor Barasertib and CPT.

RESULTS

In this study, we found that Aurora B was strongly expressed in endometrial stromal cells after implantation. Additionally, Aurora B was remarkably up regulated in the stromal cells of oil-induced deciduomoa and in vitro decidualization. As an Aurora B specific inhibitor, Barasertib significantly inhibits the mRNA expression of Prl8a2, a marker of mouse decidualization. Furthermore, the protein levels of p-Plk1, Survivin and p-Cdk1 were inhibited by Barasertib. CPT-induced DNA damage suppressed Aurkb (encodes Aurora B) expression, thus resulting in polyploidization.

CONCLUSION

Our data shows that Aurora B is expressed in decidual stromal cells of implantation sites and plays a key role for mouse decidualization. The protein of Plk1, Survivn, and Cdk1 may participate in formation of decidual cell polyploidization during mouse decidualization.

Salidroside Supplementation Affects In Vitro Maturation and Preimplantation Embryonic Development by Promoting Meiotic Resumption

Salidroside (Sal) possesses several pharmacological activities, such as antiaging, and anti-inflammatory, antioxidant, anticancer activities, and proliferation-promoting activities, but the effects of Sal on oocytes have rarely been reported. In the present study, we evaluated the beneficial effects of Sal, which is mainly found in the roots of Rhodiola. Porcine cumulus oocyte complexes were cultured in IVM medium supplemented (with 250 μmol/L) with Sal or not supplemented with Sal. The maturation rate in the Sal group increased from 88.34 ± 4.32% to 94.12 ± 2.29%, and the blastocyst rate in the Sal group increased from 30.35 ± 3.20% to 52.14 ± 7.32% compared with that in the control group. The experimental groups showed significant improvements in the cumulus expansion area. Sal reduced oocyte levels of reactive oxygen species (ROS) and enhanced intracellular GSH levels. Sal supplementation enhanced the mitochondrial membrane potential (MMP), ATP level, and mtDNA copy number, which shows that Sal enhances the cytoplasmic maturation of oocytes. Oocytes in the Sal group exhibited slowed apoptosis and reduced DNA breakage. Cell cycle signals and oocyte meiosis play important roles in oocyte maturation. The mRNA expressions of the MAPK pathway and MAPK phosphorylation increased significantly in the Sal group. The mRNA expression of the oocyte meiosis gene also increased significantly. These results show that Sal enhances the nuclear maturation of oocytes. Moreover, Sal increased the number of blastocyst cells, the proliferation of blastocysts, and the expressions of pluripotency genes. Sal down-regulated apoptosis-related genes and the apoptotic cell rate of blastocysts. In summary, our results demonstrate that Sal is helpful to improving the quality of porcine oocytes in vitro, and their subsequent embryonic development.

The origin and possible mechanism of embryonic cell-free DNA release in spent embryo culture media: a review

The presence of cell-free DNA in spent embryo culture media (SECM) has unveiled its possible utilization for embryonic ploidy determination, opening new frontiers for the development of a non-invasive pre-implantation genetic screening technique. While a growing number of studies have shown a high concordance between genetic screening using cell-free DNA (cfDNA) and trophectoderm (TE), the mechanism pertaining to the release of cfDNA in SECM is largely unknown. This review aims to evaluate research evidence on the origin and possible mechanisms for the liberations of embryonic DNA in SECM, including findings on the self-correction abilities of embryos which might contribute to the presence of cfDNA. Several databases including EMBASE, PUBMED, and SCOPUS were used to retrieve original articles, reviews, and opinion papers. The keywords used for the search were related to the origins and release mechanism of cfDNA. cfDNA in SECM originates from embryonic cells and, at some levels, non-embryonic cells such as maternal DNA and exogenous foreign DNA. The apoptotic pathway has been demonstrated to eliminate aneuploid cells in developing mosaic embryos which might culminate to the release of cfDNA in SECM. Nonetheless, there is a recognized need for exploring other pathways such as cross-talk molecules called extracellular vesicles (EVs) made of small, round bi-layer membranes. During in vitro development, embryos physiologically and actively expel EVs containing not only protein and microRNA but also embryonic DNA, hence, potentially releasing cfDNA of embryonic origin into SECM through EVs.

Age-dependent integrity of the meiotic spindle assembly checkpoint in females requires Aurora kinase B

A hallmark of advanced maternal age is a significant increase in meiotic chromosome segregation errors, resulting in early miscarriages and congenital disorders. These errors most frequently occur during meiosis I (MI). The spindle assembly checkpoint (SAC) prevents chromosome segregation errors by arresting the cell cycle until proper chromosome alignment is achieved. Unlike in mitosis, the SAC in oocytes is desensitized, allowing chromosome segregation in the presence of improperly aligned chromosomes. Whether SAC integrity further deteriorates with advancing maternal age, and if this decline contributes to increased segregation errors remains a fundamental question. In somatic cells, activation of the SAC depends upon Aurora kinase B (AURKB), which functions to monitor kinetochore–microtubule attachments and recruit SAC regulator proteins. In mice, oocyte‐specific deletion of AURKB (Aurkb cKO) results in an increased production of aneuploid metaphase II‐arrested eggs and premature age‐related infertility. Here, we aimed to understand the cause of the short reproductive lifespan and hypothesized that SAC integrity was compromised. In comparing oocytes from young and sexually mature Aurkb cKO females, we found that SAC integrity becomes compromised rapidly with maternal age. We show that the increased desensitization of the SAC is driven by reduced expression of MAD2, ZW10 and Securin proteins, key contributors to the SAC response pathway. The reduced expression of these proteins is the result of altered protein homeostasis, likely caused by the accumulation of reactive oxygen species. Taken together, our results demonstrate a novel function for AURKB in preserving the female reproductive lifespan possibly by protecting oocytes from oxidative stress.

Plasticity of the human preimplantation embryo: developmental dogmas, variations on themes and self-correction

BACKGROUND

IVF for the treatment of infertility offers unique opportunities to observe human preimplantation development. Progress in time-lapse technology (TLT) and preimplantation genetic testing (PGT) has greatly expanded our knowledge of developmental patterns leading to a healthy pregnancy or developmental failure. These technologies have also revealed unsuspected plastic properties of the preimplantation embryo, at macromolecular, cellular and multicellular levels.

OBJECTIVE AND RATIONALE

This review focuses on the emerging concept of plasticity of the human embryo as revealed by recent evidence derived from TLT and PGT, calling for an updated and more precise redefinition of the boundaries between normal and abnormal development.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed English-language original articles and reviews concerning human preimplantation development. Cross-searches were performed by adopting 'fertilisation', 'pronucleus', 'cleavage', 'multinucleation', 'compaction', 'embryo', 'preimplantation genetic testing', 'aneuploidy', mosaicism', 'micromanipulation', 'time-lapse microscopy' and 'IVF/assisted reproduction' as main terms. The most relevant publications, i.e. those concerning major phenomena occurring during normal and abnormal development-with a focus on the human species-were assessed and discussed critically.

OUTCOMES

Advances in TLT and PGT have revealed an astonishing plasticity and self-correction ability of the human preimplantation embryo in vitro. At fertilisation, an abnormal number of pronuclei do not always result in the formation of an aneuploid blastocyst. Animal studies and preliminary human observations indicate that combining of parental genomes may occur at the early cleavage stage, if not at fertilisation. Multinucleation occurs with much higher prevalence than previously thought and may be corrected at later cleavage stages. Irregular cleavage (multichotomous, direct, rapid and reverse cleavages) can generate chromosome segregation abnormalities that often lead to developmental arrest, but that sporadically may be confined to cells excluded from the blastocyst, and may sometimes result in viable pregnancy. Mitotic errors can generate mosaic blastocysts, but alternatively normal embryos may form from selective death or clonal depletion of aneuploid cells.

WIDER IMPLICATIONS

Deviations from developmental dogmas and the increasing evidence of plasticity of the human embryo challenge current embryological notions and suggest the need to write new rules governing cell cycle, cell determination and chromosome segregation during preimplantation development.

Mitotic syndicates Aurora Kinase B (AURKB) and mitotic arrest deficient 2 like 2 (MAD2L2) in cohorts of DNA damage response (DDR) and tumorigenesis

Aurora Kinase B (AURKB) and Mitotic Arrest Deficient 2 Like 2 (MAD2L2) are emerging anticancer therapeutic targets. AURKB and MAD2L2 are the least well studied members of their protein families, compared to AURKA and MAD2L1. Both AURKB and MAD2L2 play a critical role in mitosis, cell cycle checkpoint, DNA damage response (DDR) and normal physiological processes. However, the oncogenic roles of AURKB and MAD2L2 in tumorigenesis and genomic instability have also been reported. DDR acts as an arbitrator for cell fate by either repairing the damage or directing the cell to self-destruction. While there is strong evidence of interphase DDR, evidence of mitotic DDR is just emerging and remains largely unelucidated. To date, inhibitors of the DDR components show effective anti-cancer roles. Contrarily, long-term resistance towards drugs that target only one DDR target is becoming a challenge. Targeting interactions between protein-protein or protein-DNA holds prominent therapeutic potential. Both AURKB and MAD2L2 play critical roles in the success of mitosis and their emerging roles in mitotic DDR cannot be ignored. Small molecule inhibitors for AURKB are in clinical trials. A few lead compounds towards MAD2L2 inhibition have been discovered. Targeting mitotic DDR components and their interaction is emerging as a potent next generation anti-cancer therapeutic target. This can be done by developing small molecule inhibitors for AURKB and MAD2L2, thereby targeting DDR components as anti-cancer therapeutic targets and/or targeting mitotic DDR. This review focuses on AURKB and MAD2L2 prospective synergy to deregulate the p53 DDR pathway and promote favourable conditions for uncontrolled cell proliferation.

High Dosages of Equine Chorionic Gonadotropin Exert Adverse Effects on the Developmental Competence of IVF-Derived Mouse Embryos and Cause Oxidative Stress-Induced Aneuploidy

Gonadotropins play vital roles in the regulation of female reproductive ability and fertility. Our study aimed to determine the effects of superovulation induced by increasing doses of equine chorionic gonadotropin [eCG; also referred to as pregnant mare serum gonadotropin (PMSG)] on the developmental competence of mouse embryos and on aneuploidy formation during in vitro fertilization (IVF). eCG dose-dependently enhanced the oocyte yield from each mouse. Administration of 15 IU eCG significantly reduced the fertilization rate and the formation of four-cell embryos and blastocysts and increased the risk of chromosome aneuploidy. The IVF-derived blastocysts in the 15 IU eCG treatment group had the fewest total cells, inner cell mass (ICM) cells and trophectoderm (TE) cells. Moreover, more blastocysts and fewer apoptotic cells were observed in the 0, 5, and 10 IU eCG treatment groups than in the 15 IU eCG treatment group. We also investigated reactive oxygen species (ROS) levels and variations in several variables: mitochondrial membrane potential (MMP); active mitochondria; mitochondrial superoxide production; adenosine triphosphate (ATP) content; spindle structures; chromosome karyotypes; microfilament distribution; and the expression of Aurora B [an important component of the chromosomal passenger complex (CPC)], the spindle assembly checkpoint (SAC) protein mitotic arrest deficient 2 like 1 (MAD2L1), and the DNA damage response (DDR) protein γH2AX. Injection of 15 IU eCG increased ROS levels, rapidly reduced MMP, increased active mitochondria numbers and mitochondrial superoxide production, reduced ATP content, increased abnormal spindle formation rates, and induced abnormalities in chromosome number and microfilament distribution, suggesting that a high dose of eCG might alter developmental competence and exert negative effects on IVF-obtained mouse embryos. Additionally, the appearance of γH2AX and the significantly increased expression of Aurora B and MAD2L1 suggested that administration of relatively high doses of eCG caused Aurora B-mediated SAC activation triggered by ROS-induced DNA damage in early mouse IVF-derived embryos for self-correction of aneuploidy formation. These findings improve our understanding of the application of gonadotropins and provide a theoretical basis for gonadotropin treatment.

Aurora B prevents aneuploidy via MAD2 during the first mitotic cleavage in oxidatively damaged embryos

OBJECTIVES

A high rate of chromosome aneuploidy is exhibited in in vitro fertilization (IVF)-derived embryos. Our previous experiments suggested that reactive oxygen species (ROS) can activate Mad2, a key protein in the spindle assembly checkpoint (SAC), and delay the first mitotic, providing time to prevent the formation of embryonic aneuploidy. We aimed to determine whether mitotic kinase Aurora B was involved in the SAC function to prevent aneuploidy in IVF-derived embryos.

MATERIALS AND METHODS

We analysed aneuploidy formation and repair during embryo pre-implantation via 4',6-diamidino-2-phenylindole (DAPI) staining and karyotype analysis. We assessed Aurora B activation by immunofluorescence and investigated the effect of Aurora B inhibition on embryo injury-related variables, such as embryonic development, ROS levels, mitochondrial membrane potential and γH2AX-positive expression.

RESULTS

We observed the expression and phosphorylation of Thr232 in Aurora B in oxidative stress-induced zygotes. Moreover, inhibition of Aurora B caused chromosome mis-segregation, abnormal spindle structures, abnormal chromosome number and reduced expression of Mad2 in IVF embryos. Our results suggest that Aurora B causes mitotic arrest and participates in SAC via Mad2 and H3S10P, which is required for self-correction of aneuploidies.

CONCLUSIONS

We demonstrate here that oxidative stress-induced DNA damage triggers Aurora B-mediated activation of SAC, which prevents aneuploidy at the first mitotic cleavage in early mouse IVF embryos.