Accumulation of Securin on Spindle During Female Meiosis I.

Chromosome segregation during female meiosis is frequently incorrect with severe consequences including termination of further development or severe disorders, such as Down syndrome. Accurate chromosome segregation requires tight control of a protease called separase, which facilitates the separation of sister chromatids by cohesin cleavage. There are several control mechanisms in place, including the binding of specific protein inhibitor securin, phosphorylation by cyclin-dependent kinase 1 (CDK1), and complex with SGO2 and MAD2 proteins. All these mechanisms restrict the activation of separase for the time when all chromosomes are properly attached to the spindle. In our study, we focused on securin and compared the expression profile of endogenous protein with exogenous securin, which is widely used to study chromosome segregation. We also compared the dynamics of securin proteolysis in meiosis I and meiosis II. Our study revealed that the expression of both endogenous and exogenous securin in oocytes is compartmentalized and that this protein accumulates on the spindle during meiosis I. We believe that this might have a direct impact on the regulation of separase activity in the vicinity of the chromosomes.

Aneuploidy during the onset of mouse embryo development.

Aneuploidy is the most frequent single cause leading into the termination of early development in human and animal reproduction. Although the mouse is frequently used as a model organism for studying the aneuploidy, we have only incomplete information about the frequency of numerical chromosomal aberrations throughout development, usually limited to a particular stage or assumed from the occurrence of micronuclei. In our study, we systematically scored aneuploidy in in vivo mouse embryos, from zygotes up to 16-cell stage, using kinetochore counting assay. We show here that the frequency of aneuploidy per blastomere remains relatively similar from zygotes until 8-cell embryos and then increases in 16-cell embryos. Due to the accumulation of blastomeres, aneuploidy per embryo increases gradually during this developmental period. Our data also revealed that the aneuploidy from zygotes and 2-cell embryos does not propagate further into later developmental stages, suggesting that embryos suffering from aneuploidy are eliminated at this stage. Experiments with reconstituted live embryos revealed, that hyperploid blastomeres survive early development, although they exhibit slower cell cycle progression and suffer frequently from DNA fragmentation and cell cycle arrest.

Cyclin A1 in Oocytes Prevents Chromosome Segregation And Anaphase Entry.

In several species, including Xenopus, mouse and human, two members of cyclin A family were identified. Cyclin A2, which is ubiquitously expressed in dividing cells and plays role in DNA replication, entry into mitosis and spindle assembly, and cyclin A1, whose function is less clear and which is expressed in spermatocytes, leukemia cells and in postmitotic multiciliated cells. Deletion of the gene showed that cyclin A1 is essential for male meiosis, but nonessential for female meiosis. Our results revealed, that the cyclin A1 is not only dispensable in oocytes, we show here that its expression is in fact undesirable in these cells. Our data demonstrate that the APC/C and proteasome in oocytes are unable to target sufficiently cyclin A1 before anaphase, which leads into anaphase arrest and direct inhibition of separase. The cyclin A1-induced cell cycle arrest is oocyte-specific and the presence of cyclin A1 in early embryos has no effect on cell cycle progression or chromosome division. Cyclin A1 is therefore not only an important cell cycle regulator with biased expression in germline, being essential for male and damaging for female meiosis, its persistent expression during anaphase in oocytes shows fundamental differences between APC/C function in oocytes and in early embryos.