Deprotection of centromeric cohesin at meiosis II requires APC/C activity but not kinetochore tension.

Genome haploidization involves sequential loss of cohesin from chromosome arms and centromeres during two meiotic divisions. At centromeres, cohesin's Rec8 subunit is protected from separase cleavage at meiosis I and then deprotected to allow its cleavage at meiosis II. Protection of centromeric cohesin by shugoshin-PP2A seems evolutionarily conserved. However, deprotection has been proposed to rely on spindle forces separating the Rec8 protector from cohesin at metaphase II in mammalian oocytes and on APC/C-dependent destruction of the protector at anaphase II in yeast. Here, we have activated APC/C in the absence of sister kinetochore biorientation at meiosis II in yeast and mouse oocytes, and find that bipolar spindle forces are dispensable for sister centromere separation in both systems. Furthermore, we show that at least in yeast, protection of Rec8 by shugoshin and inhibition of separase by securin are both required for the stability of centromeric cohesin at metaphase II. Our data imply that related mechanisms preserve the integrity of dyad chromosomes during the short metaphase II of yeast and the prolonged metaphase II arrest of mammalian oocytes.

Mitochondrial DNA disease: new options for prevention.

Very recently, two papers have presented intriguing data suggesting that prevention of transmission of human mitochondrial DNA (mtDNA) disease is possible. [Craven, L., Tuppen, H.A., Greggains, G.D., Harbottle, S.J., Murphy, J.L., Cree, L.M., Murdoch, A.P., Chinnery, P.F., Taylor, R.W., Lightowlers, R.N. et al. (2010) Pronuclear transfer in human embryos to prevent transmission of mitochondrial DNA disease. Nature, 465, 82-85. Tachibana, M., Sparman, M., Sritanaudomchai, H., Ma, H., Clepper, L., Woodward, J., Li, Y., Ramsey, C., Kolotushkina, O. and Mitalipov, S. (2009) Mitochondrial gene replacement in primate offspring and embryonic stem cells. Nature, 461, 367-372.] These recent advances raise hopes for families with mtDNA disease; however, the successful translational of these techniques to clinical practice will require further research to test for safety and to maximize efficacy. Furthermore, in the UK, amendment to the current legislation will be required. Here, we discuss the clinical and scientific background, studies we believe are important to establish safety and efficacy of the techniques and some of the potential concerns about the use of these approaches.

Tex19.1 inhibits the N-end rule pathway and maintains acetylated SMC3 cohesin and sister chromatid cohesion in oocytes.

Age-dependent oocyte aneuploidy, a major cause of Down syndrome, is associated with declining sister chromatid cohesion in postnatal oocytes. Here we show that cohesion in postnatal mouse oocytes is regulated by Tex19.1. We show Tex19.1-/- oocytes have defects maintaining chiasmata, missegregate their chromosomes during meiosis, and transmit aneuploidies to the next generation. Furthermore, we show that mouse Tex19.1 inhibits N-end rule protein degradation mediated by its interacting partner UBR2, and that Ubr2 itself has a previously undescribed role in negatively regulating the acetylated SMC3 subpopulation of cohesin in mitotic somatic cells. Lastly, we show that acetylated SMC3 is associated with meiotic chromosome axes in mouse oocytes, and that this population of cohesin is specifically depleted in the absence of Tex19.1. These findings indicate that Tex19.1 regulates UBR protein activity to maintain acetylated SMC3 and sister chromatid cohesion in postnatal oocytes and prevent aneuploidy from arising in the female germline.

Therapeutic potential of somatic cell nuclear transfer for degenerative disease caused by mitochondrial DNA mutations.

Induced pluripotent stem cells (iPSCs) hold much promise in the quest for personalised cell therapies. However, the persistence of founder cell mitochondrial DNA (mtDNA) mutations limits the potential of iPSCs in the development of treatments for mtDNA disease. This problem may be overcome by using oocytes containing healthy mtDNA, to induce somatic cell nuclear reprogramming. However, the extent to which somatic cell mtDNA persists following fusion with human oocytes is unknown. Here we show that human nuclear transfer (NT) embryos contain very low levels of somatic cell mtDNA. In light of a recent report that embryonic stem cells can be derived from human NT embryos, our results highlight the therapeutic potential of NT for mtDNA disease, and underscore the importance of using human oocytes to pursue this goal.

Cyclin A2 is required for sister chromatid segregation, but not separase control, in mouse oocyte meiosis.

In meiosis, two specialized cell divisions allow the separation of paired chromosomes first, then of sister chromatids. Separase removes the cohesin complex holding sister chromatids together in a stepwise manner from chromosome arms in meiosis I, then from the centromere region in meiosis II. Using mouse oocytes, our study reveals that cyclin A2 promotes entry into meiosis, as well as an additional unexpected role; namely, its requirement for separase-dependent sister chromatid separation in meiosis II. Untimely cyclin A2-associated kinase activity in meiosis I leads to precocious sister separation, whereas inhibition of cyclin A2 in meiosis II prevents it. Accordingly, endogenous cyclin A is localized to kinetochores throughout meiosis II, but not in anaphase I. Additionally, we found that cyclin B1, but not cyclin A2, inhibits separase in meiosis I. These findings indicate that separase-dependent cohesin removal is differentially regulated by cyclin B1 and A2 in mammalian meiosis.