The meiotic spindle and chromosomes in oocytes
Accurate segregation of chromosomal DNA is essential for life. A failure or error in this process during somatic divisions could result in cell death or aneuploidy. Furthermore, chromosome segregation in oocytes is error-prone in humans, and mis-segregation is a major cause of infertility, miscarriages and birth defects. The chromosome segregation machinery in oocytes shares many similarities with these in somatic divisions, but also has notable differences. In spite of its importance for human health, little is known about the molecular pathways which set up the chromosome segregation machinery in oocytes. Defining these molecular pathways is crucial to understand error-prone chromosome segregation in human oocytes. Furthermore, evidence indicates that these apparent oocyte-specific pathways also operate in mitosis, although less prominently, to ensure the accuracy of chromosome segregation. Therefore uncovering the molecular basis of these pathways is also important to understand how somatic cells avoid chromosome instability, a contributing factor for cancer development.
To understand the molecular pathways which set up the chromosome segregation machinery in oocytes, we take advantage of Drosophila oocytes as a "discovery platform" because of their similarity to mammalian oocytes and suitability for a genetics-led mechanistic analysis. In Drosophila oocytes, as in human oocytes, meiotic chromosomes form a compact cluster called the karyosome within the nucleus. Later, meiotic chromosomes assemble a bipolar spindle without centrosomes in the large volume of the cytoplasm, and establish bipolar attachment. We have identified a number of genes defective in chromosome organisation and/or spindle formation in oocytes.
From studying the karyosome, we found the histone demethylase Kdm5/Lid plays an important role forming the karyosome and stable synaptonemal complex, independently of its catalytic activity. In addition, we uncovered a novel regulatory loop which controls interaction between the nuclear pore and chromatin in oocytes and somatic cells.
For bipolar spindle formation, we found that the phospho-docking 14-3-3 protein is crucial for spatial regulation of a spindle protein. It suppresses microtubule binding of the kinesin-14 Ncd in the large cytoplasm of oocytes, and this suppression is locally removed by the Aurora B kinase that acts as a chromosomal signal. We also showed that Sentin- EB1 actively prevents microtubule plus ends from forming stable kinetochore attachments during spindle formation to facilitate bipolar attachment of homologous chromosomes in Drosophila oocytes.
Beaven, R., Bastos, R.N., Spanos, C., Rome, P., Cullen, C.F., Rappsilber, J., Giet, R., Goshima, G., and Ohkura, H. (2017). 14-3-3 regulation of Ncd reveals a new mechanism for targeting proteins to the spindle in oocytes. J. Cell Biol. 216, 3029-3039.
Breuer, M., and Ohkura, H. (2015). A negative regulatory loop within the nuclear pore complex controls global chromatin organization. Genes Dev. 29, 1789-1794.
Głuszek, A.A., Cullen, C.F., Li, W., Battaglia, R.A., Radford, S.J., Costa, M.F., McKim, K.S., Goshima, G., and Ohkura, H. (2015). The microtubule catastrophe promoter Sentin delays stable kinetochore-microtubule attachment in oocytes. J. Cell Biol. 211, 1113-1120.