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 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 to a genetics-led mechanistic analysis. In Drosophila oocytes, like in human oocytes, meiotic chromosomes form a compact cluster called the karyosome within the nucleus. Later, meiotic chromosomes assemble the spindle without centrosomes, establish bipolar attachment and congress within the spindle. We have identified a number of genes defective in chromosome organisation and/or spindle formation in oocytes.
From the study of the karyosome, we found that the nuclear pore plays a major role in global chromatin organisation in oocytes and somatic cells, and identified a novel regulatory system within the pore which controls the chromatin attachment state to the nuclear envelope. Furthermore, we found the histone demethylase Kdm5/Lid is important form the karyosome and stable synaptonemal complex. Surprisingly, Kdm5/Lid executes these functions independently of its catalytic activity. We also showed that microtubule plus ends are actively prevented from forming stable attachments to kinetochores during spindle formation in Drosophila oocytes. The microtubule catastrophe-promoting complex Sentin-EB1 is responsible for this delay in attachment, and facilitates bipolar attachment of homologous chromosomes.
A. Time-lapse images of Drosophila oocytes expressing Rod-GFP and Rcc1-mCherry. Unattached kinetochores or kinetochore microtubules (arrowheads) are marked with Rod, and chromosomes are marked with Rcc1.
B. Kymographs of the same oocytes.
C. The frequency of weakly (open bars), moderately (half filled) and strongly (filled) attached kinetochores judged by Rod- GFP. Robust attachment of microtubules to kinetochore is established earlier in the sentin mutant than in wild type.
D. The approximate timing of cytological events in oocytes. Nuclear envelope breakdown (NEB), kinetochore (KT), microtubule (MT). Scale bars=10 µm.
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Breuer, M., and Ohkura, H. (2015). A negative regulatory loop within the nuclear pore complex controls global chromatin organization.
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Zhaunova, L., Ohkura, H., and Breuer M. (2016) Kdm5/Lid regulates chromosome architecture in meiotic prophase I independently of its histone demethylase activity. PLoS Genet 12, e1006241.