Molecular remodelling of the spindle architecture during metaphase arrest in oocytes
Costa, Mariana Fernandes Alves
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Oocytes of most species assemble and maintain a functional bipolar spindle in the absence of centrosomes. Strikingly, after bipolar spindle formation, oocytes arrest in metaphase for several hours before fertilisation. How the dynamic spindle maintains its bipolarity during this long arrest is poorly understood. I hypothesise that the bipolar spindle is stably maintained by changes in the distribution of microtubule-associated proteins (MAPs) on the spindle during the long oocyte arrest. To test this, I generated transgenic flies expressing GFP-tagged microtubule-associated proteins (MAPs), and found that 13 out of 24 proteins change localisation between early and late oocytes. I refer to these changes in MAP localisation after establishment of bipolarity as “spindle maturation”. In order to identify the molecular mechanisms triggering MAP relocalisation, I manipulated the kinase activity of the cell cycle regulator Cdk1 by over-expressing non-degradable cyclin A or B, the major activators of Cdk1. Their expression prevented re-localisation of distinct sets of MAPs, and disrupted spindle bipolarity and accurate chromosome segregation in oocytes. Kinesin-6 Pavarotti/MKlp1 localised strongly to the spindle equator in late oocytes, whilst nearly always absent from this region in early oocytes. The localisation of Pavarotti to the spindle equator in late oocytes was reduced when cyclin B is over-expressed in oocytes, suggesting a role for Cdk1/cyclin B complex in regulating Pavarotti localisation. Indeed, a Pavarotti/Mklp1 mutant non-phosphorylatable by Cdk1 prematurely localised to the meiotic spindle and disrupted spindle bipolarity. Moreover, removal of Pavarotti from the metaphase-I spindle by RNAi induced spindle defects in oocytes. Therefore, it is likely that the microtubule cross-linking activity of Pavarotti enhances the stability of the metaphase-I spindle during the long arrest. Consistent with this, I found that the microtubule density in the spindle equator is higher in late oocytes. Altogether, I propose that remodelling the molecular architecture of the spindle during the long oocyte arrest is important to stabilise the bipolar spindle without centrosomes.