Summary of key activities carried out by the Programme

We previously reported the asymmetric behavior of centrosomes in larval neuroblasts (NBs) of Drosophila (Rebollo et al. 07). One feature of this asymmetry is the invariant position of the NB’s microtubule organizing centre (MTOC) that can be used to predict the orientation of the next mitosis. Using live cell imaging we show now that the MTOC functions to determine cortical polarity orientation (Januschke and Gonzalez, JCB in press). Cortical polarity is set up at an ectopic position when microtubules are transiently depolymerised. When microtubule dynamics are restored this ectopic axis of cortical polarity then drives spindle orientation repositioning the MTOC in the NB. This new position coincides with the site of the cortex where the apical crescent forms in following mitoses. Hence, cortical polarity is established in a cell autonomous manner. The study further reveals that centrosomes are required for cortical polarity memory (Januschke and Gonzalez, JCB in press). NBs are specified during embryogenesis, undergo a phase of quiescence and resume division during larval stages. In striking contrast to larval NBs, spindles of embryonic NBs are thought to always assemble orthogonally to the polarity axis and later rotate to align with it. Using two-photon confocal microscopy on living embryonic NBs, we were able now to solve this discrepancy and demonstrated that embryonic NBs also use the pre-alignment mode. (Rebollo et al. 09; highlighted by the faculty of 1000). Importantly, embryonic NBs, like larval NBs, have a prominent apical MTOC; it is therefore likely that the MTOC operates in the polarity memory throughout the entire live time of NBs.

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