Cullin-5 regulates nuclear positioning and reveals insights on the sensing of the nuclear-to-cytoplasmic ratio in Drosophila embryogenesis

Summary In most metazoans, early embryonic development is characterized by rapid division cycles which pause before gastrulation at the mid-blastula transition (MBT). 1 These early cleavage divisions are accompanied by cytoskeletal rearrangements which ensure proper nuclear positioning. Yet, the molecular mechanisms controlling nuclear positioning are not fully elucidated. In Drosophila , early embryogenesis unfolds in a multinucleated syncytium, and nuclei rapidly move across the anterior-posterior (AP) axis at cell cycles 4-6 in a process driven by actomyosin contractility and cytoplasmic flows. 2,3 Previously, shackleton ( shkl ) mutants were identified in which this axial spreading is impaired. 4 Here, we show that shkl mutants carry mutations in the cullin-5 ( cul-5 ) gene. Live imaging experiments show that Cul-5 is downstream of the cell cycle but required for cortical actomyosin contractility. The nuclear spreading phenotype of cul-5 mutants can be rescued by reducing Src activity genetically, suggesting that a major target of Cul-5 is Src kinase. cul-5 mutants display gradients of nuclear density across the AP axis at the MBT which we exploit to study cell cycle control as a function of the N/C ratio. We found that the N/C ratio is sensed collectively in neighborhoods of about 100μm and such collective sensing is required for a precise MBT in which all the nuclei in the embryo pause their division cycle. Moreover, we found that the response to the N/C ratio is slightly graded along the AP axis. These two features can be linked to the spatiotemporal regulation of Cdk1 activity. Collectively, our results reveal a new pathway controlling nuclear spreading and provide a quantitative dissection of how nuclear cycles respond to the N/C ratio.