Asymmetrical microtubule organizing centers drive posterior nuclei positioning in Ciona notochord development

As the central organelle of cell, nucleus positioning affects diverse cell behaviors, including cell proliferation, migration, deformation, and polarity establishment, playing important roles in tissue and organ morphogenesis. However, the driving mechanisms underlying the asymmetrical positioning of nuclei in organ morphogenesis and embryogenesis remain elusive. Here, we described a process of posterior nucleus positioning in ascidian Ciona notochord development. Chemical inhibitor experiments demonstrated that the nucleus positioning in notochord cells is a microtubule-dependent behavior. Intriguingly, a special anterior and posterior dual microtubule-organizing center (MTOC) system was observed around notochord cell nuclei. Quantification of the microtubule intensity on both MTOCs showed a higher density of microtubule in the posterior one during the nuclear movement, indicating that the microtubule generates an asymmetrical force driving posterior nuclear movement. Overexpression of a Klarsicht, ANC1, and Syne Homology protein (KASH), which links microtubule and nucleus, disrupted the polarized localization of nuclei, demonstrating a "tug-of-war" mechanism that drives nuclei movement through the competition between a pair of MTOCs in Ciona notochord. Our findings thus present a novel mechanism of microtubule-driven nuclei asymmetrical positioning, providing insights into how nuclei positioning arranged in tissue and organ morphogenesis.