Asymmetrical microtubule-organizing centers drive posterior positioning of nuclei in <i>Ciona</i> notochord development

The nucleus is the central organelle of the cell, and positioning of the nucleus 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 asymmetrical positioning of nuclei in organ morphogenesis and embryogenesis remain elusive. Here, we describe a process of posterior nucleus positioning during notochord development in the ascidian Ciona. Chemical inhibitor experiments demonstrated that positioning of the nucleus in notochord cells is a microtubule-dependent process. Intriguingly, a special dual anterior and posterior microtubule-organizing center (MTOC) system was observed around notochord cell nuclei. Quantification of the microtubule intensity of both MTOCs revealed a higher density of microtubules in the posterior MTOC during nuclear movement, indicating that the microtubules generate an asymmetrical force driving posterior nuclear movement. Overexpression of the Klarsicht, ANC1 and Syne Homology (KASH) domain-containing protein KASH1, which links microtubules to the nucleus, disrupted the polarized localization of nuclei, demonstrating a 'tug-of-war' mechanism in which competition between a pair of MTOCs drives movement of nuclei in Ciona notochord cells. Our findings thus highlight a novel mechanism of microtubule-driven asymmetrical nuclear positioning, providing insights into how the positions of nuclei are arranged in tissue and organ morphogenesis.