The polarity protein Pard3 is required for centrosome positioning during neurulation.

Microtubules are essential regulators of cell polarity, architecture and motility. The organization of the microtubule network is context-specific. In non-polarized cells, microtubules are anchored to the centrosome and form radial arrays. In most epithelial cells, microtubules are noncentrosomal, align along the apico-basal axis and the centrosome templates a cilium. It follows that cells undergoing mesenchyme-to-epithelium transitions must reorganize their microtubule network extensively, yet little is understood about how this process is orchestrated. In particular, the pathways regulating the apical positioning of the centrosome are unknown, a central question given the role of cilia in fluid propulsion, sensation and signaling. In zebrafish, neural progenitors undergo progressive epithelialization during neurulation, and thus provide a convenient in vivo cellular context in which to address this question. We demonstrate here that the microtubule cytoskeleton gradually transitions from a radial to linear organization during neurulation and that microtubules function in conjunction with the polarity protein Pard3 to mediate centrosome positioning. Pard3 depletion results in hydrocephalus, a defect often associated with abnormal cerebrospinal fluid flow that has been linked to cilia defects. These findings thus bring to focus cellular events occurring during neurulation and reveal novel molecular mechanisms implicated in centrosome positioning.