08-P015 Human ASPM participates in cleavage furrow orientation and cytokinesis

During development of the vertebrate CNS, a precise control of planar orientation of the mitotic spindle of dividing neuroepithelial progenitors regulates the equal partitioning of apical attachment sites. This is necessary to maintain sister cells within the neuroepithelial structure. We have previously demonstrated an essential role for the G-protein regulator LGN in this process. Here we use three-dimensional time lapse imaging of dividing cells in the chick neuroepithelium and describe that the mitotic spindle follows a stereotypical biphasic rotation movement, which first drives the spindle to align with the neuroepithelial surface, then maintains it in this plane while leaving it free to revolve around the apico-basal axis of the cell. We show that Gai subunits, LGN and the dynein/dynactin interactor NuMA localize as nesting ring-shaped cortical domains on the lateral membrane of dividing cells. Gai-GDP recruits LGN, which in turn recruits NuMA to the cell cortex. LGN functions as a Gai-GDP sensor that changes conformation and is able to recruit NuMA only above a certain threshold of cortical Gai-GDP concentration. In absence of these molecules, spindle rotation is lost or erratic and the plane of division is randomized. We propose that a gradient of Gai-GDP is formed within the overall homogeneous cortical Gai distribution. A peak of Gai-GDP on the lateral cell cortex is used as a cue to define the progressively narrower rings of Gai, Gai-GDP/LGN and Gai-GDP/LGN/NuMA complexes on the lateral cell cortex. The latter domain controls astral microtubule stabilization to orients the spindle.