Spatiotemporal regulation of LGN/NuMA and astral microtubules generate mirror symmetric spindle rotations

The positioning of the mitotic spindle controls the size, content, and position of daughter cells within embryos and tissues. A major spindle positioning mechanism is “cortical pulling” whereby the membrane-bound complex LGN/NuMA/Dynein captures astral microtubules and pulls centrosomes toward the cell cortex. Cytoplasmic dynein and astral growth tend to counteract cortical pulling and position spindles at the cell center. It remains unclear how these opposing forces cooperate. Here we examine the ascidian embryo, where spindles of two germ line cells rotate toward one another causing divisions which are both mirror symmetric and unequal. We find that this spindle behavior is governed by transient enrichment of LGN and NuMA and enhanced cortical pulling at the shared cell contact. Inhibition of the LGN/NuMA complex disrupts spindle alignment, unequal cleavage, and mirror symmetry. Temporal analysis shows that cortical pulling force initates at anaphase when there is a sharp increase in astral microtubule length. These results point towards two phases of spindle positioning forces, with cytoplasmic pulling and cortical pulling operating sequentially during early and late mitosis.