Abstract
Before fertilization, mammalian oocyte undergoes an asymmetric division which depends on eccentric positioning of the spindle at the oocyte cortex to form a polar body and an egg. Since the centriole is absent and, as a result, the polar array microtubules are not fully developed in oocytes, microtubules have seldom been considered as required for eccentric positioning of the spindle, while actin-related forces have instead been proposed to be primarily responsible for this process. However, the existing models are largely conflicting and the underlying mechanism of asymmetric division is still elusive. Here we show that poly(ADP-ribose) (PAR) is enriched at mouse oocyte cortical area throughout meiosis. Specific removal of cortical PAR results in an ectopic spindle and a failure of asymmetric division. During spindle migration, when the spindle deviates from the center of oocyte by a pushing force of cytoplasmic actin, the short polar array microtubules emanating from the juxtacortical spindle pole extend to the cortex and penetrate into cortical PAR, docking and stabilizing the spindle at the cortex which facilitates the asymmetric division. This process depends on the affinity between PAR and microtubule-associated proteins such as Spindly, which contributes to a physical link for cortical PAR and the spindle. Notably, fusing a PAR-binding domain to end-binding protein 3, a plus-end tracking protein at the polar array microtubules, restores the asymmetric division of oocytes with Spindly knockdown. Thus, our work demonstrates a comprehensive mechanism for oocyte spindle positioning and asymmetric division.