The PP2A Inhibitor I2PP2A Is Essential for Sister Chromatid Segregation in Oocyte Meiosis II

Abstract

Haploid gametes are generated through two consecutive meiotic divisions, with the segregation of chromosome pairs in meiosis I and sister chromatids in meiosis II. Separase-mediated stepwise removal of cohesion, first from chromosome arms and later from the centromere region, is a prerequisite for maintaining sister chromatids together until their separation in meiosis II [1]. In all model organisms, centromeric cohesin is protected from separase-dependent removal in meiosis I through the activity of PP2A-B56 phosphatase, which is recruited to centromeres by shugoshin/MEI-S332 (Sgo) [2-5]. How this protection of centromeric cohesin is removed in meiosis II is not entirely clear; we find that all the PP2A subunits remain colocalized with the cohesin subunit Rec8 at the centromere of metaphase II chromosomes. Here, we show that sister chromatid separation in oocytes depends on a PP2A inhibitor, namely I2PP2A. I2PP2A colocalizes with the PP2A enzyme at centromeres at metaphase II, independently of bipolar attachment. When I2PP2A is depleted, sister chromatids fail to segregate during meiosis II. Our findings demonstrate that in oocytes I2PP2A is essential for faithful sister chromatid segregation by mediating deprotection of centromeric cohesin in meiosis II.

PP2A regulates kinetochore-microtubule attachment during meiosis I in oocyte

“…Figure S1C and S1D). Immunoblotting on GV stage oocytes and immunofluorescence on chromosome spreads confirmed the absence of PP2Ac proteins in DKO oocytes 30 (Supplementary information, Figure S1E and F). Ovaries from DKO females exhibited superovulatory responses normally after induction but the 1 st polar body extrusion (PBE) rate in the DKO eggs was low (Supplementary information, Figure S1G-I).…”

“…30. Briefly, oocytes at indicated time points after GVBD were treated with Tyrode's acid (Sigma T1788) to remove the zona pellucida.…”

Tension-dependent removal of pericentromeric shugoshin is an indicator of sister chromosome biorientation

“…While we cannot currently distinguish between these models, these observations demonstrate that sister kinetochore biorientation is unlikely to be sufficient for the deprotection of cohesion, and additional mechanisms must contribute. Indeed, in mouse oocytes, the PP2A inhibitor I2PP2A/Set1b colocalizes with Rec8 only in meiosis II, and its depletion prevents sister chromatid segregation during meiosis II (Chambon et al 2013). Therefore, although suppression of sister chromatid biorientation facilitates the maintenance of pericentromeric cohesion during meiosis I, its deprotection during meiosis II is likely to require additional factors.…”

Molecular causes of aneuploidy in mammalian eggs

“…Sgo2 localizes to centromeres, and it is thought that its association with the protein phosphatase 2A (PP2A) prevents local phosphorylation of the Rec8 component of cohesin (Xu et al, 2009), an event that is thought to sensitize Rec8 to the actions of separase. One of the most recent exciting developments in mouse oocytes is the discovery of how Rec8 is deprotected in MII, through centromeric recruitment of a PP2A inhibitor (Chambon et al, 2013). What is intriguing is that the recruitment of the inhibitor of PP2A, known as IPP2A (also known as SET, PHAP-II or TAF1b), to the centromere is not dependent on the presence of a dyad, because the same recruitment is induced even when bivalent integrity is maintained at MI completion, such that a MII spindle is assembled with bivalents.…”