Initial spindle positioning at the oocyte center protects against incorrect kinetochore-microtubule attachment and aneuploidy in mice

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>Aneuploidy is the leading genetic cause of miscarriage and infertility in women and occurs frequently in oocytes. Spindle formation and positioning are two critical events that must be regulated tightly to avoid erroneous chromosome segregation. Following nuclear envelope breakdown (NEBD), the spindle is assembled centrally before migrating towards the cortex to allow the first asymmetric division. The biological significance of the primary central positioning of the spindle is unknown. Because the spindle forms where NEBD occurs, whether positioned at the center or at the cortex of the cell, full-grown GV (prophase I) oocytes were collected from CF-1 mice (6-8 weeks old) and sorted according to the position of the GV into three groups: central, intermediate, and peripheral. Approximately 50% of the cells exhibited a central GV position, while 25% of cells showed a peripheral GV position. These proportions were similar to those obtained by histological evaluation of ovarian oocytes in 6-8 week mice, but not to those of mice aged less than 5 weeks, which tended to have a majority of peripherally positioned GVs. When peripheral GV oocytes were matured <jats:italic>in vitro</jats:italic>, GVs (88%) migrated towards the center and NEBD (and spindle assembly) occurred either at the center of the cell or during migration. The percentages of NEBD and polar body (PB) extrusion did not vary significantly among groups. Importantly, peripheral GV oocytes had a significant increase of abnormal K-MT attachments at metaphase I (Met I) and showed a significant increase of aneuploidy at metaphase II (Met II) when compared to central GV oocytes. Interestingly, peripheral GV oocytes that were able to achieve full GV relocation to the center were half as likely to experience aneuploidy when compared to peripheral GV oocytes unable to achieve relocation. These results indicate that preferential central spindle formation is an insurance mechanism to protect against incorrect K-MT attachments and aneuploidy.</jats:p>

Microtubule organizing centers regulate spindle positioning in mouse oocytes

“…7). These opposing forces would be essential to position the spindle centrally during pro-Met I and to prevent premature spindle migration, a risk factor for aneuploidy 5 . Our model is consistent with three sets of observations: 1) mcMTOCs are exclusively localized asymmetrically, opposite the site of PB extrusion (the side of F-actin enrichment); 2) mcMTOCs undergo a significant decrease in number and volume during late Met I and Ana I/Telo I, allowing the F-actin mediated force to extrude the PB; and 3) nocodazole-mediated MT depolymerization advances the timing of chromosome migration to the cortex, which takes place at a relatively higher speed 12,18 whereas increasing mcMTOC numbers delays chromosome migration to the cortex, which occurs at a relatively reduced speed compared to controls.…”

“…At the age of puberty, gonadotropin cues allow prophase I-arrested oocytes to resume MI evident by breakdown of the nuclear envelope (NEBD) and formation of a central bipolar spindle [2][3][4] . The central positioning of the spindle is required to establish proper kinetochore-microtubule (MT) attachments and to protect against aneuploidy 5 . The position of the spindle dictates the plane of cell division 6 and therefore, in contrast to mitotic cells where a centrally positioned spindle allows symmetrical cell division, the meiotic spindle must migrate towards the cortex for the highly asymmetric meiotic divisions 6,7 .…”