Transient exposure to the Eg5 kinesin inhibitor monastrol leads to syntelic orientation of chromosomes and aneuploidy in mouse oocytes.

Aneuploidy may result from abnormalities in the biochemical pathways and cellular organelles associated with chromosome segregation. Monastrol is a reversible, cell-permeable, non-tubulin interacting inhibitor of the mitotic kinesin Eg5 motor protein which is required for assembling and maintaining the mitotic spindle. Monastrol can also impair centrosome separation and induce monoastral spindles in mammalian somatic cells. The ability of monastrol to alter kinesin Eg5 and centrosome activities and spindle geometry may lead to abnormal chromosome segregation. Mouse oocytes were exposed to 0 (control), 15, 30, and 45 microg/ml monastrol in vitro for 6 h during meiosis I and subsequently cultured for 17 h in monastrol-free media prior to cytogenetic analysis of metaphase II oocytes. A subset of oocytes was cultured for 5 h prior to processing cells for meiotic I spindle analysis. Monastrol retarded oocyte maturation by significantly (P < 0.05) decreasing germinal vesicle breakdown and increasing the frequencies of arrested metaphase I oocytes. Also, significant (P < 0.05) increases in the frequencies of monoastral spindles and chromosome displacement from the metaphase plate were found in oocytes during meiosis I. In metaphase II oocytes, monastrol significantly (P < 0.05) increased the frequencies of premature centromere separation and aneuploidy. These findings suggest that abnormal meiotic spindle geometry predisposes oocytes to aneuploidy.