Abstract
Mammalian oocyte maturation is governed by an interaction between protein kinases, phosphatases and proteasases. Maturation promoting factor (MPF) is a serine/threonine kinase heterodimer composed of a catalytic cdc2/cdk1 subunit and a regulatory cyclin B1 subunit.1 Cyclin B1 undergoes rapid turnover via degradation in the ubiquitin-proteasome pathway (UPP) followed by de novo synthesis. A high level of MPF causes metaphase arrest, then UPP degradation of cyclin B1 allows the oocyte to exit metaphase I (MI). Proteasomes have been localised in pig and rat oocytes. Although in both species they are associated with the MI and MII meiotic spindles, immunostaining at the germinal vesicle (GV) stage is different with perinuclear staining in rat GV and intra-GV staining in the pig. This may suggest different roles for proteasomes prior to GV breakdown (GVBD) in different species. This study used confocal laser scanning immunohistochemistry with a specific antiserum against the 20S proteolytic ‘core’ of the 26S proteasome to reveal proteasomes in murine oocytes undergoing maturation in vitro. In the mouse, proteasomes were associated with the meiotic spindle, similar to observations in pig and rat oocytes and preliminary studies in the bovine suggest a similar immunolocalisation. Cyclin B1 also accumulates around the spindle during meiosis.1 This suggests that proteasomes are prevented from degrading cyclin B1 until the MI-AI and MII-AII transitions. Immunolabelling showed Fam, a deubiquitinylating enzyme (also known as Usp9x) was localised at the spindle during MI and MII. This suggests a link between Fam, the UPP and the spindle assembly checkpoint to prevent cyclin B1 degradation until required. (1)Huo LJ, Fan HY, Zhong ZS, Chen DY, Schatten H, Sun QY. (2004). Ubiquitin-proteasome pathway modulates mouse oocyte meiotic maturation and fertilization via regulation of MAPK cascade and cyclin B1 degradation. Mech. Dev. 121, 1275–1287.
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