Meiotic Regulation of TPX2 Protein Levels Governs Cell Cycle Progression in Mouse Oocytes

Stéphane Brunet,Bernard Maro,Julien Dumont,Kazuhisa Kinoshita,Pascale Hikal,Karen W Lee,Oliver J Gruss,Marie-Hélène Verlhac,Kevin G Hardwick

doi:10.1371/journal.pone.0003338

Abstract

Formation of female gametes requires acentriolar spindle assembly during meiosis. Mitotic spindles organize from centrosomes and via local activation of the RanGTPase on chromosomes. Vertebrate oocytes present a RanGTP gradient centred on chromatin at all stages of meiotic maturation. However, this gradient is dispensable for assembly of the first meiotic spindle. To understand this meiosis I peculiarity, we studied TPX2, a Ran target, in mouse oocytes. Strikingly, TPX2 activity is controlled at the protein level through its accumulation from meiosis I to II. By RNAi depletion and live imaging, we show that TPX2 is required for spindle assembly via two distinct functions. It controls microtubule assembly and spindle pole integrity via the phosphorylation of TACC3, a regulator of MTOCs activity. We show that meiotic spindle formation in vivo depends on the regulation of at least a target of Ran, TPX2, rather than on the regulation of the RanGTP gradient itself.

Highlights

The assembly of a functional spindle is critical for accurate chromosome segregation
TPX2 expression was first analyzed by immunoblot during meiotic maturation from prophase I to the metaphase II arrest using an antibody directed against human TPX2 (Fig 1A)
TPX2 accumulated after germinal vesicle breakdown (GVBD, equivalent to nuclear envelope breakdown) until the metaphase arrest of meiosis II (Fig 1B)

Summary

Introduction

The assembly of a functional spindle is critical for accurate chromosome segregation. At the end of meiosis, oocytes undergo two successive divisions (meiosis I and II) to form haploid gametes During these divisions, meiotic spindles must ensure successively the segregation of homologous chromosomes and sister chromatids. Homologous chromosome missegregation in meiosis I is a major source of embryonic aneuploidy in mammals, and accounts for most spontaneous abortion and birth defects in human [1,2]. This peculiarity of meiosis I is poorly understood at the molecular level and the mouse oocyte constitutes the model of choice for its study. Understanding the mechanisms that govern the formation of meiotic spindles in mammals remains an important and challenging goal for developmental and cell biologists

Methods

Results

Conclusion