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

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.

The APC/C in female mammalian meiosis I

“…Notably, however, sixfold over-expression of TPX2 did not augment and indeed disrupted, spindle assembly leading to multipolar spindles (Brunet et al 2008) Figure 2 The APC/C in acentrosomal spindle assembly during MI and progression to MII. The schematic diagram depicts two stages in acentrosomal spindle assembly in mouse oocytes, an early stage when spindles are roughly spherical and the late stage when spindles have acquired a bipolar phenotype (Schuh & Ellenberg 2007, Gui & Homer 2012.…”

“…Two proteins that are APC/C Cdh1 substrates, targeting protein for the Xenopus kinesin xklp2 (TPX2) and hepatoma up-regulated protein (HURP), have been shown to be important for stable acentrosomal bipolar spindle formation (Brunet et al 2008, Breuer et al 2010. In mouse oocytes, spindle assembly and spindle pole integrity are dependent on TPX2, probably through N-terminal region-dependent transforming acidic coiled-coil 3 (TACC3) phosphorylation acting via Aurora A kinase (Brunet et al 2008). HURP is important for bipolarisation in mouse oocytes by stabilising a central microtubule array that acts as a scaffold for sorting MTOCs into two distinct poles (Breuer et al 2010).…”

“…The schematic therefore incorporates APC/C Cdh1 targets that act as 'spindle assembly factors', which may or may not turn out to include Hec1/cyclin B2. Although TPX2 and HURP are known to promote acentrosomal spindle assembly (Brunet et al 2008, Breuer et al 2010 and are potential APC/C Cdh1 substrates, in mouse oocytes, they may not be subject to direct APC/C Cdh1 regulation (Holt et al 2012). Following the establishment of kMt attachments, SAC-mediated inhibition dissipates, thereby allowing activation of APC/C Cdc20 , which could then be tasked with degrading not only cyclin B1 and securin but also cyclin A2 and cyclin B2 (Touati et al 2012, Gui & Homer 2013.…”

Mouse oocyte, a paradigm of cancer cell

“…The small GTPase Ran, present in a gradient around the chromosomes in the oocyte, locally activates spindle assembly factors to promote microtubule nucleation and stabilization. 17,51 Then the chromosomes congress toward the metaphase plate, showing highly oscillatory movements until the very end of prometaphase, 52 K-fibers being established only late in meiosis I 33,34,49,53 (Fig. 3).…”

Phospho-Signaling at Oocyte Maturation and Fertilization: Set Up for Embryogenesis and Beyond Part II. Kinase Regulators and Substrates

“…At the physiological conditions, TPX2 is essential for microtubule nucleation around chromatin (Brunet et al 2004). TPX2 is required for spindle assembly and spindle pole integrity in mouse oocyte maturation (Brunet et al 2008). In Xenopus oocyte, activation of the centrosomal Aurora A by TPX2 is required during spindle assembly (Sardon et al 2008).…”