The Ran GTPase Mediates Chromatin Signaling to Control Cortical Polarity during Polar Body Extrusion in Mouse Oocytes

Abstract

The molecular basis for asymmetric meiotic divisions in mammalian oocytes that give rise to mature eggs and polar bodies remains poorly understood. Previous studies demonstrated that the asymmetrically positioned meiotic chromosomes provide the cue for cortical polarity in mouse oocytes. Here we show that the chromatin-induced cortical response can be fully reconstituted by injecting DNA-coated beads into metaphase II-arrested eggs. The injected DNA beads induce a cortical actin cap, surrounded by a myosin II ring, in a manner that depends on the number of beads and their distance from the cortex. The Ran GTPase plays a critical role in this process, because dominant-negative and constitutively active Ran mutants disrupt DNA-induced cortical polarization. The Ran-mediated signaling to the cortex is independent of the spindle but requires cortical myosin II assembly. We hypothesize that a Ran(GTP) gradient serves as a molecular ruler to interpret the asymmetric position of the meiotic chromatin.

Actin cytoskeleton in cell polarity and asymmetric division during mouse oocyte maturation

“…The size and strength of the cortical actin cap correspond quantitatively to the DNA amount on the beads and the beads-cortex distance [Deng et al, 2007]. This ''at-a-distance'' effect may be similar to the role of chromosomes in spindle/MTs organization, where chromatin controls the activity of multiple MT regulators through a Ran GTPase gradient [Zheng, 2004;Kalab and Heald, 2008].…”

“…The Ran GTPase gradient is established through the spatially separated actions of the Ran GEF and Ran GAP: the guanine nucleotide exchange factor (GEF) of Ran, RCC1 is associated with the chromatin, while the GTPase activating protein (GAP) is diffused throughout the cytoplasm. Supporting an involvement of the Ran gradient in chromatin-regulated cortical polarity, dominant-negative and constitutively active Ran mutants both disrupt the chromatin induced cortical polarization [Deng et al, 2007]. This finding led to the hypothesis that the Ran GTPase gradient serves as a molecular ruler during cortical polarization at the MII arrested stage.…”

“…However, unlike in mitotic cells, actomyosin ring assembly and contraction appear to be regulated distinctly in mouse oocytes, as these two events are temporally separated [Deng et al, 2007;Elbaz et al, 2010]. For example, the actomyosin ring forms upon stimulation from chromatin signal during the MII arrest, whereas ring contraction does not occur until anaphase onset after sperm entry.…”

“…In MII mouse oocytes, the chromosomes provide the cue for the organization of cortical actomyosin cap [Maro et al, 1984;Longo and Chen, 1985;Verlhac et al, 2000;Deng et al, 2007] (Fig. 2).…”

“…In addition to eccentric spindle positioning, a second event critical for asymmetric division in oocyte maturation is the establishment of cortical polarity: an F-actinenriched cortical domain surrounded by a myosin II ring assembles above the meiotic spindle in late metaphase I and in metaphase II arrested oocytes [Brunet and Maro, 2005;Deng et al, 2007] (Fig. 1).…”