ASYMMETRIC DIVISIONS IN OOCYTES

Marie-Hélène Verlhac group

Asymmetric division is a widespread mechanism promoting cell diversity. Oocytes from higher eukaryotes undergo asymmetric divisions in size, leading to the formation of a large cell, the oocyte and two small ones, the polar bodies. This allows most of the maternal stores to be retained in the oocyte, a vital condition for further embryo development. We study meiotic spindle morphogenesis and positioning, key events ensuring the asymmetry of meiotic divisions in oocytes. In somatic cells, the spindle axis is determined by the position of the two opposing centrosomes before entry into mitosis. In these cells, movement and orientation of the spindle are in close relationship with the polarity axis of the mother cell, and occurs via astral microtubules emanating from the centrosomes located at spindle poles. However, prophase oocytes do not have a pre-defined polarity, and meiotic spindles organize in the absence of canonical centrosomes, thus in absence of their associated astral microtubules.

First asymmetric division: one polar body is extruded (upper right corner) and an anastral barrel-shaped second meiotic spindle has reformed below the cortex.

Spindles are highly dynamic structures, which assemble around chromosomes and distribute them equally into daughter cells. Errors in spindle formation can lead to aneuploidies, responsible for various forms of cancer in the case of somatic cells, or trisomies in the case of oocytes. It is therefore essential that bipolar spindle assembly occurs correctly. Two structures cooperate in this process: the centrosomes and the chromosomes. The chromosome pathway consists in part on local accumulation of the GTPase, Ran, around the chromosomes, acting as a local switch for spindle assembly. This pathway should be predominant in oocytes, because they lack canonical centrosomes. Unexpectedly, we have discovered that even though RanGTP accumulates in the vicinity of chromosomes at all stages of meiotic divisions and creates a broad gradient, it is not strictly required for meiosis I spindle to segregate chromosomes efficiently (Dumont, JCB 2007).

A broad gradient of RanGTP can be visualized using a specific FRET-based probe. Local RanGTP enrichment follows chromosome (green) movement to the cortex at all stages of meiotic divisions (Dumont, JCB, 2007).

We have shown that a major target of Ran, TPX2, accumulates progressively in meiosis I, potentially explaining the difference in sensitivity to alteration of the RanGTP gradient between meiosis I and II (Brunet, PLos One, 2008). These differences in spindle assembly could potentially explain why meiosis I is more error prone than meiosis II.
Spindle positioning to the cortex in mouse oocytes does not depend on astral microtubules, but requires the presence of Formin-2 nucleated actin filaments (Verlhac, Curr Biol 2000; Dumont, Dev Biol 2007). Oocytes from Formin-2 deficient mice present chromosomes that remain centrally located, do not extrude polar bodies and when fertilized, lead to aneuploid embryos. We, and others, have shown that Formin-2 organizes a highly dynamic F-actin cytoplasmic meshwork, essential for spindle migration (Azoury, Curr Biol 2008). This F-actin meshwork anchors chromosomes and the spindle in the cortex via a spindle-like structure.

The F-actin spindle is anchored by one pole into the cortex in meiosis I.

Our model might seem exotic, yet we have recently identified modes of spindle assembly and positioning, predominant in these cells, which remind properties of some cancer cells. Proper spindle assembly in oocytes requires the sorting and coalescence of multiple MTOCs (Microtubule Organizing Centers; Breuer, JCB 2010), a process reminiscent of the clustering of multiple centrosomes of some cancer cells. Uncovering essential players and their mode of regulation in the actin-based spindle morphogenesis/orientation mechanism present in a germ cell, the mouse oocyte, might prove to be relevant for the finding of potential therapeutic targets that act specifically on cancer cells and not on all dividing cells in the organism.

Selected Publications

- Verlhac M.-H. (2011), Spindle positioning: going against the actin flow. Nat Cell Biol, 12: 1183-1185.

- Azoury J., Lee K.W., Georget V., Hikal P. & Verlhac M.-H. (2011), Symmetry breaking in mouse oocytes requires transient F-actin meshwork destabilization. Development, 138:2903-2908.

- Brunet S. & Verlhac M.-H. (2011), Positioning to get out of Meiosis: the asymmetry of division. Hum Reprod Update, 17: 68-75.

- Verlhac M.-H., Terret M.E. & Pintard L. (2010), Control of the oocyte-to-embryo transition by the ubiquitin-proteolytic system in mouse and C. elegans. Curr Opin Cell Biol, 22: 758-763.

- Breuer M., Kolano A., Kwon M., Li C.-C., Tsai T.-F., Pellman D., Brunet S. & Verlhac M.-H. (2010), HURP permits MTOC sorting for robust meiotic spindle bipolarity, similar to extra-centrosome-clustering in cancer cells. J Cell Biol, 191:1251-1260.

- Azoury J., Lee K.W., Georget V., Rassinier P., Leader B. & Verlhac M.-H. (2008), Spindle positioning in mouse oocytes relies on a dynamic meshwork of actin filaments. Curr Biol, 18: 1514-1519

- Brunet S., Dumont J., Lee K. W., Kinoshita K., Hikal P., Gruss O., Maro B. & Verlhac.M.-H. (2008), Meiotic regulation of TPX2 protein levels governs cell cycle progression in mouse oocytes. PLos One, 3: e3338.

- Dumont J., Petri S., Pellegrin F., Terret M.-E., Bohnsack M.T. , Rassinier P., Georget V., Kalab P., Gruss O.J. & Verlhac M.-H. (2007), A centriole and RanGTP independent spindle assembly pathway in meiosis I of vertebrate oocytes. J Cell Biol, 176: 295-305.

- Dumont J., Million K., Sunderland K., Rassinier P., Lim H., Leader B. & Verlhac M.-H. (2007), Formin-2 is required for spindle migration and for the late steps of cytokinesis in mouse oocytes. Dev Biol, 301: 254-265.

- Dumont J., Umbhauer M., Rassinier P., Hanauer A. & Verlhac M.-H. (2005), p90Rsk is not involved in CSF arrest in mouse oocytes. J Cell Biol, 169: 227-231.

- Terret M.E., Wassmann K., Waizenegger I., Maro B., Peters J.-M. & Verlhac M.-H. (2003), The meiosis I to meiosis II transition in mouse oocytes requires separase activity. Curr Biol, 13: 1797-1802.

- Terret M.E., Lefebvre C., Djiane A., Rassinier P., Moreau J., Maro B. & Verlhac M.-H. 2003. DOC1R, a MAP Kinase substrate that control microtubule organization of metaphase II arrested mouse oocytes. Development, 130: 5169-5177.

- Lefebvre C., Terret M.E., Djiane A., Rassinier P., Maro B. & Verlhac M.–H. (2002), Meiotic spindle stability depends on MISS, a new MAPK substrate. J Cell Biol, 157: 603-613.

- Verlhac M.-H., Lefebvre C., Guillaud P., Rassinier P. & B. Maro (2000), Asymmetric division in mouse oocytes: with or without Mos. Curr Biol, 10: 1303-1306.

People

Director :
Verlhac Marie-Hélène, DR2 CNRS

Researchers :
Brunet Stéphane, CR1 INSERM
Terret Marie-Emilie, CR1 INSERM

Technical Staff :
Queguiner Isabelle, AI CdF

Post-docs and Students :
Almonacid Maria, Post-doc Ligue
Kolano Agnieszka, Post-doc ANR
Luksza Malgorzata, PhD UPMC
Chaigne Agathe, PhD ENS/UPMC
Dumont Ludovic, M2 Paris XI