Actin brings chromosomes up to speed during oocyte maturation†

Aneuploidy is a leading cause of early embryonic death, including in humans [1], and most often arises from errors during either the first or second meiotic divisions of the oocyte. Prior to ovulation, the chromosomes of the oocyte are dispersed within the nucleus. Upon the initiation of meiotic maturation, the final stage of oocyte development prior to fertilization, the chromosomes assemble on the first meiotic spindle, which is then translocated to the periphery of the oocyte, just underneath the cortex. The ensuing first meiotic division and cytokinesis segregate homologous chromosomes between the oocyte and first polar body, and those remaining in the oocyte become aligned on a newly formed second meiotic spindle. The oocyte then arrests at metaphase II, until fertilization activates it to complete the second meiotic division. Actin plays an indispensable role of course in cytokinesis following meiosis and mitosis. New research by Mogessie and Schuh [2] now identifies an unexpected additional role for actin in ensuring proper segregation of the chromosomes during the meiotic divisions. Previous work by Schuh lab and others using mouse oocytes had identified a dense cytoplasmic meshwork of actin fibers as well as an actin cage surrounding the spindle, and had shown that actin controls the translocation of the first meiotic spindle to the cortex [3, 4]. In this new work, Mogessie and Schuh detected actin fibers within the spindle in oocytes of several mammalian species, where they were generally oriented parallel to the microtubular array. These fibers, which the authors term spindle actin, became detectable as the first meiotic spindle assembled and were most prominent at first meiotic anaphase. Intrigued by this finding, the authors undertook experiments to uncover the function of spindle actin. To do so, they used a well-characterized drug, cytochalasin D (CCD), to disrupt actin fibers pharmacologically and mice lacking a protein known as Formin-2 (Fmn2), which nucleates the assembly of actin filaments. Oocytes of Fmn2−/− mice can undergo the first meiotic division but fail to complete cytokinesis [5]. In both cases, Mogessie and Schuh observed little effect on the assembly of the first meiotic spindle or the initiation of the first division. However, the chromosomes moved relatively slowly toward the spindle poles and, importantly, one or more chromosomes were often observed to move polewards even more slowly than the mass of chromosomes (Figure 1). Such “lagging” chromosomes may not be captured correctly during cytokinesis, leading to an aneuploid oocyte. Actin is required for accurate chromosomal segregation during meiosis in oocytes. (Upper panel) Actin fibers (black) are present in the cortex, in a cytoplasmic mesh, and in the spindle. K-fiber microtubules in the spindle are shown in green. Homologous chromosomes are segregated during anaphase I. (Middle panel) When oocyte actin fibers are depleted, fewer spindle K-fibers are present and some chromosomes move abnormally slowly to their respective poles (lagging chromosomes, arrow), potentially generating an aneuploid oocyte. (Lower panel) Increasing spindle actin is associated with an increase in spindle K-fibers and similar chromosomal segregation defects. The authors then examined the second meiotic division. Adding CCD to the culture medium just as oocytes were executing the first division inhibited alignment of the chromosomes on the second metaphase plate. Misaligned chromosomes were observed even when CCD was added to oocytes after they had reached metaphase II, indicating that chromosomal alignment depends on a dynamic actin-dependent process. Similar results were observed using oocytes from the Fmn2−/− mice. Intriguingly, abnormal chromosomal alignment was not observed during meiosis I; the authors suggest that alignment may be actin independent at this stage. Consistent with the effects observed at metaphase I, when the authors parthenogenetically activated the metaphase II oocytes, they observed lagging chromosomes. Having shown that disrupting actin increases the likelihood of lagging chromosomes during both the first and second meiotic divisions, the authors then asked how this might happen. Two kinds of microtubules are found in meiotic, as well as mitotic, spindles. Some run between the poles of the spindles, whereas others run from the chromosome kinetochores (hence, they are called K-fibers) to each spindle pole. Chromosome separation is achieved by shortening of the K-fibers (anaphase A) and lengthening of the interpole fibers (anaphase B); in the mouse oocyte, in contrast to most cell types, anaphase B precedes anaphase A [6]. To visualize the K-fibers, the authors employed an old trick [7]—chilling the oocytes, which depolymerizes the interpole fibers, but not the K-fibers, permitting them to be more easily visualized. They found that the intensity of K-fibers was reduced by 20%–25% at metaphase I in the Fmn2−/− and CCD-treated oocytes and by ∼40% at metaphase II in the CCD-treated oocytes. These observations link actin to the K-fibers. But what is the link? Recent studies have highlighted interactions between microtubules and microfilaments and identified proteins able to bind to both [8]. To test a possible link, the authors targeted an actin-stabilizing protein domain to the spindle. This enriched the spindle actin, as expected. Strikingly, it also caused an increase in the intensity of K-fibers, and, crucially, these oocytes showed an increased frequency of lagging chromosomes (Figure 1). Thus, decreasing or increasing actin levels leads to decreased or increased intensity of K-fibers, either of which increases the probability of lagging chromosomes—more generally, of chromosomal segregation errors—during meiotic divisions. How is actin controlling K-fiber density? The authors favor a model that spindle actin coordinates the organization of the spindle K-fibers. Proteins that bind to both actin and tubulin—including formins [9]—would be candidates to mediate such interactions. They point out, however, that other explanations may account for their findings. Both CCD and the absence of Formin-2 would disrupt the entire cytoplasmic actin mesh. Intriguingly, recent work has shown that the positioning of the first meiotic spindle at the oocyte cortex is acutely sensitive to the cortical tension [10]. This tension is established by an actin-regulated process and, when it is too high or too low, the positioning of the spindle is disrupted. It might be speculated that, as observed for spindle positioning [10], the density of the actin mesh within the cytoplasm plays a crucial role in setting up an environment that permits proper spindle function—too dense or too sparse can cause problems. The work of Mogessie and Schuh sets up many questions for future work. Why is actin required to maintain chromosome alignment at metaphase II, but not at metaphase I? The answer may be related to the relative stability of the metaphase II condition, where oocytes remain arrested until activation. What fraction of the spindle K-fibers interact with spindle actin? Why do some or most chromosomes escape the lagging phenotype? The answers should bring us closer to understanding some of the complexities of the mechanism devised by nature to ensure that each oocyte enters into union with a sperm carrying precisely one haploid set of chromosomes to contribute to the new individual.

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