Summary Aneuploidy arising early in development is the leading genetic cause of birth defects and developmental disabilities in humans. Most errors in chromosome number originate from the egg, and maternal age is well established as the key risk factor. Although the importance of this problem for reproductive health is widely recognized, the underlying molecular basis for age-related aneuploidy in female meiosis is unknown. Here we show that weakened chromosome cohesion is a leading cause of aneuploidy in oocytes in a natural aging mouse model. We find that sister kinetochores are farther apart at both Metaphase I and II, indicating reduced centromere cohesion. Moreover, levels of the meiotic cohesin protein REC8 are severely reduced on chromosomes in oocytes from old mice. To test whether cohesion defects lead to the observed aneuploidies, we monitored chromosome segregation dynamics at Anaphase I in live oocytes and counted chromosomes in the resulting Metaphase II eggs. About 90% of age-related aneuploidies are best explained by weakened centromere cohesion. Together, these results demonstrate that the maternal age-associated increase in aneuploidy is often due to a failure to effectively replace cohesin proteins that are lost from chromosomes during aging.

Aurora B kinase (AURKB) is the catalytic subunit of the chromosomal passenger complex (CPC), an essential regulator of chromosome segregation. In mitosis, the CPC is required to regulate kinetochore microtubule (K-MT) attachments, the spindle assembly checkpoint, and cytokinesis. Germ cells express an AURKB homolog, AURKC, which can also function in the CPC. Separation of AURKB and AURKC function during meiosis in oocytes by conventional approaches has not been successful. Therefore, the meiotic function of AURKC is still not fully understood. Here, we describe an ATP-binding-pocket-AURKC mutant, that when expressed in mouse oocytes specifically perturbs AURKC-CPC and not AURKB-CPC function. Using this mutant we show for the first time that AURKC has functions that do not overlap with AURKB. These functions include regulating localized CPC activity and regulating chromosome alignment and K-MT attachments at metaphase of meiosis I (Met I). We find that AURKC-CPC is not the sole CPC complex that regulates the spindle assembly checkpoint in meiosis, and as a result most AURKC-perturbed oocytes arrest at Met I. A small subset of oocytes do proceed through cytokinesis normally, suggesting that AURKC-CPC is not the sole CPC complex during telophase I. But, the resulting eggs are aneuploid, indicating that AURKC is a critical regulator of meiotic chromosome segregation in female gametes. Taken together, these data suggest that mammalian oocytes contain AURKC to efficiently execute meiosis I and ensure high-quality eggs necessary for sexual reproduction.

Sporulation in Saccharomyces cerevisiae is a highly regulated process wherein a diploid cell gives rise to four haploid gametes. In this study we show that histone H4 Ser1 is phosphorylated (H4 S1ph) during sporulation, starting from mid-sporulation and persisting to germination, and is temporally distinct from earlier meiosis-linked H3 S10ph involved in chromosome condensation. A histone H4 S1A substitution mutant forms aberrant spores and has reduced sporulation efficiency. Deletion of sporulation-specific yeast Sps1, a member of the Ste20 family of kinases, nearly abolishes the sporulation-associated H4 S1ph modification. H4 S1ph may promote chromatin compaction, since deletion of SPS1 increases accessibility to antibody immunoprecipitation; furthermore, either deletion of Sps1 or an H4 S1A substitution results in increased DNA volume in nuclei within spores. We find H4 S1ph present during Drosophila melanogaster and mouse spermatogenesis, and similar to yeast, this modification extends late into sperm differentiation relative to H3 S10ph. Thus, H4 S1ph may be an evolutionarily ancient histone modification to mark the genome for gamete-associated packaging.[Keywords: Saccharomyces cerevisiae; fly and mouse spermatogenesis; genome compaction; histone H4 phosphorylation; kinase; yeast sporulation] Genetic and epigenetic information is transferred to a new cell generation through the gametogenesis process. Dramatic changes in chromatin structure occur during both metazoan spermatogenesis and yeast sporulation involving DNA compaction. In addition, spermatogenesis requires removal of most canonical histones and substitution with histone variants and histone replacement proteins (Govin et al. 2004; Kimmins and SassoneCorsi 2005).The nucleosome is the fundamental repeating unit of chromatin and harbors an octamer of basic histone proteins (two copies of dimeric H3/H4 and H2A/H2B) wrapped by ∼147 base pairs (bp) of DNA (Luger et al. 1997). Nucleosomes pack into higher-order chromatin structures, whose precise architectures are not understood. Post-translational modifications (PTMs) of histones (including acetylation, phosphorylation, methylation, and ubiquitylation) regulate chromatin function and contribute to its folding. PTMs occur in distinct patterns and in diverse cellular pathways. For example, H3 S10ph correlates with both mitotic/meiotic chromosome condensation and transcriptional activation (Nowak and Corces 2004). Chromosome condensation includes a large number of possibly redundant histone phosphorylation marks, including S10 (Hendzel et al. 1997), T3 (Polioudaki et al. 2004), T11 (Preuss et al. 2003, S28 (Goto et al. 1999S28 (Goto et al. , 2002 within H3, S1 within H4 (Barber et al. 2004), S1 within H2A (Barber et al. 2004), and S10 in H2B (Ahn et al. 2005b). Phosphorylation at H3 S10 has been causally linked to mitotic

Aurora kinases are highly conserved, essential regulators of cell division. Two Aurora kinase isoforms, A and B (AURKA and AURKB), are expressed ubiquitously in mammals, whereas a third isoform, Aurora C (AURKC), is largely restricted to germ cells. Because AURKC is very similar to AURKB, based on sequence and functional analyses, why germ cells express AURKC is unclear. We report that Aurkc −/− females are subfertile, and that AURKB function declines as development progresses based on increasing severity of cytokinesis failure and arrested embryonic development. Furthermore, we find that neither Aurkb nor Aurkc is expressed after the one-cell stage, and that AURKC is more stable during maturation than AURKB using fluorescently tagged reporter proteins. In addition, Aurkc mRNA is recruited during maturation. Because maturation occurs in the absence of transcription, posttranscriptional regulation of Aurkc mRNA, coupled with the greater stability of AURKC protein, provides a means to ensure sufficient Aurora kinase activity, despite loss of AURKB, to support both meiotic and early embryonic cell divisions. These findings suggest a model for the presence of AURKC in oocytes: that AURKC compensates for loss of AURKB through differences in both message recruitment and protein stability.A urora kinases are highly conserved cell-cycle regulators with essential roles in chromosome segregation. There are three Aurora kinases in mammals: Aurora kinases A and B (AURKA or -B) are ubiquitously expressed and their functions have been extensively studied, whereas AURKC is largely limited to germ cells (1-3); many human cancer cell lines express AURKC (4) and some somatic tissues express AURKC at low levels (5-7). It is not clear, however, why germ cells require a third AURK. Because isoforms can have different functions, it is tempting to speculate that AURKC exists because its mitotic counterparts simply cannot execute unique features of meiosis.One unique feature of meiosis is the generation of haploid gametes from diploid precursor cells by a reductional chromosome segregation during meiosis I (MI) followed by an equational division at meiosis II (MII) without an intervening round of DNA replication. In oocytes, another unique feature is that meiosis is not a continuous process because there is a growth period during a prolonged arrest at prophase I, followed by a cell division cycle during oocyte maturation, and a second arrest at metaphase of MII, until fertilization, which triggers completion of MII. Furthermore, proteins in the oocyte must support the first mitotic cell cycles of the embryo before zygotic genome activation. Despite these obvious differences, several observations suggest that AURKC may not have a specialized function. AURKB and AURKC are highly similar in sequence (61% identical), and AURKC can functionally compensate for loss of AURKB when ectopically expressed in somatic cells (8, 9). Furthermore, embryos that lack AURKB can develop to but not beyond the blastocyst, as long as AURKC is present, consis...

SUMMARY The elevated incidence of aneuploidy in human oocytes warrants study of the molecular mechanisms regulating proper chromosome segregation. The Aurora kinases are a well-conserved family of serine/threonine kinases that are involved in proper chromosome segregation during mitosis and meiosis. Here we report the expression and localization of all three Aurora kinase homologs, AURKA, AURKB, and AURKC, during meiotic maturation of mouse oocytes. AURKA, the most abundantly expressed homolog, localizes to the spindle poles during meiosis I (MI) and meiosis II (MII), whereas AURKB is concentrated at kinetochores, specifically at metaphase of MI (Met I). The germ cell-specific homolog, AURKC, is found along the entire length of chromosomes during both meiotic divisions. Maturing oocytes in the presence of the small molecule pan-Aurora kinase inhibitor, ZM447439 results in defects in meiotic progression and chromosome alignment at both Met I and Met II. Over-expression of AURKB, but not AURKA or AURKC, rescues the chromosome alignment defect suggesting that AURKB is the primary Aurora kinase responsible for regulating chromosome dynamics during meiosis in mouse oocytes.