Meiotic Kinetochores Fragment into Multiple Lobes upon Cohesin Loss in Aging Eggs

Abstract

SummaryChromosome segregation errors during female meiosis are a leading cause of pregnancy loss and human infertility. The segregation of chromosomes is driven by interactions between spindle microtubules and kinetochores. Kinetochores in mammalian oocytes are subjected to special challenges: they need to withstand microtubule pulling forces over multiple hours and are built on centromeric chromatin that in humans is decades old. In meiosis I, sister kinetochores are paired and oriented toward the same spindle pole. It is well established that they progressively separate from each other with advancing female age. However, whether aging also affects the internal architecture of centromeres and kinetochores is currently unclear. Here, we used super-resolution microscopy to study meiotic centromere and kinetochore organization in metaphase-II-arrested eggs from three mammalian species, including humans. We found that centromeric chromatin decompacts with advancing maternal age. Kinetochores built on decompacted centromeres frequently lost their integrity and fragmented into multiple lobes. Fragmentation extended across inner and outer kinetochore regions and affected over 30% of metaphase-II-arrested (MII) kinetochores in aged women and mice, making the lobular architecture a prominent feature of the female meiotic kinetochore. We demonstrate that a partial cohesin loss, as is known to occur in oocytes with advancing maternal age, is sufficient to trigger centromere decompaction and kinetochore fragmentation. Microtubule pulling forces further enhanced the fragmentation and shaped the arrangement of kinetochore lobes. Fragmented kinetochores were frequently abnormally attached to spindle microtubules, suggesting that kinetochore fragmentation could contribute to the maternal age effect in mammalian eggs.

Single cell analysis reveals the impact of age and maturation stage on the human oocyte transcriptome

“…Advanced maternal age has been shown to be directly related to an increased rate of aneuploidies, with cohesin loss and centromeric abnormalities being the most studied underlying causes (Tachibana-Konwalski et al, 2010;Burkhardt et al, 2016;Smoak et al, 2016;Toth and Jessberger, 2016;Gruhn et al, 2019;Zielinska et al, 2019). Strikingly, in our data, among the top hits of biological processes affected by maternal age in IVM-MII oocytes are chromosome and chromatid segregation-related terms (Supplementary Table 5).…”

“…In this study we have shown that age as well as BMI affect key pathways at the RNAlevel, which are involved in oocyte maturation and function such as chromosome segregation, mitochondria, RNA-metabolism and translation. While many age-related oocyte defects, as for example chromosome segregation errors, have been attributed to low protein turnover during meiotic arrest (Tachibana-Konwalski et al, 2010;Burkhardt et al, 2016;Smoak et al, 2016;Toth and Jessberger, 2016;Gruhn et al, 2019;Zielinska et al, 2019), it has been less appreciated that as well the transcripts related to centromeric-or cohesin-associated factors are misregulated during oocyte ageing. Maturing oocytes go through a dramatic rewiring of gene expression dynamics, which includes phases of global transcriptional and translational repression and selective RNA polyadenylation and RNA degradation (Clarke, 2018;De La Fuente, 2018).…”

“…It is unclear to which degree proteins like REC8 and CENP-A are turned over in human oocytes over time. Furthermore, studies in mouse and human suggest that age-related chromosome segregation errors could be due to gradual loss of cohesion and kinetochore compaction (Burkhardt et al, 2016;Smoak et al, 2016;Toth and Jessberger, 2016;Gruhn et al, 2019;Zielinska et al, 2019) and altered microtubule dynamics leading to non-disjunction events (Nakagawa and FitzHarris, 2017). In addition, mitochondria-related defects (May-Panloup et al, 2016;Almansa-Ordonez et al, 2020) and epigenetic changes affecting gene expression (Ge et al, 2015;Chamani and Keefe, 2019) contribute to the agerelated oocyte quality decline.…”

The proteomic landscape of centromeric chromatin reveals an essential role for the Ctf19CCANcomplex in meiotic kinetochore assembly

“…We propose that such kinetochore plasticity is necessary to perform distinct functions at different stages of meiosis. Loss of kinetochore integrity may contribute to age-related decline in female fertility, where gametogenesis is protracted (Patel et al, 2015;Zielinska et al, 2015Zielinska et al, , 2019. Our comprehensive analysis of meiotic kinetochore composition provides an extensive resource for the discovery of mechanisms underlying the specialised functions of kinetochores throughout meiosis.…”

Low tolerance for transcriptional variation at cohesin genes is accompanied by functional links to disease-relevant pathways

“…In meiotic cohesin, SMC1A is replaced by SMC1B; STAG1/2 by STAG3; and RAD21 by REC8 or RAD21L1 1 . Mutations in meiotic cohesin subunits are associated with infertility in men 11 , chromosome segregation errors and Primary Ovarian Insufficiency in women 12 .…”