The conserved kinetochore protein shugoshin protects centromeric cohesion during meiosis

Hybrid female sterility due to cohesin protection errors in oocytes

Hybrid incompatibility can lead to lethality and sterility of F1 hybrids, contributing to speciation. Here we found that female hybrids between Mus musculus domesticus and Mus spicilegus mice are sterile due to the failure of homologous chromosome separation in oocyte meiosis I, producing aneuploid eggs. This non-separation phenotype was driven by the mis- localization of the cohesin protector, SGO2, along the chromosome arms instead of its typical centromeric enrichment, resulting in cohesin over-protection. The upstream kinase, BUB1, showed a significantly higher activity in hybrid oocytes, explaining SGO2 mis-targeting along the chromosome arm. Higher BUB1 activity was not observed in mitosis, consistent with viable hybrid mice. Cohesion defects were also evident in hybrid mice from another genus, Peromyscus , wherein cohesin protection is weakened. Defective cohesion in oocytes is a leading cause of reduced fertility especially with advanced maternal age. Our work provides evidence that a major cause of human infertility may play a positive role in promoting mammalian speciation.

The dependence of shugoshin on Bub1-kinase activity is dispensable for the maintenance of spindle assembly checkpoint response in Cryptococcus neoformans

During chromosome segregation, the spindle assembly checkpoint (SAC) detects errors in kinetochore-microtubule attachments. Timely activation and maintenance of the SAC until defects are corrected is essential for genome stability. Here, we show that shugoshin (Sgo1), a conserved tension-sensing protein, ensures the maintenance of SAC signals in response to unattached kinetochores during mitosis in a basidiomycete budding yeast Cryptococcus neoformans. Sgo1 maintains optimum levels of Aurora B kinase Ipl1 and protein phosphatase 1 (PP1) at kinetochores. The absence of Sgo1 results in the loss of Aurora BIpl1 with a concomitant increase in PP1 levels at kinetochores. This leads to a premature reduction in the kinetochore-bound Bub1 levels and early termination of the SAC signals. Intriguingly, the kinase function of Bub1 is dispensable for shugoshin's subcellular localization. Sgo1 is predominantly localized to spindle pole bodies (SPBs) and along the mitotic spindle with a minor pool at kinetochores. In the absence of proper kinetochore-microtubule attachments, Sgo1 reinforces the Aurora B kinaseIpl1-PP1 phosphatase balance, which is critical for prolonged maintenance of the SAC response.

Length-sensitive partitioning of Caenorhabditis elegans meiotic chromosomes responds to proximity and number of crossover sites

Sensing and control of size are critical for cellular function and survival. A striking example of size sensing occurs during meiosis in the nematode Caenorhabditis elegans. C. elegans chromosomes compare the lengths of the two chromosome "arms" demarcated by the position of their single off-center crossover, and they differentially modify these arms to ensure that sister chromatid cohesion is lost specifically on the shorter arm in the first meiotic division, while the longer arm maintains cohesion until the second division. While many of the downstream steps leading to cohesion loss have been characterized, the length-sensing process itself remains poorly understood. Here, we have used cytological visualization of short and long chromosome arms, combined with quantitative microscopy, live imaging, and simulations, to investigate the principles underlying length-sensitive chromosome partitioning. By quantitatively analyzing short-arm designation patterns on fusion chromosomes carrying multiple crossovers, we develop a model in which a short-arm-determining factor originates at crossover designation sites, diffuses within the synaptonemal complex, and accumulates within crossover-bounded chromosome segments. We demonstrate experimental support for a critical assumption of this model: that crossovers act as boundaries to diffusion within the synaptonemal complex. Further, we develop a discrete simulation based on our results that recapitulates a wide variety of observed partitioning outcomes in both wild-type and previously reported mutants. Our results suggest that the concentration of a diffusible factor is used as a proxy for chromosome length, enabling the correct designation of short and long arms and proper segregation of chromosomes.

Quantitative proteomics and phosphoproteomics profiling of meiotic divisions in the fission yeast Schizosaccharomyces pombe

In eukaryotes, chromosomal DNA is equally distributed to daughter cells during mitosis, whereas the number of chromosomes is halved during meiosis. Despite considerable progress in understanding the molecular mechanisms that regulate mitosis, there is currently a lack of complete understanding of the molecular mechanisms regulating meiosis. Here, we took advantage of the fission yeast Schizosaccharomyces pombe, for which highly synchronous meiosis can be induced, and performed quantitative proteomics and phosphoproteomics analyses to track changes in protein expression and phosphorylation during meiotic divisions. We compared the proteomes and phosphoproteomes of exponentially growing mitotic cells with cells harvested around meiosis I, or meiosis II in strains bearing either the temperature-sensitive pat1-114 allele or conditional ATP analog-sensitive pat1-as2 allele of the Pat1 kinase. Comparing pat1-114 with pat1-as2 also allowed us to investigate the impact of elevated temperature (25 °C versus 34 °C) on meiosis, an issue that sexually reproducing organisms face due to climate change. Using TMTpro 18plex labeling and phosphopeptide enrichment strategies, we performed quantification of a total of 4673 proteins and 7172 phosphosites in S. pombe. We found that the protein level of 2680 proteins and the rate of phosphorylation of 4005 phosphosites significantly changed during progression of S. pombe cells through meiosis. The proteins exhibiting changes in expression and phosphorylation during meiotic divisions were represented mainly by those involved in the meiotic cell cycle, meiotic recombination, meiotic nuclear division, meiosis I, centromere clustering, microtubule cytoskeleton organization, ascospore formation, organonitrogen compound biosynthetic process, carboxylic acid metabolic process, gene expression, and ncRNA processing, among others. In summary, our findings provide global overview of changes in the levels and phosphorylation of proteins during progression of S. pombe cells through meiosis at normal and elevated temperatures, laying the groundwork for further elucidation of the functions and importance of specific proteins and their phosphorylation in regulating meiotic divisions in this yeast.

Systemic low-dose anti-fibrotic treatment attenuates ovarian aging in the mouse

The female reproductive system is one of the first to age in humans, resulting in infertility and endocrine disruptions. The aging ovary assumes a fibro-inflammatory milieu which negatively impacts gamete quantity and quality as well as ovulation. Here, we tested whether the systemic delivery of anti-inflammatory (Etanercept) or anti-fibrotic (Pirfenidone) drugs attenuates ovarian aging in mice. We first evaluated the ability of these drugs to decrease the expression of fibro-inflammatory genes in primary ovarian stromal cells treated with a pro-fibrotic or a pro-inflammatory stimulus. Whereas Etanercept did not block Tnf expression in ovarian stromal cells, Pirfenidone significantly reduced Col1a1 expression. We then tested Pirfenidone in vivo where the drug was delivered systemically via mini-osmotic pumps for 6 weeks. Pirfenidone mitigated the age-dependent increase in ovarian fibrosis without impacting overall health parameters. Ovarian function was improved in Pirfenidone-treated mice as evidenced by increased follicle and corpora lutea number, AMH levels, and improved estrous cyclicity. Transcriptomic analysis revealed that Pirfenidone treatment resulted in an upregulation of reproductive function-related genes at 8.5 months and a downregulation of inflammatory genes at 12 months of age. These findings demonstrate that reducing the fibroinflammatory ovarian microenvironment improves ovarian function, thereby supporting modulating the ovarian environment as a therapeutic avenue to extend reproductive longevity.

Molecular mechanism and functional significance of Wapl interaction with the Cohesin complex

The ring-shaped Cohesin complex, consisting of core subunits Smc1, Smc3, Scc1, and SA2 (or its paralog SA1), topologically entraps two duplicated sister DNA molecules to establish sister chromatid cohesion in S-phase. It remains largely elusive how the Cohesin release factor Wapl binds the Cohesin complex, thereby inducing Cohesin disassociation from mitotic chromosomes to allow proper resolution and separation of sister chromatids. Here, we show that Wapl uses two structural modules containing the FGF motif and the YNARHWN motif, respectively, to simultaneously bind distinct pockets in the extensive composite interface between Scc1 and SA2. Strikingly, only when both docking modules are mutated, Wapl completely loses the ability to bind the Scc1-SA2 interface and release Cohesin, leading to erroneous chromosome segregation in mitosis. Surprisingly, Sororin, which contains a conserved FGF motif and functions as a master antagonist of Wapl in S-phase and G2-phase, does not bind the Scc1-SA2 interface. Moreover, Sgo1, the major protector of Cohesin at mitotic centromeres, can only compete with the FGF motif but not the YNARHWN motif of Wapl for binding Scc1-SA2 interface. Our data uncover the molecular mechanism by which Wapl binds Cohesin to ensure precise chromosome segregation.

Systemic low-dose anti-fibrotic treatment attenuates ovarian aging in the mouse

The female reproductive system is one of the first to age in humans, resulting in infertility and endocrine disruptions. The aging ovary assumes a fibro-inflammatory milieu which negatively impacts gamete quantity and quality as well as ovulation. Here we tested whether the systemic delivery of anti-inflammatory (Etanercept) or anti-fibrotic (Pirfenidone) drugs attenuates ovarian aging in mice. We first evaluated the ability of these drugs to decrease the expression of fibro-inflammatory genes in primary ovarian stromal cells. Whereas Etanercept did not block Tnf expression in ovarian stromal cells, Pirfenidone significantly reduced Col1a1 expression. We then tested Pirfenidone in vivo where the drug was delivered systemically via mini-osmotic pumps for 6-weeks. Pirfenidone mitigated the age-dependent increase in ovarian fibrosis without impacting overall health parameters. Ovarian function was improved in Pirfenidone-treated mice as evidenced by increased follicle and corpora lutea number, AMH levels, and improved estrous cyclicity. Transcriptomic analysis revealed that Pirfenidone treatment resulted in an upregulation of reproductive function-related genes at 8.5 months and a downregulation of inflammatory genes at 12 months of age. These findings demonstrate that reducing the fibroinflammatory ovarian microenvironment improves ovarian function, thereby supporting modulating the ovarian environment as a therapeutic avenue to extend reproductive longevity.

A non-canonical role of the inner kinetochore in regulating sister-chromatid cohesion at centromeres

The 16-subunit Constitutive Centromere-associated Network (CCAN)-based inner kinetochore is well-known for connecting centromeric chromatin to the spindle-binding outer kinetochore. Here, we report a non-canonical role for the inner kinetochore in directly regulating sister-chromatid cohesion at centromeres. We provide biochemical, X-ray crystal structure, and intracellular ectopic localization evidence that the inner kinetochore directly binds cohesin, a ring-shaped multi-subunit complex that holds sister chromatids together from S-phase until anaphase onset. This interaction is mediated by binding of the 5-subunit CENP-OPQUR sub-complex of CCAN to the Scc1-SA2 sub-complex of cohesin. Mutation in the CENP-U subunit of the CENP-OPQUR complex that abolishes its binding to the composite interface between Scc1 and SA2 weakens centromeric cohesion, leading to premature separation of sister chromatids during delayed metaphase. We further show that CENP-U competes with the cohesin release factor Wapl for binding the interface of Scc1-SA2, and that the cohesion-protecting role for CENP-U can be bypassed by depleting Wapl. Taken together, this study reveals an inner kinetochore-bound pool of cohesin, which strengthens centromeric sister-chromatid cohesion to resist metaphase spindle pulling forces.

Acetylation of Rec8 cohesin complexes regulates reductional chromosome segregation in meiosis

For establishing sister chromatid cohesion and proper chromosome segregation in mitosis in fission yeast, the acetyltransferase Eso1 plays a key role. Eso1 acetylates cohesin complexes, at two conserved lysine residues K105 and K106 of the cohesin subunit Psm3. Although Eso1 also contributes to reductional chromosome segregation in meiosis, the underlying molecular mechanisms have remained elusive. Here, we purified meiosis-specific Rec8 cohesin complexes localized at centromeres and identified a new acetylation at Psm3-K1013, which largely depends on the meiotic kinetochore factor meikin (Moa1). Our molecular genetic analyses indicate that Psm3-K1013 acetylation cooperates with canonical acetylation at Psm3-K105 and K106, and plays a crucial role in establishing reductional chromosome segregation in meiosis.

Phosphorylation of Rec8 cohesin complexes regulates mono-orientation of kinetochores in meiosis I

In meiosis I, unlike in mitosis, sister kinetochores are captured by microtubules emanating from the same spindle pole (mono-orientation) and centromeric cohesion mediated by cohesin is protected in the following anaphase I. The conserved meiosis-specific kinetochore protein meikin (Moa1 in fission yeast) associates with polo-like kinase: Plo1 and regulates both mono-orientation and cohesion protection. Although the phosphorylation of Rec8-S450 by Plo1 associated with Moa1 plays a key role in cohesion protection, how Moa1-Plo1 regulates mono-orientation remains elusive. Here, we identify Plo1 phosphorylation sites in the cohesin subunits, Rec8 and Psm3. The non-phosphorylatable mutations at these sites showed specific defects in mono-orientation. These results enabled the genetic dissection of meikin functions at the centromeres.

The close relationship between oocyte aging and telomere shortening, and possible interventions for telomere protection

As women delay childbearing due to socioeconomic reasons, understanding molecular mechanisms decreasing oocyte quantity and quality during ovarian aging becomes increasingly important. The ovary undergoes biological aging at a higher pace when compared to other organs. As is known, telomeres play crucial roles in maintaining genomic integrity, and their shortening owing to increased reactive oxygen species, consecutive cellular divisions, genetic and epigenetic alterations is associated with loss of developmental competence of oocytes. Novel interventions such as antioxidant treatments and regulation of gene expression are being investigated to prevent or rescue telomere attrition and thereby oocyte aging. Herein, potential factors and molecular mechanisms causing telomere shortening in aging oocytes were comprehensively reviewed. For the purpose of extending reproductive lifespan, possible therapeutic interventions to protect telomere length were also discussed.

A role for Shugoshin in human cilia?

We have recently described a novel role for the conserved centromeric/kinetochore protein and cohesin protector, Shugoshin, in cilia of C. elegans. Worms are unusual in that the sole Shugoshin protein ( SGO-1 ) is dispensable for chromosome segregation but required for cilia function in fully differentiated sensory neurons. Depletion of sgo-1 leads to an array of sensory defects observed in other cilia mutants with a compromised diffusion barrier. Accordingly, SGO-1 loads to the base of cilia in sensory neurons and can be observed occupying the transition zone, the critical ciliary domain that regulates trafficking in and out of ciliary compartments. Here we start to address a potential conserved role in cilia for vertebrate Shugoshin by asking whether human Shugoshin can: (1) localize to cilia and (2) rescue defects due to Shugoshin depletion in C. elegans . Our preliminary results suggest that human Shugoshin is detectable in the cilia base but show limited functional conservation when expressed in C. elegans sensory neurons.

Aurora B kinase erases monopolar microtubule-kinetochore arrays at the meiosis I-II transition

During meiosis, faithful chromosome segregation requires monopolar spindle microtubule-kinetochore arrays in MI to segregate homologous chromosomes, but bipolar in MII to segregate sister chromatids. Using fission yeasts, we found that the universal Aurora B kinase localizes to kinetochores in metaphase I and in the mid-spindle during anaphase I, as in mitosis; but in the absence of an intervening S phase, the importin α Imp1 propitiates its release from the spindle midzone to re-localize at kinetochores during meiotic interkinesis. We show that "error-correction" activity of kinetochore re-localized Aurora B becomes essential to erase monopolar arrangements from anaphase I, a prerequisite to satisfy the spindle assembly checkpoint (SAC) and to generate proper bipolar arrays at the onset of MII. This microtubule-kinetochore resetting activity of Aurora B at the MI-MII transition is required to prevent chromosome missegregation in meiosis II, a type of error often associated with birth defects and infertility in humans.

Meiosis in budding yeast

Meiosis is a specialized cell division program that is essential for sexual reproduction. The two meiotic divisions reduce chromosome number by half, typically generating haploid genomes that are packaged into gametes. To achieve this ploidy reduction, meiosis relies on highly unusual chromosomal processes including the pairing of homologous chromosomes, assembly of the synaptonemal complex, programmed formation of DNA breaks followed by their processing into crossovers, and the segregation of homologous chromosomes during the first meiotic division. These processes are embedded in a carefully orchestrated cell differentiation program with multiple interdependencies between DNA metabolism, chromosome morphogenesis, and waves of gene expression that together ensure the correct number of chromosomes is delivered to the next generation. Studies in the budding yeast Saccharomyces cerevisiae have established essentially all fundamental paradigms of meiosis-specific chromosome metabolism and have uncovered components and molecular mechanisms that underlie these conserved processes. Here, we provide an overview of all stages of meiosis in this key model system and highlight how basic mechanisms of genome stability, chromosome architecture, and cell cycle control have been adapted to achieve the unique outcome of meiosis.

Subtelomeres: hotspots of genome variation

Eukaryotic cells contain multiple types of duplicated sequences. Typical examples are tandem repeat sequences including telomeres, centromeres, rDNA genes and transposable elements. Most of these sequences are unstable; thus, their copy numbers or sequences change rapidly in the course of evolution. In this review, I will describe roles of subtelomere regions, which are located adjacent to telomeres at chromosome ends, and recent discoveries about their sequence variation.

Exploring Plant Meiosis: Insights from the Kinetochore Perspective

The central player for chromosome segregation in both mitosis and meiosis is the macromolecular kinetochore structure, which is assembled by >100 structural and regulatory proteins on centromere DNA. Kinetochores play a crucial role in cell division by connecting chromosomal DNA and microtubule polymers. This connection helps in the proper segregation and alignment of chromosomes. Additionally, kinetochores can act as a signaling hub, regulating the start of anaphase through the spindle assembly checkpoint, and controlling the movement of chromosomes during anaphase. However, the role of various kinetochore proteins in plant meiosis has only been recently elucidated, and these proteins differ in their functionality from those found in animals. In this review, our current knowledge of the functioning of plant kinetochore proteins in meiosis will be summarized. In addition, the functional similarities and differences of core kinetochore proteins in meiosis between plants and other species are discussed, and the potential applications of manipulating certain kinetochore genes in meiosis for breeding purposes are explored.

Aging and oocyte competence: A molecular cell perspective

Follicular microenvironment is paramount in the acquisition of oocyte competence, which is dependent on two interconnected and interdependent processes: nuclear and cytoplasmic maturation. Extensive research conducted in human and model systems has provided evidence that those processes are disturbed with female aging. In fact, advanced maternal age (AMA) is associated with a lower chance of pregnancy and live birth, explained by the age-related decline in oocyte quality/competence. This decline has largely been attributed to mitochondria, essential for oocyte maturation, fertilization, and embryo development; with mitochondrial dysfunction leading to oxidative stress, responsible for nuclear and mitochondrial damage, suboptimal intracellular energy levels, calcium disturbance, and meiotic spindle alterations, that may result in oocyte aneuploidy. Nuclear-related mechanisms that justify increased oocyte aneuploidy include deoxyribonucleic acid (DNA) damage, loss of chromosomal cohesion, spindle assembly checkpoint dysfunction, meiotic recombination errors, and telomere attrition. On the other hand, age-dependent cytoplasmic maturation failure is related to mitochondrial dysfunction, altered mitochondrial biogenesis, altered mitochondrial morphology, distribution, activity, and dynamics, dysmorphic smooth endoplasmic reticulum and calcium disturbance, and alterations in the cytoskeleton. Furthermore, reproductive somatic cells also experience the effects of aging, including mitochondrial dysfunction and DNA damage, compromising the crosstalk between granulosa/cumulus cells and oocytes, also affected by a loss of gap junctions. Old oocytes seem therefore to mature in an altered microenvironment, with changes in metabolites, ribonucleic acid (RNA), proteins, and lipids. Overall, understanding the mechanisms implicated in the loss of oocyte quality will allow the establishment of emerging biomarkers and potential therapeutic anti-aging strategies. This article is categorized under: Reproductive System Diseases > Molecular and Cellular Physiology.

The Caenorhabditis elegans Shugoshin regulates TAC-1 in cilia

The conserved Shugoshin (SGO) protein family is essential for mediating proper chromosome segregation from yeast to humans but has also been implicated in diverse roles outside of the nucleus. SGO's roles include inhibiting incorrect spindle attachment in the kinetochore, regulating the spindle assembly checkpoint (SAC), and ensuring centriole cohesion in the centrosome, all functions that involve different microtubule scaffolding structures in the cell. In Caenorhabditis elegans, a species with holocentric chromosomes, SGO-1 is not required for cohesin protection or spindle attachment but appears important for licensing meiotic recombination. Here we provide the first functional evidence that in C. elegans, Shugoshin functions in another extranuclear, microtubule-based structure, the primary cilium. We identify the centrosomal and microtubule-regulating transforming acidic coiled-coil protein, TACC/TAC-1, which also localizes to the basal body, as an SGO-1 binding protein. Genetic analyses indicate that TAC-1 activity must be maintained below a threshold at the ciliary base for correct cilia function, and that SGO-1 likely participates in constraining TAC-1 to the basal body by influencing the function of the transition zone 'ciliary gate'. This research expands our understanding of cellular functions of Shugoshin proteins and contributes to the growing examples of overlap between kinetochore, centrosome and cilia proteomes.

Structural basis of centromeric cohesion protection

In the early stages of mitosis, cohesin is released from chromosome arms but not from centromeres. The protection of centromeric cohesin by SGO1 maintains the sister chromatid cohesion that resists the pulling forces of microtubules until all chromosomes are attached in a bipolar manner to the mitotic spindle. Here we present the X-ray crystal structure of a segment of human SGO1 bound to a conserved surface of the cohesin complex. SGO1 binds to a composite interface formed by the SA2 and SCC1^RAD21 subunits of cohesin. SGO1 shares this binding interface with CTCF, indicating that these distinct chromosomal regulators control cohesin through a universal principle. This interaction is essential for the localization of SGO1 to centromeres and protects centromeric cohesin against WAPL-mediated cohesin release. SGO1-cohesin binding is maintained until the formation of microtubule-kinetochore attachments and is required for faithful chromosome segregation and the maintenance of a stable karyotype.

ATM signaling modulates cohesin behavior in meiotic prophase and proliferating cells

Cohesins are ancient and ubiquitous regulators of chromosome architecture and function, but their diverse roles and regulation remain poorly understood. During meiosis, chromosomes are reorganized as linear arrays of chromatin loops around a cohesin axis. This unique organization underlies homolog pairing, synapsis, double-stranded break induction, and recombination. We report that axis assembly in Caenorhabditis elegans is promoted by DNA-damage response (DDR) kinases that are activated at meiotic entry, even in the absence of DNA breaks. Downregulation of the cohesin-destabilizing factor WAPL-1 by ATM-1 promotes axis association of cohesins containing the meiotic kleisins COH-3 and COH-4. ECO-1 and PDS-5 also contribute to stabilizing axis-associated meiotic cohesins. Further, our data suggest that cohesin-enriched domains that promote DNA repair in mammalian cells also depend on WAPL inhibition by ATM. Thus, DDR and Wapl seem to play conserved roles in cohesin regulation in meiotic prophase and proliferating cells.

Separase and Roads to Disengage Sister Chromatids during Anaphase

Receiving complete and undamaged genetic information is vital for the survival of daughter cells after chromosome segregation. The most critical steps in this process are accurate DNA replication during S phase and a faithful chromosome segregation during anaphase. Any errors in DNA replication or chromosome segregation have dire consequences, since cells arising after division might have either changed or incomplete genetic information. Accurate chromosome segregation during anaphase requires a protein complex called cohesin, which holds together sister chromatids. This complex unifies sister chromatids from their synthesis during S phase, until separation in anaphase. Upon entry into mitosis, the spindle apparatus is assembled, which eventually engages kinetochores of all chromosomes. Additionally, when kinetochores of sister chromatids assume amphitelic attachment to the spindle microtubules, cells are finally ready for the separation of sister chromatids. This is achieved by the enzymatic cleavage of cohesin subunits Scc1 or Rec8 by an enzyme called Separase. After cohesin cleavage, sister chromatids remain attached to the spindle apparatus and their poleward movement on the spindle is initiated. The removal of cohesion between sister chromatids is an irreversible step and therefore it must be synchronized with assembly of the spindle apparatus, since precocious separation of sister chromatids might lead into aneuploidy and tumorigenesis. In this review, we focus on recent discoveries concerning the regulation of Separase activity during the cell cycle.

Actin limits egg aneuploidies associated with female reproductive aging

Aging-related centromeric cohesion loss underlies premature separation of sister chromatids and egg aneuploidy in reproductively older females. Here, we show that F-actin maintains chromatid association after cohesion deterioration in aged eggs. F-actin disruption in aged mouse eggs exacerbated untimely dissociation of sister chromatids, while its removal in young eggs induced extensive chromatid separation events generally only seen in advanced reproductive ages. In young eggs containing experimentally reduced cohesion, F-actin removal accelerated premature splitting and scattering of sister chromatids in a microtubule dynamics-dependent manner, suggesting that actin counteracts chromatid-pulling spindle forces. Consistently, F-actin stabilization restricted scattering of unpaired chromatids generated by complete degradation of centromeric cohesion proteins. We conclude that actin mitigates egg aneuploidies arising from age-related cohesion depletion by limiting microtubule-driven separation and dispersion of sister chromatids. This is supported by our finding that spindle-associated F-actin structures are disrupted in eggs of reproductively older females.

How germline genes promote malignancy in cancer cells

Cancers often express hundreds of genes otherwise specific to germ cells, the germline/cancer (GC) genes. Here, we present and discuss the hypothesis that activation of a "germline program" promotes cancer cell malignancy. We do so by proposing four hallmark processes of the germline: meiosis, epigenetic plasticity, migration, and metabolic plasticity. Together, these hallmarks enable replicative immortality of germ cells as well as cancer cells. Especially meiotic genes are frequently expressed in cancer, implying that genes unique to meiosis may play a role in oncogenesis. Because GC genes are not expressed in healthy somatic tissues, they form an appealing source of specific treatment targets with limited side effects besides infertility. Although it is still unclear why germ cell specific genes are so abundantly expressed in cancer, from our hypothesis it follows that the germline's reproductive program is intrinsic to cancer development.

SGOL2 promotes prostate cancer progression by inhibiting RAB1A ubiquitination

Prostate cancer is the most prevalent genitourinary malignant cancer in men worldwide. Patients with prostate cancer who progress to castration-resistant prostate cancer (CRPC) or metastatic CRPC have significantly poorer survival. Advanced prostate cancer is a clinical challenge due to the lack of effective treatment strategies. In the field of oncology, SGOL2 was an emerging and differentially expressed molecule, which enhanced the proliferation of cell populations in vitro in our studies. Mass spectrum and Co-IP validated the interaction of SGOL2 and RAB1A in a protein-protein manner. We further investigated the role of SGOL2 in the regulatory mechanism of RAB1A in prostate cancer cell lines. Furthermore, SGOL2 regulated RAB1A expression by inhibiting its ubiquitination. Rescue Experiments demonstrated that SGOL2 promoted prostate cancer cell proliferation and migration by upregulating RAB1A expression. Finally, we found that SGOL2 and RAB1A may regulate the tumor microenvironment (TME) in prostate cancer. In conclusion, our findings concluded that SGOL2 stabilized RAB1A expression to promote prostate cancer development. Both of them were of great importance in TME modulation.

SGOL2 is a novel prognostic marker and fosters disease progression via a MAD2-mediated pathway in hepatocellular carcinoma

Background

Shugoshin-like protein 2 (SGOL2) is a centromeric protein that ensures the correct and orderly process of mitosis by protecting and maintaining centripetal adhesions during meiosis and mitosis. Here, we examined the potential role of SGOL2 in cancers, especially in hepatocellular carcinoma (HCC).

Methods

One hundred ninety-nine normal adjacent tissues and 202 HCC samples were collected in this study. Human HCC cells (SK-HEP-1 and HEP-3B) were employed in the present study. Immunohistochemistry, immunofluorescence, western blot, Co-Immunoprecipitation technique, and bioinformatic analysis were utilized to assess the role of SGOL2 in HCC development process.

Results

Overexpression of SGOL2 predicted an unfavorable prognosis in HCC by The Cancer Genome Atlas database (TCGA), which were further validated in our two independent cohorts. Next, 47 differentially expressed genes positively related to both SGOL2 and MAD2 were identified to be associated with the cell cycle. Subsequently, we demonstrated that SGOL2 downregulation suppressed the malignant activities of HCC in vitro and in vivo. Further investigation showed that SGOL2 promoted tumor proliferation by regulating MAD2-induced cell-cycle dysregulation, which could be reversed by the MAD2 inhibitor M2I-1. Consistently, MAD2 upregulation reversed the knockdown effects of SGOL2-shRNA in HCC. Moreover, we demonstrated that SGOL2 regulated MAD2 expression level by forming a SGOL2-MAD2 complex, which led to cell cycle dysreuglation of HCC cells.

Conclusion

SGOL2 acts as an oncogene in HCC cells by regulating MAD2 and then dysregulating the cell cycle, providing a potential therapeutic target in HCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40364-022-00422-z.

Zombies Never Die: The Double Life Bub1 Lives in Mitosis

When eukaryotic cells enter mitosis, dispersed chromosomes move to the cell center along microtubules to form a metaphase plate which facilitates the accurate chromosome segregation. Meanwhile, kinetochores not stably attached by microtubules activate the spindle assembly checkpoint and generate a wait signal to delay the initiation of anaphase. These events are highly coordinated. Disruption of the coordination will cause severe problems like chromosome gain or loss. Bub1, a conserved serine/threonine kinase, plays important roles in mitosis. After extensive studies in the last three decades, the role of Bub1 on checkpoint has achieved a comprehensive understanding; its role on chromosome alignment also starts to emerge. In this review, we summarize the latest development of Bub1 on supporting the two mitotic events. The essentiality of Bub1 in higher eukaryotic cells is also discussed. At the end, some undissolved questions are raised for future study.

The Interplay of Cohesin and RNA Processing Factors: The Impact of Their Alterations on Genome Stability

Cohesin, a multi-subunit protein complex, plays important roles in sister chromatid cohesion, DNA replication, chromatin organization, gene expression, transcription regulation, and the recombination or repair of DNA damage. Recently, several studies suggested that the functions of cohesin rely not only on cohesin-related protein-protein interactions, their post-translational modifications or specific DNA modifications, but that some RNA processing factors also play an important role in the regulation of cohesin functions. Therefore, the mutations and changes in the expression of cohesin subunits or alterations in the interactions between cohesin and RNA processing factors have been shown to have an impact on cohesion, the fidelity of chromosome segregation and, ultimately, on genome stability. In this review, we provide an overview of the cohesin complex and its role in chromosome segregation, highlight the causes and consequences of mutations and changes in the expression of cohesin subunits, and discuss the RNA processing factors that participate in the regulation of the processes involved in chromosome segregation. Overall, an understanding of the molecular determinants of the interplay between cohesin and RNA processing factors might help us to better understand the molecular mechanisms ensuring the integrity of the genome.

Nematode chromosomes

The nematode Caenorhabditis elegans has shed light on many aspects of eukaryotic biology, including genetics, development, cell biology, and genomics. A major factor in the success of C. elegans as a model organism has been the availability, since the late 1990s, of an essentially gap-free and well-annotated nuclear genome sequence, divided among 6 chromosomes. In this review, we discuss the structure, function, and biology of C. elegans chromosomes and then provide a general perspective on chromosome biology in other diverse nematode species. We highlight malleable chromosome features including centromeres, telomeres, and repetitive elements, as well as the remarkable process of programmed DNA elimination (historically described as chromatin diminution) that induces loss of portions of the genome in somatic cells of a handful of nematode species. An exciting future prospect is that nematode species may enable experimental approaches to study chromosome features and to test models of chromosome evolution. In the long term, fundamental insights regarding how speciation is integrated with chromosome biology may be revealed.

Rice Cell Division Cycle 20s are required for faithful chromosome segregation and cytokinesis during meiosis

Chromosome segregation must be under strict regulation to maintain chromosome euploidy and stability. Cell Division Cycle 20 (CDC20) is an essential cell cycle regulator that promotes the metaphase-to-anaphase transition and functions in the spindle assembly checkpoint, a surveillance pathway that ensures the fidelity of chromosome segregation. Plant CDC20 genes are present in multiple copies, and whether CDC20s have the same functions in plants as in yeast and animals is unclear, given the potential for divergence or redundancy among the multiple copies. Here, we studied all three CDC20 genes in rice (Oryza sativa) and constructed two triple mutants by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated genome editing to explore their roles in development. Knocking out all three CDC20 genes led to total sterility but did not affect vegetative development. Loss of the three CDC20 proteins did not alter mitotic division but severely disrupted meiosis as a result of asynchronous and unequal chromosome segregation, chromosome lagging, and premature separation of chromatids. Immunofluorescence of tubulin revealed malformed meiotic spindles in microsporocytes of the triple mutants. Furthermore, cytokinesis of meiosis I was absent or abnormal, and cytokinesis II was completely prevented in all mutant microsporocytes; thus, no tetrads or pollen formed in either cdc20 triple mutant. Finally, the subcellular structures and functions of the tapetum were disturbed by the lack of CDC20 proteins. These findings demonstrate that the three rice CDC20s play redundant roles but are indispensable for faithful meiotic chromosome segregation and cytokinesis, which are required for the production of fertile microspores.

Rec8 Cohesin: A Structural Platform for Shaping the Meiotic Chromosomes

Meiosis is critically different from mitosis in that during meiosis, pairing and segregation of homologous chromosomes occur. During meiosis, the morphology of sister chromatids changes drastically, forming a prominent axial structure in the synaptonemal complex. The meiosis-specific cohesin complex plays a central role in the regulation of the processes required for recombination. In particular, the Rec8 subunit of the meiotic cohesin complex, which is conserved in a wide range of eukaryotes, has been analyzed for its function in modulating chromosomal architecture during the pairing and recombination of homologous chromosomes in meiosis. Here, we review the current understanding of Rec8 cohesin as a structural platform for meiotic chromosomes.

Effects of post-ovulatory aging on centromeric cohesin protection in murine MII oocytes

Purpose

Post-ovulatory aging causes a high frequency of aneuploidy during meiosis II in mouse oocytes, irrespective of maternal age. In this study, we evaluated the effects of post-ovulatory oocyte aging on the protection of chromosomal cohesion involved in aneuploidy and verified the relationship between PP2A or SGO2 expression and the phosphorylation level of REC8 in oocytes.

Methods

Murine ovulated oocytes were incubated for 6 or 12 h in vitro after collection and denoted as the aged group. The oocytes examined immediately after collection were used as the control group. Immunofluorescent staining was used to detect the localization of PP2A, SGO2, BUB1, AURORA B, and MAD2 in the chromosomal centromere. Immunoblotting was used to quantify the expression of proteins describe above and REC8 in the oocytes.

Results

PP2A expression involved in the de-phosphorylation of REC8 decreased over time in oocytes, suggesting a deficiency in PP2A in centromeres. This indicated an increase in the level of phosphorylated REC8, which destabilizes centromeric cohesion in oocytes. In contrast, SGO2 expression was significantly high in aged oocytes.

Conclusions

The findings show that post-ovulatory aging destabilizes the centromeric cohesin protection in oocytes and can cause aneuploidy, which is often observed in aged oocytes during meiosis II.

Bub1 and CENP-U redundantly recruit Plk1 to stabilize kinetochore-microtubule attachments and ensure accurate chromosome segregation

Bub1 is required for the kinetochore/centromere localization of two essential mitotic kinases Plk1 and Aurora B. Surprisingly, stable depletion of Bub1 by ∼95% in human cells marginally affects whole chromosome segregation fidelity. We show that CENP-U, which is recruited to kinetochores by the CENP-P and CENP-Q subunits of the CENP-O complex, is required to prevent chromosome mis-segregation in Bub1-depleted cells. Mechanistically, Bub1 and CENP-U redundantly recruit Plk1 to kinetochores to stabilize kinetochore-microtubule attachments, thereby ensuring accurate chromosome segregation. Furthermore, unlike its budding yeast homolog, the CENP-O complex does not regulate centromeric localization of Aurora B. Consistently, depletion of Bub1 or CENP-U sensitizes cells to the inhibition of Plk1 but not Aurora B kinase activity. Taken together, our findings provide mechanistic insight into the regulation of kinetochore function, which may have implications for targeted treatment of cancer cells with mutations perturbing kinetochore recruitment of Plk1 by Bub1 or the CENP-O complex.

Shugoshin ensures maintenance of the spindle assembly checkpoint response and efficient spindle disassembly

Shugoshin proteins are evolutionarily conserved across eukaryotes, with some species-specific cellular functions, ensuring the fidelity of chromosome segregation. They act as adaptors at various subcellular locales to mediate several protein-protein interactions in a spatio-temporal manner. Here, we characterize shugoshin (Sgo1) in the human fungal pathogen Candida albicans. We observe that Sgo1 retains its centromeric localization and performs its conserved functions of regulating the sister chromatid biorientation, centromeric condensin localization, and maintenance of chromosomal passenger complex (CPC). We identify novel roles of Sgo1 as a spindle assembly checkpoint (SAC) component with functions in maintaining a prolonged SAC response by retaining Mad2 and Bub1 at the kinetochores in response to improper kinetochore-microtubule attachments. Strikingly, we discover the in vivo localization of Sgo1 along the length of the mitotic spindle. Our results indicate that Sgo1 performs a hitherto unknown function of facilitating timely disassembly of the mitotic spindle in C. albicans. To summarize, this study unravels a unique functional adaptation of shugoshin in maintaining genomic stability.

A highly conserved pocket on PP2A-B56 is required for hSgo1 binding and cohesion protection during mitosis

The shugoshin proteins are universal protectors of centromeric cohesin during mitosis and meiosis. The binding of human hSgo1 to the PP2A-B56 phosphatase through a coiled-coil (CC) region mediates cohesion protection during mitosis. Here we undertook a structure function analysis of the PP2A-B56-hSgo1 complex, revealing unanticipated aspects of complex formation and function. We establish that a highly conserved pocket on the B56 regulatory subunit is required for hSgo1 binding and cohesion protection during mitosis in human somatic cells. Consistent with this, we show that hSgo1 blocks the binding of PP2A-B56 substrates containing a canonical B56 binding motif. We find that PP2A-B56 bound to hSgo1 dephosphorylates Cdk1 sites on hSgo1 itself to modulate cohesin interactions. Collectively our work provides important insight into cohesion protection during mitosis.

Meikin synergizes with shugoshin to protect cohesin Rec8 during meiosis I

The conserved meiosis-specific kinetochore regulator, meikin (Moa1 in fission yeast) plays a central role in establishing meiosis-specific kinetochore function. However, the underlying molecular mechanisms remain elusive. Here, we show how Moa1 regulates centromeric cohesion protection, a function that has been previously attributed to shugoshin (Sgo1). Moa1 is known to associate with Plo1 kinase. We explore Plo1-dependent Rec8 phosphorylation and identify a key phosphorylation site required for cohesion protection. The phosphorylation of Rec8 by Moa1-Plo1 potentiates the activity of PP2A associated with Sgo1. This leads to dephosphorylation of Rec8 at another site, which thereby prevents cleavage of Rec8 by separase.

SET levels contribute to cohesion fatigue

Chromosome instability (CIN) is a major hallmark of cancer cells and believed to drive tumor progression. Several cellular defects including weak centromeric cohesion are proposed to promote CIN, but the molecular mechanisms underlying these defects are poorly understood. In a screening for SET protein levels in various cancer cell lines, we found that most of the cancer cells exhibit higher SET protein levels than nontransformed cells, including RPE-1. Cancer cells with elevated SET often show weak centromeric cohesion, revealed by MG132-induced cohesion fatigue. Partial SET knockdown largely strengthens centromeric cohesion in cancer cells without increasing overall phosphatase 2A (PP2A) activity. Pharmacologically increased PP2A activity in these cancer cells barely ameliorates centromeric cohesion. These results suggest that compromised PP2A activity, a common phenomenon in cancer cells, may not be responsible for weak centromeric cohesion. Furthermore, centromeric cohesion in cancer cells can be strengthened by ectopic Sgo1 overexpression and weakened by SET WT, not by Sgo1-binding-deficient mutants. Altogether, these findings demonstrate that SET overexpression contributes to impaired centromeric cohesion in cancer cells and illustrate misregulated SET-Sgo1 pathway as an underlying mechanism.

Ubiquitination in Plant Meiosis: Recent Advances and High Throughput Methods

Meiosis is a specialized cell division which is essential to sexual reproduction. The success of this highly ordered process involves the timely activation, interaction, movement, and removal of many proteins. Ubiquitination is an extraordinarily diverse post-translational modification with a regulatory role in almost all cellular processes. During meiosis, ubiquitin localizes to chromatin and the expression of genes related to ubiquitination appears to be enhanced. This may be due to extensive protein turnover mediated by proteasomal degradation. However, degradation is not the only substrate fate conferred by ubiquitination which may also mediate, for example, the activation of key transcription factors. In plant meiosis, the specific roles of several components of the ubiquitination cascade-particularly SCF complex proteins, the APC/C, and HEI10-have been partially characterized indicating diverse roles in chromosome segregation, recombination, and synapsis. Nonetheless, these components remain comparatively poorly understood to their counterparts in other processes and in other eukaryotes. In this review, we present an overview of our understanding of the role of ubiquitination in plant meiosis, highlighting recent advances, remaining challenges, and high throughput methods which may be used to overcome them.

Transcriptome Analyses Throughout Chili Pepper Fruit Development Reveal Novel Insights into the Domestication Process

Chili pepper (Capsicum spp.) is an important crop, as well as a model for fruit development studies and domestication. Here, we performed a time-course experiment to estimate standardized gene expression profiles with respect to fruit development for six domesticated and four wild chili pepper ancestors. We sampled the transcriptomes every 10 days from flowering to fruit maturity, and found that the mean standardized expression profiles for domesticated and wild accessions significantly differed. The mean standardized expression was higher and peaked earlier for domesticated vs. wild genotypes, particularly for genes involved in the cell cycle that ultimately control fruit size. We postulate that these gene expression changes are driven by selection pressures during domestication and show a robust network of cell cycle genes with a time shift in expression, which explains some of the differences between domesticated and wild phenotypes.

The Elusive Structure of Centro-Chromatin: Molecular Order or Dynamic Heterogenetity?

The centromere is an essential chromatin domain required for kinetochore recruitment and chromosome segregation in eukaryotes. To perform this role, centro-chromatin adopts a unique structure that provides access to kinetochore proteins and maintains stability under tension during mitosis. This is achieved by the presence of nucleosomes containing the H3 variant CENP-A, which also acts as the epigenetic mark defining the centromere. In this review, we discuss the role of CENP-A on the structure and dynamics of centromeric chromatin. We further discuss the impact of the CENP-A binding proteins CENP-C, CENP-N, and CENP-B on modulating centro-chromatin structure. Based on these findings we provide an overview of the higher order structure of the centromere.

Centromeres are dismantled by foundational meiotic proteins Spo11 and Rec8

Meiotic processes are potentially dangerous to genome stability and could be disastrous if activated in proliferative cells. Here we show that two key meiosis-defining proteins, the topoisomerase Spo11 (which forms double-strand breaks) and the meiotic cohesin Rec8, can dismantle centromeres. This dismantlement is normally observable only in mutant cells that lack the telomere bouquet, which provides a nuclear microdomain conducive to centromere reassembly¹; however, overexpression of Spo11 or Rec8 leads to levels of centromere dismantlement that cannot be countered by the bouquet. Specific nucleosome remodelling factors mediate centromere dismantlement by Spo11 and Rec8. Ectopic expression of either protein in proliferating cells leads to the loss of mitotic kinetochores in both fission yeast and human cells. Hence, while centromeric chromatin has been characterized as extraordinarily stable, Spo11 and Rec8 challenge this stability and may jeopardize kinetochores in cancers that express meiotic proteins.

A visual atlas of meiotic protein dynamics in living fission yeast

Meiosis is a carefully choreographed dynamic process that re-purposes proteins from somatic/vegetative cell division, as well as meiosis-specific factors, to carry out the differentiation and recombination pathway common to sexually reproducing eukaryotes. Studies of individual proteins from a variety of different experimental protocols can make it difficult to compare details between them. Using a consistent protocol in otherwise wild-type fission yeast cells, this report provides an atlas of dynamic protein behaviour of representative proteins at different stages during normal zygotic meiosis in fission yeast. This establishes common landmarks to facilitate comparison of different proteins and shows that initiation of S phase likely occurs prior to nuclear fusion/karyogamy.

Functioning mechanisms of Shugoshin-1 in centromeric cohesion during mitosis

Proper regulation of centromeric cohesion is required for faithful chromosome segregation that prevents chromosomal instability. Extensive studies have identified and established the conserved protein Shugoshin (Sgo1/2) as an essential protector for centromeric cohesion. In this review, we summarize the current understanding of how Shugoshin-1 (Sgo1) protects centromeric cohesion at the molecular level. Targeting of Sgo1 to inner centromeres is required for its proper function of cohesion protection. We therefore discuss about the molecular mechanisms that install Sgo1 onto inner centromeres. At metaphase-to-anaphase transition, Sgo1 at inner centromeres needs to be disabled for the subsequent sister-chromatid segregation. A few recent studies suggest interesting models to explain how it is achieved. These models are discussed as well.

Chiasmata and the kinetochore component Dam1 are crucial for elimination of erroneous chromosome attachments and centromere oscillation at meiosis I

Establishment of proper chromosome attachments to the spindle requires elimination of erroneous attachments, but the mechanism of this process is not fully understood. During meiosis I, sister chromatids attach to the same spindle pole (mono-oriented attachment), whereas homologous chromosomes attach to opposite poles (bi-oriented attachment), resulting in homologous chromosome segregation. Here, we show that chiasmata that link homologous chromosomes and kinetochore component Dam1 are crucial for elimination of erroneous attachments and oscillation of centromeres between the spindle poles at meiosis I in fission yeast. In chiasma-forming cells, Mad2 and Aurora B kinase, which provides time for attachment correction and destabilizes erroneous attachments, respectively, caused elimination of bi-oriented attachments of sister chromatids, whereas in chiasma-lacking cells, they caused elimination of mono-oriented attachments. In chiasma-forming cells, in addition, homologous centromere oscillation was coordinated. Furthermore, Dam1 contributed to attachment elimination in both chiasma-forming and chiasma-lacking cells, and drove centromere oscillation. These results demonstrate that chiasmata alter attachment correction patterns by enabling error correction factors to eliminate bi-oriented attachment of sister chromatids, and suggest that Dam1 induces elimination of erroneous attachments. The coincidental contribution of chiasmata and Dam1 to centromere oscillation also suggests a potential link between centromere oscillation and attachment elimination.

Surprising phenotypic diversity of cancer-associated mutations of Gly 34 in the histone H3 tail

Sequencing of cancer genomes has identified recurrent somatic mutations in histones, termed oncohistones, which are frequently poorly understood. Previously we showed that fission yeast expressing only the H3.3G34R mutant identified in aggressive pediatric glioma had reduced H3K36 trimethylation and acetylation, increased genomic instability and replicative stress, and defective homology-dependent DNA damage repair. Here we show that surprisingly distinct phenotypes result from G34V (also in glioma) and G34W (giant cell tumors of bone) mutations, differentially affecting H3K36 modifications, subtelomeric silencing, genomic stability; sensitivity to irradiation, alkylating agents, and hydroxyurea; and influencing DNA repair. In cancer, only 1 of 30 alleles encoding H3 is mutated. Whilst co-expression of wild-type H3 rescues most G34 mutant phenotypes, G34R causes dominant hydroxyurea sensitivity, homologous recombination defects, and dominant subtelomeric silencing. Together, these studies demonstrate the complexity associated with different substitutions at even a single residue in H3 and highlight the utility of genetically tractable systems for their analysis.

Fission Yeast Methylenetetrahydrofolate Reductase Ensures Mitotic and Meiotic Chromosome Segregation Fidelity

Methylenetetrahydrofolate reductase (MTHFR) is a key enzyme in the folate metabolic pathway, and its loss of function through polymorphisms is often associated with human conditions, including cancer, congenital heart disease, and Down syndrome. MTHFR is also required in the maintenance of heterochromatin, a crucial determinant of genomic stability and precise chromosomal segregation. Here, we characterize the function of a fission yeast gene met11⁺, which encodes a protein that is highly homologous to the mammalian MTHFR. We show that, although met11⁺ is not essential for viability, its disruption increases chromosome missegregation and destabilizes constitutive heterochromatic regions at pericentromeric, sub-telomeric and ribosomal DNA (rDNA) loci. Transcriptional silencing at these sites were disrupted, which is accompanied by the reduction in enrichment of histone H3 lysine 9 dimethylation (H3K9me2) and binding of the heterochromatin protein 1 (HP1)-like Swi6. The met11 null mutant also dominantly disrupts meiotic fidelity, as displayed by reduced sporulation efficiency and defects in proper partitioning of the genetic material during meiosis. Interestingly, the faithful execution of these meiotic processes is synergistically ensured by cooperation among Met11, Rec8, a meiosis-specific cohesin protein, and the shugoshin protein Sgo1, which protects Rec8 from untimely cleavage. Overall, our results suggest a key role for Met11 in maintaining pericentromeric heterochromatin for precise genetic inheritance during mitosis and meiosis.

Meiosis Progression and Recombination in Holocentric Plants: What Is Known?

Differently from the common monocentric organization of eukaryotic chromosomes, the so-called holocentric chromosomes present many centromeric regions along their length. This chromosomal organization can be found in animal and plant lineages, whose distribution suggests that it has evolved independently several times. Holocentric chromosomes present an advantage: even broken chromosome parts can be correctly segregated upon cell division. However, the evolution of holocentricity brought about consequences to nuclear processes and several adaptations are necessary to cope with this new organization. Centromeres of monocentric chromosomes are involved in a two-step cohesion release during meiosis. To deal with that holocentric lineages developed different adaptations, like the chromosome remodeling strategy in Caenorhabditis elegans or the inverted meiosis in plants. Furthermore, the frequency of recombination at or around centromeres is normally very low and the presence of centromeric regions throughout the entire length of the chromosomes could potentially pose a problem for recombination in holocentric organisms. However, meiotic recombination happens, with exceptions, in those lineages in spite of their holocentric organization suggesting that the role of centromere as recombination suppressor might be altered in these lineages. Most of the available information about adaptations to meiosis in holocentric organisms is derived from the animal model C. elegans. As holocentricity evolved independently in different lineages, adaptations observed in C. elegans probably do not apply to other lineages and very limited research is available for holocentric plants. Currently, we still lack a holocentric model for plants, but good candidates may be found among Cyperaceae, a large angiosperm family. Besides holocentricity, chiasmatic and achiasmatic inverted meiosis are found in the family. Here, we introduce the main concepts of meiotic constraints and adaptations with special focus in meiosis progression and recombination in holocentric plants. Finally, we present the main challenges and perspectives for future research in the field of chromosome biology and meiosis in holocentric plants.

Differential requirement for Bub1 and Bub3 in regulation of meiotic versus mitotic chromosome segregation

Accurate chromosome segregation depends on the proper attachment of kinetochores to spindle microtubules before anaphase onset. The Ipl1/Aurora B kinase corrects improper attachments by phosphorylating kinetochore components and so releasing aberrant kinetochore–microtubule interactions. The localization of Ipl1 to kinetochores in budding yeast depends upon multiple pathways, including the Bub1–Bub3 pathway. We show here that in meiosis, Bub3 is crucial for correction of attachment errors. Depletion of Bub3 results in reduced levels of kinetochore-localized Ipl1 and concomitant massive chromosome missegregation caused by incorrect chromosome–spindle attachments. Depletion of Bub3 also results in shorter metaphase I and metaphase II due to premature localization of protein phosphatase 1 (PP1) to kinetochores, which antagonizes Ipl1-mediated phosphorylation. We propose a new role for the Bub1–Bub3 pathway in maintaining the balance between kinetochore localization of Ipl1 and PP1, a balance that is essential for accurate meiotic chromosome segregation and timely anaphase onset.

Why is oocyte aneuploidy increased with maternal aging?

One of the main causes of pregnancy failure and fetus abortion is oocyte aneuploidy, which is increased with maternal aging. Numerous possible causes of oocyte aneuploidy in aged women have been proposed, including cross-over formation defect, cohesin loss, spindle deformation, spindle assembly checkpoint malfunction, microtubule-kinetochore attachment failure, kinetochore mis-orientation, mitochondria dysfunction-induced increases in reactive oxygen species, protein over-acetylation, and DNA damage. However, it still needs to be answered if these aneuploidization factors have inherent relations, and how to prevent chromosome aneuploidy in aged oocytes. Epidemiologically, oocyte aneuploidy has been found to be weakly associated with higher homocysteine concentrations, obesity, ionizing radiation and even seasonality. In this review, we summarize the research progress and present an integrated view of oocyte aneuploidization.

The horse as a natural model to study reproductive aging-induced aneuploidy and weakened centromeric cohesion in oocytes

Aneuploidy of meiotic origin is a major contributor to age-related subfertility and an increased risk of miscarriage in women. Although age-related aneuploidy has been studied in rodents, the mare may be a more appropriate animal model to study reproductive aging. Similar to women, aged mares show reduced fertility and an increased incidence of early pregnancy loss; however, it is not known whether aging predisposes to aneuploidy in equine oocytes. We evaluated the effect of advanced mare age on (1) gene expression for cohesin components, (2) incidence of aneuploidy and (3) chromosome centromere cohesion (measured as the distance between sister kinetochores) in oocytes matured in vitro. Oocytes from aged mares showed reduced gene expression for the centromere cohesion stabilizing protein, Shugoshin 1. Moreover, in vitro matured oocytes from aged mares showed a higher incidence of aneuploidy and premature sister chromatid separation, and weakened centromeric cohesion. We therefore propose the mare as a valid model for studying effects of aging on centromeric cohesion; cohesion loss predisposes to disintegration of bivalents and premature separation of sister chromatids during the first meiotic division, leading to embryonic aneuploidy; this probably contributes to the reduced fertility and increased incidence of pregnancy loss observed in aged mares.

Random chromosome distribution in the first meiosis of F1 disomic substitution line 2R(2D) x rye hybrids (ABDR, 4× = 28) occurs without bipolar spindle assembly

The assembly of the microtubule-based spindle structure in plant meiosis remains poorly understood compared with our knowledge of mitotic spindle formation. One of the approaches in our understanding of microtubule dynamics is to study spindle assembly in meiosis of amphyhaploids. Using immunostaining with phH3Ser10, CENH3 and α-tubulin-specific antibodies, we studied the chromosome distribution and spindle organisation in meiosis of F₁ 2R(2D)xR wheat-rye hybrids (genome structure ABDR, 4× = 28), as well as in wheat and rye mitosis and meiosis. At the prometaphase of mitosis, spindle assembly was asymmetric; one half of the spindle assembled before the other, with simultaneous chromosome alignment in the spindle mid-zone. At diakinesis in wheat and rye, microtubules formed a pro-spindle which was subsequently disassembled followed by a bipolar spindle assembly. In the first meiosis of hybrids 2R(2D)xR, a bipolar spindle was not found and the kinetochore microtubules distributed the chromosomes. Univalent chromosomes are characterised by a monopolar orientation and maintenance of sister chromatid and centromere cohesion. Presence of bivalents did not affect the formation of a bipolar spindle. Since the central spindle was absent, phragmoplast originates from "interpolar" microtubules generated by kinetochores. Cell plate development occurred with a delay. However, meiocytes in meiosis II contained apparently normal bipolar spindles. Thus, we can conclude that: (1) cohesion maintenance in centromeres and between arms of sister chromatids may negatively affect bipolar spindle formation in the first meiosis; (2) 2R/2D rye/wheat chromosome substitution affects the regulation of the random chromosome distribution in the absence of a bipolar spindle.