Overexpression of SETβ, a protein localizing to centromeres, causes precocious separation of chromatids during the first meiosis of mouse oocytes

PPP4C facilitates homologous recombination DNA repair by dephosphorylating PLK1 during early embryo development

Mammalian early embryo cells have complex DNA repair mechanisms to maintain genomic integrity, and homologous recombination (HR) plays the main role in response to double-strand DNA breaks (DSBs) in these cells. Polo-like kinase 1 (PLK1) participates in the HR process and its overexpression has been shown to occur in a variety of human cancers. Nevertheless, the regulatory mechanism of PLK1 remains poorly understood, especially during the S and G2 phase. Here, we show that protein phosphatase 4 catalytic subunit (PPP4C) deletion causes severe female subfertility due to accumulation of DNA damage in oocytes and early embryos. PPP4C dephosphorylated PLK1 at the S137 site, negatively regulating its activity in the DSB response in early embryonic cells. Depletion of PPP4C induced sustained activity of PLK1 when cells exhibited DNA lesions that inhibited CHK2 and upregulated the activation of CDK1, resulting in inefficient loading of the essential HR factor RAD51. On the other hand, when inhibiting PLK1 in the S phase, DNA end resection was restricted. These results demonstrate that PPP4C orchestrates the switch between high-PLK1 and low-PLK1 periods, which couple the checkpoint to HR.

Critical Functions of PP2A-Like Protein Phosphotases in Regulating Meiotic Progression

Meiosis is essential to the continuity of life in sexually-reproducing organisms through the formation of haploid gametes. Unlike somatic cells, the germ cells undergo two successive rounds of meiotic divisions after a single cycle of DNA replication, resulting in the decrease in ploidy. In humans, errors in meiotic progression can cause infertility and birth defects. Post-translational modifications, such as phosphorylation, ubiquitylation and sumoylation have emerged as important regulatory events in meiosis. There are dynamic equilibrium of protein phosphorylation and protein dephosphorylation in meiotic cell cycle process, regulated by a conservative series of protein kinases and protein phosphatases. Among these protein phosphatases, PP2A, PP4, and PP6 constitute the PP2A-like subfamily within the serine/threonine protein phosphatase family. Herein, we review recent discoveries and explore the role of PP2A-like protein phosphatases during meiotic progression.

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.

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.

A PP2A-B56-Centered View on Metaphase-to-Anaphase Transition in Mouse Oocyte Meiosis I

Meiosis is required to reduce to haploid the diploid genome content of a cell, generating gametes-oocytes and sperm-with the correct number of chromosomes. To achieve this goal, two specialized cell divisions without intermediate S-phase are executed in a time-controlled manner. In mammalian female meiosis, these divisions are error-prone. Human oocytes have an exceptionally high error rate that further increases with age, with significant consequences for human fertility. To understand why errors in chromosome segregation occur at such high rates in oocytes, it is essential to understand the molecular players at work controlling these divisions. In this review, we look at the interplay of kinase and phosphatase activities at the transition from metaphase-to-anaphase for correct segregation of chromosomes. We focus on the activity of PP2A-B56, a key phosphatase for anaphase onset in both mitosis and meiosis. We start by introducing multiple roles PP2A-B56 occupies for progression through mitosis, before laying out whether or not the same principles may apply to the first meiotic division in oocytes, and describing the known meiosis-specific roles of PP2A-B56 and discrepancies with mitotic cell cycle regulation.

Priming chromatin for segregation: functional roles of mitotic histone modifications

Posttranslational modifications (PTMs) of histone proteins are important for various cellular processes including regulation of gene expression and chromatin structure, DNA damage response and chromosome segregation. Here we comprehensively review mitotic histone PTMs, in particular phosphorylations, and discuss their interplay and functions in the control of dynamic protein-protein interactions as well as their contribution to centromere and chromosome structure and function during cell division. Histone phosphorylations can create binding sites for mitotic regulators such as the chromosomal passenger complex, which is required for correction of erroneous spindle attachments and chromosome bi-orientation. Other histone PTMs can alter the structural properties of nucleosomes and the accessibility of chromatin. Epigenetic marks such as lysine methylations are maintained during mitosis and may also be important for mitotic transcription as well as bookmarking of transcriptional states to ensure the transmission of gene expression programs through cell division. Additionally, histone phosphorylation can dissociate readers of methylated histones without losing epigenetic information. Through all of these processes, mitotic histone PTMs play a functional role in priming the chromatin for faithful chromosome segregation and preventing genetic instability, one of the characteristic hallmarks of cancer cells.

Protein phosphatase 6 is a key factor regulating spermatogenesis

Protein phosphatase 6 (PP6) is a member of the PP2A-like subfamily, which plays a critical role in many fundamental cellular processes. We recently reported that PP6 is essential for female fertility. Here, we report that PP6 is involved in meiotic recombination and that germ cell-specific deletion of PP6 by Stra8-Cre causes defective spermatogenesis. The PP6-deficient spermatocytes were arrested at the pachytene stage and defects in DSB repair and crossover formation were observed, indicating that PP6 facilitated meiotic double-stranded breaks (DSB) repair. Further investigations revealed that depletion of PP6 in the germ cells affected chromatin relaxation, which was dependent on MAPK pathway activity, consequently preventing programmed DSB repair factors from being recruited to proper positions on the chromatin. Taken together, our results demonstrate that PP6 has an important role in meiotic recombination and male fertility.

Deletion of Ck2β gene causes germ cell development arrest and azoospermia in male mice

Objectives

In humans, non‐obstructive azoospermia (NOA) is a major cause of male infertility. However, the aetiology of NOA is largely unknown. Previous studies reported that protein CK2β was abundantly and broadly expressed in spermatogenic cells. Here, we investigate whether protein CK2β participates in spermatogenesis.

Materials and Methods

In this study, we separated spermatogenic cells using STA‐PUT velocity sedimentation, analysed the expression pattern of protein CK2β by immunoblotting, specifically deleted Ck2β gene in early‐stage spermatogenic cells by crossing Ck2β^fl mice with Stra8‐Cre⁺ mice and validated the knockout efficiency by quantitative RT‐PCR and immunoblotting. The phenotypes of Ck2β^fl/Δ;SCre⁺ mice were studied by immunohistochemistry and immunofluorescence. The molecular mechanisms of male germ cell development arrest were elucidated by immunoblotting and TUNEL assay.

Results

Ablation of Ck2β gene triggered excessive germ cell apoptosis, germ cell development arrest, azoospermia and male infertility. Inactivation of Ck2β gene caused distinctly reduced expression of Ck2α′ gene and CK2α′ protein.

Conclusions

Ck2β is a vital gene for germ cell survival and male fertility in mice.

Aurora B kinase activity is regulated by SET/TAF1 on Sgo2 at the inner centromere

Phosphorylation of kinetochore proteins destabilizes improper kinetochore–microtubule attachments. Asai et al. find that SET/TAF1, an inhibitor of the PP2A phosphatase, binds shugoshin 2 and corrects erroneous kinetochore–microtubule attachment by maintaining Aurora B kinase activity. Therefore, SET has a key role in establishing chromosome bi-orientation by balancing Aurora B and PP2A activity.

The accurate regulation of phosphorylation at the kinetochore is essential for establishing chromosome bi-orientation. Phosphorylation of kinetochore proteins by the Aurora B kinase destabilizes improper kinetochore–microtubule attachments, whereas the phosphatase PP2A has a counteracting role. Imbalanced phosphoregulation leads to error-prone chromosome segregation and aneuploidy, a hallmark of cancer cells. However, little is known about the molecular events that control the balance of phosphorylation at the kinetochore. Here, we show that localization of SET/TAF1, an oncogene product, to centromeres maintains Aurora B kinase activity by inhibiting PP2A, thereby correcting erroneous kinetochore–microtubule attachment. SET localizes at the inner centromere by interacting directly with shugoshin 2, with SET levels declining at increased distances between kinetochore pairs, leading to establishment of chromosome bi-orientation. Moreover, SET overexpression induces chromosomal instability by disrupting kinetochore–microtubule attachment. Thus, our findings reveal the novel role of SET in fine-tuning the phosphorylation level at the kinetochore by balancing the activities of Aurora B and PP2A.

SET binding to Sgo1 inhibits Sgo1-cohesin interactions and promotes chromosome segregation

At anaphase onset, Sgo1 function of cohesion protection must be disabled to allow timely chromosome segregation, but how this is achieved is not fully understood. Here, we show that SET, a known PP2A inhibitor, directly binds to a domain in Sgo1 in close proximity to the cohesin-binding motif. The Sgo1-cohesin binding can be disrupted by SET in a dose-dependent manner in vitro as well as by SET overexpression in cells, suggesting that SET is also an inhibitor to the Sgo1-cohesin binding. Furthermore, the SET binding-deficient Sgo1 mutant fully supports centromeric cohesion protection but delays chromosome segregation, suggesting that the SET-Sgo1 binding is required for timely chromosome segregation. Moreover, overexpression of SET WT, not the Sgo1 binding-deficient mutant, exacerbates the occurrence of cohesion fatigue in MG132-arrested cells. Conversely, SET depletion delays it. Thus, we propose that a major function of SET during mitosis is to disrupt the Sgo1-cohesin interaction, thereby promoting centromeric cohesion de-protection and timely chromosome segregation at anaphase onset.

The cohesion establishment factor Esco1 acetylates α-tubulin to ensure proper spindle assembly in oocyte meiosis

Esco1 has been reported to function as a cohesion establishment factor that mediates chromosome cohesion and segregation in mitotic cells. However, its exact roles in meiosis have not been clearly defined. Here, we document that Esco1 is expressed and localized to both the nucleus and cytoplasm during mouse oocyte meiotic maturation. Depletion of Esco1 by siRNA microinjection causes the meiotic progression arrest with a severe spindle abnormality and chromosome misalignment, which is coupled with a higher incidence of the erroneous kinetochore–microtubule attachments and activation of spindle assembly checkpoint. In addition, depletion of Esco1 leads to the impaired microtubule stability shown by the weakened resistance ability to the microtubule depolymerizing drug nocodazole and the decreased level of acetylated α-tubulin. Conversely, overexpression of Esco1 causes hyperacetylation of α-tubulin and spindle defects. Moreover, we find that Esco1 binds to α-tubulin and is required for its acetylation. The reduced acetylation level of α-tubulin in Esco1-depleted oocytes can be restored by the ectopic expression of exogenous wild-type Esco1 but not enzymatically dead Esco1-G768D. Purified wild-type Esco1 instead of mutant Esco1-G768D acetylates the synthesized peptide of α-tubulin in vitro. Collectively, our data assign a novel function to Esco1 as a microtubule regulator during oocyte meiotic maturation beyond its conventional role in chromosome cohesion.

Cyclin-dependent kinase 1-mediated phosphorylation of SET at serine 7 is essential for its oncogenic activity

SE translocation (SET), an inhibitor of protein phosphatase 2A (PP2A), plays important roles in mitosis and possesses oncogenic activity in several types of cancer. However, little is known regarding its regulation. Here we reveal a novel phosphorylation site of SET isoform 1, and we have determined its biological significance in tumorigenesis. We found that the mitotic kinase cyclin-dependent kinase 1 (CDK1) phosphorylates SET isoform 1 in vitro and in vivo at serine 7 during antitubulin drug-induced mitotic arrest and normal mitosis. SET deletion resulted in massive multipolar spindles, chromosome misalignment and missegregation, and centrosome amplification during mitosis. Moreover, mitotic phosphorylation of SET isoform 1 is required for cell migration, invasion, and anchorage-independent growth in vitro and tumorigenesis in xenograft animal models. We further documented that SET phosphorylation affects Akt activity. Collectively, our findings suggest that SET isoform 1 promotes oncogenesis in a mitotic phosphorylation-dependent manner.

CDK1-mediated phosphorylation at H2B serine 6 is required for mitotic chromosome segregation

Faithful mitotic chromosome segregation is required for the maintenance of genomic stability. We discovered the phosphorylation of histone H2B at serine 6 (H2B S6ph) as a new chromatin modification site and found that this modification occurs during the early mitotic phases at inner centromeres and pericentromeric heterochromatin. This modification is directly mediated by cyclin B1-associated CDK1, and indirectly by Aurora B, and is antagonized by PP1-mediated dephosphorylation. H2B S6ph impairs chromatin binding of the histone chaperone SET (I2PP2A), which is important for mitotic fidelity. Injection of phosphorylation-specific H2B S6 antibodies in mitotic cells caused anaphase defects with impaired chromosome segregation and incomplete cytokinesis. As H2B S6ph is important for faithful chromosome separation, this modification may contribute to the prevention chromosomal instability and aneuploidy which frequently occur in cancer cells.

Rad9a is involved in chromatin decondensation and post-zygotic embryo development in mice

Zygotic chromatin undergoes extensive reprogramming immediately after fertilization. It is generally accepted that maternal factors control this process. However, little is known about the underlying mechanisms. Here we report that maternal RAD9A, a key protein in DNA damage response pathway, is involved in post-zygotic embryo development, via a mouse model with conditional depletion of Rad9a alleles in oocytes of primordial follicles. Post-zygotic losses originate from delayed zygotic chromatin decondensation after depletion of maternal RAD9A. Pronucleus formation and DNA replication of most mutant zygotes are therefore deferred, which subsequently trigger the G2/M checkpoint and arrest development of most mutant zygotes. Delayed zygotic chromatin decondensation could also lead to increased reabsorption of post-implantation mutant embryos. In addition, our data indicate that delayed zygotic chromatin decondensation may be attributed to deferred epigenetic modification of histone in paternal chromatin after fertilization, as fertilization and resumption of secondary meiosis in mutant oocytes were both normal. More interestingly, most mutant oocytes could not support development beyond one-cell stage after parthenogenetic activation. Therefore, RAD9A may also play an important role in maternal chromatin reprogramming. In summary, our data reveal an important role of RAD9A in zygotic chromatin reprogramming and female fertility.

GnRH regulates the expression of its receptor accessory protein SET in pituitary gonadotropes

Reproductive function is under the control of the neurohormone GnRH, which activates a G-protein-coupled receptor (GnRHR) expressed in pituitary gonadotrope cells. GnRHR activates a complex signaling network to regulate synthesis and secretion of the two gonadotropin hormones, luteinizing hormone and follicle-stimulating hormone, both regulating gametogenesis and steroidogenesis in gonads. Recently, in an attempt to identify the mechanisms underlying GnRHR signaling plasticity, we identified the first interacting partner of GnRHR, the proto-oncogene SET. We showed that SET binds to intracellular domains of GnRHR to enhance its coupling to cAMP pathway in αT3-1 gonadotrope cells. Here, we demonstrate that SET protein is rapidly regulated by GnRH, which increases SET phosphorylation state and decreases dose-dependently SET protein level. Our results highlight a post-translational regulation of SET protein involving the proteasome pathway. We determined that SET phosphorylation upon GnRH stimulation is mediated by PKC and that PKC mediates GnRH-induced SET down-regulation. Phosphorylation on serine 9 targets SET for degradation into the proteasome. Furthermore, a non-phosphorylatable SET mutant on serine 9 is resistant to GnRH-induced down-regulation. Altogether, these data suggest that GnRH-induced SET phosphorylation on serine 9 mediates SET protein down-regulation through the proteasome pathway. Noteworthy, SET down-regulation was also observed in response to pulsatile GnRH stimulation in LβT2 gonadotrope cells as well as in vivo in prepubertal female mice supporting its physiological relevance. In conclusion, this study highlights a regulation of SET protein by the neurohormone GnRH and identifies some of the mechanisms involved.

Ablation of beta subunit of protein kinase CK2 in mouse oocytes causes follicle atresia and premature ovarian failure

Premature ovarian failure (POF), a major cause of female infertility, is a complex disorder, but the molecular mechanisms underlying the disorder are only poorly understood. Here we report that protein kinase CK2 contributes to maintaining follicular survival through PI3K/AKT pathway and DNA damage response pathway. Targeted deletion of CK2β in mouse oocytes from the primordial follicle stage resulted in female infertility, which was attributed to POF incurring by massive follicle atresia. Downregulated PI3K/AKT signaling was found after CK2β deletion, indicated by reduced level of phosphorylated AKT (S473, T308, and S129) and altered AKT targets related to cell survival. Further studies discovered that CK2β-deficient oocytes showed enhanced γH2AX signals, indicative of accumulative unrepaired DSBs, which activated CHK2-dependant p53 and p63 signaling. The suppressed PI3K/AKT signaling and failed DNA damage response signaling probably contribute to large-scale oocyte loss and eventually POF. Our findings provide important new clues for elucidating the mechanisms underlying follicle atresia and POF.

Mps1 kinase-dependent Sgo2 centromere localisation mediates cohesin protection in mouse oocyte meiosis I

A key feature of meiosis is the step-wise removal of cohesin, the protein complex holding sister chromatids together, first from arms in meiosis I and then from the centromere region in meiosis II. Centromeric cohesin is protected by Sgo2 from Separase-mediated cleavage, in order to maintain sister chromatids together until their separation in meiosis II. Failures in step-wise cohesin removal result in aneuploid gametes, preventing the generation of healthy embryos. Here, we report that kinase activities of Bub1 and Mps1 are required for Sgo2 localisation to the centromere region. Mps1 inhibitor-treated oocytes are defective in centromeric cohesin protection, whereas oocytes devoid of Bub1 kinase activity, which cannot phosphorylate H2A at T121, are not perturbed in cohesin protection as long as Mps1 is functional. Mps1 and Bub1 kinase activities localise Sgo2 in meiosis I preferentially to the centromere and pericentromere respectively, indicating that Sgo2 at the centromere is required for protection.In meiosis I centromeric cohesin is protected by Sgo2 from Separase-mediated cleavage ensuring that sister chromatids are kept together until their separation in meiosis II. Here the authors demonstrate that Bub1 and Mps1 kinase activities are required for Sgo2 localisation to the centromere region.

The effect of DHEA on apoptosis and cohesin levels in oocytes in aged mice

Female fertility declines with age as the number of ovarian follicles decreases and aneuploidy increases. Degradation of the cohesin complex might be responsible for age-related aneuploidy. Dehydroepiandrosterone (DHEA) can improve the ovarian reserve and reduce the rate of aneuploidy, but the relationship between DHEA and cohesin levels in oocytes is still unknown. The aim of the current study was to evaluate the effect of the supplement DHEA on ovarian function, including the number of follicles and cohesin levels in oocytes. C57BL/6J mice at 3 weeks, 6 weeks, 12 weeks, 6 months, and 10 months of age were used to obtain a systematic view into follicle apoptosis and cohesin levels in oocytes. Nine-month-old C57BL/6J mice were administered saline (n = 5), 17β-estradiol (100 µg/kg per day, n = 5), or DHEA (5mg/Kg per day, n = 5). After 4 weeks, aged mice were weighed and sacrificed, and ovarian tissue samples were prepared. Anti-VASA staining and HE staining were used to count the number of follicles. Anti-γH2AX staining and TUNEL were used to measure follicle apoptosis and immunofluorescent staining was used to detect the levels of three oocyte cohesin subunits: REC8, SMC1β, and SMC3. Administration of the supplements 17β-estradiol and DHEA to aged mice increased the number of primordial and primary follicles and decreased the age-related apoptosis of follicles. Levels of the cohesin subunits REC8 and SMC1β declined with age, but DHEA and 17β-estradiol tended to delay that decline. The supplement DHEA increased the number of primordial and primary follicles in aged mice by inhibiting follicle apoptosis and tended to delay the decrease in cohesin levels in oocytes.

Phospho-H1 Decorates the Inter-chromatid Axis and Is Evicted along with Shugoshin by SET during Mitosis

Precise control of sister chromatid separation during mitosis is pivotal to maintaining genomic integrity. Yet, the regulatory mechanisms involved are not well understood. Remarkably, we discovered that linker histone H1 phosphorylated at S/T18 decorated the inter-chromatid axial DNA on mitotic chromosomes. Sister chromatid resolution during mitosis required the eviction of such H1S/T18ph by the chaperone SET, with this process being independent of and most likely downstream of arm-cohesin dissociation. SET also directed the disassembly of Shugoshins in a polo-like kinase 1-augmented manner, aiding centromere resolution. SET ablation compromised mitotic fidelity as evidenced by unresolved sister chromatids with marked accumulation of H1S/T18ph and centromeric Shugoshin. Thus, chaperone-assisted eviction of linker histones and Shugoshins is a fundamental step in mammalian mitotic progression. Our findings also elucidate the functional implications of the decades-old observation of mitotic linker histone phosphorylation, serving as a paradigm to explore the role of linker histones in bio-signaling processes.

CENP-A regulates chromosome segregation during the first meiosis of mouse oocytes

Proper chromosome separation in both mitosis and meiosis depends on the correct connection between kinetochores of chromosomes and spindle microtubules. Kinetochore dysfunction can lead to unequal distribution of chromosomes during cell division and result in aneuploidy, thus kinetochores are critical for faithful segregation of chromosomes. Centromere protein A (CENP-A) is an important component of the inner kinetochore plate. Multiple studies in mitosis have found that deficiencies in CENP-A could result in structural and functional changes of kinetochores, leading to abnormal chromosome segregation, aneuploidy and apoptosis in cells. Here we report the expression and function of CENP-A during mouse oocyte meiosis. Our study found that microinjection of CENP-A blocking antibody resulted in errors of homologous chromosome segregation and caused aneuploidy in eggs. Thus, our findings provide evidence that CENP-A is critical for the faithful chromosome segregation during mammalian oocyte meiosis.

Protein Phosphatase 6 Protects Prophase I-Arrested Oocytes by Safeguarding Genomic Integrity

Mammalian oocytes are arrested at prophase of the first meiotic division in the primordial follicle pool for months, even years, after birth depending on species, and only a limited number of oocytes resume meiosis, complete maturation, and ovulate with each reproductive cycle. We recently reported that protein phosphatase 6 (PP6), a member of the PP2A-like subfamily, which accounts for cellular serine/threonine phosphatase activity, functions in completing the second meiosis. Here, we generated mutant mice with a specific deletion of Ppp6c in oocytes from the primordial follicle stage by crossing Ppp6c^F/F mice with Gdf9-Cre mice and found that Ppp6c^F/F; GCre+ mice are infertile. Depletion of PP6c caused folliculogenesis defects and germ cell loss independent of the traditional AKT/mTOR pathway, but due to persistent phosphorylation of H2AX (a marker of double strand breaks), increased susceptibility to DNA damage and defective DNA repair, which led to massive oocyte elimination and eventually premature ovarian failure (POF). Our findings uncover an important role for PP6 as an indispensable guardian of genomic integrity of the lengthy prophase I oocyte arrest, maintenance of primordial follicle pool, and thus female fertility.

Formation of haploid gametes from diploid germ cells requires a specialized reductive cell division known as meiosis. In contrast to male meiosis that takes place continuously, a unique feature of female meiosis in mammals is the long arrest in meiosis I, which lasts up to 50 years in humans. Because the size of the germ cell pool determines the reproductive lifespan of females, it is important to discover mechanisms preserving the germ cell pool during the lengthy meiotic arrest. In this study, we examined the physiological role of a member of the PP2A-like serine/threonine phosphatase subfamily, protein phosphatase 6, in mouse oocytes during ovarian follicular development. This is the first study linking PP6 to the maintenance of the female germ cell pool and fertility. We find PP6 is an indispensable protector of arrested oocytes by safeguarding genomic integrity during their dormancy in the mouse ovary.

PP2A regulates kinetochore-microtubule attachment during meiosis I in oocyte

Studies using in vitro cultured oocytes have indicated that the protein phosphatase 2A (PP2A), a major serine/threonine protein phosphatase, participates in multiple steps of meiosis. Details of oocyte maturation regulation by PP2A remain unclear and an in vivo model can provide more convincing information. Here, we inactivated PP2A by mutating genes encoding for its catalytic subunits (PP2Acs) in mouse oocytes. We found that eliminating both PP2Acs caused female infertility. Oocytes lacking PP2Acs failed to complete 1(st) meiotic division due to chromosome misalignment and abnormal spindle assembly. In mitosis, PP2A counteracts Aurora kinase B/C (AurkB/C) to facilitate correct kinetochore-microtubule (KT-MT) attachment. In meiosis I in oocyte, we found that PP2Ac deficiency destabilized KT-MT attachments. Chemical inhibition of AurkB/C in PP2Ac-null oocytes partly restored the formation of lateral/merotelic KT-MT attachments but not correct KT-MT attachments. Taken together, our findings demonstrate that PP2Acs are essential for chromosome alignments and regulate the formation of correct KT-MT attachments in meiosis I in oocytes.

Casein kinase 1 (α, δ and ε) localize at the spindle poles, but may not be essential for mammalian oocyte meiotic progression

CK1 (casein kinase 1) is a family of serine/threonine protein kinase that is ubiquitously expressed in eukaryotic organism. CK1 members are involved in the regulation of many cellular processes. Particularly, CK1 was reported to phosphorylate Rec8 subunits of cohesin complex and regulate chromosome segregation in meiosis in budding yeast and fission yeast. (1-3) Here we investigated the expression, subcellular localization and potential functions of CK1α, CK1δ and CK1ε during mouse oocyte meiotic maturation. We found that CK1α, CK1δ and CK1ε all concentrated at the spindle poles and co-localized with γ-tubulin in oocytes at both metaphase I (MI) and metaphase II (MII) stages. However, depletion of CK1 by RNAi or overexpression of wild type or kinase-dead CK1 showed no effects on either spindle organization or chromosome segregation during oocyte meiotic maturation. Thus, CK1 is not the kinase that phosphorylates Rec8 cohesin in mammalian oocytes, and CK1 may not be essential for spindle organization and meiotic progression although they localize at spindle poles.

High-fat diets exaggerate endocrine and metabolic phenotypes in a rat model of DHEA-induced PCOS

Polycystic ovary syndrome (PCOS) is a complex endocrine and metabolic disorder with unclear etiology and unsatisfactory management. Effects of diets on the phenotype of PCOS were not fully understood. In the present study, we applied 45 and 60% high-fat diets (HFDs) on a rat model of PCOS induced by postnatal DHEA injection. We found that both DHEA and DHEA+HFDs rats exhibited reproductive abnormalities, including hyperandrogenism, irregular cycles and polycystic ovaries. The addition of HFDs, especially 60% HFDs, exaggerated morphological changes of ovaries and a number of metabolic changes, including increased body weight and body fat content, impaired glucose tolerance and increased serum insulin levels. Results from qPCR showed that DHEA-induced increased expression of hypothalamic androgen receptor and LH receptor were reversed by the addition of 60% HFDs. In contrast, the ovarian expression of LH receptor and insulin receptor mRNA was upregulated only with the addition of 60% HFDs. These findings indicated that DHEA and DHEA+HFDs might influence PCOS phenotypes through distinct mechanisms: DHEA affects the normal function of hypothalamus-pituitary-ovarian axis through LH, whereas the addition of HFDs exaggerated endocrine and metabolic dysfunction through ovarian responses to insulin-related mechanisms. We concluded that the addition of HFDs yielded distinct phenotypes of DHEA-induced PCOS and could be used for studies on both reproductive and metabolic features of the syndrome.

Loss of protein phosphatase 6 in oocytes causes failure of meiosis II exit and impaired female fertility

Dynamic protein phosphorylation and dephosphorylation, mediated by a conserved cohort of protein kinases and phosphatases, regulate cell cycle progression. Among the well-known PP2A-like protein phosphatases, protein phosphatase 6 (PP6) has been analyzed in mammalian mitosis, and Aurora A has recently been identified as its key substrate. However, the functions of PP6 in meiosis are still entirely unknown. To identify the physiological role of PP6 in female gametogenesis, Ppp6c(F/F) mice were first generated and crossed with Zp3-Cre mice to selectively disrupt Ppp6c expression in oocytes. Here, we report for the first time that PP6c is dispensable for oocyte meiotic maturation but essential for exit from meiosis II (MII) after fertilization. Depletion of PP6c caused an abnormal MII spindle and disrupted MII cytokinesis, resulting in zygotes with high risk of aneuploidy and defective early embryonic development, and thus severe subfertility. We also reveal that PP6 inactivation interferes with MII spindle formation and MII exit owing to increased Aurora A activity, and that Aurora A inhibition with MLN8237 can rescue the PP6c depletion phenotype. In conclusion, our findings uncover a hitherto unknown role for PP6 as an indispensable regulator of oocyte meiosis and female fertility.

Eviction of linker histone H1 by NAP-family histone chaperones enhances activated transcription

Background

In the Metazoan nucleus, core histones assemble the genomic DNA to form nucleosome arrays, which are further compacted into dense chromatin structures by the linker histone H1. The extraordinary density of chromatin creates an obstacle for accessing the genetic information. Regulation of chromatin dynamics is therefore critical to cellular homeostasis, and histone chaperones serve as prominent players in these processes. In the current study, we examined the role of specific histone chaperones in negotiating the inherently repressive chromatin structure during transcriptional activation.

Results

Using a model promoter, we demonstrate that the human nucleosome assembly protein family members hNap1 and SET/Taf1β stimulate transcription in vitro during pre-initiation complex formation, prior to elongation. This stimulatory effect is dependent upon the presence of activators, p300, and Acetyl-CoA. We show that transcription from our chromatin template is strongly repressed by H1, and that both histone chaperones enhance RNA synthesis by overcoming H1-induced repression. Importantly, both hNap1 and SET/Taf1β directly bind H1, and function to enhance transcription by evicting the linker histone from chromatin reconstituted with H1. In vivo studies demonstrate that SET/Taf1β, but not hNap1, strongly stimulates activated transcription from the chromosomally-integrated model promoter, consistent with the observation that SET/Taf1β is nuclear, whereas hNap1 is primarily cytoplasmic. Together, these observations indicate that SET/Taf1β may serve as a critical regulator of H1 dynamics and gene activation in vivo.

Conclusions

These studies uncover a novel function for SET that mechanistically couples transcriptional derepression with H1 dynamics. Furthermore, they underscore the significance of chaperone-dependent H1 displacement as an essential early step in the transition of a promoter from a dense chromatin state into one that is permissive to transcription factor binding and robust activation.

CRL4-DCAF1 ubiquitin E3 ligase directs protein phosphatase 2A degradation to control oocyte meiotic maturation

Oocyte meiosis is a specialized cell cycle that gives rise to fertilizable haploid gametes and is precisely controlled in various dimensions. We recently found that E3 ubiquitin ligase CRL4 is required for female fertility by regulating DNA hydroxymethylation to maintain oocyte survival and to promote zygotic genome reprogramming. However, not all phenotypes of CRL4-deleted oocytes could be explained by this mechanism. Here we show that CRL4 controls oocyte meiotic maturation by proteasomal degradation of protein phosphatase 2A scaffold subunit, PP2A-A. Oocyte-specific deletion of DDB1 or DCAF1 (also called VPRBP) results in delayed meiotic resumption and failure to complete meiosis I along with PP2A-A accumulation. DCAF1 directly binds to and results in the poly-ubiquitination of PP2A-A. Moreover, combined deletion of Ppp2r1a rescues the meiotic defects caused by DDB1/DCAF1 deficiency. These results provide in vivo evidence that CRL4-directed PP2A-A degradation is physiologically essential for regulating oocyte meiosis and female fertility.

The E3 ubiquitin ligase CRL4 regulates oocyte survival through hydroxymethylation of genomic DNA. Here Yu et al. show that CRL4 is also required for oocytes to complete meiosis I by mediating the poly-ubiquitination and proteasomal degradation of the cell cycle regulator protein phosphatase 2A-A subunit.

Scaffold subunit Aalpha of PP2A is essential for female meiosis and fertility in mice

Ppp2r1a encodes the scaffold subunit Aalpha of protein phosphatase 2A (PP2A), which is an important and ubiquitously expressed serine threonine phosphatase family and plays a critical role in many fundamental cellular processes. To identify the physiological role of PP2A in female germ cell meiosis, we selectively disrupted Ppp2r1a expression in oocytes by using the Cre-Loxp conditional knockout system. Here we report for the first time that oocyte-specific deletion of Ppp2r1a led to severe female subfertility without affecting follicle survival, growth, and ovulation. PP2A-Aalpha was essential for regulating oocyte meiotic maturation because depletion of PP2A-Aalpha facilitated germinal vesicle breakdown, causing elongation of the MII spindle and precocious separation of sister chromatids. The resulting eggs had high risk of aneuploidy, though they could be fertilized, leading to defective embryonic development and thus subfertility. Our findings provide strong evidence that PP2A-Aalpha within the oocyte plays an indispensable role in oocyte meiotic maturation, though it is dispensable for folliculogenesis in the mouse ovary.

Histone chaperone NAP1 mediates sister chromatid resolution by counteracting protein phosphatase 2A

Chromosome duplication and transmission into daughter cells requires the precisely orchestrated binding and release of cohesin. We found that the Drosophila histone chaperone NAP1 is required for cohesin release and sister chromatid resolution during mitosis. Genome-wide surveys revealed that NAP1 and cohesin co-localize at multiple genomic loci. Proteomic and biochemical analysis established that NAP1 associates with the full cohesin complex, but it also forms a separate complex with the cohesin subunit stromalin (SA). NAP1 binding to cohesin is cell-cycle regulated and increases during G2/M phase. This causes the dissociation of protein phosphatase 2A (PP2A) from cohesin, increased phosphorylation of SA and cohesin removal in early mitosis. PP2A depletion led to a loss of centromeric cohesion. The distinct mitotic phenotypes caused by the loss of either PP2A or NAP1, were both rescued by their concomitant depletion. We conclude that the balanced antagonism between NAP1 and PP2A controls cohesin dissociation during mitosis.

Sister chromatid segregation in meiosis II: deprotection through phosphorylation

Meiotic divisions (meiosis I and II) are specialized cell divisions to generate haploid gametes. The first meiotic division with the separation of chromosomes is named reductional division. The second division, which takes place immediately after meiosis I without intervening S-phase, is equational, with the separation of sister chromatids, similar to mitosis. This meiotic segregation pattern requires the two-step removal of the cohesin complex holding sister chromatids together: cohesin is removed from chromosome arms that have been subjected to homologous recombination in meiosis I and from the centromere region in meiosis II. Cohesin in the centromere region is protected from removal in meiosis I, but this protection has to be removed--deprotected--for sister chromatid segregation in meiosis II. Whereas the mechanisms of cohesin protection are quite well understood, the mechanisms of deprotection have been largely unknown until recently. In this review I summarize our current knowledge on cohesin deprotection.