Dynamics of histone H3 phosphorylation at threonine 3 during meiotic maturation in mouse oocytes

Dissecting the roles of Haspin and VRK1 in histone H3 phosphorylation during mitosis

Protein kinases that phosphorylate histones are ideally-placed to influence the behavior of chromosomes during cell division. Indeed, a number of conserved histone phosphorylation events occur prominently during mitosis and meiosis in most eukaryotes, including on histone H3 at threonine-3 (H3T3ph). At least two kinases, Haspin and VRK1 (NHK-1/ballchen in Drosophila), have been proposed to carry out this modification. Phosphorylation of H3 by Haspin has defined roles in mitosis, but the significance of VRK1 activity towards histones in dividing cells has been unclear. Here, using in vitro kinase assays, KiPIK screening, RNA interference, and CRISPR/Cas9 approaches, we were unable to substantiate a direct role for VRK1, or its paralogue VRK2, in the phosphorylation of threonine-3 or serine-10 of Histone H3 in mitosis, although loss of VRK1 did slow cell proliferation. We conclude that the role of VRKs, and their more recently identified association with neuromuscular disease and importance in cancers of the nervous system, are unlikely to involve mitotic histone kinase activity. In contrast, Haspin is required to generate H3T3ph during mitosis.

Haspin participates in AURKB recruitment to centromeres and contributes to chromosome congression in male mouse meiosis

Chromosome segregation requires that centromeres properly attach to spindle microtubules. This essential step regulates the accuracy of cell division and therefore must be precisely regulated. One of the main centromeric regulatory signaling pathways is the Haspin-H3T3ph-chromosomal passenger complex (CPC) cascade, which is responsible for the recruitment of the CPC to the centromeres. In mitosis, Haspin kinase phosphorylates histone H3 at threonine 3 (H3T3ph), an essential epigenetic mark that recruits the CPC, whose catalytic component is Aurora B kinase. However, the centromeric Haspin-H3T3ph-CPC pathway remains largely uncharacterized in mammalian male meiosis. We have analyzed Haspin functions by either its chemical inhibition in cultured spermatocytes using LDN-192960, or the ablation of Haspin gene in Haspin-/-. Our studies suggest that Haspin kinase activity is required for proper chromosome congression during both meiotic divisions and for the recruitment of Aurora B and kinesin MCAK to meiotic centromeres. However, the absence of H3T3ph histone mark does not alter Borealin and SGO2 centromeric localization. These results add new and relevant information regarding the regulation of the Haspin-H3T3ph-CPC pathway and centromere function during meiosis.

Post-Translational Modifications in Oocyte Maturation and Embryo Development

Mammalian oocyte maturation and embryo development are unique biological processes regulated by various modifications. Since de novo mRNA transcription is absent during oocyte meiosis, protein-level regulation, especially post-translational modification (PTM), is crucial. It is known that PTM plays key roles in diverse cellular events such as DNA damage response, chromosome condensation, and cytoskeletal organization during oocyte maturation and embryo development. However, most previous reviews on PTM in oocytes and embryos have only focused on studies of Xenopus laevis or Caenorhabditis elegans eggs. In this review, we will discuss the latest discoveries regarding PTM in mammalian oocytes maturation and embryo development, focusing on phosphorylation, ubiquitination, SUMOylation and Poly(ADP-ribosyl)ation (PARylation). Phosphorylation functions in chromosome condensation and spindle alignment by regulating histone H3, mitogen-activated protein kinases, and some other pathways during mammalian oocyte maturation. Ubiquitination is a three-step enzymatic cascade that facilitates the degradation of proteins, and numerous E3 ubiquitin ligases are involved in modifying substrates and thus regulating oocyte maturation, oocyte-sperm binding, and early embryo development. Through the reversible addition and removal of SUMO (small ubiquitin-related modifier) on lysine residues, SUMOylation affects the cell cycle and DNA damage response in oocytes. As an emerging PTM, PARlation has been shown to not only participate in DNA damage repair, but also mediate asymmetric division of oocyte meiosis. Each of these PTMs and external environments is versatile and contributes to distinct phases during oocyte maturation and embryo development.

HASPIN kinase mediates histone deacetylation to regulate oocyte meiotic maturation in pigs

HASPIN kinase-catalyzed phosphorylation of histone H3 on threonine 3 (H3T3p) directs the activity and localization of chromosomal passenger complex (CPC) and spindle assembly checkpoint (SAC) to regulate chromosome condensation and segregation in both mitosis and meiosis. However, the function of HASPIN kinase in the meiotic maturation of porcine oocytes is not yet known. Here, we found that HASPIN mRNA is constantly expressed in porcine oocyte maturation and subsequent early embryo development. H3T3p is highly enriched on chromosomes at germinal vesicle breakdown (GVBD) stage and thereafter maintains a low level in progression through metaphase I (MI) to metaphase II (MII). Correspondingly, H3T3p was completely abolished in oocytes treated with an inhibitor of HASPIN kinase. Functionally, inhibition of HASPIN activity led to a significant reduction in the rate of oocyte meiotic maturation and the limited cumulus expansion. Additionally, HASPIN inhibition caused both spindle disorganization and chromosome misalignment in oocytes at MI and MII stage. Importantly, HASPIN inhibition severely prevented deacetylation of several highly conserved lysine (K) residues of histone H3 and H4 including H3K9, H3K14, H4K5, H4K8, H4K12 and H4K16 on the metaphase chromosomes during oocyte meiotic maturation. Taken together, these results demonstrate that HASPIN kinase regulates porcine oocyte meiotic maturation via modulating histone deacetylation.

A Peak of H3T3 Phosphorylation Occurs in Synchrony with Mitosis in Sea Urchin Early Embryos

The sea urchin embryo provides a valuable system to analyse the molecular mechanisms orchestrating cell cycle progression and mitosis in a developmental context. However, although it is known that the regulation of histone activity by post-translational modification plays an important role during cell division, the dynamics and the impact of these modifications have not been characterised in detail in a developing embryo. Using different immuno-detection techniques, we show that the levels of Histone 3 phosphorylation at Threonine 3 oscillate in synchrony with mitosis in Sphaerechinus granularis early embryos. We present, in addition, the results of a pharmacological study aimed at analysing the role of this key histone post-translational modification during sea urchin early development.

H4S1ph, an alternative epigenetic marker for sperm maturity

Histone phosphorylation, an epigenetic post-translational modification, plays essential roles in male gamete chromatin compaction during spermatogenesis and sperm maturity. Previously, we studied the epigenetic marker of phosphorylated serine 1 of histone H2A and H4 (HS1ph) during spermatogenesis in mice and crabs, which was shown to be closely related to the sperm maturity. To further investigate the correlation between phosphorylated serine 1 of histone H4 (H4S1ph) and sperm maturation, a comparison study was conducted in this work between the healthy and the precocious crabs. It was discovered that the distribution of H4S1ph was similar for the two groups of crabs during spermatogenesis before maturity, but totally different in the sperm nuclei. H4S1ph vanished in the nuclei of healthy crab spermatozoa mostly, while retained in the precocious crabs just like what it was in elongated spermatid of both kinds of crabs. The results showed that a high level of H4S1ph conservation was closely associated with immaturity and might indicate inefficient fertility of male precocious crabs. Thus, H4S1ph was suggested to be an epigenetic marker of sperm maturity.

Structural basis for histone H3K4me3 recognition by the N-terminal domain of the PHD finger protein Spp1

Saccharomyces cerevisiae Spp1, a plant homeodomain (PHD) finger containing protein, is a critical subunit of the histone H3K4 methyltransferase complex of proteins associated with Set1 (COMPASS). The chromatin binding affinity of the PHD finger of Spp1 has been proposed to modulate COMPASS activity. During meiosis, Spp1 plays another role in promoting programmed double-strand break (DSB) formation by binding H3K4me3 via its PHD finger and interacting with a DSB protein, Mer2. However, how the Spp1 PHD finger performs site-specific readout of H3K4me3 is still not fully understood. In the present study, we determined the crystal structure of the highly conserved Spp1 N-terminal domain (Sc_Spp1_NTD) in complex with the H3K4me3 peptide. The structure shows that Sc_Spp1_NTD comprises a PHD finger responsible for methylated H3K4 recognition and a C3H-type zinc finger necessary to ensure the overall structural stability. Our isothermal titration calorimetry results show that binding of H3K4me3 to Sc_Spp1_NTD is mildly inhibited by H3R2 methylation, weakened by H3T6 phosphorylation, and abrogated by H3T3 phosphorylation. This histone modification cross-talk, which is conserved in the Saccharomyces pombe and mammalian orthologs of Sc_Spp1 in vitro, can be rationalized structurally and might contribute to the roles of Spp1 in COMPASS activity regulation and meiotic recombination.

CFP1 coordinates histone H3 lysine-4 trimethylation and meiotic cell cycle progression in mouse oocytes

Trimethylation of histone H3 on lysine-4 (H3K4me3) is associated with gene-regulatory elements, but its transcription-independent function in cell division is unclear. CxxC-finger protein-1 (CFP1) is a major mediator of H3K4 trimethylation in mouse oocytes. Here we report that oocyte-specific knockout of Cxxc1, inhibition of CFP1 function, or abrogation of H3K4 methylation in oocytes each causes a delay of meiotic resumption as well as metaphase I arrest owing to defective spindle assembly and chromosome misalignment. These phenomena are partially attributed to insufficient phosphorylation of histone H3 at threonine-3. CDK1 triggers cell division–coupled degradation and inhibitory phosphorylation of CFP1. Preventing CFP1 degradation and phosphorylation causes CFP1 accumulation on chromosomes and impairs meiotic maturation and preimplantation embryo development. Therefore, CFP1-mediated H3K4 trimethylation provides 3a permission signal for the G2–M transition. Dual inhibition of CFP1 removes the SETD1–CFP1 complex from chromatin and ensures appropriate chromosome configuration changes during meiosis and mitosis.

The transcription-independent function of trimethylation of histone H3 (H3K4me) in cell division is unclear. Here, Heng-Yu Fan and colleagues report that CFP1, a subunit of the H3K4 methyltransferase, is required for oocyte meiosis, being phosphorylated and degraded during cell cycle transition.

Polo-like kinase 1 inhibition results in misaligned chromosomes and aberrant spindles in porcine oocytes during the first meiotic division

Polo-like kinase 1 (Plk1), a type of serine/threonine protein kinase, has been implicated in various functions in the regulation of mitotic processes. However, these kinase's roles in meiotic division are not fully understood, particularly in the meiotic maturation of porcine oocytes. In this study, the expression and spatiotemporal localization of Plk1 were initially assessed in the meiotic process of pig oocytes by utilizing Western blotting with immunofluorescent staining combined with confocal microscopy imaging technique. The results showed that Plk1 was expressed and exhibited a dynamic subcellular localization throughout the meiotic process. After germinal vesicle breakdown (GVBD), Plk1 was detected prominently around the condensed chromosomes and subsequently exhibited a similar subcellular localization to α-tubulin throughout subsequent meiotic phases, with particular enrichment being observed near spindle poles at MI and MII. Inhibition of Plk1 via a highly selective inhibitor, GSK461364, led to the failure of first polar body extrusion in porcine oocytes, with the majority of the treated oocytes being arrested in GVBD. Further subcellular structure examination results indicated that Plk1 inhibition caused the great majority of oocytes with spindle abnormalities and chromosome misalignment during the first meiotic division. The results of this study illustrate that Plk1 is critical for the first meiotic division in porcine oocytes through its influence on spindle organization and chromosome alignment, which further affects the ensuing meiotic cell cycle progression.

Haspin inhibition reveals functional differences of interchromatid axis-localized AURKB and AURKC

Use of mouse oocytes that only express Aurora kinase B as the catalytic subunit of the chromosomal passenger complex (CPC) provides evidence indicating differential capacities of AURKB– and AURKC–CPC complexes at a distinct localization.

Aneuploidy is the leading genetic abnormality contributing to infertility, and chromosome segregation errors are common during female mammalian meiosis I (MI). Previous results indicate that haspin kinase regulates resumption of meiosis from prophase arrest, chromosome condensation, and kinetochore–microtubule attachments during early prometaphase of MI. Here we report that haspin inhibition in late prometaphase I causes acceleration of MI, bypass of the spindle assembly checkpoint (SAC), and loss of interchromatid axis–localized Aurora kinase C. Meiotic cells contain a second chromosomal passenger complex (CPC) population, with Aurora kinase B (AURKB) bound to INCENP. Haspin inhibition in oocytes from Aurkc^−/− mice, where AURKB is the sole CPC kinase, does not alter MI completion timing, and no change in localization of the SAC protein, MAD2, is observed. These data suggest that AURKB on the interchromatid axis is not needed for SAC activation and illustrate a key difference between the functional capacities of the two AURK homologues.

Haspin kinase regulates microtubule-organizing center clustering and stability through Aurora kinase C in mouse oocytes

Meiotic oocytes lack classic centrosomes and, therefore, bipolar spindle assembly depends on clustering of acentriolar microtubule-organizing centers (MTOCs) into two poles. However, the molecular mechanism regulating MTOC assembly into two poles is not fully understood. The kinase haspin (also known as GSG2) is required to regulate Aurora kinase C (AURKC) localization at chromosomes during meiosis I. Here, we show that inhibition of haspin perturbed MTOC clustering into two poles and the stability of the clustered MTOCs. Furthermore, we show that AURKC localizes to MTOCs in mouse oocytes. Inhibition of haspin perturbed the localization of AURKC at MTOCs, and overexpression of AURKC rescued the MTOC-clustering defects in haspin-inhibited oocytes. Taken together, our data uncover a role for haspin as a regulator of bipolar spindle assembly by regulating AURKC function at acentriolar MTOCs in oocytes.

Dynamics of histone H2A, H4 and HS1ph during spermatogenesis with a focus on chromatin condensation and maturity of spermatozoa

Histones and histone phosphorylation play vital roles during animal spermatogenesis and spermatozoa maturation. The dynamic distribution of histones H2A and H4 and phosphorylated H2A and H4 at serine 1 (HS1ph) was explored in mammalian and Decapoda germ cells, with a special focus on the distribution of H2A, H4 and HS1ph between mouse condensed spermatozoa chromatin and crab non-condensed spermatozoa chromatin. The distribution of histone marks was also analysed in mature spermatozoa with different chromatin structures. Histone H2A and H4 marks were closely associated with the relatively loose chromatin structure in crab spermatozoa. The significant decrease in the HS1ph signal during spermatogenesis suggests that eliminating most of these epigenetic marks in the nucleusis closely associated with spermatozoa maturity.

Aurora B inhibitor barasertib prevents meiotic maturation and subsequent embryo development in pig oocytes

Barasertib, a highly selective Aurora B inhibitor, has been widely used in a variety of cells to investigate the role of Aurora B kinase, which has been implicated in various functions in the mitotic process. However, effects of barasertib on the meiotic maturation process are not fully understood, particularly in porcine oocyte meiotic maturation. In the present study, the effects of barasertib on the meiotic maturation and developmental competence of pig oocytes were investigated, and the possible roles of Aurora B were also evaluated in porcine oocytes undergoing meiosis. Initially, we examined the expression and subcellular localization of Aurora B using Western blot analysis and immunofluorescent staining. Aurora B was found to express and exhibit specific dynamic intracellular localization during porcine oocyte meiotic maturation. Aurora B was observed around the chromosomes after germinal vesicle breakdown. Then it was transferred to the spindle region after metaphase I stage, and was particularly concentrated at the central spindles at telophase I stage. barasertib treatment resulted in the failure of polar body extrusion in pig oocytes, with a larger percentage of barasertib-treated oocytes remaining at the pro-metaphase I stage. Additional results reported that barasertib treatment had no effect on chromosome condensation but resulted in a significantly higher percentage of the treated oocytes with aberrant spindles and misaligned chromosomes during the first meiotic division. In addition, inhibition of Aurora B with lower concentrations of barasertib during pig oocyte meiotic maturation decreased the subsequent embryo developmental competence. Thus, these results illustrate that barasertib has significant effects on porcine oocyte meiotic maturation and subsequent development through Aurora B inhibition, and this regulation is related to its effects on spindle formation and chromosome alignment during the first meiotic division in porcine oocytes.

H3 Thr3 phosphorylation is crucial for meiotic resumption and anaphase onset in oocyte meiosis

Haspin-catalyzed histone H3 threonine 3 (Thr3) phosphorylation facilitates chromosomal passenger complex (CPC) docking at centromeres, regulating indirectly chromosome behavior during somatic mitosis. It is not fully known about the expression and function of H3 with phosphorylated Thr3 (H3T3-P) during meiosis in oocytes. In this study, we investigated the expression and sub-cellular distribution of H3T3-P, as well as its function in mouse oocytes during meiotic division. Western blot analysis revealed that H3T3-P expression was only detected after germinal vesicle breakdown (GVBD), and gradually increased to peak level at metaphase I (MI), but sharply decreased at metaphase II (MII). Immunofluorescence showed H3T3-P was only brightly labeled on chromosomes after GVBD, with relatively high concentration across the whole chromosome axis from pro-metaphase I (pro-MI) to MI. Specially, H3T3-P distribution was exclusively limited to the local space between sister centromeres at MII stage. Haspin inhibitor, 5-iodotubercidin (5-ITu), dose- and time-dependently blocked H3T3-P expression in mouse oocytes. H3T3-P inhibition delayed the resumption of meiosis (GVBD) and chromatin condensation. Moreover, the loss of H3T3-P speeded up the meiotic transition to MII of pro-MI oocytes in spite of the presence of non-aligned chromosomes, even reversed MI-arrest induced with Nocodazole. The inhibition of H3T3-P expression distinguishably damaged MAD1 recruitment on centromeres, which indicates the spindle assembly checkpoint was impaired in function, logically explaining the premature onset of anaphase I. Therefore, Haspin-catalyzed histone H3 phosphorylation is essential for chromatin condensation and the following timely transition from meiosis I to meiosis II in mouse oocytes during meiotic division.