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D S Bogolyubov - One of the best experts on this subject based on the ideXlab platform.

  • karyosphere Karyosome a peculiar structure of the oocyte nucleus
    International Review of Cell and Molecular Biology, 2018
    Co-Authors: D S Bogolyubov
    Abstract:

    Abstract The karyosphere, aka the Karyosome, is a meiosis-specific structure that represents a “knot” of condensed chromosomes joined together in a limited volume of the oocyte nucleus. The karyosphere is an evolutionarily conserved but morphologically rather “multifaceted” structure. It forms at the diplotene stage of meiotic prophase in many animals, from hydra and Drosophila to human. Karyosphere formation is generally linked with transcriptional silencing of the genome. It is believed that karyosphere/Karyosome is a prerequisite for proper completion of meiotic divisions and further development. Here, a brief review on the karyosphere features in some invertebrates and vertebrates is provided. Special emphasis is made on terminology, since current discrepancies in this field may lead to confusions. In particular, it is proposed to distinguish the karyosphere with a capsule and the Karyosome (a karyosphere devoid of a capsule). The “inverted” karyospheres are also considered, in which the chromosomes situate externally to an extrachromosomal structure (e.g., in human oocytes).

  • Localization of the chromatin-remodeling protein ATRX in the oocyte nucleus of some insects
    Cell and Tissue Biology, 2017
    Co-Authors: I. S. Stepanova, D S Bogolyubov
    Abstract:

    A comparative study of nuclear distribution of the chromatin-remodeling protein ATRX in the oocytes of three species of insects in which the oocyte nucleus at the diplotene stage differs in structure, has been carried out using fluorescent and immunoelectron microscopy. In tóhe oocyte nucleus of the tenebrionid beetles, Tribolium castaneum and Tenebrio molitor , ATRX preferably associates with the karyosphere (Karyosome) that represents a result of concentration of the condensed chromosomes in a limited volume of the nucleus. In the oocyte nucleus of the house cricket, Acheta domesticus , in which a karyosphere does not form, the protein ATRX is distributed in the entire nuclear volume in association with the chromatin. The fact of ATRX presence in the extrachromosomal structures of the insect oocyte nucleus, the karyosphere capsule and specific nuclear bodies, is documented for the first time.

  • The karyosphere capsule in Tribolium castaneum oocytes
    Cell and Tissue Biology, 2014
    Co-Authors: F. M. Batalova, D S Bogolyubov
    Abstract:

    The structure and composition of the karyosphere (Karyosome) capsule of the laboratory insect, Tribolium castaneum , were studied with the use of electron and immunoelectron microscopy. Eight stages that characterized the period of oocyte growth have been established basing on the study of nuclear structure dynamics. At the diplotene stage, T. castaneum oocyte chromosomes are being united early to form a compact karyosphere; however, prominent chromatin condensation does not occur at the same time. The process of karyosphere formation is accompanied by the development of a spacious extrachromosomal capsule surrounding chromatin. The capsule consists of a fibrous material of different morphological types. The most prominent molecular components of T. castaneum karyosphere capsule are represented by the proteins of nuclear matrix, including F-actin and lamin B. Apart from the structural proteins, immunocytochemical approach allowed revealing Sm proteins of small nuclear (sn) RNPs and “mature” snRNAs with 2,2,7-trimetyl guanosine (TMG) cap at the 5’-end of their molecules. These data may serve as a base for further broadening of the conception about the functions of the karyosphere capsule as a specialized oocyte nuclear domain. We believe that T. castaneum karyosphere capsule plays not only a structural role, but may be involved directly in the processes related to gene expression.

Hiroyuki Ohkura - One of the best experts on this subject based on the ideXlab platform.

  • Kdm5/Lid Regulates Chromosome Architecture in Meiotic Prophase I Independently of Its Histone Demethylase Activity.
    PLOS Genetics, 2016
    Co-Authors: Liudmila Zhaunova, Hiroyuki Ohkura, Manuel Breuer
    Abstract:

    During prophase of the first meiotic division (prophase I), chromatin dynamically reorganises to recombine and prepare for chromosome segregation. Histone modifying enzymes are major regulators of chromatin structure, but our knowledge of their roles in prophase I is still limited. Here we report on crucial roles of Kdm5/Lid, one of two histone demethylases in Drosophila that remove one of the trimethyl groups at Lys4 of Histone 3 (H3K4me3). In the absence of Kdm5/Lid, the synaptonemal complex was only partially formed and failed to be maintained along chromosome arms, while localisation of its components at centromeres was unaffected. Kdm5/Lid was also required for Karyosome formation and homologous centromere pairing in prophase I. Although loss of Kdm5/Lid dramatically increased the level of H3K4me3 in oocytes, catalytically inactive Kdm5/Lid can rescue the above cytological defects. Therefore Kdm5/Lid controls chromatin architecture in meiotic prophase I oocytes independently of its demethylase activity.

  • the conserved kinase srpk regulates Karyosome formation and spindle microtubule assembly in drosophila oocytes
    Journal of Cell Science, 2012
    Co-Authors: Fiona C Cullen, Nina Vogt, Hiroyuki Ohkura
    Abstract:

    In Drosophila oocytes, after the completion of recombination, meiotic chromosomes form a compact cluster called the Karyosome within the nucleus, and later assemble spindle microtubules without centrosomes. Although these oocyte-specific phenomena are also observed in humans, their molecular basis is not well understood. Here, we report essential roles for the conserved kinase SRPK in both Karyosome formation and spindle microtubule assembly in oocytes. We have identified a female-sterile srpk mutant through a cytological screen for Karyosome defects. Unlike most Karyosome mutants, the Karyosome defect is independent of the meiotic recombination checkpoint. Heterochromatin clustering found within the wild-type Karyosome is disrupted in the mutant. Strikingly, a loss of SRPK severely prevents microtubule assembly for acentrosomal spindles in mature oocytes. Subsequently, bi-orientation and segregation of meiotic chromosomes are also defective. Therefore, this study demonstrates new roles of this conserved kinase in two independent meiotic steps specific to oocytes.

  • The meiotic recombination checkpoint suppresses NHK-1 kinase to prevent reorganisation of the oocyte nucleus in Drosophila.
    PLOS Genetics, 2010
    Co-Authors: Oscar M Lancaster, Manuel Breuer, C. Fiona Cullen, Hiroyuki Ohkura
    Abstract:

    The meiotic recombination checkpoint is a signalling pathway that blocks meiotic progression when the repair of DNA breaks formed during recombination is delayed. In comparison to the signalling pathway itself, however, the molecular targets of the checkpoint that control meiotic progression are not well understood in metazoans. In Drosophila, activation of the meiotic checkpoint is known to prevent formation of the Karyosome, a meiosis-specific organisation of chromosomes, but the molecular pathway by which this occurs remains to be identified. Here we show that the conserved kinase NHK-1 (Drosophila Vrk-1) is a crucial meiotic regulator controlled by the meiotic checkpoint. An nhk-1 mutation, whilst resulting in Karyosome defects, does so independent of meiotic checkpoint activation. Rather, we find unrepaired DNA breaks formed during recombination suppress NHK-1 activity (inferred from the phosphorylation level of one of its substrates) through the meiotic checkpoint. Additionally DNA breaks induced by X-rays in cultured cells also suppress NHK-1 kinase activity. Unrepaired DNA breaks in oocytes also delay other NHK-1 dependent nuclear events, such as synaptonemal complex disassembly and condensin loading onto chromosomes. Therefore we propose that NHK-1 is a crucial regulator of meiosis and that the meiotic checkpoint suppresses NHK-1 activity to prevent oocyte nuclear reorganisation until DNA breaks are repaired.

  • nhk 1 phosphorylates baf to allow Karyosome formation in the drosophila oocyte nucleus
    Journal of Cell Biology, 2007
    Co-Authors: Oscar M Lancaster, Fiona C Cullen, Hiroyuki Ohkura
    Abstract:

    Accurate chromosome segregation in meiosis requires dynamic changes in chromatin organization. In Drosophila melanogaster, upon completion of recombination, meiotic chromosomes form a single, compact cluster called the Karyosome in an enlarged oocyte nucleus. This clustering is also found in humans; however, the mechanisms underlying Karyosome formation are not understood. In this study, we report that phosphorylation of barrier to autointegration factor (BAF) by the conserved kinase nucleosomal histone kinase-1 (NHK-1; Drosophila Vrk1) has a critical function in Karyosome formation. We find that the noncatalytic domain of NHK-1 is crucial for its kinase activity toward BAF, a protein that acts as a linker between chromatin and the nuclear envelope. A reduction of NHK-1 or expression of nonphosphorylatable BAF results in ectopic association of chromosomes with the nuclear envelope in oocytes. We propose that BAF phosphorylation by NHK-1 disrupts anchorage of chromosomes to the nuclear envelope, allowing Karyosome formation in oocytes. These data provide the first mechanistic insight into how the Karyosome forms.

Fiona C Cullen - One of the best experts on this subject based on the ideXlab platform.

  • the conserved kinase srpk regulates Karyosome formation and spindle microtubule assembly in drosophila oocytes
    Journal of Cell Science, 2012
    Co-Authors: Fiona C Cullen, Nina Vogt, Hiroyuki Ohkura
    Abstract:

    In Drosophila oocytes, after the completion of recombination, meiotic chromosomes form a compact cluster called the Karyosome within the nucleus, and later assemble spindle microtubules without centrosomes. Although these oocyte-specific phenomena are also observed in humans, their molecular basis is not well understood. Here, we report essential roles for the conserved kinase SRPK in both Karyosome formation and spindle microtubule assembly in oocytes. We have identified a female-sterile srpk mutant through a cytological screen for Karyosome defects. Unlike most Karyosome mutants, the Karyosome defect is independent of the meiotic recombination checkpoint. Heterochromatin clustering found within the wild-type Karyosome is disrupted in the mutant. Strikingly, a loss of SRPK severely prevents microtubule assembly for acentrosomal spindles in mature oocytes. Subsequently, bi-orientation and segregation of meiotic chromosomes are also defective. Therefore, this study demonstrates new roles of this conserved kinase in two independent meiotic steps specific to oocytes.

  • nhk 1 phosphorylates baf to allow Karyosome formation in the drosophila oocyte nucleus
    Journal of Cell Biology, 2007
    Co-Authors: Oscar M Lancaster, Fiona C Cullen, Hiroyuki Ohkura
    Abstract:

    Accurate chromosome segregation in meiosis requires dynamic changes in chromatin organization. In Drosophila melanogaster, upon completion of recombination, meiotic chromosomes form a single, compact cluster called the Karyosome in an enlarged oocyte nucleus. This clustering is also found in humans; however, the mechanisms underlying Karyosome formation are not understood. In this study, we report that phosphorylation of barrier to autointegration factor (BAF) by the conserved kinase nucleosomal histone kinase-1 (NHK-1; Drosophila Vrk1) has a critical function in Karyosome formation. We find that the noncatalytic domain of NHK-1 is crucial for its kinase activity toward BAF, a protein that acts as a linker between chromatin and the nuclear envelope. A reduction of NHK-1 or expression of nonphosphorylatable BAF results in ectopic association of chromosomes with the nuclear envelope in oocytes. We propose that BAF phosphorylation by NHK-1 disrupts anchorage of chromosomes to the nuclear envelope, allowing Karyosome formation in oocytes. These data provide the first mechanistic insight into how the Karyosome forms.

N. H. Swellengrebel - One of the best experts on this subject based on the ideXlab platform.

  • Normal and Abnormal Morphology of Trypanosoma lewisi, in the blood of the Rat
    Parasitology, 2020
    Co-Authors: N. H. Swellengrebel
    Abstract:

    The development of T. lewisi in the blood of the rat is accompanied by a division of the nucleus which is more like an amitosis than a mitosis.The division of the blepharoplast, always preceded by the division of a Karyosome-like granule, might be called a promitosis (sensu: Nägler, 1909).It is not proved that autogamy or parthenogenesis takes place in rats' blood or in cultures. The evidence brought forward by Prowazek and by Moore, Breinl and Hindle proves only the existence of chromidia; neither are Schilling's recent proofs suggestive.Under abnormal conditions hypertrophy takes place affecting the nucleus and the blepharoplast, with subsequent formation of chromidia and final fragmentation. Similar stages found in the rat-louse have been described as sexual cells.Considering these results in connection with previous observations of Strickland and myself, we may conclude that as yet nothing whatever indicates the occurrence of any sexual process in the life-cycle of T. levrisi, both in the vertebrate and invertebrate hosts.

  • Fixation and Staining of Trypanosoma lewisi
    Parasitology, 2020
    Co-Authors: N. H. Swellengrebel
    Abstract:

    Since the publication of Moore and Breinl's paper (1908) a new method of wet fixation has been introduced into the technique of protozoological research. It is generally stated that the old method of drying the blood films and fixing them afterwards in absolute alcohol, destroys the minute details of nuclear and protoplasmatic structure; that the preparations made in this way are wholly misleading and the structures do not correspond to those of the living Trypanosomes. This becomes evident when we compare the figures of Moore and Breinl and of Rosenbusch (1909), who studied Tr. lewisi with the aid of wet fixation in Flemming's liquid, with those of other authors (Prowazek (1905), Wenyon (1908) etc.) who studied the same subject. The structure of the nucleus is very dissimilar with the two methods. The big Karyosome observed by the first authors cannot be found with dry fixation; then only one or more minute granules are to be seen.

Nicole Lebrasseur - One of the best experts on this subject based on the ideXlab platform.