Chromosome Segregation: Is the Spindle All About Microtubules?

HDAC8 drives spindle organization during meiotic maturation of porcine oocytes

OBJECTIVES

Histone deacetylase 8 (HDAC8) is one of the class I HDAC family proteins, which participates in the neuronal disorders, parasitic/viral infections, tumorigenesis and many other biological processes. However, its potential function during female germ cell development has not yet been fully understood.

MATERIALS AND METHODS

HDAC8-targeting siRNA was microinjected into GV oocytes to deplete HDAC8. PCI-34051 was used to inhibit the enzyme activity of HDAC8. Immunostaining, immunoblotting and fluorescence intensity quantification were applied to assess the effects of HDAC8 depletion or inhibition on the oocyte meiotic maturation, spindle/chromosome structure, γ-tubulin dynamics and acetylation level of α-tubulin.

RESULTS

We observed that HDAC8 was localized in the nucleus at GV stage and then translocated to the spindle apparatus from GVBD to M II stages in porcine oocytes. Depletion of HDAC8 led to the oocyte meiotic failure by showing the reduced polar body extrusion rate. In addition, depletion of HDAC8 resulted in aberrant spindle morphologies and misaligned chromosomes due to the defective recruitment of γ-tubulin to the spindle poles. Notably, these meiotic defects were photocopied by inhibition of HDAC8 activity using its specific inhibitor PCI-34051. However, inhibition of HDAC8 did not affect microtubule stability as assessed by the acetylation level of α-tubulin.

CONCLUSIONS

Collectively, our findings demonstrate that HDAC8 acts as a regulator of spindle assembly during porcine oocyte meiotic maturation.

Fbxo30 regulates chromosome segregation of oocyte meiosis

As the female gamete, meiotic oocytes provide not only half of the genome but also almost all stores for fertilization and early embryonic development. Because de novo mRNA transcription is absent in oocyte meiosis, protein-level regulations, especially the ubiquitin proteasome system, are more crucial. As the largest family of ubiquitin E3 ligases, Skp1-Cullin-F-box complexes recognize their substrates via F-box proteins with substrate-selected specificity. However, the variety of F-box proteins and their unknown substrates hinder our understanding of their functions. In this report, we find that Fbxo30, a new member of F-box proteins, is enriched in mouse oocytes, and its expression level declines substantially after the metaphase of the first meiosis (MI). Notably, depletion of Fbxo30 causes significant chromosome compaction accompanied by chromosome segregation failure and arrest at the MI stage, and this arrest is not caused by over-activation of spindle assembly checkpoint. Using immunoprecipitation and mass spectrometric analysis, we identify stem-loop-binding protein (SLBP) as a novel substrate of Fbxo30. SLBP overexpression caused by Fbxo30 depletion results in a remarkable overload of histone H3 on chromosomes that excessively condenses chromosomes and inhibits chromosome segregation. Our finding uncovers an unidentified pathway-controlling chromosome segregation and cell progress.

Microtubules: From understanding their dynamics to using them as potential therapeutic targets

Microtubules (MT) and actin microfilaments are dynamic cytoskeleton components involved in a range of intracellular processes. MTs play a role in cell division, beating of cilia and flagella, and intracellular transport. Over the past decades, much knowledge has been gained regarding MT function and structure, and its role in underlying disease progression. This makes MT potential therapeutic targets for various disorders. Disturbances in MT and their associated proteins are the underlying cause of diseases such as Alzheimer's disease, cancer, and several genetic diseases. Some of the advances in the field of MT research, as well as the potenti G beta gamma, is needed al uses of MT-targeting agents in various conditions have been reviewed here.

Oocyte stage-specific effects of MTOR determine granulosa cell fate and oocyte quality in mice

MTOR (mechanistic target of rapamycin), an integrator of pathways important for cellular metabolism, proliferation, and differentiation, is expressed at all stages of oocyte development. Primordial oocytes constitute a nonproliferating, nongrowing reserve of potential eggs maintained for the entire reproductive lifespan of mammalian females. Using conditional knockouts, we determined the role of MTOR in both primordial and growing oocytes. MTOR-dependent pathways in primordial oocytes are not needed to sustain the viability of the primordial oocyte pool or their recruitment into the cohort of growing oocytes but are essential later for maintenance of oocyte genomic integrity, sustaining ovarian follicular development, and fertility. In growing oocytes, MTOR-dependent pathways are required for processes that promote completion of meiosis and enable embryonic development.

MTOR (mechanistic target of rapamycin) is a widely recognized integrator of signals and pathways key for cellular metabolism, proliferation, and differentiation. Here we show that conditional knockout (cKO) of Mtor in either primordial or growing oocytes caused infertility but differentially affected oocyte quality, granulosa cell fate, and follicular development. cKO of Mtor in nongrowing primordial oocytes caused defective follicular development leading to progressive degeneration of oocytes and loss of granulosa cell identity coincident with the acquisition of immature Sertoli cell-like characteristics. Although Mtor was deleted at the primordial oocyte stage, DNA damage accumulated in oocytes during their later growth, and there was a marked alteration of the transcriptome in the few oocytes that achieved the fully grown stage. Although oocyte quality and fertility were also compromised when Mtor was deleted after oocytes had begun to grow, these occurred without overtly affecting folliculogenesis or the oocyte transcriptome. Nevertheless, there was a significant change in a cohort of proteins in mature oocytes. In particular, down-regulation of PRC1 (protein regulator of cytokinesis 1) impaired completion of the first meiotic division. Therefore, MTOR-dependent pathways in primordial or growing oocytes differentially affected downstream processes including follicular development, sex-specific identity of early granulosa cells, maintenance of oocyte genome integrity, oocyte gene expression, meiosis, and preimplantation developmental competence.