Shugoshins function as a guardian for chromosomal stability in nuclear division

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.

High SGO2 predicted poor prognosis and high therapeutic value of lung adenocarcinoma and promoted cell proliferation, migration, invasion, and epithelial-to-mesenchymal transformation

Background: Shugoshin 2 (SGO2), a component of the cell division cohesion complex, is involved in both mitotic and meiotic processes. Despite being overexpressed in various malignant tumors and is associated with poor prognosis, its exact role in lung adenocarcinoma (LUAD) and its biological effects on lung cancer cells are not well understood. Methods: The transcriptomics data and clinical information for LUAD were obtained from TCGA and GEO, and DEGs associated with prognostic risk factors were screened using Cox regression analysis and chi-square testing. Identify these gene functions using correlation heatmaps, protein interaction networks (PPIs), and KEGG enrichment assays. The expression of SGO2 in tissues was verified by PCR and IHC, and the prognostic value of SGO2 in LUAD was evaluated by survival analysis. In addition, the effects of SGO2 knockdown on lung cancer cell proliferation, migration, invasion, and epithelial-to-mesenchymal transition (EMT) were studied in vitro. After that, the TIMER database and single-sample GSEA (ssGSEA) analysis were used to investigate the correlation between SGO2 and immune infiltration. Finally, the tumor mutational burden (TMB) of different SGO2 clusters and the efficacy of the two clusters in multiple treatments were evaluated. Results: High-risk genes associated with poor prognosis in LUAD are involved in cell cycle regulation and proliferation. Among these genes, SGO2 exhibited high expression in LUAD and corresponded with the TNM stage. Furthermore, the knockdown of SGO2 led to a decrease in the proliferation, migration, invasion, and EMT processes of lung cancer cells. Notably, high SGO2 expression may have poorer anti-tumor immunity and may therefore be more suitable for immunotherapy to re-establish immune function, while its high expression with a higher TMB could enable LUAD to benefit from multiple therapies. Conclusion: Our findings suggest that SGO2 may be a promising prognostic biomarker for LUAD, particularly in regulating the cell cycle and benefiting from multiple therapies.

Sustained Shugoshin 1 downregulation reduces tumor growth and metastasis in a mouse xenograft tumor model of triple-negative breast cancer

BACKGROUND

Triple-negative breast cancer (TBNC) is an aggressive breast cancer subtype with a poor prognosis. Shugoshin-1 (SGO1) protects chromatids from early separation. Previous studies from our group have demonstrated that transient SGO1 downregulation suppresses early stages of metastasis (the epithelial-to-mesenchymal transition, or EMT, cell invasion, and cell migration) in TNBC cells. Thus, the inhibition of SGO1 activity may represent a potential therapeutic intervention against cancers that progress to metastasis. Therefore, we aimed to investigate the effects of sustained shRNA-mediated SGO1 downregulation on tumor growth and metastasis in TBNC. To that end, female NOD-SCID Gamma (NSG) mice were injected with 2.5 × 10⁶ shRNA Control (n = 10) or shRNA SGO1 (n = 10) MDA-MB-231 cells. After eight weeks, the number of mice with metastasis to the lymph nodes was calculated. Primary and metastatic tumors, as well as lung and liver tissue, were harvested, measured, sectioned, and stained with hematoxylin and eosin (H&E) stain.

RESULTS

Tumor growth and metastasis to the lymph nodes and lungs were significantly reduced in the shRNA SGO1-treated mice group, while metastasis to the liver tends to be lower in cells with downregulated SGO1, but it did not reach statistical significance. Furthermore, sustained SGO1 downregulation significantly reduced cell proliferation, cell migration, and invasion which correlated with lower levels of Snail, Slug, MMP2, MMP3, and MMP9.

CONCLUSION

The supression of SGO1 activity in TNBC harboring dysregulated expression of SGO1 may be a potential target for preventing breast cancer growth and metastasis.

Parallel pathways for recruiting effector proteins determine centromere drive and suppression

Selfish centromere DNA sequences bias their transmission to the egg in female meiosis. Evolutionary theory suggests that centromere proteins evolve to suppress costs of this "centromere drive." In hybrid mouse models with genetically different maternal and paternal centromeres, selfish centromere DNA exploits a kinetochore pathway to recruit microtubule-destabilizing proteins that act as drive effectors. We show that such functional differences are suppressed by a parallel pathway for effector recruitment by heterochromatin, which is similar between centromeres in this system. Disrupting the kinetochore pathway with a divergent allele of CENP-C reduces functional differences between centromeres, whereas disrupting heterochromatin by CENP-B deletion amplifies the differences. Molecular evolution analyses using Murinae genomes identify adaptive evolution in proteins in both pathways. We propose that centromere proteins have recurrently evolved to minimize the kinetochore pathway, which is exploited by selfish DNA, relative to the heterochromatin pathway that equalizes centromeres, while maintaining essential functions.

Role of E2Fs and mitotic regulators controlled by E2Fs in the epithelial to mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex cellular process in which epithelial cells acquire mesenchymal properties. EMT occurs in three biological settings: development, wound healing and fibrosis, and tumor progression. Despite occurring in three independent biological settings, EMT signaling shares some molecular mechanisms that allow epithelial cells to de-differentiate and acquire mesenchymal characteristics that confer cells invasive and migratory capacity to distant sites. Here we summarize the molecular mechanism that delineates EMT and we will focus on the role of E2 promoter binding factors (E2Fs) in EMT during tumor progression. Since the E2Fs are presently undruggable due to their control in numerous pivotal cellular functions and due to the lack of selectivity against individual E2Fs, we will also discuss the role of three mitotic regulators and/or mitotic kinases controlled by the E2Fs (NEK2, Mps1/TTK, and SGO1) in EMT that can be useful as drug targets.

Impact statement

The study of the epithelial to mesenchymal transition (EMT) is an active area of research since it is one of the early intermediates to invasion and metastasis—a state of the cancer cells that ultimately kills many cancer patients. We will present in this review that besides their canonical roles as regulators of proliferation, unregulated expression of the E2F transcription factors may contribute to cancer initiation and progression to metastasis by signaling centrosome amplification, chromosome instability, and EMT. Since our discovery that the E2F activators control centrosome amplification and mitosis in cancer cells, we have identified centrosome and mitotic regulators that may represent actionable targets against EMT and metastasis in cancer cells. This is impactful to all of the cancer patients in which the Cdk/Rb/E2F pathway is deregulated, which has been estimated to be most cancer patients with solid tumors.

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.

Haplo-insufficiency of both BubR1 and SGO1 accelerates cellular senescence

Background

Spindle assembly checkpoint components BubR1 and Sgo1 play a key role in the maintenance of chromosomal instability during cell division. These proteins function to block the anaphase entry until all condensed chromosomes have been attached by the microtubules emanating from both spindle poles. Haplo-insufficiency of either BubR1 or SGO1 results in enhanced chromosomal instability and tumor development in the intestine. Recent studies show that spindle checkpoint proteins also have a role in slowing down the ageing process. Therefore, we want to study whether haplo-insufficiency of both BubR1 and SGO1 accelerates cellular senescence in mice.

Methods

We took advantage of the availability of BubR1 and SGO1 knockout mice and generated primary murine embryonic fibroblasts (MEFs) with mutations in either BubR1, SGO1, or both and analyzed cellular senescence of the MEFs of various genetic backgrounds.

Results

We observed that BubR1^+/−SGO^+/− MEFs had an accelerated cellular senescence characterized by morphological changes and expressed senescence-associated β-galactosidase. In addition, compared with wild-type MEFs or MEFs with a single gene deficiency, BubR1^+/−SGO1^+/− MEFs expressed enhanced levels of p21 but not p16.

Conclusions

Taken together, our observations suggest that combined deficiency of BubR1 and Sgo1 accelerates cellular senescence.

Sgo1 is a potential therapeutic target for hepatocellular carcinoma

Shugoshin-like protein 1 (Sgo1) is an essential protein in mitosis; it protects sister chromatid cohesion and thereby ensures the fidelity of chromosome separation. We found that the expression of Sgo1 mRNA was relatively low in normal tissues, but was upregulated in 82% of hepatocellular carcinoma (HCC), and correlated with elevated alpha-fetoprotein and early disease onset of HCC. The depletion of Sgo1 reduced cell viability of hepatoma cell lines including HuH7, HepG2, Hep3B, and HepaRG. Using time-lapse microscopy, we showed that hepatoma cells were delayed and ultimately die in mitosis in the absence of Sgo1. In contrast, cell viability and mitotic progression of immortalized cells were not significantly affected. Notably, mitotic cell death induced upon Sgo1 depletion was suppressed upon inhibitions of cyclin-dependent kinase-1 and Aurora kinase-B, or the depletion of mitotic arrest deficient-2. Thus, mitotic cell death induced upon Sgo1 depletion in hepatoma cells is mediated by persistent activation of the spindle assembly checkpoint. Together, these results highlight the essential role of Sgo1 in the maintenance of a proper mitotic progression in hepatoma cells and suggest that Sgo1 is a promising oncotarget for HCC.

Meiosis: an overview of key differences from mitosis

Meiosis is the specialized cell division that generates gametes. In contrast to mitosis, molecular mechanisms and regulation of meiosis are much less understood. Meiosis shares mechanisms and regulation with mitosis in many aspects, but also has critical differences from mitosis. This review highlights these differences between meiosis and mitosis. Recent studies using various model systems revealed differences in a surprisingly wide range of aspects, including cell-cycle regulation, recombination, postrecombination events, spindle assembly, chromosome-spindle interaction, and chromosome segregation. Although a great degree of diversity can be found among organisms, meiosis-specific processes, and regulation are generally conserved.

SHUGOSHINs and PATRONUS protect meiotic centromere cohesion in Arabidopsis thaliana

In meiosis, chromosome cohesion is maintained by the cohesin complex, which is released in a two-step manner. At meiosis I, the meiosis-specific cohesin subunit Rec8 is cleaved by the protease Separase along chromosome arms, allowing homologous chromosome segregation. Next, in meiosis II, cleavage of the remaining centromere cohesin results in separation of the sister chromatids. In eukaryotes, protection of centromeric cohesion in meiosis I is mediated by SHUGOSHINs (SGOs). The Arabidopsis genome contains two SGO homologs. Here we demonstrate that Atsgo1 mutants show a premature loss of cohesion of sister chromatid centromeres at anaphase I and that AtSGO2 partially rescues this loss of cohesion. In addition to SGOs, we characterize PATRONUS which is specifically required for the maintenance of cohesion of sister chromatid centromeres in meiosis II. In contrast to the Atsgo1 Atsgo2 double mutant, patronus T-DNA insertion mutants only display loss of sister chromatid cohesion after meiosis I, and additionally show disorganized spindles, resulting in defects in chromosome segregation in meiosis. This leads to reduced fertility and aneuploid offspring. Furthermore, we detect aneuploidy in sporophytic tissue, indicating a role for PATRONUS in chromosome segregation in somatic cells. Thus, ploidy stability is preserved in Arabidopsis by PATRONUS during both meiosis and mitosis.

Chromosome segregation in plant meiosis

Faithful chromosome segregation in meiosis is essential for ploidy stability over sexual life cycles. In plants, defective chromosome segregation caused by gene mutations or other factors leads to the formation of unbalanced or unreduced gametes creating aneuploid or polyploid progeny, respectively. Accurate segregation requires the coordinated execution of conserved processes occurring throughout the two meiotic cell divisions. Synapsis and recombination ensure the establishment of chiasmata that hold homologous chromosomes together allowing their correct segregation in the first meiotic division, which is also tightly regulated by cell-cycle dependent release of cohesin and monopolar attachment of sister kinetochores to microtubules. In meiosis II, bi-orientation of sister kinetochores and proper spindle orientation correctly segregate chromosomes in four haploid cells. Checkpoint mechanisms acting at kinetochores control the accuracy of kinetochore-microtubule attachment, thus ensuring the completion of segregation. Here we review the current knowledge on the processes taking place during chromosome segregation in plant meiosis, focusing on the characterization of the molecular factors involved.

Centrosome dysfunction contributes to chromosome instability, chromoanagenesis, and genome reprograming in cancer

The unique ability of centrosomes to nucleate and organize microtubules makes them unrivaled conductors of important interphase processes, such as intracellular payload traffic, cell polarity, cell locomotion, and organization of the immunologic synapse. But it is in mitosis that centrosomes loom large, for they orchestrate, with clockmaker's precision, the assembly and functioning of the mitotic spindle, ensuring the equal partitioning of the replicated genome into daughter cells. Centrosome dysfunction is inextricably linked to aneuploidy and chromosome instability, both hallmarks of cancer cells. Several aspects of centrosome function in normal and cancer cells have been molecularly characterized during the last two decades, greatly enhancing our mechanistic understanding of this tiny organelle. Whether centrosome defects alone can cause cancer, remains unanswered. Until recently, the aggregate of the evidence had suggested that centrosome dysfunction, by deregulating the fidelity of chromosome segregation, promotes and accelerates the characteristic Darwinian evolution of the cancer genome enabled by increased mutational load and/or decreased DNA repair. Very recent experimental work has shown that missegregated chromosomes resulting from centrosome dysfunction may experience extensive DNA damage, suggesting additional dimensions to the role of centrosomes in cancer. Centrosome dysfunction is particularly prevalent in tumors in which the genome has undergone extensive structural rearrangements and chromosome domain reshuffling. Ongoing gene reshuffling reprograms the genome for continuous growth, survival, and evasion of the immune system. Manipulation of molecular networks controlling centrosome function may soon become a viable target for specific therapeutic intervention in cancer, particularly since normal cells, which lack centrosome alterations, may be spared the toxicity of such therapies.

Potential role of meiosis proteins in melanoma chromosomal instability

Melanomas demonstrate chromosomal instability (CIN). In fact, CIN can be used to differentiate melanoma from benign nevi. The exact molecular mechanisms that drive CIN in melanoma have yet to be fully elucidated. Cancer/testis antigens are a unique group of germ cell proteins that are found to be primarily expressed in melanoma as compared to benign nevi. The abnormal expression of these germ cell proteins, normally expected only in the testis and ovaries, in somatic cells may lead to interference with normal cellular pathways. Germ cell proteins that may be particularly critical in CIN are meiosis proteins. Here, we review pathways unique to meiosis with a focus on how the aberrant expression of meiosis proteins in normal mitotic cells “meiomitosis” could impact chromosomal instability in melanoma and other cancers.

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.