Paclitaxel-resistant triple negative breast cancer (TNBC) remains one of the most challenging breast cancers to treat. Here, using an epigenetic chemical probe screen, we uncover an acquired vulnerability of paclitaxel-resistant TNBC cells to protein arginine methyltransferases (PRMTs) inhibition. Analysis of cell lines and in-house clinical samples demonstrates that resistant cells evade paclitaxel killing through stabilizing mitotic chromatin assembly. Genetic or pharmacologic inhibition of PRMT5 alters RNA splicing, particularly intron retention of aurora kinases B (AURKB), leading to a decrease in protein expression, and finally results in selective mitosis catastrophe in paclitaxel-resistant cells. In addition, type I PRMT inhibition synergies with PRMT5 inhibition in suppressing tumor growth of drug-resistant cells through augmenting perturbation of AURKB-mediated mitotic signaling pathway. These findings are fully recapitulated in a patient-derived xenograft (PDX) model generated from a paclitaxel-resistant TNBC patient, providing the rationale for targeting PRMTs in paclitaxel-resistant TNBC.

Few transcription factors have been examined for their direct roles in physically connecting enhancers and promoters. Here acute degradation of Yin Yang 1 (YY1) in erythroid cells revealed its requirement for the maintenance of numerous enhancer-promoter loops, but not compartments or domains. Despite its reported ability to interact with cohesin, the formation of YY1-dependent enhancer-promoter loops does not involve stalling of cohesin-mediated loop extrusion. Integrating mitosis-to-G1-phase dynamics, we observed partial retention of YY1 on mitotic chromatin, predominantly at gene promoters, followed by rapid rebinding during mitotic exit, coinciding with enhancer-promoter loop establishment. YY1 degradation during the mitosis-to-G1-phase interval revealed a set of enhancer-promoter loops that require YY1 for establishment during G1-phase entry but not for maintenance in interphase, suggesting that cell cycle stage influences YY1\'s architectural function. Thus, as revealed here for YY1, chromatin architectural functions of transcription factors can vary in their interplay with CTCF and cohesin as well as by cell cycle stage.

Breast cancer (BC) is one of the most common types of cancer among women worldwide. Lycorine (Lycoris radiata), a small molecule derived from the traditional Chinese herb Amaryllidaceae plants, has appeared potential effect on inhibiting the growth of cancer cells and inducing apoptosis in various types of cancer with minor side effects. To discuss the therapeutic effects and molecular mechanisms of lycorine on BC established by lycorine-treated S180 tumour-bearing mice in vivo. Furthermore, both the mitotic and microtubule assembly dynamics genes were performed by qPCR assays, and the protein expression associated with mitotic arrest was investigated by western blot. Lycorine was demonstrated to reduce sarcoma growth of S180 tumour-bearing mice and inhibit the proliferation of MCF-7 cells in concentration-dependent manner. Moreover, lycorine induced M phase cell cycle arrest via interfering with the mitotic apparatus regulated the expression of 20 genes and 15 proteins in cell cycle progression. Furthermore, this study confirmed that the potential effect of lycorine on BC might be mediated by cell cycle arrest in M phase for the first time. These results would be the consequence of exploitation of lycorine as a potential drug for BC therapy, however further preclinical and clinical studies are still needed.

BACKGROUND: Chromosome stability is crucial for homeostasis of pluripotent stem cells (PSCs) and early-stage embryonic development. Chromosomal defects may raise carcinogenic risks in regenerative medicine when using PSCs as original materials. However, the detailed mechanism regarding PSCs chromosome stability maintenance is not fully understood.
METHODS: Mouse embryonic stem cells (line D3) and human embryonic stem cells (line H9) were cultured under standard conditions. To confirm the loading of RetSat protein on mitotic chromosomes of PSCs, immunostaining was performed in PSCs spontaneous differentiation assay and iPSC reprogramming assay from mouse embryonic fibroblasts (MEFs), respectively. In addition, qPCR, immunoprecipitation, LC-MS/MS and immunoblotting were used to study the expression of RetSat, and interactions of RetSat with cohesin/condensin components. RNA sequencing and teratoma formation assay was conducted to evaluate the carcinogenic risk of mouse embryonic stem cells with RetSat deletion.
RESULTS: We reported a PSC high-expressing gene, RetSat, plays key roles in chromosome stabilization. We identified RetSat protein localizing onto mitotic chromosomes specifically in stemness positive cells such as embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). We found dramatic chromosome instability, e.g. chromosome bridging, lagging and interphase micronuclei in mouse and human ESCs when down regulating RetSat. RetSat knock-out mouse ESCs upregulated cancer associated gene pathways, and displayed higher tumorigenic capacities in teratoma formation assay. Mechanistically, we confirmed that RetSat interacts with cohesin/condensin components Smc1a and Nudcd2. RetSat deletion impaired the chromosome loading dosage of Smc1a, Smc3 and Nudcd2.
CONCLUSIONS: In summary, we reported RetSat to be a key stabilizer of chromosome condensation in pluripotent stem cells. This highlights the crucial roles of RetSat in early-stage embryonic development, and potential value of RetSat as an effective biomarker for assessing the quality of pluripotent stem cells.

The acentrosomal spindle apparatus has kinetochore fibers organized and converged toward opposite poles; however, mechanisms underlying the organization of these microtubule fibers into an orchestrated bipolar array were largely unknown. Kinesin-14D is one of the four classes of Kinesin-14 motors that are conserved from green algae to flowering plants. In Arabidopsis thaliana, three Kinesin-14D members displayed distinct cell cycle-dependent localization patterns on spindle microtubules in mitosis. Notably, Kinesin-14D1 was enriched on the midzone microtubules of prophase and mitotic spindles and later persisted in the spindle and phragmoplast midzones. The kinesin-14d1 mutant had kinetochore fibers disengaged from each other during mitosis and exhibited hypersensitivity to the microtubule-depolymerizing herbicide oryzalin. Oryzalin-treated kinesin-14d1 mutant cells had kinetochore fibers tangled together in collapsed spindle microtubule arrays. Kinesin-14D1, unlike other Kinesin-14 motors, showed slow microtubule plus end-directed motility, and its localization and function were dependent on its motor activity and the novel malectin-like domain. Our findings revealed a Kinesin-14D1-dependent mechanism that employs interpolar microtubules to regulate the organization of kinetochore fibers for acentrosomal spindle morphogenesis.

Heterochromatin de-condensation in companion gametic cells is conserved in both plants and animals. In plants, microspore undergoes asymmetric pollen mitosis (PMI) to produce a vegetative cell (VC) and a generative cell (GC). Subsequently, the GC undergoes pollen mitosis (PMII) to produce two sperm cells (SC). Consistent with heterochromatin de-condensation in the VC, H3K9me2, a heterochromatin mark, is barely detected in VC. However, how H3K9me2 is differentially regulated during pollen mitosis remains unclear. Here, we show that H3K9me2 is gradually evicted from the VC since PMI but remain unchanged in the GC and SC. ARID1, a pollen-specific transcription factor that facilitates PMII, promotes H3K9me2 maintenance in the GC/SC but slows down its eviction in the VC. The genomic targets of ARID1 mostly overlaps with H3K9me2 loci, and ARID1 recruits H3K9 methyltransferase SUVH6. Our results uncover that differential pattern of H3K9me2 between two cell types is regulated by ARID1 during pollen mitosis.

The ring-shaped Cohesin complex, consisting of core subunits Smc1, Smc3, Scc1, and SA2 (or its paralog SA1), topologically entraps two duplicated sister DNA molecules to establish sister chromatid cohesion in S-phase. It remains largely elusive how the Cohesin release factor Wapl binds the Cohesin complex, thereby inducing Cohesin disassociation from mitotic chromosomes to allow proper resolution and separation of sister chromatids. Here, we show that Wapl uses two structural modules containing the FGF motif and the YNARHWN motif, respectively, to simultaneously bind distinct pockets in the extensive composite interface between Scc1 and SA2. Strikingly, only when both docking modules are mutated, Wapl completely loses the ability to bind the Scc1-SA2 interface and release Cohesin, leading to erroneous chromosome segregation in mitosis. Surprisingly, Sororin, which contains a conserved FGF motif and functions as a master antagonist of Wapl in S-phase and G2-phase, does not bind the Scc1-SA2 interface. Moreover, Sgo1, the major protector of Cohesin at mitotic centromeres, can only compete with the FGF motif but not the YNARHWN motif of Wapl for binding Scc1-SA2 interface. Our data uncover the molecular mechanism by which Wapl binds Cohesin to ensure precise chromosome segregation.

UFMylation is a highly conserved ubiquitin-like post-translational modification that catalyzes the covalent linkage of UFM1 to its target proteins. This modification plays a critical role in the maintenance of endoplasmic reticulum proteostasis, DNA damage response, autophagy, and transcriptional regulation. Mutations in UFM1, as well as in its specific E1 enzyme UBA5 and E2 enzyme UFC1, have been genetically linked to microcephaly. Our previous research unveiled the important role of UFMylation in regulating mitosis. However, the underlying mechanisms have remained unclear due to the limited identification of substrates. In this study, we identified Eg5, a motor protein crucial for mitotic spindle assembly and maintenance, as a novel substrate for UFMylation and identified Lys564 as the crucial UFMylation site. UFMylation did not alter its transcriptional level, phosphorylation level, or protein stability, but affected the mono-ubiquitination of Eg5. During mitosis, Eg5 and UFM1 co-localize at the centrosome and spindle apparatus, and defective UFMylation leads to diminished spindle localization of Eg5. Notably, the UFMylation-defective Eg5 mutant (K564R) exhibited shorter spindles, metaphase arrest, spindle checkpoint activation, and a failure of cell division in HeLa cells. Overall, Eg5 UFMylation is essential for proper spindle organization, mitotic progression, and cell proliferation.

BACKGROUND: Aurora kinase B (Aurora-B), a member of the chromosomal passenger complex, is involved in correcting kinetochore-microtubule (KT-MT) attachment errors and regulating sister chromatid condensation and cytoplasmic division during mitosis.
CONCLUSIONS: However, few reviews have discussed its mechanism in oocyte meiosis and the differences between its role in mitosis and meiosis. Therefore, in this review, we summarize the localization, recruitment, activation, and functions of Aurora-B in mitosis and oocyte meiosis. The accurate regulation of Aurora-B is essential for ensuring accurate chromosomal segregation and correct KT-MT attachments. Aurora-B regulates the stability of KT-MT attachments by competing with cyclin-dependent kinase 1 to control the phosphorylation of the SILK and RVSF motifs on kinetochore scaffold 1 and by competing with protein phosphatase 1 to influence the phosphorylation of NDC80 which is the substrate of Aurora-B. In addition, Aurora-B regulates the spindle assembly checkpoint by promoting the recruitment and activation of mitotic arrest deficient 2.
CONCLUSIONS: This review provides a theoretical foundation for elucidating the mechanism of cell division and understanding oocyte chromosomal aneuploidy.

BACKGROUND: The mitosis-to-meiosis switch during spermatogenesis requires dynamic changes in gene expression. However, the regulation of meiotic transcriptional and post-transcriptional machinery during this transition remains elusive.
RESULTS: We report that methyltransferase-like protein 16 (METTL16), an N6-methyladenosine (m6A) writer, is required for mitosis-to-meiosis transition during spermatogenesis. Germline conditional knockout of Mettl16 in male mice impairs spermatogonial differentiation and meiosis initiation. Mechanistically, METTL16 interacts with splicing factors to regulate the alternative splicing of meiosis-related genes such as Stag3. Ribosome profiling reveals that the translation efficiency of many meiotic genes is dysregulated in METTL16-deficient testes. m6A-sequencing shows that ablation of METTL16 causes upregulation of the m6A-enriched transcripts and downregulation of the m6A-depleted transcripts, similar to Meioc and/or Ythdc2 mutants. Further in vivo and in vitro experiments demonstrate that the methyltransferase activity site (PP185-186AA) of METTL16 is necessary for spermatogenesis.
CONCLUSIONS: Our findings support a molecular model wherein the m6A writer METTL16-mediated alternative splicing and translation efficiency regulation are required to control the mitosis-to-meiosis germ cell fate decision in mice, with implications for understanding meiosis-related male fertility disorders.