Aurora A promotes chromosome congression by activating the condensin-dependent pool of KIF4A

Transcriptomics and proteomics reveal associations between myometrium and intrauterine adhesions

BACKGROUND

Intrauterine adhesions (IUAs) is a gynecological condition with a poor therapeutic prognosis, that severely threatens the fertility and the reproductive physiology and psychological health of women. Our previous research on the use of umbilical cord mesenchymal stem cells (HUCMSCs) for treating IUAs revealed that CM-Dil-labelled HUCMSCs were barely distributed in the endometrial epithelium. Instead, these cells were predominantly found in the myometrium, with no statistically significant difference in distribution compared to the endometrial stromal cells. Therefore, we aimed to explore the associations between the myometrium and IUAs.

METHODS

Eight patients with moderate and 5 severe lesional IUAs were included in the experimental group. The control group included 7 patients whose inner and outer myometrium were normal. We used H&E, Masson's trichrome and immunohistochemical staining to obtain the pathological features of the tissues. Transcriptomic and proteomic analyses were conducted to identify differentially expressed genes, proteins and enrichment pathways.

RESULTS

Both IUAs lesion tissues expressed the smooth muscle markers α-SMA and H-caldesmon, and there was no significant difference between severe IUAs tissue and normal myometrium (p > 0.05). Transcriptomic and proteomic data revealed that genes and proteins involved in cell mitosis, such as KIF14, KIF4A, and CIT, were downregulated in both IUAs lesion tissues compared with the inner myometrium (p < 0.05). Additionally, some genes or proteins that participate in activating the complement-coagulation cascade system and extracellular matrix (ECM) degradation also significantly differed (p < 0.05).

CONCLUSIONS

Transcriptomic and proteomic data revealed a correlation between endometrial injury and the myometrium. These findings preliminarily revealed that the myometrium possibly contributes to the aetiology and progression of IUAs through dual mechanisms. On the one hand, the myometrium inhibits endometrial regeneration by suppressing the cell mitogenic pathway. On the other hand, it promotes fibrosis by activating the complement-coagulation cascade system and inhibiting the ECM degradation pathway. These new findings increase our understanding of the pathogenesis of IUAs and potentially contribute to the application of precision clinical treatment for IUAs.

HIV-1 Vif disrupts phosphatase feedback regulation at the kinetochore, leading to a pronounced pseudo-metaphase arrest

Virion Infectivity Factor (Vif) of the Human Immunodeficiency Virus type 1 (HIV-1) targets and degrades cellular APOBEC3 proteins, key regulators of intrinsic and innate antiretroviral immune responses, thereby facilitating HIV-1 infection. While Vif's role in degrading APOBEC3G is well-studied, Vif is also known to cause cell cycle arrest, but the detailed nature of Vif's effects on the cell cycle has yet to be delineated. In this study, we employed high-temporal resolution single-cell live imaging and super-resolution microscopy to monitor individual cells during Vif-induced cell cycle arrest. Our findings reveal that Vif does not affect the G2/M boundary as previously thought. Instead, Vif triggers a unique and robust pseudo-metaphase arrest, distinct from the mild prometaphase arrest induced by Vpr. During this arrest, chromosomes align properly and form the metaphase plate, but later lose alignment, resulting in polar chromosomes. Notably, Vif, unlike Vpr, significantly reduces the levels of both Protein Phosphatase 1 (PP1) and 2 A (PP2A) at kinetochores, which regulate chromosome-microtubule interactions. These results unveil a novel role for Vif in kinetochore regulation that governs the spatial organization of chromosomes during mitosis.

Molecular mechanism targeting condensin for chromosome condensation

Genomes are organised into DNA loops by the Structural Maintenance of Chromosomes (SMC) proteins. SMCs establish functional chromosomal sub-domains for DNA repair, gene expression and chromosome segregation, but how SMC activity is specifically targeted is unclear. Here, we define the molecular mechanism targeting the condensin SMC complex to specific chromosomal regions in budding yeast. A conserved pocket on the condensin HAWK subunit Ycg1 binds to chromosomal receptors carrying a related motif, CR1. In early mitosis, CR1 motifs in receptors Sgo1 and Lrs4 recruit condensin to pericentromeres and rDNA, to facilitate sister kinetochore biorientation and rDNA condensation, respectively. We additionally find that chromosome arm condensation begins as sister kinetochores come under tension, in a manner dependent on the Ycg1 pocket. We propose that multiple CR1-containing proteins recruit condensin to chromosomes and identify several additional candidates based on their sequence. Overall, we uncover the molecular mechanism that targets condensin to functionalise chromosomal domains to achieve accurate chromosome segregation during mitosis.

Molecular mechanism of condensin I activation by KIF4A

During mitosis, the condensin I and II complexes compact chromatin into chromosomes. Loss of the chromokinesin, KIF4A, results in reduced condensin I association with chromosomes, but the molecular mechanism behind this phenotype is unknown. In this study, we reveal that KIF4A binds directly to the human condensin I HAWK subunit, NCAPG, via a conserved disordered short linear motif (SLiM) located in its C-terminal tail. KIF4A competes for NCAPG binding to an overlapping site with SLiMs at the N-terminus of NCAPH and the C-terminus of NCAPD2, which mediate two auto-inhibitory interactions within condensin I. Consistently, the KIF4A SLiM peptide alone is sufficient to stimulate ATPase and DNA loop extrusion activities of condensin I. We identify similar SLiMs in the known yeast condensin interactors, Sgo1 and Lrs4, which bind yeast condensin subunit, Ycg1, the equivalent HAWK to NCAPG. Our findings, together with previous work on condensin II and cohesin, demonstrate that SLiM binding to the NCAPG-equivalent HAWK subunit is a conserved mechanism of regulation in SMC complexes.

HIV-1 Vif disrupts phosphatase feedback regulation at the kinetochore, leading to a pronounced pseudo-metaphase arrest

Virion Infectivity Factor (Vif) of the Human Immunodeficiency Virus type 1 (HIV-1) targets and degrades cellular APOBEC3 proteins, key regulators of intrinsic and innate antiretroviral immune responses, thereby facilitating HIV-1 infection. While Vif's role in degrading APOBEC3G is well-studied, Vif is also known to cause cell cycle arrest, but the detailed nature of Vif's effects on the cell cycle has yet to be delineated. In this study, we employed high-temporal single-cell live imaging and super-resolution microscopy to monitor individual cells during Vif-induced cell cycle arrest. Our findings reveal that Vif does not affect the G2/M boundary as previously thought. Instead, Vif triggers a unique and robust pseudo-metaphase arrest, distinct from the mild prometaphase arrest induced by Vpr. During this arrest, chromosomes align properly and form the metaphase plate, but later lose alignment, resulting in polar chromosomes. Notably, Vif, unlike Vpr, significantly reduces the levels of both Protein Phosphatase 1 (PP1) and 2A (PP2A) at kinetochores, which regulate chromosome-microtubule interactions. These results unveil a novel role for Vif in kinetochore regulation that governs the spatial organization of chromosomes during mitosis.

Regulation of minimal spindle midzone organization by mitotic kinases

During cell division, the microtubule cytoskeleton undergoes dramatic cell cycle-driven reorganizations of its architecture. Coordinated by changes in the phosphorylation patterns of a multitude of microtubule associated proteins, the mitotic spindle first self-assembles to capture the chromosomes and then reorganizes in anaphase as the chromosomes are segregated. A key protein for this reorganization is PRC1 which is differentially phosphorylated by the mitotic kinases CDK1 and PLK1. How the phosphorylation state of PRC1 orchestrates spindle reorganization is not understood mechanistically. Here, we reconstitute in vitro the transition between metaphase and anaphase-like microtubule architectures triggered by the changes in PRC1 phosphorylation. We find that whereas PLK1 regulates its own recruitment by PRC1, CDK1 controls the affinity of PRC1 for antiparallel microtubule binding. Dephosphorylation of CDK1-phosphorylated PRC1 is required and sufficient to trigger the reorganization of a minimal anaphase midzone in the presence of the midzone length controlling kinesin KIF4A. These results demonstrate how phosphorylation-controlled affinity changes regulate the architecture of active microtubule networks, providing new insight into the mechanistic underpinnings of the cell cycle-driven reorganization of the central spindle during mitosis.

Unraveling the rhizobial infection thread

Most legumes can form an endosymbiotic association with soil bacteria called rhizobia, which colonize specialized root structures called nodules where they fix nitrogen. To colonize nodule cells, rhizobia must first traverse the epidermis and outer cortical cell layers of the root. In most legumes, this involves formation of the infection thread, an intracellular structure that becomes colonized by rhizobia, guiding their passage through the outer cell layers of the root and into the newly formed nodule cells. In this brief review, we recount the early research milestones relating to the rhizobial infection thread and highlight two relatively recent advances in the symbiotic infection mechanism, the eukaryotically conserved 'MYB-AUR1-MAP' mitotic module, which links cytokinesis mechanisms to intracellular infection, and the discovery of the 'infectosome' complex, which guides infection thread growth. We also discuss the potential intertwining of the two modules and the hypothesis that cytokinesis served as a foundation for intracellular infection of symbiotic microbes.

Phosphorylation controls spatial and temporal activities of motor-PRC1 complexes to complete mitosis

During mitosis, spindle architecture alters as chromosomes segregate into daughter cells. The microtubule crosslinker protein regulator of cytokinesis 1 (PRC1) is essential for spindle stability, chromosome segregation and completion of cytokinesis, but how it recruits motors to the central spindle to coordinate the segregation of chromosomes is unknown. Here, we combine structural and cell biology approaches to show that the human CENP-E motor, which is essential for chromosome capture and alignment by microtubules, binds to PRC1 through a conserved hydrophobic motif. This binding mechanism is also used by Kinesin-4 Kif4A:PRC1. Using in vitro reconstitution, we demonstrate that CENP-E slides antiparallel PRC1-crosslinked microtubules. We find that the regulation of CENP-E -PRC1 interaction is spatially and temporally coupled with relocalization to overlapping microtubules in anaphase. Finally, we demonstrate that the PRC1-microtubule motor interaction is essential in anaphase to control chromosome partitioning, retain central spindle integrity and ensure cytokinesis. Taken together our findings reveal the molecular basis for the cell cycle regulation of motor-PRC1 complexes to couple chromosome segregation and cytokinesis.

PP6 regulation of Aurora A-TPX2 limits NDC80 phosphorylation and mitotic spindle size

Amplification of the mitotic kinase Aurora A or loss of its regulator protein phosphatase 6 (PP6) have emerged as drivers of genome instability. Cells lacking PPP6C, the catalytic subunit of PP6, have amplified Aurora A activity, and as we show here, enlarged mitotic spindles which fail to hold chromosomes tightly together in anaphase, causing defective nuclear structure. Using functional genomics to shed light on the processes underpinning these changes, we discover synthetic lethality between PPP6C and the kinetochore protein NDC80. We find that NDC80 is phosphorylated on multiple N-terminal sites during spindle formation by Aurora A-TPX2, exclusively at checkpoint-silenced, microtubule-attached kinetochores. NDC80 phosphorylation persists until spindle disassembly in telophase, is increased in PPP6C knockout cells, and is Aurora B-independent. An Aurora-phosphorylation-deficient NDC80-9A mutant reduces spindle size and suppresses defective nuclear structure in PPP6C knockout cells. In regulating NDC80 phosphorylation by Aurora A-TPX2, PP6 plays an important role in mitotic spindle formation and size control and thus the fidelity of cell division.

Regulation of the mitotic chromosome folding machines

Over the last several years enormous progress has been made in identifying the molecular machines, including condensins and topoisomerases that fold mitotic chromosomes. The discovery that condensins generate chromatin loops through loop extrusion has revolutionized, and energized, the field of chromosome folding. To understand how these machines fold chromosomes with the appropriate dimensions, while disentangling sister chromatids, it needs to be determined how they are regulated and deployed. Here, we outline the current understanding of how these machines and factors are regulated through cell cycle dependent expression, chromatin localization, activation and inactivation through post-translational modifications, and through associations with each other, with other factors and with the chromatin template itself. There are still many open questions about how condensins and topoisomerases are regulated but given the pace of progress in the chromosome folding field, it seems likely that many of these will be answered in the years ahead.

Phosphoregulation of Kinesins Involved in Long-Range Intracellular Transport

Kinesins, the microtubule-dependent mechanochemical enzymes, power a variety of intracellular movements. Regulation of Kinesin activity and Kinesin-Cargo interactions determine the direction, timing and flux of various intracellular transports. This review examines how phosphorylation of Kinesin subunits and adaptors influence the traffic driven by Kinesin-1, -2, and -3 family motors. Each family of Kinesins are phosphorylated by a partially overlapping set of serine/threonine kinases, and each event produces a unique outcome. For example, phosphorylation of the motor domain inhibits motility, and that of the stalk and tail domains induces cargo loading and unloading effects according to the residue and context. Also, the association of accessory subunits with cargo and adaptor proteins with the motor, respectively, is disrupted by phosphorylation. In some instances, phosphorylation by the same kinase on different Kinesins elicited opposite outcomes. We discuss how this diverse range of effects could manage the logistics of Kinesin-dependent, long-range intracellular transport.

MKLP2 functions in early mitosis to ensure proper chromosome congression

Mitotic kinesin-like protein 2 (MKLP2) is a motor protein with a well-established function in promoting cytokinesis. However, our results with siRNAs targeting MKLP2 and small molecule inhibitors of MKLP2 (MKLP2i) suggested a function earlier in mitosis, prior to anaphase. In this study we provide direct evidence that MKLP2 facilitates chromosome congression in prometaphase. We employed live imaging to observe HeLa cells with fluorescently tagged histones treated with MKLP2i and discovered a pronounced chromosome congression defect. We show that MKLP2 facilitates error correction as inhibited cells had a significant increase in unstable, syntelic kinetochore-microtubule attachments. We find that the aberrant attachments are accompanied by elevated Aurora Kinase (A/B) activity and phosphorylation of the downstream target, pHEC1 (Ser 55). Lastly, we show that MKLP2 inhibition results in aneuploidy, confirming that MKLP2 safeguards cells against chromosomal instability.

Protein regulator of cytokinesis 1 regulates chromosome dynamics and cytoplasmic division during mouse oocyte meiotic maturation and early embryonic development

In contrast to the homeokinesis of mitosis, asymmetric division of cytoplasm is the conspicuous feature of meiosis in mammalian oocytes. Protein regulator of cytokinesis 1 (PRC1) is an important regulator during mitotic spindle assembly and cytoplasmic division, but its functions in oocyte meiosis and early embryo development have not been fully elucidated. In this study, we detected PRC1 expression and localization and revealed a nuclear, spindle midzone-related dynamic pattern throughout meiotic and mitotic progressions. Treatment of oocytes with the reagents taxol or nocodazole disturbed the distribution of PRC1 in metaphase II oocytes. Further, PRC1 depletion led to failure of first polar body (PB1) extrusion and spindle migration, aneuploidy and defective kinetochore-microtubule attachment and spindle assembly. Overexpression of PRC1 resulted in PB1 extrusion failure, aneuploidy and serious defects of spindle assembly. To investigate PRC1 function in early embryos, we injected Prc1 morpholino into zygotes and 2-cell stage embryos. Depletion of PRC1 in zygotes impaired 4-cell, morula and blastocyst formation. Loss of PRC1 in single or double blastomeres in 2-cell stage embryos significantly impaired cell division, indicating its indispensable role in early embryo development. Co-immunoprecipitation showed that PRC1 interacts with polo-like kinase 1 (PLK1), and functional knockdown and rescue experiments demonstrated that PRC1 recruits PLK1 to the spindle midzone to regulate cytoplasmic division during meiosis. Finally, kinesin family member 4 knockdown downregulates PRC1 expression and leads to PRC1 localization failure. Taken together, our data suggest PRC1 plays an important role during oocyte maturation and early embryonic development by regulating chromosome dynamics and cytoplasmic division.

Mitotic chromosomes

Our understanding of the structure and function of mitotic chromosomes has come a long way since these iconic objects were first recognized more than 140 years ago, though many details remain to be elucidated. In this chapter, we start with the early history of chromosome studies and then describe the path that led to our current understanding of the formation and structure of mitotic chromosomes. We also discuss some of the remaining questions. It is now well established that each mitotic chromatid consists of a central organizing region containing a so-called "chromosome scaffold" from which loops of DNA project radially. Only a few key non-histone proteins and protein complexes are required to form the chromosome: topoisomerase IIα, cohesin, condensin I and condensin II, and the chromokinesin KIF4A. These proteins are concentrated along the axis of the chromatid. Condensins I and II are primarily responsible for shaping the chromosome and the scaffold, and they produce the loops of DNA by an ATP-dependent process known as loop extrusion. Modelling of Hi-C data suggests that condensin II adopts a spiral staircase arrangement with an extruded loop extending out from each step in a roughly helical pattern. Condensin I then forms loops nested within these larger condensin II loops, thereby giving rise to the final compaction of the mitotic chromosome in a process that requires Topo IIα.

From the cytoskeleton to the nucleus: An integrated view on early spindle assembly

Accurate chromosome segregation requires a complete restructuring of cellular organization. Microtubules remodel to assemble a mitotic spindle and the actin cytoskeleton rearranges to form a stiff actomyosin cortex. These cytoplasmic events must be spatially and temporally coordinated with mitotic chromosome condensation and nuclear envelope permeabilization, in order to ensure mitotic timing and fidelity. Here, we discuss the main cytoskeletal and nuclear events that occur during mitotic entry in proliferating animal cells, focusing on their coordinated contribution for early mitotic spindle assembly. We will also explore recent progress in understanding their regulatory biochemical and mechanical pathways.

The tyrosine kinase v-Src modifies cytotoxicities of anticancer drugs targeting cell division

v-Src oncogene causes cell transformation through its strong tyrosine kinase activity. We have revealed that v-Src-mediated cell transformation occurs at a low frequency and it is attributed to mitotic abnormalities-mediated chromosome instability. v-Src directly phosphorylates Tyr-15 of cyclin-dependent kinase 1 (CDK1), thereby causing mitotic slippage and reduction in Eg5 inhibitor cytotoxicity. However, it is not clear whether v-Src modifies cytotoxicities of the other anticancer drugs targeting cell division. In this study, we found that v-Src restores cancer cell viability reduced by various microtubule-targeting agents (MTAs), although v-Src does not alter cytotoxicity of DNA-damaging anticancer drugs. v-Src causes mitotic slippage of MTAs-treated cells, consequently generating proliferating tetraploid cells. We further demonstrate that v-Src also restores cell viability reduced by a polo-like kinase 1 (PLK1) inhibitor. Interestingly, treatment with Aurora kinase inhibitor strongly induces cell death when cells express v-Src. These results suggest that the v-Src modifies cytotoxicities of anticancer drugs targeting cell division. Highly activated Src-induced resistance to MTAs through mitotic slippage might have a risk to enhance the malignancy of cancer cells through the increase in chromosome instability upon chemotherapy using MTAs.

Upregulation of centromere protein M promotes tumorigenesis: A potential predictive target for cancer in humans

Centromere protein M (CENPM), a protein required for chromosome separation, is involved in in mitosis. However, little has been reported about the roles of CENPM in various types of cancer. The present study identified that the mRNA expression levels of CENPM were significantly upregulated in 14 types of human cancer and identified a positive association between CENPM mRNA expression and patient mortality using the Oncomine, Gene Expression Profiling Interactive Analysis, Human Protein Atlas and Kaplan‑Meier Plotter databases. A protein interaction network constructed with CENPM‑interacting genes obtained from the cBioPortal demonstrated that nine genes participating in the cell cycle served key roles in the function of CENPM. Cell cycle analysis, reverse transcription‑quantitative polymerase chain reaction, a Cell Counting Kit‑8‑based proliferation assay and a terminal deoxynucleotidyl transferase dUTP nick end labelling assay further revealed the tumorigenic and carcinogenic roles of CENPM in vitro. In addition, it was identified that the mRNA expression levels of five of the nine identified genes were significantly associated with CENPM in MCF7 cells and that CENPM was rarely mutated among various types of human cancer. In conclusion, the data from the present study revealed that CENPM exerted its pro‑tumorigenic function by regulating cell cycle‑associated protein expression and suggested that CENPM could be used as a prognostic marker for breast cancer.

A dynamic charge-charge interaction modulates PP2A:B56 substrate recruitment

The recruitment of substrates by the ser/thr protein phosphatase 2A (PP2A) is poorly understood, limiting our understanding of PP2A-regulated signaling. Recently, the first PP2A:B56 consensus binding motif, LxxIxE, was identified. However, most validated LxxIxE motifs bind PP2A:B56 with micromolar affinities, suggesting that additional motifs exist to enhance PP2A:B56 binding. Here, we report the requirement of a positively charged motif in a subset of PP2A:B56 interactors, including KIF4A, to facilitate B56 binding via dynamic, electrostatic interactions. Using molecular and cellular experiments, we show that a conserved, negatively charged groove on B56 mediates dynamic binding. We also discovered that this positively charged motif, in addition to facilitating KIF4A dephosphorylation, is essential for condensin I binding, a function distinct and exclusive from PP2A-B56 binding. Together, these results reveal how dynamic, charge-charge interactions fine-tune the interactions mediated by specific motifs, providing a new framework for understanding how PP2A regulation drives cellular signaling.