Inhibition of the ATR-DNAPKcs-RB axis drives G1/S-phase transition and sensitizes triple-negative breast cancer (TNBC) to DNA holliday junctions.

Targeting the DNA damage response (DDR) is a promising strategy in oncotherapy, as most tumor cells are sensitive to excess damage due to their repair defects. Ataxia telangiectasia mutated and RAD3-related protein (ATR) is a damage response signal transduction sensor, and its therapeutic potential in tumor cells needs to be precisely investigated. Herein, we identified a new axis that could be targeted by ATR inhibitors to decrease the DNA-dependent protein kinase catalytic subunit (DNAPKcs), downregulate the expression of the retinoblastoma (RB), and drive G1/S-phase transition. Four-way DNA Holliday junctions (FJs) assembled in this process could trigger S-phase arrest and induce lethal chromosome damage in RB-positive triple-negative breast cancer (TNBC) cells. Furthermore, these unrepaired junctions also exerted toxic effects to RB-deficient TNBC cells when the homologous recombination repair (HRR) was inhibited. This study proposes a precise strategy for treating TNBC by targeting the DDR and extends our understanding of ATR and HJ in tumor treatment.

Human α-defensin 6 (HD6) suppresses CRC proliferation and metastasis through abolished EGF/EGFR signaling pathway.

The incidence of colorectal cancer (CRC) has increased significantly in the past decade. Early diagnosis and new therapeutics are still urgently needed for CRC in clinical practice. Human α-defensin 6 (HD6) plays a defense role against microbes in the gastrointestinal tract. However, the role and mechanism of HD6 in CRC is still unresolved. Specimens from CRC patients with higher HD6 showed better outcomes. Overexpressed HD6 in CRC cells caused a reduction of cell proliferative, migratory, and invasive ability in vitro and in vivo. HD6-overexpressed caused S phase arrest through changes in cyclin-A and B and CDK2 levels. In addition, serpine-1 may be negatively regulated by HD6 altering the translocation of c-Jun N-terminal kinases (JNK), extracellular regulated protein kinases (ERK), and p38. Higher HD6 and lower serpine-1 levels in CRC patients reflected better outcomes. Finally, we found that HD6 interacts directly with epidermal growth factor receptor (EGFR) by co-immunoprecipitated assay. EGF treatment caused an increase of the level of serpine-1 and pEGFR levels and then increased growth activity in HD6 overexpressing cells. Together, our study shows that HD6 may compete with EGF to bind to EGFR and interrupt cancer progression in CRC. We believe these findings may give new insights for HD6 in CRC therapy.

Structural characterisation of the Chaetomium thermophilum Chl1 helicase.

Chl1 is a member of the XPD family of 5'-3' DNA helicases, which perform a variety of roles in genome maintenance and transmission. They possess a variety of unique structural features, including the presence of a highly variable, partially-ordered insertion in the helicase domain 1. Chl1 has been shown to be required for chromosome segregation in yeast due to its role in the formation of persistent chromosome cohesion during S-phase. Here we present structural and biochemical data to show that Chl1 has the same overall domain organisation as other members of the XPD family, but with some conformational alterations. We also present data suggesting the insert domain in Chl1 regulates its DNA binding.

PLEKHA4 Promotes Wnt/ß-Catenin Signaling-Mediated G1-S Transition and Proliferation in Melanoma.

Despite recent promising advances in targeted therapies and immunotherapies, patients with melanoma incur substantial mortality. In particular, inhibitors targeting BRAF-mutant melanoma can lead to resistance, and no targeted therapies exist for NRAS-mutant melanoma, motivating the search for additional therapeutic targets and vulnerable pathways. Here we identify a regulator of Wnt/ß-catenin signaling, PLEKHA4, as a factor required for melanoma proliferation and survival. PLEKHA4 knockdown in vitro decreased Dishevelled levels, attenuated Wnt/ß-catenin signaling, and blocked progression through the G1-S cell-cycle transition. In mouse xenograft and allograft models, inducible PLEKHA4 knockdown attenuated tumor growth in BRAF- and NRAS-mutant melanomas and exhibited an additive effect with the clinically used inhibitor encorafenib in a BRAF-mutant model. As an E3 ubiquitin ligase regulator with both lipid- and protein-binding partners, PLEKHA4 presents several opportunities for targeting with small molecules. Our work identifies PLEKHA4 as a promising drug target for melanoma and clarifies a controversial role for Wnt/ß-catenin signaling in the control of melanoma proliferation. SIGNIFICANCE: This study establishes that melanoma cell proliferation requires the protein PLEKHA4 to promote pathologic Wnt signaling for proliferation, highlighting PLEKHA4 inhibition as a new avenue for the development of targeted therapies.

Checkpoint inhibition of origin firing prevents inappropriate replication outside of S-phase.

Checkpoints maintain the order of cell cycle events during DNA damage or incomplete replication. How the checkpoint response is tailored to different phases of the cell cycle remains poorly understood. The S-phase checkpoint for example results in the slowing of replication, which in budding yeast occurs by Rad53-dependent inhibition of the initiation factors Sld3 and Dbf4. Despite this, we show here that Rad53 phosphorylates both of these substrates throughout the cell cycle at the same sites as in S-phase, suggesting roles for this pathway beyond S-phase. Indeed, we show that Rad53-dependent inhibition of Sld3 and Dbf4 limits re-replication in G2/M, preventing gene amplification. In addition, we show that inhibition of Sld3 and Dbf4 in G1 prevents premature initiation at all origins at the G1/S transition. This study redefines the scope of the 'S-phase checkpoint' with implications for understanding checkpoint function in cancers that lack cell cycle controls.

A new linear cyclin docking motif that mediates exclusively S-phase CDK-specific signaling.

Cyclin-dependent kinases (CDKs), the master regulators of cell division, are activated by different cyclins at different cell cycle stages. In addition to being activators of CDKs, cyclins recognize various linear motifs to target CDK activity to specific proteins. We uncovered a cyclin docking motif, NLxxxL, that contributes to phosphorylation-dependent degradation of the CDK inhibitor Far1 at the G1/S stage in the yeast Saccharomyces cerevisiae. This motif is recognized exclusively by S-phase CDK (S-CDK) Clb5/6-Cdc28 and is considerably more potent than the conventional RxL docking motif. The NLxxxL and RxL motifs were found to overlap in some target proteins, suggesting that cyclin docking motifs can evolve to switch from one to another for fine-tuning of cell cycle events. Using time-lapse fluorescence microscopy, we show how different docking connections temporally control phosphorylation-driven target degradation. This also revealed a differential function of the phosphoadaptor protein Cks1, as Cks1 docking potentiated degron phosphorylation of RxL-containing but not of NLxxxL-containing substrates. The NLxxxL motif was found to govern S-cyclin-specificity in multiple yeast CDK targets including Fin1, Lif1, and Slx4, suggesting its wider importance.

Baicalein flavone targets cisplatin resistant human pancreatic cancer cells via inducing S-phase cell cycle arrest, inhibition of cell migration and invasion, caspase activation and mitochondrial-dependent apoptosis.

PURPOSE: Pancreatic cancer (PC) is a lethal disease of the alimentary system and is ranked 4th in cancer-related deaths in United States. PC has a poor prognosis and limited therapeutic options. The main purpose of the current study was to demonstrate the anticancer effects of the naturally occurring Baicalein flavone in human cisplatin-resistant pancreatic carcinoma cell line CAPAN-2. METHODS: Cell viability was examined via MTT cell proliferative assay. Mitochondrial-mediated apoptosis was examined through DAPI and annexin V/propidium iodide (PI) staining using fluorescence microscopy along with estimation of apoptosis-related protein expressions like caspase-3, Bax, Bcl-2 for which western blot was used. Next, wound-healing and transwell assays were performed to find out the effects of Baicalein on cell migration and invasion, respectively. RESULTS: The results showed that Baicalein induced dose-dependent and selective anticancer effects in CAPAN-2 PC cancer cells with much less cytotoxicity to normal HTRET-HPNE cells. The antiproliferative effects of Baicalein were due to apoptosis induction as the number of apoptotic cells increased on increasing doses of the test molecule. Western blotting analysis revealed that the expressions of caspase-3 and Bcl-2 were decreased and Bax was increased. The test molecule also induced S-phase cell cycle arrest in PC cells with decreasing the cyclin-B1 expressions. Cell migration and invasion analysis revealed that Baicalein induced dose-dependent suppression in migration and invasion of CAPAN-2 PC cell line. CONCLUSION: Baicalein is a potential anticancer agent against PC cells and can be considered for PC systemic therapy provided more toxicological and in vivo studies are carried out.

Adeno-associated virus-mediated gene delivery promotes S-phase entry-independent precise targeted integration in cardiomyocytes.

Post-mitotic cardiomyocytes have been considered to be non-permissive to precise targeted integration including homology-directed repair (HDR) after CRISPR/Cas9 genome editing. Here, we demonstrate that direct delivery of large amounts of transgene encoding guide RNA (gRNA) and repair template DNA via intra-ventricular injection of adeno-associated virus (AAV) promotes precise targeted genome replacement in adult murine cardiomyocytes expressing Cas9. Neither systemic injection of AAV nor direct injection of adenovirus promotes targeted integration, suggesting that high copy numbers of single-stranded transgenes are required in cardiomyocytes. Notably, AAV-mediated targeted integration in cardiomyocytes both in vitro and in vivo depends on the Fanconi anemia pathway, a key component of the single-strand template repair mechanism. In human cardiomyocytes differentiated from induced pluripotent stem cells, AAV-mediated targeted integration fluorescently labeled Mlc2v protein after differentiation, independently of DNA synthesis, and enabled real-time detection of sarcomere contraction in monolayered beating cardiomyocytes. Our findings provide a wide range of applications for targeted genome replacement in non-dividing cardiomyocytes.

E2F-dependent transcription determines replication capacity and S phase length.

DNA replication timing is tightly regulated during S-phase. S-phase length is determined by DNA synthesis rate, which depends on the number of active replication forks and their velocity. Here, we show that E2F-dependent transcription, through E2F6, determines the replication capacity of a cell, defined as the maximal amount of DNA a cell can synthesise per unit time during S-phase. Increasing or decreasing E2F-dependent transcription during S-phase increases or decreases replication capacity, and thereby replication rates, thus shortening or lengthening S-phase, respectively. The changes in replication rate occur mainly through changes in fork speed without affecting the number of active forks. An increase in fork speed does not induce replication stress directly, but increases DNA damage over time causing cell cycle arrest. Thus, E2F-dependent transcription determines the DNA replication capacity of a cell, which affects the replication rate, controlling the time it takes to duplicate the genome and complete S-phase.

An advanced cell cycle tag toolbox reveals principles underlying temporal control of structure-selective nucleases.

Cell cycle tags allow to restrict target protein expression to specific cell cycle phases. Here, we present an advanced toolbox of cell cycle tag constructs in budding yeast with defined and compatible peak expression that allow comparison of protein functionality at different cell cycle phases. We apply this technology to the question of how and when Mus81-Mms4 and Yen1 nucleases act on DNA replication or recombination structures. Restriction of Mus81-Mms4 to M phase but not S phase allows a wildtype response to various forms of replication perturbation and DNA damage in S phase, suggesting it acts as a post-replicative resolvase. Moreover, we use cell cycle tags to reinstall cell cycle control to a deregulated version of Yen1, showing that its premature activation interferes with the response to perturbed replication. Curbing resolvase activity and establishing a hierarchy of resolution mechanisms are therefore the principal reasons underlying resolvase cell cycle regulation.

The acetyltransferase Eco1 elicits cohesin dimerization during S phase.

Cohesin is a DNA-associated protein complex that forms a tripartite ring controlling sister chromatid cohesion, chromosome segregation and organization, DNA replication, and gene expression. Sister chromatid cohesion is established by the protein acetyltransferase Eco1, which acetylates two conserved lysine residues on the cohesin subunit Smc3 and thereby ensures correct chromatid separation in yeast (Saccharomyces cerevisiae) and other eukaryotes. However, the consequence of Eco1-catalyzed cohesin acetylation is unknown, and the exact nature of the cohesive state of chromatids remains controversial. Here, we show that self-interactions of the cohesin subunits Scc1/Rad21 and Scc3 occur in a DNA replication-coupled manner in both yeast and human cells. Using cross-linking MS-based and in vivo disulfide cross-linking analyses of purified cohesin, we show that a subpopulation of cohesin may exist as dimers. Importantly, upon temperature-sensitive and auxin-induced degron-mediated Eco1 depletion, the cohesin-cohesin interactions became significantly compromised, whereas deleting either the deacetylase Hos1 or the Eco1 antagonist Wpl1/Rad61 increased cohesin dimer levels by ∼20%. These results indicate that cohesin dimerizes in the S phase and monomerizes in mitosis, processes that are controlled by Eco1, Wpl1, and Hos1 in the sister chromatid cohesion-dissolution cycle. These findings suggest that cohesin dimerization is controlled by the cohesion cycle and support the notion that a double-ring cohesin model operates in sister chromatid cohesion.

Mec1 Is Activated at the Onset of Normal S Phase by Low-dNTP Pools Impeding DNA Replication.

The Mec1 and Rad53 kinases play a central role during acute replication stress in budding yeast. They are also essential for viability in normal growth conditions, but the signal that activates the Mec1-Rad53 pathway in the absence of exogenous insults is currently unknown. Here, we show that this pathway is active at the onset of normal S phase because deoxyribonucleotide triphosphate (dNTP) levels present in G1 phase may not be sufficient to support processive DNA synthesis and impede DNA replication. This activation can be suppressed experimentally by increasing dNTP levels in G1 phase. Moreover, we show that unchallenged cells entering S phase in the absence of Rad53 undergo irreversible fork collapse and mitotic catastrophe. Together, these data indicate that cells use suboptimal dNTP pools to detect the onset of DNA replication and activate the Mec1-Rad53 pathway, which in turn maintains functional forks and triggers dNTP synthesis, allowing the completion of DNA replication.

Using activity time windows and logical representation to reduce the complexity of biological network models: G1/S checkpoint pathway with DNA damage.

Biological systems are difficult to understand complex systems. Scientists continue to create models to simulate biological systems but these models are complex too; for this reason, new reduction methods to simplify complex biological models into simpler ones are increasingly needed. In this paper, we present a way of reducing complex quantitative (continuous) models into logical models based on time windows of system activity and logical (Boolean) models. Time windows were used to define slow and fast activity areas. We use the proposed approach to reduce a continuous ODE model into a logical model describing the G1/S checkpoint with and without DNA damage as a case study. We show that the temporal unfolding of this signalling system can be broken down into three time windows where only two display high level of activity and the other shows little or no activity. The two active windows represent a cell committing to cell cycle and making the G1/S transition, respectively, the two most important high level functions of cell cycle in the G1 phase. Therefore, we developed two models to represent these time windows to reduce time complexity and used Boolean approach to reduce interaction complexity in the ODE model in the respective time windows. The developed reduced models correctly produced the commitment to cell cycle and G1/S transfer through the expected behavior of signalling molecules involved in these processes. As most biological models have a large number of fast reactions and a relatively smaller number of slow reactions, we believe that the proposed approach could be suitable for representing many, if not all biological signalling networks. The approach presented in this study greatly helps in simplifying complex continuous models (ODE models) into simpler models. Moreover, it will also assist scientists build models concentrating on understanding and representing system behavior rather than setting values for a large number of kinetic parameters.

Cell Cycle-Dependent Switch of TopBP1 Functions by Cdk2 and Akt.

Cdk2-dependent TopBP1-treslin interaction is critical for DNA replication initiation. However, it remains unclear how this association is terminated after replication initiation is finished. Here, we demonstrate that phosphorylation of TopBP1 by Akt coincides with cyclin A activation during S and G2 phases and switches the TopBP1-interacting partner from treslin to E2F1, which results in the termination of replication initiation. Premature activation of Akt in G1 phase causes an early switch and inhibits DNA replication. TopBP1 is often overexpressed in cancer and can bypass control by Cdk2 to interact with treslin, leading to enhanced DNA replication. Consistent with this notion, reducing the levels of TopBP1 in cancer cells restores sensitivity to a Cdk2 inhibitor. Together, our study links Cdk2 and Akt pathways to the control of DNA replication through the regulation of TopBP1-treslin interaction. These data also suggest an important role for TopBP1 in driving abnormal DNA replication in cancer.

Beta1- and Beta2-Adrenoceptors Expression Patterns in Human Non-small Cell Lung Cancer: Relationship with Cancer Histology.

Assessment of Beta-AR protein expression on tumour tissues might be a plausible strategy to select cancer patients who can benefit from Beta-blockers therapy. The aim of this study is to evaluate the differences between resected tissue specimens from primary lung cancer (adenocarcinoma (ADC) and squamous cell carcinoma (SCC)) in terms of expression pattern of Beta1- and Beta2-AR in both tumour and adjacent surrounding non-tumour tissue. This retrospective study was based on the analysis of 80 patients with histologically confirmed diagnosis of primary Non-Small Cell Lung Cancer (NSCLC) who received surgical treatment. The cases were carefully selected in order to obtain the most homogeneous sample in terms of histologic subtype (40 ADCs and 40 SCCs) and clinical stage (10 each). Beta1- and Beta2-AR expression was determined by immunohistochemistry and the staining evaluated by semi-quantitative scoring using the H-score method. In our NSCLC series, Beta1- and Beta2-AR are differentially expressed. Beta1-AR expression is present at low levels in both SCC and ADC. Likewise, when compared with the matched surrounding non-tumour tissues, Beta1-AR expression level was significantly lower in both histologic subtypes. Conversely, Beta2-AR is highly expressed in both histologic subtypes, but clearly highly expressed in ADC when compared with SCC and with their matched surrounding non-tumour tissue. Overall, this clinicopathological study highlights the differential expression of Beta1- and Beta2-AR in ADC and SCC. Repurposing non-selective Beta-blockers in oncologic setting might be a suitable therapeutic strategy for lung ADC. Graphical abstract.

LncRNA LINC00974 Upregulates CDK6 to Promote Cell Cycle Progression in Gastric Carcinoma.

Background: It is known that LINC00974 is an oncogenic long noncoding RNA in liver cancer. Results: The authors observed in this study that LINC00974 was upregulated in gastric cancer (GC) and positively correlated with CDK6. Survival analysis showed that high levels of LINC00974 and CDK6 predicted poor survival. In GC tissues, LINC00974 and CDK6 were positively correlated. In GC cells, LINC00974 overexpression led to upregulated, whereas LINC00974 siRNA silencing led to downregulated CDK6. Analysis of cell cycle progression and proliferation showed that LINC00974 and CDK6 overexpression promoted and siRNA silencing inhibited G1-S transition and cell proliferation. Conclusion: Therefore, LINC00974 upregulates CDK6 to promote cell cycle progression in GC.

Recruitment of Rec8, Pds5 and Rad61/Wapl to meiotic homolog pairing, recombination, axis formation and S-phase.

We have explored the meiotic roles of cohesin modulators Pds5 and Rad61/Wapl, in relation to one another, and to meiotic kleisin Rec8, for homolog pairing, all physically definable steps of recombination, prophase axis length and S-phase progression, in budding yeast. We show that Pds5 promotes early steps of recombination and thus homolog pairing, and also modulates axis length, with both effects independent of a sister chromatid. [Pds5+Rec8] promotes double-strand break formation, maintains homolog bias for crossover formation and promotes S-phase progression. Oppositely, the unique role of Rad61/Wapl is to promote non-crossover recombination by releasing [Pds5+Rec8]. For this effect, Rad61/Wapl probably acts to maintain homolog bias by preventing channeling into sister interactions. Mysteriously, each analyzed molecule has one role that involves neither of the other two. Overall, the presented findings suggest that Pds5's role in maintenance of sister chromatid cohesion during the mitotic prophase-analogous stage of G2/M is repurposed during meiosis prophase to promote interactions between homologs.

Conditioned medium derived from human amniotic stem cells delays H2O2­induced premature senescence in human dermal fibroblasts.

Stem cells derived from human amniotic membrane (hAM) are promising targets in regenerative medicine. A previous study focused on human amniotic stem cells in skin wound and scar­free healing. The present study aimed to investigate whether hydrogen peroxide (H2O2)­induced senescence of human dermal fibroblasts (hDFs) was influenced by the anti­aging effect of conditioned medium (CdM) derived from human amniotic stem cells. First, the biological function of two types of amniotic stem cells, namely human amniotic epithelial cells (hAECs) and human amniotic mesenchymal stem cells (hAMSCs), on hDFs was compared. The results of cell proliferation and wound healing assays showed that CdM promoted cell proliferation and migration. In addition, CdM from hAECs and hAMSCs significantly promoted proliferation of senescent hDFs induced by H2O2. These results indicated that CdM protects cells from damage caused by H2O2. Treatment with CdM decreased senescence­associated ß­galactosidase activity and improved the entry of proliferating cells into the S phase. Simultaneously, it was found that CdM increased the activity of superoxide dismutase and catalase and decreased malondialdehyde by reducing H2O2­induced intracellular reactive oxygen species production. It was found that CdM downregulated H2O2­stimulated 8­hydroxydeoxyguanosine and γ­H2AX levels and decreased the expression of the senescence­associated proteins p21 and p16. In conclusion, the findings indicated that the paracrine effects derived from human amniotic stem cells aided delaying oxidative stress­induced premature senescence.

Selenoprotein T Promotes Proliferation and G1-to-S Transition in SK-N-SH Cells: Implications in Parkinson's Disease.

BACKGROUND: Selenium is prioritized to the brain mainly for selenoprotein expression. Selenoprotein T (SELENOT) protects dopaminergic, postmitotic neurons in a mouse model of Parkinson's disease (PD). OBJECTIVE: We hypothesized a proliferative role of SELENOT in neural cells. METHODS: To assess SELENOT status in PD, sedated male C57BL/6 mice at 10-12 wk of age were injected with 6-hydroxydopamine in neurons, and human peripheral blood mononuclear cells were isolated from 9 healthy subjects (56% men, 68-y-old) and 11 subjects with PD (64% men, 63-y-old). Dopaminergic neural progenitor-like SK-N-SH cells with transient SELENOT overexpression or knockdown were maintained in the presence or absence of the antioxidant N-acetyl-l-cysteine and the calcium channel blocker nimodipine. Cell cycle, proliferation, and signaling parameters were determined by immunoblotting, qPCR, and flow cytometry. RESULTS: SELENOT mRNA abundance was increased (P < 0.05) in SK-N-SH cells treated with 1-methyl-4-phenylpyridinium iodide (3.5-fold) and peripheral blood mononuclear cells from PD patients (1.6-fold). Likewise, SELENOT was expressed in tyrosine hydroxylase-positive dopaminergic neurons of 6-hydroxydopamine-injected mice. Knockdown of SELENOT in SK-N-SH cells suppressed (54%; P < 0.05) 5-ethynyl-2'-deoxyuridine incorporation but induced (17-47%; P < 0.05) annexin V-positive cells, CASPASE-3 cleavage, and G1/S cell cycle arrest. SELENOT knockdown and overexpression increased (88-120%; P < 0.05) and reduced (37-42%; P < 0.05) both forkhead box O3 and p27, but reduced (51%; P < 0.05) and increased (1.2-fold; P < 0.05) cyclin-dependent kinase 4 protein abundance, respectively. These protein changes were diminished by nimodipine or N-acetyl-l-cysteine treatment (24 h) at steady-state levels. While the N-acetyl-l-cysteine treatment did not influence the reduction in the amount of calcium (13%; P < 0.05) by SELENOT knockdown, the nimodipine treatment reversed the decreased amount of reactive oxygen species (33%; P < 0.05) by SELENOT overexpression. CONCLUSIONS: These cellular and mouse data link SELENOT to neural proliferation, expanding our understanding of selenium protection in PD.

Inhibition of spindle extension through the yeast S phase checkpoint is coupled to replication fork stability and the integrity of centromeric DNA.

Budding yeast treated with hydroxyurea (HU) activate the S phase checkpoint kinase Rad53, which prevents DNA replication forks from undergoing aberrant structural transitions and nuclease processing. Rad53 is also required to prevent premature extension of the mitotic spindle that assembles during a HU-extended S phase. Here we present evidence that checkpoint restraint of spindle extension is directly coupled to Rad53 control of replication fork stability. In budding yeast, centromeres are flanked by replication origins that fire in early S phase. Mutations affecting the Zn2+-finger of Dbf4, an origin activator, preferentially reduce centromere-proximal origin firing in HU, corresponding with suppression of rad53 spindle extension. Inactivating Exo1 nuclease or displacing centromeres from origins provides a similar suppression. Conversely, short-circuiting Rad53 targeting of Dbf4, Sld3, and Dun1, substrates contributing to fork stability, induces spindle extension. These results reveal spindle extension in HU-treated rad53 mutants is a consequence of replication fork catastrophes at centromeres. When such catastrophes occur, centromeres become susceptible to nucleases, disrupting kinetochore function and spindle force balancing mechanisms. At the same time, our data indicate centromere duplication is not required to stabilize S phase spindle structure, leading us to propose a model for how monopolar kinetochore-spindle attachments may contribute to spindle force balance in HU.