PICH and cotargeted Plk1 coordinately maintain prometaphase chromosome arm architecture

PICH impacts the spindle assembly checkpoint via its DNA translocase and SUMO-interaction activities

Either inhibiting or stabilizing SUMOylation in mitosis causes defects in chromosome segregation, suggesting that dynamic mitotic SUMOylation of proteins is critical to maintain integrity of the genome. Polo-like kinase 1-interacting checkpoint helicase (PICH), a mitotic chromatin remodeling enzyme, interacts with SUMOylated chromosomal proteins via three SUMO-interacting motifs (SIMs) to control their association with chromosomes. Using cell lines with conditional PICH depletion/PICH replacement, we revealed mitotic defects associated with compromised PICH functions toward SUMOylated chromosomal proteins. Defects in either remodeling activity or SIMs of PICH delayed mitotic progression caused by activation of the spindle assembly checkpoint (SAC) indicated by extended duration of Mad1 foci at centromeres. Proteomics analysis of chromosomal SUMOylated proteins whose abundance is controlled by PICH activity identified candidate proteins to explain the SAC activation phenotype. Among the identified candidates, Bub1 kinetochore abundance is increased upon loss of PICH. Our results demonstrated a novel relationship between PICH and the SAC, where PICH directly or indirectly affects Bub1 association at the kinetochore and impacts SAC activity to control mitosis.

PICH, A protein that maintains genomic stability, can promote tumor growth

Genomic instability is regardedas a hallmark of cancer cells. It can be presented in many ways, among which chromosome instability has received attention. Ultrafine anaphase bridges are a typeof chromatin bridges, the untimely resolution of which can also lead to chromosome instability. PICH can play a role in maintaining chromosome stability by regulating chromosome morphologyand resolving ultrafine anaphase bridges. Recently, PICH has been found to be overexpressed in various cancers. Overexpression of PICH is related to the proliferation of tumors and poor prognosis. In this article, we consider that PICH can maintain genomic stability by regulating appropriate chromosome structure, ensuring proper chromosome segregation, and facilitating replication fork reversal. We summarize how PICH regulates chromosome stability, how PICH resolves Ultrafine anaphase bridges with other proteins, and how PICH promotes tumor progression.

The interplay of the translocase activity and protein recruitment function of PICH in ultrafine anaphase bridge resolution and genomic stability

Incomplete sister centromere decatenation results in centromeric ultrafine anaphase bridges (UFBs). PICH (PLK1-interacting checkpoint helicase), a DNA translocase, plays a crucial role in UFB resolution by recruiting UFB-binding proteins and stimulating topoisomerase IIα. However, the involvement of distinct PICH functions in UFB resolution remains ambiguous. Here, we demonstrate that PICH depletion in non-transformed diploid cells induces DNA damage, micronuclei formation, p53 activation, G1-phase delay and cell death. Whole-genome sequencing reveals that segregation defects induced by PICH depletion cause chromosomal rearrangements, including translocations and inversions, emphasizing its significance in preserving genomic integrity. Furthermore, a PICH mutant that impairs UFB recruitment of BLM and RIF1 partially inhibits UFB resolution while a translocase-inactive mutant (PICHK128A) fails to resolve UFBs. Notably, expression of PICHK128A inhibits single-stranded UFB formation and induces hypocondensed chromosomes. We propose that PICH's translocase activity plays a dual role in promoting UFB resolution by facilitating the generation of single-stranded UFBs and stimulating topoisomerase IIα.

ERCC6L facilitates the onset of mammary neoplasia and promotes the high malignance of breast cancer by accelerating the cell cycle

BACKGROUND

Breast cancer (BC) is the leading cause of morbidity and the second leading cause of death among female malignant tumors. Although available drugs have been approved for the corresponding breast cancer subtypes (ER-positive, HER2+) currently, there are still no effective targeted drugs or treatment strategies for metastatic breast cancer or triple-negative breast cancer that lack targets. Therefore, it is urgent to discover new potential targets. ERCC6L is an essential protein involved in chromosome separation during cell mitosis. However, the effect of ERCC6L on the tumorigenesis and progression of breast cancer is unclear.

METHODS AND RESULTS

Here, we found that ERCC6L was highly expressed in breast cancer, especially in TNBC, which was closely related to poor outcomes of patients. An ERCC6L conditional knockout mouse model was first established in this study, and the results confirmed that ERCC6L was required for the development of the mammary gland and the tumorigenesis and progression of mammary gland cancers. In in vitro cell culture, ERCC6L acted as a tumor promoter in the malignant progression of breast cancer cells. Overexpression of ERCC6L promoted cell proliferation, migration and invasion, while knockdown of ERCC6L caused the opposite results. Mechanistically, ERCC6L accelerated the cell cycle by regulating the G2/M checkpoint signalling pathway. Additionally, we demonstrated that there is an interaction between ERCC6L and KIF4A, both of which are closely related factors in mitosis and are involved in the malignant progression of breast cancer.

CONCLUSIONS

We first demonstrated that ERCC6L deficiency can significantly inhibit the occurrence and development of mammary gland tumors. ERCC6L was found to accelerate the cell cycle by regulating the p53/p21/CDK1/Cyclin B and PLK/CDC25C/CDK1/Cyclin B signalling pathways, thereby promoting the malignant progression of breast cancer cell lines. There was a direct interaction between KIF4A and ERCC6L, and both are closely associated with mitosis and contribute to growth and metastasis of breast tumor. To sum up, our results suggest that ERCC6L may be used as a promising target for the treatment of BC.

PLK-1 Interacting Checkpoint Helicase, PICH, Mediates Cellular Oxidative Stress Response

Cells respond to oxidative stress by elevating the levels of antioxidants, signaling, and transcriptional regulation, often implemented by chromatin remodeling proteins. The study presented here shows that the expression of PICH, a Rad54-like helicase belonging to the ATP-dependent chromatin remodeling protein family, is upregulated during oxidative stress in HeLa cells. We also show that PICH regulates the expression of Nrf2, a transcription factor regulating antioxidant response in both the absence and presence of oxidative stress. The overexpression of PICH in PICH-depleted cells restored Nrf2 as well as antioxidant gene expression. In turn, Nrf2 regulated the expression of PICH in the presence of oxidative stress. ChIP experiments showed that PICH is present on the Nrf2 as well as antioxidant gene promoters, suggesting that the protein might be regulating the expression of these genes directly by binding to the DNA sequences. In addition, Nrf2 and histone acetylation (H3K27ac) also played a role in activating transcription in the presence of oxidative stress. Both Nrf2 and H3K27ac were found to be present on PICH and antioxidant promoters. Their occupancy was dependent on the PICH expression as fold enrichment was found to be decreased in PICH-depleted cells. PICH ablation led to the reduced expression of Nrf2 and impaired antioxidant response, leading to increased ROS content and thus showing PICH is essential for the cell to respond to oxidative stress.

Overexpression of ERCC6L correlates with poor prognosis and confers malignant phenotypes of lung adenocarcinoma

Excision repair cross‑complementation group 6 like (ERCC6L) has been reported to be upregulated in a variety of malignant tumors and plays a critical oncogenic role. However, the role and molecular mechanism of ERCC6L in lung adenocarcinoma (LUAD) remain unclear, and were therefore investigated in the present study. Clinical data of patients with LUAD were obtained and bioinformatics analysis was performed to investigate the expression characteristics, prognostic value, and biological function of ERCC6L. In addition, cell function experiments were performed to detect the effect of ERCC6L silencing on the biological behavior of LUAD cells. The results revealed that ERCC6L expression was significantly higher in LUAD tissues vs. normal lung tissues and closely associated with nodal invasion, advanced clinical stage and survival in LUAD. Overexpression of ERCC6L was an independent prognostic biomarker of overall survival, progression‑free interval, and disease‑specific survival in patients with LUAD. DNA amplification and low methylation levels of ERCC6L suggested regulation at both the genetic and epigenetic levels. The most significant positive genes co‑expressed with ERCC6L were mainly enriched in the cell cycle signaling pathway. The major functions of ERCC6L in LUAD cells were positively correlated with the cell cycle, DNA damage, DNA repair, proliferation, invasion and epithelial‑mesenchymal transition (EMT). Knockdown of ERCC6L inhibited the proliferative, migratory and invasive abilities of A549 and PC9 cells. It also promoted cell apoptosis, and led to cell cycle arrest in the S phase. ERCC6L may regulate the EMT process through the Wnt/β‑catenin and Wnt/Notch 3 signaling pathways, thus regulating the tumorigenesis and progression of LUAD. The overexpression of ERCC6L may be a biological indicator for the diagnosis and prognosis of LUAD. ERCC6L may be a novel molecular target for the treatment of lung cancer.

Single-Molecule Micromanipulation and Super-Resolution Imaging Resolve Nanodomains Underlying Chromatin Folding in Mitotic Chromosomes

The folding of interphase chromatin into highly compact mitotic chromosomes is one of the most recognizable changes during the cell cycle. However, the structural organization underlying this drastic compaction remains elusive. Here, we combine several super resolution methods, including structured illumination microscopy (SIM), binding-activated localization microscopy (BALM), and atomic force microscopy (AFM), to examine the structural details of the DNA within the mitotic chromosome, both in the native state and after up to 30-fold extension using single-molecule micromanipulation. Images of native chromosomes reveal widespread ∼125 nm compact granules (CGs) throughout the metaphase chromosome. However, at maximal extensions, we find exclusively ∼90 nm domains (mitotic nanodomains, MNDs) that are unexpectedly resistant to extensive forces of tens of nanonewtons. The DNA content of the MNDs is estimated to be predominantly ∼80 kb, which is comparable to the size of the inner loops predicted by a recent nested loop model of the mitotic chromosome. With this DNA content, the total volume expected of the human genome assuming closely packed MNDs is nearly identical to what is observed. Thus, altogether, these results suggest that these mechanically stable MNDs, and their higher-order assembly into CGs, are the dominant higher-level structures that underlie the compaction of chromatin from interphase to metaphase.

Regulation of mitotic chromosome architecture and resolution of ultrafine anaphase bridges by PICH

To ensure genome stability, chromosomes need to undergo proper condensation into two linked sister chromatids from prophase to prometaphase, followed by equal segregation at anaphase. Emerging evidence has shown that persistent DNA entanglements connecting the sister chromatids lead to the formation of ultrafine anaphase bridges (UFBs). If UFBs are not resolved soon after anaphase, they can induce chromosome missegregation. PICH (PLK1-interacting checkpoint helicase) is a DNA translocase that localizes on chromosome arms, centromeres and UFBs. It plays multiple essential roles in mitotic chromosome organization and segregation. PICH also recruits other associated proteins to UFBs, and together they mediate UFB resolution. Here, the proposed mechanism behind PICH's functions in chromosome organization and UFB resolution will be discussed. We summarize the regulation of PICH action at chromosome arms and centromeres, how PICH recognizes UFBs and recruits other UFB-associated factors, and finally how PICH promotes UFB resolution together with other DNA processing enzymes.

PICH regulates the abundance and localization of SUMOylated proteins on mitotic chromosomes

Proper chromosome segregation is essential for faithful cell division and if not maintained results in defective cell function caused by the abnormal distribution of genetic information. Polo-like kinase 1-interacting checkpoint helicase (PICH) is a DNA translocase essential for chromosome bridge resolution during mitosis. Its function in resolving chromosome bridges requires both DNA translocase activity and ability to bind chromosomal proteins modified by the small ubiquitin-like modifier (SUMO). However, it is unclear how these activities cooperate to resolve chromosome bridges. Here, we show that PICH specifically disperses SUMO2/3 foci on mitotic chromosomes. This PICH function is apparent toward SUMOylated topoisomerase IIα (TopoIIα) after inhibition of TopoIIα by ICRF-193. Conditional depletion of PICH using the auxin-inducible degron (AID) system resulted in the retention of SUMO2/3-modified chromosomal proteins, including TopoIIα, indicating that PICH functions to reduce the association of these proteins with chromosomes. Replacement of PICH with its translocase-deficient mutants led to increased SUMO2/3 foci on chromosomes, suggesting that the reduction of SUMO2/3 foci requires the remodeling activity of PICH. In vitro assays showed that PICH specifically attenuates SUMOylated TopoIIα activity using its SUMO-binding ability. Taking the results together, we propose a novel function of PICH in remodeling SUMOylated proteins to ensure faithful chromosome segregation.

The CINs of Polo-Like Kinase 1 in Cancer

Simple Summary

Many alterations specific to cancer cells have been investigated as targets for targeted therapies. Chromosomal instability is a characteristic of nearly all cancers that can limit response to targeted therapies by ensuring the tumor population is not genetically homogenous. Polo-like Kinase 1 (PLK1) is often up regulated in cancers and it regulates chromosomal instability extensively. PLK1 has been the subject of much pre-clinical and clinical studies, but thus far, PLK1 inhibitors have not shown significant improvement in cancer patients. We discuss the numerous roles and interactions of PLK1 in regulating chromosomal instability, and how these may provide an avenue for identifying targets for targeted therapies. As selective inhibitors of PLK1 showed limited clinical success, we also highlight how genetic interactions of PLK1 may be exploited to tackle these challenges.

Abstract

Polo-like kinase 1 (PLK1) is overexpressed near ubiquitously across all cancer types and dysregulation of this enzyme is closely tied to increased chromosomal instability and tumor heterogeneity. PLK1 is a mitotic kinase with a critical role in maintaining chromosomal integrity through its function in processes ranging from the mitotic checkpoint, centrosome biogenesis, bipolar spindle formation, chromosome segregation, DNA replication licensing, DNA damage repair, and cytokinesis. The relation between dysregulated PLK1 and chromosomal instability (CIN) makes it an attractive target for cancer therapy. However, clinical trials with PLK1 inhibitors as cancer drugs have generally displayed poor responses or adverse side-effects. This is in part because targeting CIN regulators, including PLK1, can elevate CIN to lethal levels in normal cells, affecting normal physiology. Nevertheless, aiming at related genetic interactions, such as synthetic dosage lethal (SDL) interactions of PLK1 instead of PLK1 itself, can help to avoid the detrimental side effects associated with increased levels of CIN. Since PLK1 overexpression contributes to tumor heterogeneity, targeting SDL interactions may also provide an effective strategy to suppressing this malignant phenotype in a personalized fashion.

Upregulation of ERCC6L is associated with tumor progression and unfavorable prognosis in hepatocellular carcinoma

Background

The oncogenic role of excision repair cross-complementation group 6-like (ERCC6L) has been revealed in several cancers recently, but little is known about its expression and function in hepatocellular carcinoma (HCC).

Methods

Utilizing public data from Human Protein Atlas (HPA) and The Cancer Genome Atlas (TCGA) databases, ERCC6L dysregulation in HCC and its clinical significance were determined by t-test and Chi-square test. Comprehensive survival analyses (such as nomogram, Cox regression model and Kaplan-Meier analysis) were performed to assess prognostic value of ERCC6L for HCC patients. Integrated bioinformatics analyses [including copy number alterations (CNA), DNA methylation, miRNA prediction and gene set enrichment analysis (GSEA)] were conducted to explore the mechanisms and biological roles underlying ERCC6L dysregulation in HCC.

Results

ERCC6L upregulation was identified in HCC tissues compared to normal controls (P<0.05). In addition, overexpression of ERCC6L not only correlated with elevated alpha fetoprotein (AFP), vascular invasion (VI), and advanced histologic grade and TNM stage, but also had an independent prognostic value for the poorer overall survival (OS) and recurrence-free survival (RFS) of HCC patients (all P<0.05). Besides, nomogram integrating ERCC6L expression and TNM stage showed superior prognostic ability than that of TNM stage (P<0.05). Moreover, ERCC6L promoter hypomethylation and miR-5589 downregulation in HCC might result in ERCC6L overexpression (all P<0.05). Furthermore, eight biological pathways (including the DNA replication, cell cycle and p53 pathways) related to ERCC6L upregulation in HCC were found to be enriched by GSEA, and ERCC6L upregulation was positively correlated with PLK1 (polo-like kinase 1) expression and TP53 mutation in HCC, which preliminarily shed light on the roles of ERCC6L in HCC.

Conclusions

ERCC6L may serve as a promising prognostic indicator and therapeutic target for HCC patients.

Loss of PICH promotes chromosome instability and cell death in triple-negative breast cancer

Triple-negative breast cancer (TNBC), defined by the lack of expression of estrogen, progesterone, and ERBB2 receptors, has the worst prognosis of all breast cancers. It is difficult to treat owing to a lack of effective molecular targets. Here, we report that the growth of TNBC cells is exceptionally dependent on PICH, a DNA-dependent ATPase. Clinical samples analysis showed that PICH is highly expressed in TNBC compared to other breast cancer subtypes. Importantly, its high expression correlates with higher risk of distal metastasis and worse clinical outcomes. Further analysis revealed that PICH depletion selectively impairs the proliferation of TNBC cells, but not that of luminal breast cancer cells, in vitro and in vivo. In addition, knockdown of PICH in TNBC cells induces the formation of chromatin bridges and lagging chromosomes in anaphase, frequently resulting in micronucleation or binucleation, finally leading to mitotic catastrophe and apoptosis. Collectively, our findings show the dependency of TNBC cells on PICH for faithful chromosome segregation and the clinical potential of PICH inhibition to improve treatment of patients with high-risk TNBC.

ERCC6L promotes cell growth and invasion in human colorectal cancer

Excision repair cross-complementation group 6 like (ERCC6L), a recently discovered DNA helicase, has been demonstrated to be highly expressed in a variety of human cancer types. However, the precise role of ERCC6L in colorectal cancer (CRC) remains unclear. The current study aimed to investigate the potential role of ERCC6L in the development and progression of CRC. Reverse transcription-quantitative polymerase chain reaction, western blot analysis and immunohistochemistry were used to detect the expression level of ERCC6L in 30 matched pairs of CRC and adjacent noncancerous tissues. The function of ERCC6L in cell proliferation, cycle, apoptosis, invasion and colony formation was examined in CRC cell lines. ERCC6L was revealed to be highly expressed in CRC tissues and cell lines compared with normal controls (P<0.05). The expression level of ERCC6L was significantly associated with tumor size (P<0.05), but not with other clinical features, including age, gender, differentiation and clinical stage. It was identified that reducing ERCC6L expression using small interfering RNA significantly inhibited the proliferation and colony-forming ability of CRC cell lines. Flow cytometric analysis demonstrated that ERCC6L knockdown in CRC cells inhibited cell cycle progression and increased the number of cells in the G0/G1 phase without affecting apoptosis. Furthermore, ERCC6L knockdown markedly decreased the number of invading CRC cells compared with control cells. These results suggest that ERCC6L promotes the growth and invasion of CRC cells, and ERCC6L may be a potential new target for cancer therapy.

The Unresolved Problem of DNA Bridging

Accurate duplication and transmission of identical genetic information into offspring cells lies at the heart of a cell division cycle. During the last stage of cellular division, namely mitosis, the fully replicated DNA molecules are condensed into X-shaped chromosomes, followed by a chromosome separation process called sister chromatid disjunction. This process allows for the equal partition of genetic material into two newly born daughter cells. However, emerging evidence has shown that faithful chromosome segregation is challenged by the presence of persistent DNA intertwining structures generated during DNA replication and repair, which manifest as so-called ultra-fine DNA bridges (UFBs) during anaphase. Undoubtedly, failure to disentangle DNA linkages poses a severe threat to mitosis and genome integrity. This review will summarize the possible causes of DNA bridges, particularly sister DNA inter-linkage structures, in an attempt to explain how they may be processed and how they influence faithful chromosome segregation and the maintenance of genome stability.

ERCC6L that is up-regulated in high grade of renal cell carcinoma enhances cell viability in vitro and promotes tumor growth in vivo potentially through modulating MAPK signalling pathway

Renal cell carcinoma (RCC), which is one of the most diagnosed urological malignancies worldwide, is usually associated with abnormality in both genetic and cellular processes. In the present study, through analyzing The Cancer Genome Atlas (TCGA) dataset, we screened out ERCC6L as a candidate gene that is potentially related to the development of RCC based on its increased expression in ccRCC tissues compared with normal kidney tissues as well as its possible relevance to cancer prognosis. Evidence indicates that ERCC6L is an indispensable component of mammalian cell mitosis, while it fails to disclose the role of ERCC6L in tumorigenesis. By using RT-PCR, it was confirmed that the mRNA expression of ERCC6L was upregulated in RCC tissues as compared to normal controls in 28 pared samples. In addition, the immunohistochemistry study in a tissue microarray (TMA) containing 150 ccRCC samples showed that the staining score of ERCC6L was positively correlated with the Fuhrman grade of cancers. Next, when the expression of ERCC6L was lowered by specific shRNA, the cell viability was significantly inhibited in 786-O and Caki-1 cells, while the apoptosis was induced accordingly. At the same time, RCC cells those were transfected with shRNA targeting to ERCC6L grew significantly slower than parental cells in immunodeficient mice. These results consistently suggest that ERCC6L may play a role in regulating the cell viability of RCC both in vitro and in vivo. Further, gene expression microarray analysis followed by the validating western blot after knocking down ERCC6L expression in 786-O cells highlighted the involvement of MAPK signaling pathway in regulation of ERCC6L on cellular process of RCC. In conclusion, the present study suggests a likely promoting role of ERCC6L on the development of RCC. Thus, further study to explore the potential utility of ERCC6L as a novel therapeutic target of RCC is clearly needed.

ERCC6L, a DNA helicase, is involved in cell proliferation and associated with survival and progress in breast and kidney cancers

By analyzing 4987 cancer transcriptomes from The Cancer Genome Atlas (TCGA), we identified that excision repair cross-complementation group 6 like (ERCC6L), a newly discovered DNA helicase, is highly expressed in 12 solid cancers. However, its role and mechanism in tumorigenesis are largely unknown. In this study, we found that ERCC6L silencing by small interring RNA (siRNA) or short hairpin RNA (shRNA) significantly inhibited the proliferation of breast (MCF-7, MDA-MB-231) and kidney cancer cells (786-0). Furthermore, ERCC6L silencing induced cell cycle arrest at G0/G1 phase without affecting apoptosis. We then performed RNA sequencing (RNA-seq) analysis after ERCC6L silencing and identified that RAB31 was markedly downregulated at both the transcriptional and translational levels. Its downstream protein, phosphorylated MAPK and CDK2 were also inhibited by ERCC6L silencing. The xenograft experiment showed that silencing of ERCC6L strikingly inhibited tumor growth from the 7th day after xenograft in nude mice. In addition, higher ERCC6L expression was found to be significantly associated with worse clinical survival in breast and kidney cancers. In conclusion, our results suggest that ERCC6L may stimulates cancer cell proliferation by promoting cell cycle through a way of RAB31-MAPK-CDK2, and it could be a potential biomarker for cancer prognosis and target for cancer treatment.

Knotty Problems during Mitosis: Mechanistic Insight into the Processing of Ultrafine DNA Bridges in Anaphase

To survive and proliferate, cells have to faithfully segregate their newly replicated genomic DNA to the two daughter cells. However, the sister chromatids of mitotic chromosomes are frequently interlinked by so-called ultrafine DNA bridges (UFBs) that are visible in the anaphase of mitosis. UFBs can only be detected by the proteins bound to them and not by staining with conventional DNA dyes. These DNA bridges are presumed to represent entangled sister chromatids and hence pose a threat to faithful segregation. A failure to accurately unlink UFB DNA results in chromosome segregation errors and binucleation. This, in turn, compromises genome integrity, which is a hallmark of cancer. UFBs are actively removed during anaphase, and most known UFB-associated proteins are enzymes involved in DNA repair in interphase. However, little is known about the mitotic activities of these enzymes or the exact DNA structures present on UFBs. We focus on the biology of UFBs, with special emphasis on their underlying DNA structure and the decatenation machineries that process UFBs.

Regulatory functional territory of PLK-1 and their substrates beyond mitosis

Polo-like kinase 1 (PLK-1) is a well-known (Ser/Thr) mitotic protein kinase and is considered as a proto-oncogene. As hyper-activation of PLK-1 is broadly associated with poor prognosis and cancer progression, it is one of the most extensively studied mitotic kinases. During mitosis, PLK-1 regulates various cell cycle events, such as spindle pole maturation, chromosome segregation and cytokinesis. However, studies have demonstrated that the role of PLK-1 is not only restricted to mitosis, but PLK-1 can also regulate other vital events beyond mitosis, including transcription, translation, ciliogenesis, checkpoint adaptation and recovery, apoptosis, chromosomes dynamics etc. Recent reviews have tried to define the regulatory role of PLK-1 during mitosis progression and tumorigenesis, but its' functional role beyond mitosis is still largely unexplored. PLK-1 can regulate the activity of many proteins that work outside of its conventional territory. The dysregulation of these proteins can cause diseases such as Alzheimer's disease, tumorigenesis etc. and may also lead to drug resistance. Thus, in this review, we discussed the versatile role of PLK-1 and tried to collect data to validate its' functional role in cell cycle regulation apart from mitosis.

PICH promotes mitotic chromosome segregation: Identification of a novel role in rDNA disjunction

PICH is an SNF2-family DNA translocase that appears to play a role specifically in mitosis. Characterization of PICH in human cells led to the initial discovery of "ultra-fine DNA bridges" (UFBs) that connect the 2 segregating DNA masses in the anaphase of mitosis. These bridge structures, which arise from specific regions of the genome, are a normal feature of anaphase but had escaped detection previously because they do not stain with commonly used DNA dyes. Nevertheless, UFBs are important for genome maintenance because defects in UFB resolution can lead to cytokinesis failure. We reported recently that PICH stimulates the unlinking (decatenation) of entangled DNA by Topoisomerase IIα (Topo IIα), and is important for the resolution of UFBs. We also demonstrated that PICH and Topo IIα co-localize at the rDNA (rDNA). In this Extra View article, we discuss the mitotic roles of PICH and explore further the role of PICH in the timely segregation of the rDNA locus.

Dynamic Phosphorylation of NudC by Aurora B in Cytokinesis

Nuclear distribution protein C (NudC) is a mitotic regulator that plays a role in cytokinesis. However, how NudC is regulated during cytokinesis remains unclear. Here, we show that NudC is phosphorylated by Aurora B, a kinase critical for cell abscission. NudC is co-localized with Aurora B at the midbody and co-immunoprecipitated with Aurora B in mitosis. Inhibition of Aurora B by ZM447439 reduced NudC phosphorylation, suggesting that NudC is an Aurora B substrate in vivo. We identified T40 on NudC as an Aurora B phosphorylation site. NudC depletion resulted in cytokinesis failure with a dramatic elongation of the intercellular bridge between daughter cells, sustained Aurora B activity at the midbody, and reduced cell abscission. These cytokinetic defects can be rescued by the ectopic expression of wild-type NudC. Reconstitution with T40A phospho-defective NudC was found to rescue the cytokinesis defect. In contrast, reconstitution with the T40D phospho-mimetic NudC was inefficient in supporting the completion of cytokinesis. These results suggest that that dynamic phosphorylation of NudC by Aurora B regulates cytokinesis.

PICH promotes sister chromatid disjunction and co-operates with topoisomerase II in mitosis

PICH is a SNF2 family DNA translocase that binds to ultra-fine DNA bridges (UFBs) in mitosis. Numerous roles for PICH have been proposed from protein depletion experiments, but a consensus has failed to emerge. Here, we report that deletion of PICH in avian cells causes chromosome structural abnormalities, and hypersensitivity to an inhibitor of Topoisomerase II (Topo II), ICRF-193. ICRF-193-treated PICH^−/− cells undergo sister chromatid non-disjunction in anaphase, and frequently abort cytokinesis. PICH co-localizes with Topo IIα on UFBs and at the ribosomal DNA locus, and the timely resolution of both structures depends on the ATPase activity of PICH. Purified PICH protein strongly stimulates the catalytic activity of Topo II in vitro. Consistent with this, a human PICH^−/− cell line exhibits chromosome instability and chromosome condensation and decatenation defects similar to those of ICRF-193-treated cells. We propose that PICH and Topo II cooperate to prevent chromosome missegregation events in mitosis.

During mitosis the translocase PICH binds to ultrafine bridges formed from DNA catenanes that are unresolved by topoisomerase II. In this study, the authors show that PICH stimulates toposiomerase II activity and that they cooperate to resolve these structures.

SUMOylation regulates polo-like kinase 1-interacting checkpoint helicase (PICH) during mitosis

Mitotic SUMOylation has an essential role in faithful chromosome segregation in eukaryotes, although its molecular consequences are not yet fully understood. In Xenopus egg extract assays, we showed that poly(ADP-ribose) polymerase 1 (PARP1) is modified by SUMO2/3 at mitotic centromeres and that its enzymatic activity could be regulated by SUMOylation. To determine the molecular consequence of mitotic SUMOylation, we analyzed SUMOylated PARP1-specific binding proteins. We identified Polo-like kinase 1-interacting checkpoint helicase (PICH) as an interaction partner of SUMOylated PARP1 in Xenopus egg extract. Interestingly, PICH also bound to SUMOylated topoisomerase IIα (TopoIIα), a major centromeric small ubiquitin-like modifier (SUMO) substrate. Purified recombinant human PICH interacted with SUMOylated substrates, indicating that PICH directly interacts with SUMO, and this interaction is conserved among species. Further analysis of mitotic chromosomes revealed that PICH localized to the centromere independent of mitotic SUMOylation. Additionally, we found that PICH is modified by SUMO2/3 on mitotic chromosomes and in vitro. PICH SUMOylation is highly dependent on protein inhibitor of activated STAT, PIASy, consistent with other mitotic chromosomal SUMO substrates. Finally, the SUMOylation of PICH significantly reduced its DNA binding capability, indicating that SUMOylation might regulate its DNA-dependent ATPase activity. Collectively, our findings suggest a novel SUMO-mediated regulation of the function of PICH at mitotic centromeres.

Rice SNF2 family helicase ENL1 is essential for syncytial endosperm development

The endosperm of cereal grains represents the most important source of human nutrition. In addition, the endosperm provides many investigatory opportunities for biologists because of the unique processes that occur during its ontogeny, including syncytial development at early stages. Rice endospermless 1 (enl1) develops seeds lacking an endosperm but carrying a functional embryo. The enl1 endosperm produces strikingly enlarged amoeboid nuclei. These abnormal nuclei result from a malfunction in mitotic chromosomal segregation during syncytial endosperm development. The molecular identification of the causal gene revealed that ENL1 encodes an SNF2 helicase family protein that is orthologous to human Plk1-Interacting Checkpoint Helicase (PICH), which has been implicated in the resolution of persistent DNA catenation during anaphase. ENL1-Venus (enhanced yellow fluorescent protein (YFP)) localizes to the cytoplasm during interphase but moves to the chromosome arms during mitosis. ENL1-Venus is also detected on a thread-like structure that connects separating sister chromosomes. These observations indicate the functional conservation between PICH and ENL1 and confirm the proposed role of PICH. Although ENL1 dysfunction also affects karyokinesis in the root meristem, enl1 plants can grow in a field and set seeds, indicating that its indispensability is tissue-dependent. Notably, despite the wide conservation of ENL1/PICH among eukaryotes, the loss of function of the ENL1 ortholog in Arabidopsis (CHR24) has only marginal effects on endosperm nuclei and results in normal plant development. Our results suggest that ENL1 is endowed with an indispensable role to secure the extremely rapid nuclear cycle during syncytial endosperm development in rice.

How unfinished business from S-phase affects mitosis and beyond

The eukaryotic cell cycle is conventionally viewed as comprising several discrete steps, each of which must be completed before the next one is initiated. However, emerging evidence suggests that incompletely replicated, or unresolved, chromosomes from S-phase can persist into mitosis, where they present a potential threat to the faithful segregation of sister chromatids. In this review, we provide an overview of the different classes of loci where this 'unfinished S-phase business' can lead to a variety of cytogenetically distinct DNA structures throughout the various steps of mitosis. Furthermore, we discuss the potential ways in which cells might not only tolerate this inevitable aspect of chromosome biology, but also exploit it to assist in the maintenance of genome stability.

NudC deacetylation regulates mitotic progression

Mitosis is largely driven by posttranslational modifications of proteins. Recent studies suggest that protein acetylation is prevalent in mitosis, but how protein acetylation/deacetylation regulates mitotic progression remains unclear. Nuclear distribution protein C (NudC), a conserved protein that regulates cell division, was previously shown to be acetylated. We found that NudC acetylation was decreased during mitosis. Using mass spectrometry analysis, we identified K39 to be an acetylation site on NudC. Reconstitution of NudC-deficient cells with wild-type or K39R acetylation-defective NudC rescued mitotic phenotypes, including chromosome misalignment, chromosome missegregation, and reduced spindle width, observed after NudC protein knockdown. In contrast, the K39Q acetylation-mimetic NudC was unable to rescue these mitotic phenotypes, suggesting that NudC deacetylation is important for mitotic progression. To examine proteins that may play a role in NudC deacetylation during mitosis, we found that NudC co-localizes on the mitotic spindle with the histone deacetylase HDAC3, an HDAC shown to regulate mitotic spindle stability. Further, NudC co-immunoprecipitates with HDAC3 and loss of function of HDAC3 either by protein knockdown or inhibition with a small molecule inhibitor increased NudC acetylation. These observations suggest that HDAC3 may be involved in NudC deacetylation during mitosis. Cells with NudC or HDAC3 knockdown exhibited overlapping mitotic abnormalities, including chromosomes arranged in a “dome-like” configuration surrounding a collapsed mitotic spindle. Our studies suggest that NudC acetylation/deacetylation regulates mitotic progression and NudC deacetylation, likely through HDAC3, is critical for spindle function and chromosome congression.

PICH: a DNA translocase specially adapted for processing anaphase bridge DNA

The Plk1-interacting checkpoint helicase (PICH) protein localizes to ultrafine anaphase bridges (UFBs) in mitosis alongside a complex of DNA repair proteins, including the Bloom's syndrome protein (BLM). However, very little is known about the function of PICH or how it is recruited to UFBs. Using a combination of microfluidics, fluorescence microscopy, and optical tweezers, we have defined the properties of PICH in an in vitro model of an anaphase bridge. We show that PICH binds with a remarkably high affinity to duplex DNA, resulting in ATP-dependent protein translocation and extension of the DNA. Most strikingly, the affinity of PICH for binding DNA increases with tension-induced DNA stretching, which mimics the effect of the mitotic spindle on a UFB. PICH binding also appears to diminish force-induced DNA melting. We propose a model in which PICH recognizes and stabilizes DNA under tension during anaphase, thereby facilitating the resolution of entangled sister chromatids.

Plk1 is upregulated in androgen-insensitive prostate cancer cells and its inhibition leads to necroptosis

Castration-resistant prostate cancer (PCa) is refractory to hormone therapy and new strategies for treatment are urgently needed. We found that androgen-insensitive (AI) PCa cells, LNCaP-AI, are reprogrammed to upregulate the mitotic kinase Plk1 (Polo-like kinase 1) and other M-phase cell-cycle proteins, which may underlie AI PCa growth. In androgen-depleted media, LNCaP-AI cells showed exquisite sensitivity to growth inhibition by subnanomolar concentrations of a small molecule inhibitor of Plk1, BI2536, suggesting that these cells are dependent on Plk1 for growth. In contrast, the androgen-responsive parental LNCaP cells showed negligible responses to BI2536 treatment under the same condition. BI2536 treatment of LNCaP-AI cells resulted in an increase in cell death marker PARP-1 (polymerase-1) but did not activate caspase-3, an apoptosis marker, suggesting that the observed cell death was caspase-independent. BI2536-treated LNCaP-AI cells formed multinucleated giant cells that contain clusters of nuclear vesicles indicative of mitotic catastrophe. Live-cell time-lapse imaging revealed that BI2536-treated giant LNCaP-AI cells underwent necroptosis, as evidenced by 'explosive' cell death and partial reversal of cell death by a necroptosis inhibitor. Our studies suggest that LNCaP-AI cells underwent reprogramming in both their cell growth and cell death pathways, rendering them highly sensitive to Plk1 inhibition that induces necroptosis. Harnessing necroptosis through Plk1 inhibition may be explored for therapeutic intervention of castration-resistant PCa.

Aurora B is regulated by acetylation/deacetylation during mitosis in prostate cancer cells

Protein acetylation has been implicated in playing an important role during mitotic progression. Aurora B kinase is known to play a critical role in mitosis. However, whether Aurora B is regulated by acetylation is not known. Using IP with an anti-acetyl lysine antibody, we identified Aurora B as an acetylated protein in PC3 prostate cancer cells. Knockdown of HDAC3 or inhibiting HDAC3 deacetylase activity led to a significant increase (P<0.01 and P<0.05, respectively) in Aurora B acetylation as compared to siLuc or vehicle-treated controls. Increased Aurora B acetylation is correlated with a 30% reduction in Aurora B kinase activity in vitro and resulted in significant defects in Aurora B-dependent mitotic processes, including kinetochore-microtubule attachment and chromosome congression. Furthermore, Aurora B transiently interacts with HDAC3 at the kinetochore-microtubule interface of congressing chromosomes during prometaphase. This window of interaction corresponded with a transient but significant reduction (P=0.02) in Aurora B acetylation during early mitosis. Together, these results indicate that Aurora B is more active in its deacetylated state and further suggest a new mechanism by which dynamic acetylation/deacetylation acts as a rheostat to fine-tune Aurora B activity during mitotic progression.

Dissecting the phenotypes of Plk1 inhibition in cancer cells using novel kinase inhibitory chemical CBB2001

Polo-like kinase 1 (Plk1) is a mitotic serine/threonine kinase and its kinase activity is closely interrelated to cell cycle progression, various types of cancer development and often correlates with poor prognosis. Thus, it is of prime importance to characterize the phenotypes of Plk1 inhibition in cells for drug development and clinical application. Here, we report a novel kinase inhibitory chemical, CBB2001, which specifically inhibited Plk1 kinase activity in vitro with an IC(50) of 0.39 μM. In cervical carcinoma HeLa cells, we found that treatment of CBB2001 caused mitotic cell cycle arrest (EC(50)=0.72 μM) and induction of 'polo' cells (EC(50)=0.32 μM). Interestingly, the cell cycle arrest induced by CBB2001 was associated with accumulation of Plk1 (EC(50)=0.61 μM) and Geminin (EC(50)=0.43 μM) proteins, but distinct from the phenotypes induced by Aurora kinase inhibitors. The inhibitory effects of CBB2001 were phenocopied by RNA interferences of Plk1. We also confirmed the cell cycle inhibitory effects of CBB2001 in other cancer cells. Moreover, CBB2001 inhibited the growth of HeLa cells with an IC(50) of 0.85 μM in MTT assays, which is better than that of reported Plk1 inhibitory chemicals ON01910 (IC(50)=6.46 μM) and LFM-A13 (IC(50)=37.36 μM). CBB2001 also inhibited mouse xenograft tumor growth. Furthermore, CBB2001 inhibited mitotic exit and delayed degradation of APC/C substrates, Geminin, Cyclin B1 and Aurora A. These specific phenotypes may serve as specific features for Plk1 inhibition and for Plk1-based clinic trials.

POLO ensures chromosome bi-orientation by preventing and correcting erroneous chromosome-spindle attachments

Correct chromosome segregation during cell division requires bi-orientation at the mitotic spindle. Cells possess mechanisms to prevent and correct inappropriate chromosome attachment. Sister kinetochores assume a 'back-to-back' geometry on chromosomes that favors amphitelic orientation but the regulation of this process and molecular components are unknown. Abnormal chromosome-spindle interactions do occur but are corrected through the activity of Aurora B, which destabilizes erroneous attachments. Here, we address the role of Drosophila POLO in chromosome-spindle interactions and show that, unlike inhibition of its activity, depletion of the protein results in bipolar spindles with most chromosomes forming stable attachments with both sister kinetochores bound to microtubules from the same pole in a syntelic orientation. This is partly the result of impaired localization and activity of Aurora B but also of an altered centromere organization with abnormal distribution of centromeric proteins and shorter interkinetochore distances. Our results suggests that POLO is required to promote amphitelic attachment and chromosome bi-orientation by regulating both the activity of the correction mechanism and the architecture of the centromere.

On the regulation, function, and localization of the DNA-dependent ATPase PICH

The putative chromatin remodeling enzyme Plk1-interacting checkpoint helicase (PICH) was discovered as an interaction partner and substrate of the mitotic kinase Plk1. During mitosis PICH associates with centromeres and kinetochores and, most interestingly, constitutes a robust marker for ultrafine DNA bridges (UFBs) that connect separating chromatids in anaphase cells. The precise roles of PICH remain to be clarified. Here, we have used antibody microinjection and siRNA-rescue experiments to study PICH function and localization during M phase progression, with particular emphasis on the role of the predicted ATPase domain and the regulation of PICH localization by Plk1. We show that interference with PICH function results in chromatin bridge formation and micronucleation and that ATPase activity is critical for PICH function. Interestingly, an intact ATPase domain of PICH is required for prevention of chromatin bridge formation but not for UFB resolution, and quantitative analyses of UFB and chromatin bridge frequencies suggest that these structures are of different etiologies. We also show that the ATPase activity of PICH is required for temporal and spatial control of PICH localization to chromatin and that Plk1 likely controls PICH localization through phosphorylation of proteins distinct from PICH itself. This work strengthens the view that PICH is an important, Plk1-regulated enzyme, whose ATPase activity is essential for maintenance of genome integrity. Although not required for the spindle assembly checkpoint, PICH is clearly important for faithful chromosome segregation.

Contrasting roles of condensin I and condensin II in mitotic chromosome formation

In vertebrates, two condensin complexes exist, condensin I and condensin II, which have differing but unresolved roles in organizing mitotic chromosomes. To dissect accurately the role of each complex in mitosis, we have made and studied the first vertebrate conditional knockouts of the genes encoding condensin I subunit CAP-H and condensin II subunit CAP-D3 in chicken DT40 cells. Live-cell imaging reveals highly distinct segregation defects. CAP-D3 (condensin II) knockout results in masses of chromatin-containing anaphase bridges. CAP-H (condensin I)-knockout anaphases have a more subtle defect, with chromatids showing fine chromatin fibres that are associated with failure of cytokinesis and cell death. Super-resolution microscopy reveals that condensin-I-depleted mitotic chromosomes are wider and shorter, with a diffuse chromosome scaffold, whereas condensin-II-depleted chromosomes retain a more defined scaffold, with chromosomes more stretched and seemingly lacking in axial rigidity. We conclude that condensin II is required primarily to provide rigidity by establishing an initial chromosome axis around which condensin I can arrange loops of chromatin.

New insights into the formation and resolution of ultra-fine anaphase bridges

Recent data indicate an unexpected requirement for proteins that were hitherto considered to be dedicated to DNA repair to facilitate the faithful disjunction of sister chromatids in anaphase. These include the Bloom's syndrome gene product, BLM and its partners, as well as a number of proteins that are important for preventing Fanconi anemia (FA) in man. As part of an analysis of the roles of these proteins in mitosis, we identified a novel class of anaphase bridge structure, called an ultra-fine anaphase bridge (UFB). These UFBs are also defined by the presence of a SNF2 family protein called PICH. In this review, we will discuss the possible sources of UFBs, and how the BLM, PICH and FA proteins might serve to process these structures in order to maintain genome stability.

PICH and BLM limit histone association with anaphase centromeric DNA threads and promote their resolution

The helicase proteins PICH and BLM localize to ultrafine DNA threads between separating sister chromatids. It now appears they cooperate to remove histones from these anaphase DNA bridges, to allow their stretching and unravelling without breakage.

Centromeres nucleate the formation of kinetochores and are vital for chromosome segregation during mitosis. The SNF2 family helicase PICH (Plk1-interacting checkpoint helicase) and the BLM (the Bloom's syndrome protein) helicase decorate ultrafine histone-negative DNA threads that link the segregating sister centromeres during anaphase. The functions of PICH and BLM at these threads are not understood, however. Here, we show that PICH binds to BLM and enables BLM localization to anaphase centromeric threads. PICH- or BLM-RNAi cells fail to resolve these threads in anaphase. The fragmented threads form centromeric-chromatin-containing micronuclei in daughter cells. Anaphase threads in PICH- and BLM-RNAi cells contain histones and centromere markers. Recombinant purified PICH has nucleosome remodelling activities in vitro. We propose that PICH and BLM unravel centromeric chromatin and keep anaphase DNA threads mostly free of nucleosomes, thus allowing these threads to span long distances between rapidly segregating centromeres without breakage and providing a spatiotemporal window for their resolution.

Fanconi anaemia proteins are associated with sister chromatid bridging in mitosis

The maintenance of genome stability is critical for the suppression of cancer and premature ageing. The maintenance of the human genome requires hundreds of proteins involved in DNA repair, DNA replication, chromosome segregation and cell cycle checkpoint responses. A number of genetic disorders exist in man where a breakdown in genome maintenance is associated with cancer predisposition. Amongst these are Bloom's syndrome (BS) and Fanconi anaemia (FA). The BS and FA gene products co-operate in the repair of damaged DNA. In this review, we focus on interactions between BS and FA proteins that specifically occur during chromosome segregation in mitosis. The BS protein, BLM, was shown recently to define a novel class of anaphase DNA bridge structures that, in some cases, also contain FA proteins. We will discuss the possible source of these bridges and the role that FA proteins and BLM might play in their removal.

The substrates of Plk1, beyond the functions in mitosis

Polo-like kinase 1 (Plk1) is a key regulator of cell division in eukaryotic cells. In this short review, we briefly summarized the well-established functions modulated by Plk1 during mitosis. Beyond mitosis, we focused mainly on the unexpected processes in which Plk1 emerges as a critical player, including microtubule dynamics, DNA replication, chromosome dynamics, p53 regulation, and recovery from the G2 DNA-damage checkpoint. Our discussion is mainly based on the critical substrates targeted by Plk1 during these cellular events and the functional significance associated with each phosphorylation event.