Polo kinase and separase regulate the mitotic licensing of centriole duplication in human cells

Ninein isoform contributions to intracellular processes and macrophage immune function

Ninein is a multifunctional protein involved in microtubule (MT) organization and dynein/dynactin complex recruitment and activation. Several isoforms of ninein have been identified in various tissues, however, their relative contribution(s) are not clear. Here, we identify two ninein isoforms in mouse macrophages with distinct C-termini and disproportionate expression levels; a canonical ninein (nineinCAN) isoform and ninein isoform 2 (nineinISO2). Analysis of ninein pre-mRNA exon-intron boundaries revealed that nineinISO2 transcript is likely generated by two alternative splicing site selection events predicted to result in a distinct 3D structure compared to nineinCAN. We used selective and total protein knockdown experiments to assess the intracellular and functional roles of ninein in macrophages. Live cell imaging analyses of macrophages implicated both isoforms in regulating cell proliferation. MT regrowth following nocodazole depolymerization showed that both isoforms contributed to MT nucleation and structural integrity of the centrosome, as cells lacking nineinCAN or nineinISO2 contained multiple ectopic γ-tubulin foci. However, nineinCAN, but not nineinISO2, was important for the separation of duplicated centrosomes during cell division. Despite a requirement of both ninein isoforms to recruit dynein/dynactin to the centrosome, only nineinCAN was required for Golgi positioning and morphology, dynein-dependent events. We additionally found that nineinISO2 was the primary isoform required for F-actin recruitment during the internalization of IgG-opsonized particles. Our study indicates that alternative splicing promotes both redundant and differential activities for ninein in MT organization, organelle positioning, and macrophage function.

Multimodal mechanisms of human centriole engagement and disengagement

Centrioles are unique cellular structures that replicate to produce identical copies, ensuring accurate chromosome segregation during mitosis. A new centriole, the "daughter", is assembled adjacent to an existing "mother" centriole. Only after the daughter centriole is fully developed as a complete replica, does it disengage and become the core of a new functional centrosome. The mechanisms preventing precocious disengagement of the immature daughter centriole have remained unclear. Here, we identify three key mechanisms maintaining mother-daughter centriole engagement: the cartwheel, the torus, and the pericentriolar material (PCM). Among these, the torus critically establishes the characteristic orthogonal engagement. We also demonstrate that engagement mediated by the cartwheel and torus is progressively released during centriole maturation. This release involves structural changes in the daughter, known as centriole blooming and distancing, respectively. Disrupting these structural transitions blocks subsequent steps, preventing centriole disengagement and centrosome conversion in the G1 phase. This study provides a comprehensive understanding of how the maturing daughter centriole progressively disengages from its mother through multiple steps, ensuring its complete structure and conversion into an independent centrosome.

Polo-like kinase 1 maintains transcription and chromosomal accessibility during mitosis

Transcription persists at low levels in mitotic cells and plays essential roles in mitotic fidelity and chromosomal dynamics. However, the detailed regulatory network of mitotic transcription remains largely unresolved. Here, we report the novel role of Polo-like kinase 1 (Plk1) in maintaining mitotic transcription. Using 5-ethynyl uridine (5-EU) labeling of nascent RNAs, we found that Plk1 inhibition leads to significant downregulation of nascent transcription in prometaphase cells. Chromatin-localized Plk1 activity is required for transcription regulation and mitotic fidelity. Plk1 sustains global chromosomal accessibility in mitosis, especially at promoter and transcription start site (promoter-TSS) regions, facilitating transcription factor binding and ensuring proper transcriptional activity. We identified SMC4, a common subunit of condensin I and II, as a potential Plk1 substrate. Plk1 activity is fundamental to these processes across non-transformed and transformed cell lines, underscoring its critical role in cell cycle regulation. This study elucidates a novel regulatory mechanism of global mitotic transcription, advancing our understanding of cell cycle control.

Significance Statement

Cells retain a low level of transcription during mitosis, while the regulatory network and specific contributions of mitotic transcription are not well understood.We identify Polo-like kinase 1 (Plk1) as a novel regulator of mitotic transcription, crucial for chromosome condensation, genome accessibility, and maintaining mitotic fidelity.This study enhances our understanding of Plk1's multifaceted role in mitotic progression, advancing cell cycle regulation knowledge, and informing new cancer therapies' development.

Betrayal From the Core: Centriolar and Cytoskeletal Subversion by Infectious Pathogens

Microbes and parasites have evolved several means to evade and usurp the host cellular machinery to mediate pathogenesis. Being the major microtubule-organizing center (MTOC) of the cell, the centrosome is targeted by multiple viral and nonviral pathogens to mediate their assembly and trafficking within the host cell. This review examines the consequence of such targeting to the centrosome and associated cytoskeletal machinery. We have also amassed a substantial body of evidence of viruses utilizing the cilia within airway epithelium to mediate infection and the hijacking of host cytoskeletal machinery for efficient entry, replication, and egress. While infections have been demonstrated to induce structural, functional, and numerical aberrations in centrosomes, and induce ciliary dysfunction, current literature increasingly supports the notion of a pro-viral role for these organelles. Although less explored, the impact of bacterial and parasitic pathogens on these structures has also been addressed very briefly. Mechanistically, the molecular pathways responsible for these effects remain largely uncharacterized in many instances. Future research focusing on the centriolar triad comprising the centrosome, cilia, and centriolar satellites will undoubtedly provide vital insights into the tactics employed by infectious agents to subvert the host centriole and cytoskeleton-based machinery.

Securin regulates the spatiotemporal dynamics of separase

Separase regulates multiple aspects of the metaphase-to-anaphase transition. Separase cleaves cohesin to allow chromosome segregation and localizes to vesicles to promote exocytosis. The anaphase-promoting complex/cyclosome (APC/C) activates separase by ubiquitinating its inhibitory chaperone, securin, triggering its degradation. How this pathway controls the exocytic function of separase is unknown. During meiosis I, securin is degraded over several minutes, while separase rapidly relocalizes from kinetochore structures at the spindle and cortex to sites of action on chromosomes and vesicles at anaphase onset. The loss of cohesin coincides with the relocalization of separase to the chromosome midbivalent at anaphase onset. APC/C depletion prevents separase relocalization, while securin depletion causes precocious separase relocalization. Expression of non-degradable securin inhibits chromosome segregation, exocytosis, and separase localization to vesicles but not to the anaphase spindle. We conclude that APC/C-mediated securin degradation controls separase localization. This spatiotemporal regulation will impact the effective local concentration of separase for more precise targeting of substrates in anaphase.

Balancing Plk1 activity levels: The secret of synchrony between the cell and the centrosome cycle

The accuracy of cell division requires precise regulation of the cellular machinery governing DNA/genome duplication, ensuring its equal distribution among the daughter cells. The control of the centrosome cycle is crucial for the formation of a bipolar spindle, ensuring error-free segregation of the genome. The cell and centrosome cycles operate in close synchrony along similar principles. Both require a single duplication round in every cell cycle, and both are controlled by the activity of key protein kinases. Nevertheless, our comprehension of the precise cellular mechanisms and critical regulators synchronizing these two cycles remains poorly defined. Here, we present our hypothesis that the spatiotemporal regulation of a dynamic equilibrium of mitotic kinases activities forms a molecular clock that governs the synchronous progression of both the cell and the centrosome cycles.

Amplified centrosomes-more than just a threat

Centrosomes are major organizing components of the tubulin-based cytoskeleton. In recent years, we have gained extensive knowledge about their structure, biogenesis, and function from single cells, cell-cell interactions to tissue homeostasis, including their role in human diseases. Centrosome abnormalities are linked to, among others primary microcephaly, birth defects, ciliopathies, and tumorigenesis. Centrosome amplification, a state where two or more centrosomes are present in the G1 phase of the cell cycle, correlates in cancer with karyotype alterations, clinical aggressiveness, and lymph node metastasis. However, amplified centrosomes also appear in healthy tissues and, independent of their established role, in multi-ciliation. One example is the liver where hepatocytes carry amplified centrosomes owing to whole-genome duplication events during organogenesis. More recently, amplified centrosomes have been found in neuronal progenitors and several cell types of hematopoietic origin in which they enhance cellular effector functions. These findings suggest that extra centrosomes do not necessarily pose a risk for genome integrity and are harnessed for physiological processes. Here, we compare established and emerging 'non-canonical functions' of amplified centrosomes in cancerous and somatic cells and discuss their role in cellular physiology.

Dynamic Mitotic Localization of the Centrosomal Kinases CDK1, Plk, AurK, and Nek2 in Dictyostelium amoebae

The centrosome of the amoebozoan model Dictyostelium discoideum provides the best-established model for an acentriolar centrosome outside the Opisthokonta. Dictyostelium exhibits an unusual centrosome cycle, in which duplication is initiated only at the G2/M transition and occurs entirely during the M phase. Little is known about the role of conserved centrosomal kinases in this process. Therefore, we have generated knock-in strains for Aurora (AurK), CDK1, cyclin B, Nek2, and Plk, replacing the endogenous genes with constructs expressing the respective green fluorescent Neon fusion proteins, driven by the endogenous promoters, and studied their behavior in living cells. Our results show that CDK1 and cyclin B arrive at the centrosome first, already during G2, followed by Plk, Nek2, and AurK. Furthermore, CDK1/cyclin B and AurK were dynamically localized at kinetochores, and AurK in addition at nucleoli. The putative roles of all four kinases in centrosome duplication, mitosis, cytokinesis, and nucleolar dynamics are discussed.

Polo-like kinase 1 prevents excess microtubule polymerization in C. elegans oocytes to ensure faithful meiosis

Sexual reproduction relies on meiosis, a specialized cell division program that produces haploid gametes. Oocytes of most organisms lack centrosomes, and therefore chromosome segregation is mediated by acentrosomal spindles. Here, we explore the role of Polo-like kinase 1 (PLK-1) in C. elegans oocytes, revealing mechanisms that ensure the fidelity of this unique form of cell division. Previously, PLK-1 was shown to be required for nuclear envelope breakdown and chromosome segregation in oocytes. We now find that PLK-1 is also required for establishing and maintaining acentrosomal spindle organization and for preventing excess microtubule polymerization in these cells. Additionally, our studies revealed an unexpected new role for this essential kinase. While PLK-1 is known to be required for centrosome maturation during mitosis, we found that removal of this kinase from oocytes caused premature recruitment of pericentriolar material to the sperm-provided centrioles following fertilization. Thus, PLK-1 suppresses centrosome maturation during oocyte meiosis, which is opposite to its role in mitosis. Taken together, our work reveals multiple new roles for PLK-1 in oocytes, identifying PLK-1 as a key player that promotes faithful acentrosomal meiosis.

Live Cell Monitoring of Separase Activity, a Key Enzymatic Reaction for Chromosome Segregation, with Chimeric FRET-Based Molecular Sensor upon Cell Cycle Progression

Separase is a key cysteine protease in the separation of sister chromatids through the digestion of the cohesin ring that inhibits chromosome segregation as a trigger of the metaphase-anaphase transition in eukaryotes. Its activity is highly regulated by binding with securin and cyclinB-CDK1 complex. These bindings prevent the proteolytic activity of separase until the onset of anaphase. Chromosome missegregation and aneuploidy are frequently observed in malignancies. However, there are some difficulties in biochemical examinations due to the instability of separase in vitro and the fact that few spatiotemporal resolution approaches exist for monitoring live separase activity throughout mitotic processes. Here, we have developed FRET-based molecular sensors, including GFP variants, with separase-cleavable sequences as donors and covalently attached fluorescent dyes as acceptor molecules. These are applicable to conventional live cell imaging and flow cytometric analysis because of efficient live cell uptake. We investigated the performance of equivalent molecular sensors, either localized or not localized inside the nucleus under cell cycle control, using flow cytometry. Synchronized cell cycle progression rendered significant separase activity detections in both molecular sensors. We obtained consistent outcomes with localized molecular sensor introduction and cell cycle control by fluorescent microscopic observations. We thus established live cell separase activity monitoring systems that can be used specifically or statistically, which could lead to the elucidation of separase properties in detail.

Lasiokaurin Regulates PLK1 to Induce Breast Cancer Cell G2/M Phase Block and Apoptosis

Aim of the study: To investigate the anti-tumor effects of Lasiokaurin on breast cancer and explore its underlying molecular mechanism. Materials and methods: In this study, MTT assay, plate colony formation assays, soft agar assay, and EdU assay were employed to evaluate the anti-proliferation effects of LAS. Apoptosis and cell cycle distribution were detected by flow cytometry. The molecular mechanism was predicted by performing RNA sequencing and verified by using immunoblotting assays. Breast cancer organiods derived from patient-derived xenografts model and MDA-MB-231 xenograft mouse model were established to assess the effect of LAS. Results: Our study showed that LAS treatment significantly suppressed cell viability of 5 breast cancer cell lines, with the IC50 value of approximately 1-5 μM. LAS also inhibitied the clonogenic ability and DNA synthesis of breast cancer cells, Moreover, LAS induced apoptosis and G2/M cell cycle arrest in SK-BR-3 and MDA-MB-231 cells. Notably, transcriptomic analysis predicted the mechanistic involvement of PLK1 in LAS-suppressed breast cancer progression. Our experiment data further verified that LAS reduced PLK1 mRNA and protein expression in breast cancer, accompanied by downregulating CDC25C and AKT phosphorylation. Ultimately, we confirmed that LAS inhibit breast cancer growth via inhibiting PLK1 pathway in vivo. Conclusions: Collectively, our findings revealed that LAS inhibits breast cancer progression via regulating PLK1 pathway, which provids scientific evidence for the use of traditional Chinese medicine in cancer therapy.

Role of chromosomal cohesion and separation in aneuploidy and tumorigenesis

Cell division is a crucial process, and one of its essential steps involves copying the genetic material, which is organized into structures called chromosomes. Before a cell can divide into two, it needs to ensure that each newly copied chromosome is paired tightly with its identical twin. This pairing is maintained by a protein complex known as cohesin, which is conserved in various organisms, from single-celled ones to humans. Cohesin essentially encircles the DNA, creating a ring-like structure to handcuff, to keep the newly synthesized sister chromosomes together in pairs. Therefore, chromosomal cohesion and separation are fundamental processes governing the attachment and segregation of sister chromatids during cell division. Metaphase-to-anaphase transition requires dissolution of cohesins by the enzyme Separase. The tight regulation of these processes is vital for safeguarding genomic stability. Dysregulation in chromosomal cohesion and separation resulting in aneuploidy, a condition characterized by an abnormal chromosome count in a cell, is strongly associated with cancer. Aneuploidy is a recurring hallmark in many cancer types, and abnormalities in chromosomal cohesion and separation have been identified as significant contributors to various cancers, such as acute myeloid leukemia, myelodysplastic syndrome, colorectal, bladder, and other solid cancers. Mutations within the cohesin complex have been associated with these cancers, as they interfere with chromosomal segregation, genome organization, and gene expression, promoting aneuploidy and contributing to the initiation of malignancy. In summary, chromosomal cohesion and separation processes play a pivotal role in preserving genomic stability, and aberrations in these mechanisms can lead to aneuploidy and cancer. Gaining a deeper understanding of the molecular intricacies of chromosomal cohesion and separation offers promising prospects for the development of innovative therapeutic approaches in the battle against cancer.

Formation and function of multiciliated cells

In vertebrates, multiciliated cells (MCCs) are terminally differentiated cells that line the airway tracts, brain ventricles, and reproductive ducts. Each MCC contains dozens to hundreds of motile cilia that beat in a synchronized manner to drive fluid flow across epithelia, the dysfunction of which is associated with a group of human diseases referred to as motile ciliopathies, such as primary cilia dyskinesia. Given the dynamic and complex process of multiciliogenesis, the biological events essential for forming multiple motile cilia are comparatively unelucidated. Thanks to advancements in genetic tools, omics technologies, and structural biology, significant progress has been achieved in the past decade in understanding the molecular mechanism underlying the regulation of multiple motile cilia formation. In this review, we discuss recent studies with ex vivo culture MCC and animal models, summarize current knowledge of multiciliogenesis, and particularly highlight recent advances and their implications.

Hexavalent Chromium Targets Securin to Drive Numerical Chromosome Instability in Human Lung Cells

Hexavalent chromium [Cr(VI)] is a known human lung carcinogen with widespread exposure in environmental and occupational settings. Despite well-known cancer risks, the molecular mechanisms of Cr(VI)-induced carcinogenesis are not well understood, but a major driver of Cr(VI) carcinogenesis is chromosome instability. Previously, we reported Cr(VI) induced numerical chromosome instability, premature centriole disengagement, centrosome amplification, premature centromere division, and spindle assembly checkpoint bypass. A key regulator of these events is securin, which acts by regulating the cleavage ability of separase. Thus, in this study we investigated securin disruption by Cr(VI) exposure. We exposed human lung cells to a particulate Cr(VI) compound, zinc chromate, for acute (24 h) and prolonged (120 h) time points. We found prolonged Cr(VI) exposure caused marked decrease in securin levels and function. After prolonged exposure at the highest concentration, securin protein levels were decreased to 15.3% of control cells, while securin mRNA quantification was 7.9% relative to control cells. Additionally, loss of securin function led to increased separase activity manifested as enhanced cleavage of separase substrates; separase, kendrin, and SCC1. These data show securin is targeted by prolonged Cr(VI) exposure in human lung cells. Thus, a new mechanistic model for Cr(VI)-induced carcinogenesis emerges with centrosome and centromere disruption as key components of numerical chromosome instability, a key driver in Cr(VI) carcinogenesis.

Securin Regulates the Spatiotemporal Dynamics of Separase

Separase is a key regulator of the metaphase to anaphase transition with multiple functions. Separase cleaves cohesin to allow chromosome segregation and localizes to vesicles to promote exocytosis in mid-anaphase. The anaphase promoting complex/cyclosome (APC/C) activates separase by ubiquitinating its inhibitory chaperone, securin, triggering its degradation. How this pathway controls the exocytic function of separase has not been investigated. During meiosis I, securin is degraded over several minutes, while separase rapidly relocalizes from kinetochore structures at the spindle and cortex to sites of action on chromosomes and vesicles at anaphase onset. The loss of cohesin coincides with the relocalization of separase to the chromosome midbivalent at anaphase onset. APC/C depletion prevents separase relocalization, while securin depletion causes precocious separase relocalization. Expression of non-degradable securin inhibits chromosome segregation, exocytosis, and separase localization to vesicles but not to the anaphase spindle. We conclude that APC/C mediated securin degradation controls separase localization. This spatiotemporal regulation will impact the effective local concentration of separase for more precise targeting of substrates in anaphase.

Mild replication stress causes premature centriole disengagement via a sub-critical Plk1 activity under the control of ATR-Chk1

A tight synchrony between the DNA and centrosome cycle is essential for genomic integrity. Centriole disengagement, which licenses centrosomes for duplication, occurs normally during mitotic exit. We recently demonstrated that mild DNA replication stress typically seen in cancer cells causes premature centriole disengagement in untransformed mitotic human cells, leading to transient multipolar spindles that favour chromosome missegregation. How mild replication stress accelerates the centrosome cycle at the molecular level remained, however, unclear. Using ultrastructure expansion microscopy, we show that mild replication stress induces premature centriole disengagement already in G2 via the ATR-Chk1 axis of the DNA damage repair pathway. This results in a sub-critical Plk1 kinase activity that primes the pericentriolar matrix for Separase-dependent disassembly but is insufficient for rapid mitotic entry, causing premature centriole disengagement in G2. We postulate that the differential requirement of Plk1 activity for the DNA and centrosome cycles explains how mild replication stress disrupts the synchrony between both processes and contributes to genomic instability.

Reproductive biology: A genetic recipe for parthenogenesis

New work reveals differences in oogenic gene expression between parthenogenetic and sexually reproducing Drosophila mercatorum strains. Recapitulating those changes in D. melanogaster oocytes induced parthenogenesis in this normally sexually reproducing species, providing molecular insight into how these reproductive modes arise.

Hornerin mediates phosphorylation of the polo-box domain in Plk1 by Chk1 to induce death in mitosis

The centrosome assembles a bipolar spindle for faithful chromosome segregation during mitosis. To prevent the inheritance of DNA damage, the DNA damage response (DDR) triggers programmed spindle multipolarity and concomitant death in mitosis through a poorly understood mechanism. We identified hornerin, which forms a complex with checkpoint kinase 1 (Chk1) and polo-like kinase 1 (Plk1) to mediate phosphorylation at the polo-box domain (PBD) of Plk1, as the link between the DDR and death in mitosis. We demonstrate that hornerin mediates DDR-induced precocious centriole disengagement through a dichotomous mechanism that includes sequestration of Sgo1 and Plk1 in the cytoplasm through phosphorylation of the PBD in Plk1 by Chk1. Phosphorylation of the PBD in Plk1 abolishes the interaction with Sgo1 and phosphorylation-dependent Sgo1 translocation to the centrosome, leading to precocious centriole disengagement and spindle multipolarity. Mechanistically, hornerin traps phosphorylated Plk1 in the cytoplasm. Furthermore, PBD phosphorylation inactivates Plk1 and disrupts Cep192::Aurora A::Plk1 complex translocation to the centrosome and concurrent centrosome maturation. Remarkably, hornerin depletion leads to chemoresistance against DNA damaging agents by attenuating DDR-induced death in mitosis. These results reveal how the DDR eradicates mitotic cells harboring DNA damage to ensure genome integrity during cell division.

Separase and Roads to Disengage Sister Chromatids during Anaphase

Receiving complete and undamaged genetic information is vital for the survival of daughter cells after chromosome segregation. The most critical steps in this process are accurate DNA replication during S phase and a faithful chromosome segregation during anaphase. Any errors in DNA replication or chromosome segregation have dire consequences, since cells arising after division might have either changed or incomplete genetic information. Accurate chromosome segregation during anaphase requires a protein complex called cohesin, which holds together sister chromatids. This complex unifies sister chromatids from their synthesis during S phase, until separation in anaphase. Upon entry into mitosis, the spindle apparatus is assembled, which eventually engages kinetochores of all chromosomes. Additionally, when kinetochores of sister chromatids assume amphitelic attachment to the spindle microtubules, cells are finally ready for the separation of sister chromatids. This is achieved by the enzymatic cleavage of cohesin subunits Scc1 or Rec8 by an enzyme called Separase. After cohesin cleavage, sister chromatids remain attached to the spindle apparatus and their poleward movement on the spindle is initiated. The removal of cohesion between sister chromatids is an irreversible step and therefore it must be synchronized with assembly of the spindle apparatus, since precocious separation of sister chromatids might lead into aneuploidy and tumorigenesis. In this review, we focus on recent discoveries concerning the regulation of Separase activity during the cell cycle.

In Search of a Mechanistic Link between Chlamydia trachomatis-Induced Cellular Pathophysiology and Oncogenesis

Centrosome duplication and cell cycle progression are essential cellular processes that must be tightly controlled to ensure cellular integrity. Despite their complex regulatory mechanisms, microbial pathogens have evolved sophisticated strategies to co-opt these processes to promote infection. While misregulation of these processes can greatly benefit the pathogen, the consequences to the host cell can be devastating. During infection, the obligate intracellular pathogen Chlamydia trachomatis induces gross cellular abnormalities, including supernumerary centrosomes, multipolar spindles, and defects in cytokinesis. While these observations were made over 15 years ago, identification of the bacterial factors responsible has been elusive due to the genetic intractability of Chlamydia. Recent advances in techniques of genetic manipulation now allows for the direct linking of bacterial virulence factors to manipulation of centrosome duplication and cell cycle progression. In this review, we discuss the impact, both immediate and downstream, of C. trachomatis infection on the host cell cycle regulatory apparatus and centrosome replication. We highlight links between C. trachomatis infection and cervical and ovarian cancers and speculate whether perturbations of the cell cycle and centrosome are sufficient to initiate cellular transformation. We also explore the biological mechanisms employed by Inc proteins and other secreted effector proteins implicated in the perturbation of these host cell pathways. Future work is needed to better understand the nuances of each effector's mechanism and their collective impact on Chlamydia's ability to induce host cellular abnormalities.

Piezo mechanosensory channels regulate centrosome integrity and mitotic entry

Piezo1 and 2 are evolutionarily conserved mechanosensory cation channels known to function on the cell surface by responding to external pressure and transducing a mechanically activated Ca²⁺ current. Here we show that both Piezo1 and 2 also exhibit concentrated intracellular localization at centrosomes. Both Piezo1 and 2 loss-of-function and Piezo1 activation by the small molecule Yoda1 result in supernumerary centrosomes, premature centriole disengagement, multi-polar spindles, and mitotic delay. By using a GFP, Calmodulin and M13 Protein fusion (GCaMP) Ca²⁺-sensitive reporter, we show that perturbations in Piezo modulate Ca²⁺ flux at centrosomes. Moreover, the inhibition of Polo-like-kinase 1 eliminates Yoda1-induced centriole disengagement. Because previous studies have implicated force generation by microtubules as essential for maintaining centrosomal integrity, we propose that mechanotransduction by Piezo maintains pericentrosomal Ca²⁺ within a defined range, possibly through sensing cell intrinsic forces from microtubules.

BRCA1 transports the DNA damage signal for CDDP-induced centrosome amplification through the centrosomal Aurora A

Breast cancer gene 1 (BRCA1) plays roles in DNA repair and centrosome regulation and is involved in DNA damage-induced centrosome amplification (DDICA). Here, the centrosomal localization of BRCA1 and the kinases involved in centrosome duplication were analyzed in each cell cycle phase after treatment with DNA crosslinker cisplatin (CDDP). CDDP treatment increased the centrosomal localization of BRCA1 in early S-G2 phase. BRCA1 contributed to the increased centrosomal localization of Aurora A in S phase and that of phosphorylated Polo-like kinase 1 (PLK1) in late S phase after CDDP treatment, resulting in centriole disengagement and overduplication. The increased centrosomal localization of BRCA1 and Aurora A induced by CDDP treatment involved the nuclear export of BRCA1 and BRCA1 phosphorylation by ataxia telangiectasia mutated (ATM). Patient-derived variants and mutations at phosphorylated residues of BRCA1 suppressed the interaction between BRCA1 and Aurora A, as well as the CDDP-induced increase in the centrosomal localization of BRCA1 and Aurora A. These results suggest that CDDP induces the phosphorylation of BRCA1 by ATM in the nucleus and its transport to the cytoplasm, thereby promoting the centrosomal localization Aurora A, which phosphorylates PLK1. The function of BRCA1 in the translocation of the DNA damage signal from the nucleus to the centrosome to induce centrosome amplification after CDDP treatment might support its role as a tumor suppressor.

Cartwheel disassembly regulated by CDK1-cyclin B kinase allows human centriole disengagement and licensing

Cartwheel assembly is considered the first step in the initiation of procentriole biogenesis; however, the reason for persistence of the assembled human cartwheel structure from S phase to late mitosis remains unclear. Here, we demonstrate mainly using cell synchronization, RNA interference, immunofluorescence and time-lapse-microscopy, biochemical analysis, and methods that the cartwheel persistently assembles and maintains centriole engagement and centrosome integrity during S phase to late G2 phase. Blockade of the continuous accumulation of centriolar Sas-6, a major cartwheel protein, after procentriole formation induces premature centriole disengagement and disrupts pericentriolar matrix integrity. Additionally, we determined that during mitosis, CDK1-cyclin B phosphorylates Sas-6 at T495 and S510, disrupting its binding to cartwheel component STIL and pericentriolar component Nedd1 and promoting cartwheel disassembly and centriole disengagement. Perturbation of this phosphorylation maintains the accumulation of centriolar Sas-6 and retains centriole engagement during mitotic exit, which results in the inhibition of centriole reduplication. Collectively, these data demonstrate that persistent cartwheel assembly after procentriole formation maintains centriole engagement and that this configuration is relieved through phosphorylation of Sas-6 by CDK1-cyclin B during mitosis in human cells.

Mapping separase-mediated cleavage in situ

Separase is a protease that performs critical functions in the maintenance of genetic homeostasis. Among them, the cleavage of the meiotic cohesin during meiosis is a key step in producing gametes in eukaryotes. However, the exact chromosomal localization of this proteolytic cleavage was not addressed due to the lack of experimental tools. To this end, we developed a method based on monoclonal antibodies capable of recognizing the predicted neo-epitopes produced by separase-mediated proteolysis in the RAD21 and REC8 cohesin subunits. To validate the epigenomic strategy of mapping cohesin proteolysis, anti-RAD21 neo-epitopes antibodies were used in ChIP-On-ChEPseq analysis of human cells undergoing mitotic anaphase. Second, a similar analysis applied for mapping of REC8 cleavage in germline cells in Macaque showed a correlation with a subset of alpha-satellites and other repeats, directly demonstrating that the site-specific mei-cohesin proteolysis hotspots are coincident but not identical with centromeres. The sequences for the corresponding immunoglobulin genes show a convergence of antibodies with close specificity. This approach could be potentially used to investigate cohesin ring opening events in other chromosomal locations, if applied to single cells.

Emerging roles of centrosome cohesion

The centrosome, consisting of centrioles and the associated pericentriolar material, is the main microtubule-organizing centre (MTOC) in animal cells. During most of interphase, the two centrosomes of a cell are joined together by centrosome cohesion into one MTOC. The most dominant element of centrosome cohesion is the centrosome linker, an interdigitating, fibrous network formed by the protein C-Nap1 anchoring a number of coiled-coil proteins including rootletin to the proximal end of centrioles. Alternatively, centrosomes can be kept together by the action of the minus end directed kinesin motor protein KIFC3 that works on interdigitating microtubules organized by both centrosomes and probably by the actin network. Although cells connect the two interphase centrosomes by several mechanisms into one MTOC, the general importance of centrosome cohesion, particularly for an organism, is still largely unclear. In this article, we review the functions of the centrosome linker and discuss how centrosome cohesion defects can lead to diseases.

Recent Progress on the Localization of PLK1 to the Kinetochore and Its Role in Mitosis

The accurate distribution of the replicated genome during cell division is essential for cell survival and healthy organismal development. Errors in this process have catastrophic consequences, such as birth defects and aneuploidy, a hallmark of cancer cells. PLK1 is one of the master kinases in mitosis and has multiple functions, including mitotic entry, chromosome segregation, spindle assembly checkpoint, and cytokinesis. To dissect the role of PLK1 in mitosis, it is important to understand how PLK1 localizes in the specific region in cells. PLK1 localizes at the kinetochore and is essential in spindle assembly checkpoint and chromosome segregation. However, how PLK1 localizes at the kinetochore remains elusive. Here, we review the recent literature on the kinetochore recruitment mechanisms of PLK1 and its roles in spindle assembly checkpoint and attachment between kinetochores and spindle microtubules. Together, this review provides an overview of how the local distribution of PLK1 could regulate major pathways in mitosis.

Emi2 enables centriole amplification during multiciliated cell differentiation

Massive centriole amplification during multiciliated cell (MCC) differentiation is a notable example of organelle biogenesis. This process is thought to be enabled by a derived cell cycle state, but the key cell cycle components required for centriole amplification in MCC progenitors remain poorly defined. Here, we show that emi2 (fbxo43) expression is up-regulated and acts in MCC progenitors after cell cycle exit to transiently inhibit anaphase-promoting complex/cyclosome (APC/C)cdh1 activity. We find that this inhibition is required for the phosphorylation and activation of a key cell cycle kinase, plk1, which acts, in turn, to promote different steps required for centriole amplification and basal body formation, including centriole disengagement, apical migration, and maturation into basal bodies. This emi2-APC/C-plk1 axis is also required to down-regulate gene expression essential for centriole amplification after differentiation is complete. These results identify an emi2-APC/C-plk1 axis that promotes and then terminates centriole assembly and basal body formation during MCC differentiation.

Role of Polo-like Kinases Plk1 and Plk4 in the Initiation of Centriole Duplication-Impact on Cancer

Centrosomes nucleate and anchor microtubules and therefore play major roles in spindle formation and chromosome segregation during mitosis. Duplication of the centrosome occurs, similar to DNA, only once during the cell cycle. Aberration of the centrosome number is common in human tumors. At the core of centriole duplication is the conserved polo-like kinase 4, Plk4, and two structural proteins, STIL and Sas-6. In this review, I summarize and discuss developments in our understanding of the first steps of centriole duplication and their regulation.

Microtubule and Actin Cytoskeletal Dynamics in Male Meiotic Cells of Drosophila melanogaster

Drosophila dividing spermatocytes offer a highly suitable cell system in which to investigate the coordinated reorganization of microtubule and actin cytoskeleton systems during cell division of animal cells. Like male germ cells of mammals, Drosophila spermatogonia and spermatocytes undergo cleavage furrow ingression during cytokinesis, but abscission does not take place. Thus, clusters of primary and secondary spermatocytes undergo meiotic divisions in synchrony, resulting in cysts of 32 secondary spermatocytes and then 64 spermatids connected by specialized structures called ring canals. The meiotic spindles in Drosophila males are substantially larger than the spindles of mammalian somatic cells and exhibit prominent central spindles and contractile rings during cytokinesis. These characteristics make male meiotic cells particularly amenable to immunofluorescence and live imaging analysis of the spindle microtubules and the actomyosin apparatus during meiotic divisions. Moreover, because the spindle assembly checkpoint is not robust in spermatocytes, Drosophila male meiosis allows investigating of whether gene products required for chromosome segregation play additional roles during cytokinesis. Here, we will review how the research studies on Drosophila male meiotic cells have contributed to our knowledge of the conserved molecular pathways that regulate spindle microtubules and cytokinesis with important implications for the comprehension of cancer and other diseases.

Differential Expression of Mitosis and Cell Cycle Regulatory Genes during Recovery from an Acute Respiratory Virus Infection

Acute respiratory virus infections can have profound and long-term effects on lung function that persist even after the acute responses have fully resolved. In this study, we examined gene expression by RNA sequencing in the lung tissue of wild-type BALB/c mice that were recovering from a sublethal infection with the pneumonia virus of mice (PVM), a natural rodent pathogen of the same virus family and genus as the human respiratory syncytial virus. We compared these responses to gene expression in PVM-infected mice treated with Lactobacillus plantarum, an immunobiotic agent that limits inflammation and averts the negative clinical sequelae typically observed in response to acute infection with this pathogen. Our findings revealed prominent differential expression of inflammation-associated genes as well as numerous genes and gene families implicated in mitosis and cell-cycle regulation, including cyclins, cyclin-dependent kinases, cell division cycle genes, E2F transcription factors, kinesins, centromere proteins, and aurora kinases, among others. Of particular note was the differential expression of the cell division cycle gene Cdc20b, which was previously identified as critical for the ex vivo differentiation of multi-ciliated cells. Collectively, these findings provided us with substantial insight into post-viral repair processes and broadened our understanding of the mechanisms underlying Lactobacillus-mediated protection.

Roles of RACK1 in centrosome regulation and carcinogenesis

Receptor for activated C kinase 1 (RACK1) regulates various cellular functions and signaling pathways by interacting with different proteins. Recently, we showed that RACK1 interacts with breast cancer gene 1 (BRCA1), which regulates centrosome duplication. RACK1 localizes to centrosomes and spindle poles and is involved in the proper centrosomal localization of BRCA1. The interaction between RACK1 and BRCA1 is critical for the regulation of centrosome number. In addition, RACK1 contributes to centriole duplication by regulating polo-like kinase 1 (PLK1) activity in S phase. RACK1 binds directly to PLK1 and Aurora A, promoting the phosphorylation of PLK1 and activating the Aurora A/PLK1 signaling axis. Overexpression of RACK1 causes centrosome amplification, especially in mammary gland epithelial cells, inducing overactivation of PLK1 followed by premature centriole disengagement and centriole re-duplication. Other proteins, including hypoxia-inducible factor α, von Hippel-Lindau protein, heat-shock protein 90, β-catenin, and glycogen synthase kinase-3β, interact with RACK1 and play roles in centrosome regulation. In this review, we focus on the roles and underlying molecular mechanisms of RACK1 in centrosome regulation mediated by its interaction with different proteins and the modulation of their functions.

Polo-like kinase 1 (PLK1) signaling in cancer and beyond

PLK1 is an evolutionary conserved Ser/Thr kinase that is best known for its role in cell cycle regulation and is expressed predominantly during the G2/S and M phase of the cell cycle. PLK1-mediated phosphorylation of specific substrates controls cell entry into mitosis, centrosome maturation, spindle assembly, sister chromatid cohesion and cytokinesis. In addition, a growing body of evidence describes additional roles of PLK1 beyond the cell cycle, more specifically in the DNA damage response, autophagy, apoptosis and cytokine signaling. PLK1 has an indisputable role in cancer as it controls several key transcription factors and promotes cell proliferation, transformation and epithelial-to-mesenchymal transition. Furthermore, deregulation of PLK1 results in chromosome instability and aneuploidy. PLK1 is overexpressed in many cancers, which is associated with poor prognosis, making PLK1 an attractive target for cancer treatment. Additionally, PLK1 is involved in immune and neurological disorders including Graft versus Host Disease, Huntington's disease and Alzheimer's disease. Unfortunately, newly developed small compound PLK1 inhibitors have only had limited success so far, due to low therapeutic response rates and toxicity. In this review we will highlight the current knowledge about the established roles of PLK1 in mitosis regulation and beyond. In addition, we will discuss its tumor promoting but also tumor suppressing capacities, as well as the available PLK1 inhibitors, elaborating on their efficacy and limitations.

The SON RNA splicing factor is required for intracellular trafficking structures that promote centriole assembly and ciliogenesis

Control of centrosome assembly is critical for cell division, intracellular trafficking, and cilia. Regulation of centrosome number occurs through the precise duplication of centrioles that reside in centrosomes. Here we explored transcriptional control of centriole assembly and find that the RNA splicing factor SON is specifically required for completing procentriole assembly. Whole genome mRNA sequencing identified genes whose splicing and expression are affected by the reduction of SON, with an enrichment in genes involved in the microtubule (MT) cytoskeleton, centrosome, and centriolar satellites. SON is required for the proper splicing and expression of CEP131, which encodes a major centriolar satellite protein and is required to organize the trafficking and MT network around the centrosomes. This study highlights the importance of the distinct MT trafficking network that is intimately associated with nascent centrioles and is responsible for procentriole development and efficient ciliogenesis.

A specific basal body linker protein provides the connection function for basal body inheritance in trypanosomes

Centrioles and basal bodies (CBBs) are found in physically linked pairs, and in mammalian cells intercentriole connections (G1-G2 tether and S-M linker) regulate centriole duplication and function. In trypanosomes BBs are not associated with the spindle and function in flagellum/cilia nucleation with an additional role in mitochondrial genome (kinetoplast DNA [kDNA]) segregation. Here, we describe BBLP, a BB/pro-BB (pBB) linker protein in Trypanosoma brucei predicted to be a large coiled-coil protein conserved in the kinetoplastida. Colocalization with the centriole marker SAS6 showed that BBLP localizes between the BB/pBB pair, throughout the cell cycle, with a stronger signal in the old flagellum BB/pBB pair. Importantly, RNA interference (RNAi) depletion of BBLP leads to a conspicuous splitting of the BB/pBB pair associated only with the new flagellum. BBLP RNAi is lethal in the bloodstream form of the parasite and perturbs mitochondrial kDNA inheritance. Immunogold labeling confirmed that BBLP is localized to a cytoskeletal component of the BB/pBB linker, and tagged protein induction showed that BBLP is incorporated de novo in both new and old flagella BB pairs of dividing cells. We show that the two aspects of CBB disengagement-loss of orthogonal orientation and ability to separate and move apart-are consistent but separable events in evolutionarily diverse cells and we provide a unifying model explaining centriole/BB linkage differences between such cells.

Biophysical and Quantitative Principles of Centrosome Biogenesis and Structure

The centrosome is a main orchestrator of the animal cellular microtubule cytoskeleton. Dissecting its structure and assembly mechanisms has been a goal of cell biologists for over a century. In the last two decades, a good understanding of the molecular constituents of centrosomes has been achieved. Moreover, recent breakthroughs in electron and light microscopy techniques have enabled the inspection of the centrosome and the mapping of its components with unprecedented detail. However, we now need a profound and dynamic understanding of how these constituents interact in space and time. Here, we review the latest findings on the structural and molecular architecture of the centrosome and how its biogenesis is regulated, highlighting how biophysical techniques and principles as well as quantitative modeling are changing our understanding of this enigmatic cellular organelle. Expected final online publication date for the Annual Review of Cell and Developmental Biology, Volume 37 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Ana1 helps recruit Polo to centrioles to promote mitotic PCM assembly and centriole elongation

Polo kinase (PLK1 in mammals) is a master cell cycle regulator that is recruited to various subcellular structures, often by its polo-box domain (PBD), which binds to phosphorylated S-pS/pT motifs. Polo/PLK1 kinases have multiple functions at centrioles and centrosomes, and we have previously shown that in Drosophila phosphorylated Sas-4 initiates Polo recruitment to newly formed centrioles, while phosphorylated Spd-2 recruits Polo to the pericentriolar material (PCM) that assembles around mother centrioles in mitosis. Here, we show that Ana1 (Cep295 in humans) also helps to recruit Polo to mother centrioles in Drosophila. If Ana1-dependent Polo recruitment is impaired, mother centrioles can still duplicate, disengage from their daughters and form functional cilia, but they can no longer efficiently assemble mitotic PCM or elongate during G2. We conclude that Ana1 helps recruit Polo to mother centrioles to specifically promote mitotic centrosome assembly and centriole elongation in G2, but not centriole duplication, centriole disengagement or cilia assembly. This article has an associated First Person interview with the first author of the paper.

The careful control of Polo kinase by APC/C-Ube2C ensures the intercellular transport of germline centrosomes during Drosophila oogenesis

A feature of metazoan reproduction is the elimination of maternal centrosomes from the oocyte. In animals that form syncytial cysts during oogenesis, including Drosophila and human, all centrosomes within the cyst migrate to the oocyte where they are subsequently degenerated. The importance and the underlying mechanism of this event remain unclear. Here, we show that, during early Drosophila oogenesis, control of the Anaphase Promoting Complex/Cyclosome (APC/C), the ubiquitin ligase complex essential for cell cycle control, ensures proper transport of centrosomes into the oocyte through the regulation of Polo/Plk1 kinase, a critical regulator of the integrity and activity of the centrosome. We show that novel mutations in the APC/C-specific E2, Vihar/Ube2c, that affect its inhibitory regulation on APC/C cause precocious Polo degradation and impedes centrosome transport, through destabilization of centrosomes. The failure of centrosome migration correlates with weakened microtubule polarization in the cyst and allows ectopic microtubule nucleation in nurse cells, leading to the loss of oocyte identity. These results suggest a role for centrosome migration in oocyte fate maintenance through the concentration and confinement of microtubule nucleation activity into the oocyte. Considering the conserved roles of APC/C and Polo throughout the animal kingdom, our findings may be translated into other animals.

PLK4-phosphorylated NEDD1 facilitates cartwheel assembly and centriole biogenesis initiations

Centrosome duplication occurs under strict spatiotemporal regulation once per cell cycle, and it begins with cartwheel assembly and daughter centriole biogenesis at the lateral sites of the mother centrioles. However, although much of this process is understood, how centrosome duplication is initiated remains unclear. Here, we show that cartwheel assembly followed by daughter centriole biogenesis is initiated on the NEDD1-containing layer of the pericentriolar material (PCM) by the recruitment of SAS-6 to the mother centriole under the regulation of PLK4. We found that PLK4-mediated phosphorylation of NEDD1 at its S325 amino acid residue directly promotes both NEDD1 binding to SAS-6 and recruiting SAS-6 to the centrosome. Overexpression of phosphomimicking NEDD1 mutant S325E promoted cartwheel assembly and daughter centriole biogenesis initiations, whereas overexpression of nonphosphorylatable NEDD1 mutant S325A abolished the initiations. Collectively, our results demonstrate that PLK4-regulated NEDD1 facilitates initiation of the cartwheel assembly and of daughter centriole biogenesis in mammals.

Dysregulation of the centrosome induced by BRCA1 deficiency contributes to tissue-specific carcinogenesis

Alterations in breast cancer gene 1 (BRCA1), a tumor suppressor gene, increase the risk of breast and ovarian cancers. BRCA1 forms a heterodimer with BRCA1-associated RING domain protein 1 (BARD1) and functions in multiple cellular processes, including DNA repair and centrosome regulation. BRCA1 acts as a tumor suppressor by promoting homologous recombination (HR) repair, and alterations in BRCA1 cause HR deficiency, not only in breast and ovarian tissues but also in other tissues. The molecular mechanisms underlying BRCA1 alteration-induced carcinogenesis remain unclear. Centrosomes are the major microtubule-organizing centers and function in bipolar spindle formation. The regulation of centrosome number is critical for chromosome segregation in mitosis, which maintains genomic stability. BRCA1/BARD1 function in centrosome regulation together with Obg-like ATPase (OLA1) and receptor for activating protein C kinase 1 (RACK1). Cancer-derived variants of BRCA1, BARD1, OLA1, and RACK1 do not interact, and aberrant expression of these proteins results in abnormal centrosome duplication in mammary-derived cells, and rarely in other cell types. RACK1 is involved in centriole duplication in the S phase by promoting polo-like kinase 1 activation by Aurora A, which is critical for centrosome duplication. Centriole number is higher in cells derived from mammary tissues compared with in those derived from other tissues, suggesting that tissue-specific centrosome characterization may shed light on the tissue specificity of BRCA1-associated carcinogenesis. Here, we explored the role of the BRCA1-containing complex in centrosome regulation and the effect of its deficiency on tissue-specific carcinogenesis.

14-3-3γ prevents centrosome duplication by inhibiting NPM1 function

14-3-3 proteins bind to ligands via phospho-serine containing consensus motifs. However, the molecular mechanisms underlying complex formation and dissociation between 14-3-3 proteins and their ligands remain unclear. We identified two conserved acidic residues in the 14-3-3 peptide-binding pocket (D129 and E136) that potentially regulate complex formation and dissociation. Altering these residues to alanine led to opposing effects on centrosome duplication. D129A inhibited centrosome duplication, whereas E136A stimulated centrosome amplification. These results were due to the differing abilities of these mutant proteins to form a complex with NPM1. Inhibiting complex formation between NPM1 and 14-3-3γ led to an increase in centrosome duplication and over-rode the ability of D129A to inhibit centrosome duplication. We identify a novel role of 14-3-3γ in regulating centrosome licensing and a novel mechanism underlying the formation and dissociation of 14-3-3 ligand complexes dictated by conserved residues in the 14-3-3 family.

Cep57 and Cep57L1 maintain centriole engagement in interphase to ensure centriole duplication cycle

Centrioles duplicate in interphase only once per cell cycle. Newly formed centrioles remain associated with their mother centrioles. The two centrioles disengage at the end of mitosis, which licenses centriole duplication in the next cell cycle. Therefore, timely centriole disengagement is critical for the proper centriole duplication cycle. However, the mechanisms underlying centriole engagement during interphase are poorly understood. Here, we show that Cep57 and Cep57L1 cooperatively maintain centriole engagement during interphase. Codepletion of Cep57 and Cep57L1 induces precocious centriole disengagement in interphase without compromising cell cycle progression. The disengaged daughter centrioles convert into centrosomes during interphase in a Plk1-dependent manner. Furthermore, the centrioles reduplicate and the centriole number increases, which results in chromosome segregation errors. Overall, these findings demonstrate that the maintenance of centriole engagement by Cep57 and Cep57L1 during interphase is crucial for the tight control of centriole copy number and thus for proper chromosome segregation.

TRIM37 prevents formation of centriolar protein assemblies by regulating Centrobin

TRIM37 is an E3 ubiquitin ligase mutated in Mulibrey nanism, a disease with impaired organ growth and increased tumor formation. TRIM37 depletion from tissue culture cells results in supernumerary foci bearing the centriolar protein Centrin. Here, we characterize these centriolar protein assemblies (Cenpas) to uncover the mechanism of action of TRIM37. We find that an atypical de novo assembly pathway can generate Cenpas that act as microtubule-organizing centers (MTOCs), including in Mulibrey patient cells. Correlative light electron microscopy reveals that Cenpas are centriole-related or electron-dense structures with stripes. TRIM37 regulates the stability and solubility of Centrobin, which accumulates in elongated entities resembling the striped electron dense structures upon TRIM37 depletion. Furthermore, Cenpas formation upon TRIM37 depletion requires PLK4, as well as two parallel pathways relying respectively on Centrobin and PLK1. Overall, our work uncovers how TRIM37 prevents Cenpas formation, which would otherwise threaten genome integrity.

Non-mitotic functions of polo-like kinases in cancer cells

Inhibitors of mitotic protein kinases are currently being developed as non-neurotoxic alternatives of microtubule-targeting agents (taxanes, vinca alkaloids) which provide a substantial survival benefit for patients afflicted with different types of solid tumors. Among the mitotic kinases, the cyclin-dependent kinases, the Aurora kinases, the kinesin spindle protein and Polo-like kinases (PLKs) have emerged as attractive targets of cancer therapeutics. The functions of mammalian PLK1-5 are traditionally linked to the regulation of the cell cycle and to the stress response. Especially the key role of PLK1 and PLK4 in cellular growth and proliferation, their overexpression in multiple types of human cancer and their druggability, make them appealing targets for cancer therapy. Inhibitors for PLK1 and PLK4 are currently being tested in multiple cancer trials. The clinical success of microtubule-targeting agents is attributed not solely to the induction of a mitotic arrest in cancer cells, but also to non-mitotic effects like targeting intracellular trafficking on microtubules. This raises the question whether new cancer targets like PLK1 and PLK4 regulate critical non-mitotic functions in tumor cells. In this article we summarize the important roles of PLK1-5 for the regulation of non-mitotic signaling. Due to these functions it is conceivable that inhibitors for PLK1 or PLK4 can target interphase cells, which underscores their attractive potential as cancer drug targets. Moreover, we also describe the contribution of the tumor-suppressors PLK2, PLK3 and PLK5 to cancer cell signaling outside of mitosis. These observations highlight the urgent need to develop highly specific ATP-competitive inhibitors for PLK4 and for PLK1 like the 3rd generation PLK-inhibitor Onvansertib to prevent the inhibition of tumor-suppressor PLKs in- and outside of mitosis. The remarkable feature of PLKs to encompass a unique druggable domain, the polo-box-domain (PBD) that can be found only in PLKs offers the opportunity for the development of inhibitors that target PLKs exclusively. Beyond the development of mono-specific ATP-competitive PLK inhibitors, the PBD as drug target will support the design of new drugs that eradicate cancer cells based on the mitotic and non-mitotic function of PLK1 and PLK4.

Structure and Function of the Separase-Securin Complex

Separase is a large cysteine protease in eukaryotes and has crucial roles in many cellular processes, especially chromosome segregation during mitosis and meiosis, apoptosis, DNA damage repair, centrosome disengagement and duplication, spindle stabilization and elongation. It dissolves the cohesion between sister chromatids by cleaving one of the subunits of the cohesin ring for chromosome segregation. The activity of separase is tightly controlled at many levels, through direct binding of inhibitory proteins as well as posttranslational modification. Dysregulation of separase activity is linked to cancer and genome instability, making it a target for drug discovery. One of the best-known inhibitors of separase is securin, which has been identified in yeast, plants, and animals. Securin forms a tight complex with separase and potently inhibits its catalytic activity. Recent structures of the separase-securin complex have revealed the molecular mechanism for the inhibitory activity of securin. A segment of securin is bound in the active site of separase, thereby blocking substrate binding. Securin itself is not cleaved by separase as its binding mode is not compatible with catalysis. Securin also has extensive interactions with separase outside the active site, consistent with its function as a chaperone to stabilize this enzyme.

Serum ESPL1 Can Be Used as a Biomarker for Patients With Hepatitis B Virus-Related Liver Cancer: A Chinese Case-Control Study

AIMS

To investigate the feasibility of serum extra spindle pole bodies-like 1 (ESPL1) used as a biomarker for patients with hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC).

METHODS

131 chronic HBV-infection patients were recruited and divided into HBV S gene integration, non-HBV S gene integration, chronic hepatitis B (CHB), HBV-related liver cirrhosis (LC) and HBV-related HCC group, 24 non-HBV-related HCC patients were selected as HCC control group, 30 people without HBV-infection as healthy control group. Serum ESPL1 were detected and compared.

RESULTS

ESPL1 level of integration group was significantly higher than that of non-integration group (346.7 vs 199.6 ng/ml, P = 0.000) and healthy control group (346.7 vs 41.3 ng/ml, P = 0.000). ESPL1 level of non-integration group was significantly higher than that of healthy control group (199.6 vs 41.3 ng/ml, P = 0.000); ESPL1 levels in chronic HBV-infection related groups were increased in turn according to CHB group (95.8 ng/ml), HBV-related LC group (268.2 ng/ml), HBV-related HCC group (279.9 ng/ml) and integration group (346.7 ng/ml). Except that there was no significant difference in ESPL1 levels between HBV-related LC and HCC group (P = 0.662), pairwise comparisons between other groups showed significant differences (P < 0.05). ESPL1 level of HBV-related HCC group was significantly higher than that of non-HBV-related HCC group (279.9 vs 46.6 ng/ml, P = 0.000), there was no noticeable difference between non-HBV-related HCC and healthy control group (46.6 vs 41.3 ng/ml, P = 0.848). ESPL1 level of HBV-related HCC group after resection was significantly lower than that of before resection (178.4 vs 260.8 ng/ml, P = 0.000).

CONCLUSIONS

Chronic HBV-infection patients with high ESPL1 level may indicate HBV S gene integration and is a high-risk population for HBV-related HCC. Serum ESPL1 can be used as a biomarker for screening HBV-related HCC high-risk population and monitoring recurrence.

TTK, CDC25A, and ESPL1 as Prognostic Biomarkers for Endometrial Cancer

Objective

Endometrial cancer (EC) is one of the most common malignant gynaecological tumours worldwide. This study was aimed at identifying EC prognostic genes and investigating the molecular mechanisms of these genes in EC.

Methods

Two mRNA datasets of EC were downloaded from the Gene Expression Omnibus (GEO). The GEO2R tool and Draw Venn Diagram were used to identify differentially expressed genes (DEGs) between normal endometrial tissues and EC tissues. Then, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID). Next, the protein-protein interactions (PPIs) of these DEGs were determined by the Search Tool for the Retrieval of Interacting Genes (STRING) tool and Cytoscape with Molecular Complex Detection (MCODE). Furthermore, Kaplan-Meier survival analysis was performed by UALCAN to verify genes associated with significantly poor prognosis. Next, Gene Expression Profiling Interactive Analysis (GEPIA) was used to verify the expression levels of these selected genes. Additionally, a reanalysis of the KEGG pathways was performed to understand the potential biological functions of selected genes. Finally, the associations between these genes and clinical features were analysed based on TCGA cancer genomic datasets for EC.

Results

In EC tissues, compared with normal endometrial tissues, 147 of 249 DEGs were upregulated and 102 were downregulated. A total of 64 upregulated genes were assembled into a PPI network. Next, 14 genes were found to be both associated with significantly poor prognosis and highly expressed in EC tissues. Reanalysis of the KEGG pathways found that three of these genes were enriched in the cell cycle pathway. TTK, CDC25A, and ESPL1 showed higher expression in cancers with late stage and higher tumour grade.

Conclusion

In summary, through integrated bioinformatics approaches, we found three significant prognostic genes of EC, which might be potential therapeutic targets for EC patients.

Cohesin subunit RAD21: From biology to disease

RAD21 (also known as KIAA0078, NXP1, HR21, Mcd1, Scc1, and hereafter called RAD21), an essential gene, encodes a DNA double-strand break (DSB) repair protein that is evolutionarily conserved in all eukaryotes from budding yeast to humans. RAD21 protein is a structural component of the highly conserved cohesin complex consisting of RAD21, SMC1a, SMC3, and SCC3 [STAG1 (SA1) and STAG2 (SA2) in metazoans] proteins, involved in sister chromatid cohesion. This function is essential for proper chromosome segregation, post-replicative DNA repair, and prevention of inappropriate recombination between repetitive regions. In interphase, cohesin also functions in the control of gene expression by binding to numerous sites within the genome. In addition to playing roles in the normal cell cycle and DNA DSB repair, RAD21 is also linked to the apoptotic pathways. Germline heterozygous or homozygous missense mutations in RAD21 have been associated with human genetic disorders, including developmental diseases such as Cornelia de Lange syndrome (CdLS) and chronic intestinal pseudo-obstruction (CIPO) called Mungan syndrome, respectively, and collectively termed as cohesinopathies. Somatic mutations and amplification of the RAD21 have also been widely reported in both human solid and hematopoietic tumors. Considering the role of RAD21 in a broad range of cellular processes that are hot spots in neoplasm, it is not surprising that the deregulation of RAD21 has been increasingly evident in human cancers. Herein, we review the biology of RAD21 and the cellular processes that this important protein regulates and discuss the significance of RAD21 deregulation in cancer and cohesinopathies.

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.

The centriole protein CEP76 negatively regulates PLK1 activity in the cytoplasm for proper mitotic progression

Polo-like kinase 1 (PLK1) dynamically changes its localization and plays important roles in proper mitotic progression. In particular, strict control of cytoplasmic PLK1 is needed to prevent mitotic defects. However, the regulation of cytoplasmic PLK1 is not fully understood. In this study, we show that CEP76, a centriolar protein, physically interacts with PLK1 and tightly controls the activation of cytoplasmic PLK1 during mitosis in human cells. We found that removal of centrosomes induced ectopic aggregation of PLK1, which is highly phosphorylated, in the cytoplasm during mitosis. Importantly, a targeted RNAi screen revealed that depletion of CEP76 resulted in a similar phenotype. In addition, depletion of CEP76 caused defective spindle orientation and mitotic delay. Moreover, the formation of ectopic PLK1 aggregates and defective spindle orientation were significantly suppressed by the inhibition of PLK1 kinase activity. Overall, these results demonstrate that CEP76 suppresses the aberrant activation of cytoplasmic PLK1 for proper mitotic progression.This article has an associated First Person interview with the first author of the paper.

RACK1 regulates centriole duplication through promoting the activation of polo-like kinase 1 by Aurora A

Breast cancer gene 1 (BRCA1) contributes to the regulation of centrosome number. We previously identified receptor for activated C kinase 1 (RACK1) as a BRCA1-interacting partner. RACK1, a scaffold protein that interacts with multiple proteins through its seven WD40 domains, directly binds to BRCA1 and localizes to centrosomes. RACK1 knockdown suppresses centriole duplication, whereas RACK1 overexpression causes centriole overduplication in a subset of mammary gland-derived cells. In this study, we showed that RACK1 binds directly to polo-like kinase 1 (PLK1) and Aurora A, and promotes the Aurora A-PLK1 interaction. RACK1 knockdown decreased phosphorylated PLK1 (p-PLK1) levels and the centrosomal localization of Aurora A and p-PLK1 in S phase, whereas RACK1 overexpression increased p-PLK1 level and the centrosomal localization of Aurora A and p-PLK1 in interphase, resulting in an increase of cells with abnormal centriole disengagement. Overexpression of cancer-derived RACK1 variants failed to enhance the Aurora A-PLK1 interaction, PLK1 phosphorylation and the centrosomal localization of p-PLK1. These results suggest that RACK1 functions as a scaffold protein that promotes the activation of PLK1 by Aurora A in order to promote centriole duplication.This article has an associated First Person interview with the first author of the paper.