Phosphoproteome analysis of the human mitotic spindle

Vaccinia virus subverts xenophagy through phosphorylation and nuclear targeting of p62

Autophagy is an essential degradation program required for cell homeostasis. Among its functions is the engulfment and destruction of cytosolic pathogens, termed xenophagy. Not surprisingly, many pathogens use various strategies to circumvent or co-opt autophagic degradation. For poxviruses, it is known that infection activates autophagy, which however is not required for successful replication. Even though these complex viruses replicate exclusively in the cytoplasm, autophagy-mediated control of poxvirus infection has not been extensively explored. Using the prototypic poxvirus, vaccinia virus (VACV), we show that overexpression of the xenophagy receptors p62, NDP52, and Tax1Bp1 restricts poxvirus infection. While NDP52 and Tax1Bp1 were degraded, p62 initially targeted cytoplasmic virions before being shunted to the nucleus. Nuclear translocation of p62 was dependent upon p62 NLS2 and correlated with VACV kinase mediated phosphorylation of p62 T269/S272. This suggests that VACV targets p62 during the early stages of infection to avoid destruction and further implies that poxviruses exhibit multi-layered control of autophagy to facilitate cytoplasmic replication.

Rare HER2 L796P missense mutation promotes the growth and oncogenic signaling in breast cancer cells

PURPOSE

This research aimed to find potential HER2 mutations that would have an impact on breast cancer and investigate the underlying mechanism.

EXPERIMENTAL DESIGN

This study first investigated 238 pairs of breast cancer and para-cancerous tissue samples from patients on the targeted next-generation sequencing (tNGS) platform. CCK-8 and clone formation assay were used to investigate whether the mutation exerts proliferative effects on breast cancer cells. In addition, mass spectrometry-based comparative proteomic and phosphoproteomic analyses of the mutation types and wild types of MCF-7 cell lines were carried out.

RESULTS

Among the identified mutations, a new mutation HER2 L796P promoted the proliferation of breast cancer cells and had resistance to lapatinib using CCK-8 cell proliferation assay and clone formation assay. The bioinformatic analysis showed that RAS family proteins and ERK phosphorylated proteins significantly increased in the L796P mutant cells. The Gene Ontology (GO) analysis revealed that L796P mutation affected the function of breast cancer at the level of upstream genes in the MAPK and PI3K-AKT-TOR pathways.

CONCLUSIONS AND CLINICAL RELEVANCE

This study demonstrated that a rare mutation HER2 L796P could be a potential therapeutic target for the clinical management of breast cancer.

RZZ-Spindly and CENP-E form an integrated platform to recruit dynein to the kinetochore corona

Chromosome biorientation on the mitotic spindle is prerequisite to errorless genome inheritance. CENP-E (kinesin-7) and dynein-dynactin (DD), microtubule motors with opposite polarity, promote biorientation from the kinetochore corona, a polymeric structure whose assembly requires MPS1 kinase. The corona's building block consists of ROD, Zwilch, ZW10, and the DD adaptor Spindly (RZZS). How CENP-E and DD are scaffolded and mutually coordinated in the corona remains unclear. Here, we show that when corona assembly is prevented through MPS1 inhibition, CENP-E is absolutely required to retain RZZS at kinetochores. An RZZS phosphomimetic mutant bypasses this requirement, demonstrating the existence of a second receptor for polymeric RZZS. With active MPS1, CENP-E is dispensable for corona expansion, but strictly required for physiological kinetochore accumulation of DD. Thus, we identify the corona as an integrated scaffold where CENP-E kinesin controls DD kinetochore loading for coordinated bidirectional transport of chromosome cargo.

The anaphase-promoting complex controls a ubiquitination-phosphoprotein axis in chromatin during neurodevelopment

Mutations in the degradative ubiquitin ligase anaphase-promoting complex (APC) alter neurodevelopment by impairing proteasomal protein clearance, but our understanding of their molecular and cellular pathogenesis remains limited. Here, we employ the proteomic-based discovery of APC substrates in APC mutant mouse brain and human cell lines and identify the chromosome-passenger complex (CPC), topoisomerase 2a (Top2a), and Ki-67 as major chromatin factors targeted by the APC during neuronal differentiation. These substrates accumulate in phosphorylated form, suggesting that they fail to be eliminated after mitosis during terminal differentiation. The accumulation of the CPC kinase Aurora B within constitutive heterochromatin and hyperphosphorylation of its target histone 3 are corrected in the mutant brain by pharmacologic Aurora B inhibition. Surprisingly, the reduction of Ki-67, but not H3S10ph, rescued the function of constitutive heterochromatin in APC mutant neurons. These results expand our understanding of how ubiquitin signaling regulates chromatin during neurodevelopment and identify potential therapeutic targets in APC-related disorders.

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

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

Glutaredoxin 1 protects lens epithelial cells from epithelial-mesenchymal transition by preventing casein kinase 1α S-glutathionylation during posterior capsular opacification

Oxidative stress drives protein S-glutathionylation, which regulates the structure and function of target proteins and is implicated in the pathogenesis of many diseases. Glutaredoxin 1 (Grx1), a cytoplasmic deglutathionylating enzyme, maintains a reducing environment within the cell under various conditions by reversing S-glutathionylation. Grx1 performs a wide range of antioxidant activities in the lens and prevents protein-thiol mixed disulfide accumulation, reducing protein-protein aggregation, insolubilization, and apoptosis of lens epithelial cells. Oxidative stress is related to epithelial-mesenchymal transition (EMT) during posterior capsular opacification (PCO). However, whether Grx1-regulated protein S-glutathionylation plays an essential role in PCO remains unclear. In this study, we revealed that Grx1 expression was decreased in mice following cataract surgery. Furthermore, the absence of Grx1 elevated oxidative stress and protein S-glutathionylation and aggravated EMT in both in vitro and in vivo models. Concurrently, these results could be reversed by Grx1 overexpression. Notably, liquid chromatography-tandem mass spectrometry results showed that casein kinase 1α (CK1α) was susceptible to S-glutathionylation under oxidative stress, and CK1α S-glutathionylation (CK1α-SSG) was mediated at Cys249. The absence of Grx1 upregulated CK1α-SSG, subsequently decreasing the CK1α-induced phosphorylation of β-catenin at Ser45. The consequential downregulation of degradative β-catenin and upregulation of its nuclear translocation activated the Wnt/β-catenin signaling pathway and aggravated EMT. In conclusion, the downregulated expression of Grx1 in mice following cataract surgery aggravated EMT by upregulating the extent of CK1α-SSG. To the best of our knowledge, our study is the first to report how S-glutathionylation regulates CK1α activity under oxidative stress.

A mechanism that integrates microtubule motors of opposite polarity at the kinetochore corona

Chromosome biorientation on the mitotic spindle is prerequisite to errorless genome inheritance. CENP-E (kinesin 7) and Dynein-Dynactin (DD), microtubule motors with opposite polarity, promote biorientation from the kinetochore corona, a polymeric structure whose assembly requires MPS1 kinase. The corona's building block consists of ROD, Zwilch, ZW10, and the DD adaptor Spindly (RZZS). How CENP-E and DD are scaffolded and mutually coordinated in the corona remains unclear. Here, we report near-complete depletion of RZZS and DD from kinetochores after depletion of CENP-E and the outer kinetochore protein KNL1. With inhibited MPS1, CENP-E, which we show binds directly to RZZS, is required to retain kinetochore RZZS. An RZZS phosphomimetic mutant bypasses this requirement. With active MPS1, CENP-E is dispensable for corona expansion, but strictly required for physiological kinetochore accumulation of DD. Thus, we identify the corona as an integrated scaffold where CENP-E kinesin controls DD kinetochore loading for coordinated bidirectional transport of chromosome cargo.

Insights on the Role of PGRMC1 in Mitotic and Meiotic Cell Division

During mitosis, chromosome missegregation and cytokinesis defects have been recognized as hallmarks of cancer cells. Cytoskeletal elements composing the spindle and the contractile ring and their associated proteins play crucial roles in the faithful progression of mitotic cell division. The hypothesis that PGRMC1, most likely as a part of a yet-to-be-defined complex, is involved in the regulation of spindle function and, more broadly, the cytoskeletal machinery driving cell division is particularly appealing. Nevertheless, more than ten years after the preliminary observation that PGRMC1 changes its localization dynamically during meiotic and mitotic cell division, this field of research has remained a niche and needs to be fully explored. To encourage research in this fascinating field, in this review, we will recap the current knowledge on PGRMC1 function during mitotic and meiotic cell division, critically highlighting the strengths and limitations of the experimental approaches used so far. We will focus on known interacting partners as well as new putative associated proteins that have recently arisen in the literature and that might support current as well as new hypotheses of a role for PGRMC1 in specific spindle subcompartments, such as the centrosome, kinetochores, and the midzone/midbody.

The LEM-ESCRT toolkit: Repair and maintenance of the nucleus

The eukaryotic genome is enclosed in a nuclear envelope that protects it from potentially damaging cellular activities and physically segregates transcription and translation.Transport across the NE is highly regulated and occurs primarily via the macromolecular nuclear pore complexes.Loss of nuclear compartmentalization due to defects in NPC function and NE integrity are tied to neurological and ageing disorders like Alzheimer’s, viral pathogenesis, immune disorders, and cancer progression.Recent work implicates inner-nuclear membrane proteins of the conserved LEM domain family and the ESCRT machinery in NE reformation during cell division and NE repair upon rupture in migrating cancer cells, and generating seals over defective NPCs. In this review, we discuss the recent in-roads made into defining the molecular mechanisms and biochemical networks engaged by LEM and many other integral inner nuclear membrane proteins to preserve the nuclear barrier.

Small changes in phospho-occupancy at the kinetochore-microtubule interface drive mitotic fidelity

Kinetochore protein phosphorylation promotes the correction of erroneous microtubule attachments to ensure faithful chromosome segregation during cell division. Determining how phosphorylation executes error correction requires an understanding of whether kinetochore substrates are completely (i.e., all-or-none) or only fractionally phosphorylated. Using quantitative mass spectrometry (MS), we measured phospho-occupancy on the conserved kinetochore protein Hec1 (NDC80) that directly binds microtubules. None of the positions measured exceeded ∼50% phospho-occupancy, and the cumulative phospho-occupancy changed by only ∼20% in response to changes in microtubule attachment status. The narrow dynamic range of phospho-occupancy is maintained, in part, by the ongoing phosphatase activity. Further, both Cdk1-Cyclin B1 and Aurora kinases phosphorylate Hec1 to enhance error correction in response to different types of microtubule attachment errors. The low inherent phospho-occupancy promotes microtubule attachment to kinetochores while the high sensitivity of kinetochore-microtubule attachments to small changes in phospho-occupancy drives error correction and ensures high mitotic fidelity.

Profiling the Human Phosphoproteome to Estimate the True Extent of Protein Phosphorylation

Public phosphorylation databases such as PhosphoSitePlus (PSP) and PeptideAtlas (PA) compile results from published papers or openly available mass spectrometry (MS) data. However, there is no database-level control for false discovery of sites, likely leading to the overestimation of true phosphosites. By profiling the human phosphoproteome, we estimate the false discovery rate (FDR) of phosphosites and predict a more realistic count of true identifications. We rank sites into phosphorylation likelihood sets and analyze them in terms of conservation across 100 species, sequence properties, and functional annotations. We demonstrate significant differences between the sets and develop a method for independent phosphosite FDR estimation. Remarkably, we report estimated FDRs of 84, 98, and 82% within sets of phosphoserine (pSer), phosphothreonine (pThr), and phosphotyrosine (pTyr) sites, respectively, that are supported by only a single piece of identification evidence─the majority of sites in PSP. We estimate that around 62 000 Ser, 8000 Thr, and 12 000 Tyr phosphosites in the human proteome are likely to be true, which is lower than most published estimates. Furthermore, our analysis estimates that 86 000 Ser, 50 000 Thr, and 26 000 Tyr phosphosites are likely false-positive identifications, highlighting the significant potential of false-positive data to be present in phosphorylation databases.

An integrative pan-cancer analysis reveals the oncogenic role of mutS homolog 6 (MSH6) in human tumors

There are three most important mismatch repair genes in the mismatch repair system, MSH6 is one of them and it plays an essential role in DNA mismatch repair. Several emerging cell- or animal-based studies have verified that MSH6 mutations are closely linked to the occurrence, progression or metastasis of cancer, but there is still no practicable pan-cancer analysis. On account of the available datasets of the cancer genome atlas (TCGA) and Gene expression omnibus (GEO), a comprehensive analysis of the potential carcinogenic effects of the MSH6 gene was conducted in 33 human cancers. MSH6 was highly expressed in most cancers, and the high expression of MSH6 was associated with poor overall survival prognosis of patients with multiple cancers, such as adrenocortical carcinoma. MSH6 mutations occurred in most cancers and were closely related to the prognosis of cancer patients. Increased phosphorylation levels of S227 and S830 were noted in several tumors, including breast cancer and colon cancer. MSH6 expression was also observed to be correlated with cancer-associated fibroblasts and CD8⁺ T-cells infiltration levels in various cancer types, e. g. pancreatic adenocarcinoma or testicular germ cell tumors. Furthermore, pathway enrichment analysis demonstrated that the main biological activities of MSH6 were related to ATPase activity, mismatch repair, and DNA metabolism-related functions. Altogether, our pan-cancer research has suggested that the MSH6 expression level was closely related to the carcinogenesis and prognosis of certain tumors, which helps to know the effect of MSH6 in tumorigenesis from the point of view of clinical tumor samples.

What Do We Know about Classical and Non-Classical Progesterone Receptors in the Human Female Reproductive Tract? A Review

The progesterone hormone regulates the human menstrual cycle, pregnancy, and parturition by its action via the different progesterone receptors and signaling pathways in the female reproductive tract. Progesterone actions can be exerted through classical and non-classical receptors, or even a combination of both. The former are nuclear receptors whose activation leads to transcriptional activity regulation and thus in turn leads to slower but long-lasting responses. The latter are composed of progesterone receptors membrane components (PGRMC) and membrane progestin receptors (mPRs). These receptors rapidly activate the appropriate intracellular signal transduction pathways, and they can subsequently initiate specific cell responses or even modulate genomic cell responses. This review covers our current knowledge on the mechanisms of action and the relevance of classical and non-classical progesterone receptors in female reproductive tissues ranging from the ovary and uterus to the cervix, and it exposes their crucial role in female infertility.

Exploration of the Role of the C-Terminal Domain of Human DNA Topoisomerase IIα in Catalytic Activity

Human topoisomerase IIα (TOP2A) is a vital nuclear enzyme involved in resolving knots and tangles in DNA during replication and cell division. TOP2A is a homodimer with a symmetrical, multidomain structure. While the N-terminal and core regions of the protein are well-studied, the C-terminal domain is poorly understood but is involved in enzyme regulation and is predicted to be intrinsically disordered. In addition, it appears to be a major region of post-translational modification and includes several Ser and Thr residues, many of which have not been studied for biochemical effects. Therefore, we generated a series of human TOP2A mutants where we changed specific Ser and Thr residues in the C-terminal domain to Ala, Gly, or Ile residues. We designed, purified, and examined 11 mutant TOP2A enzymes. The amino acid changes were made between positions 1272 and 1525 with 1-7 residues changed per mutant. Several mutants displayed increased levels of DNA cleavage without displaying any change in plasmid DNA relaxation or DNA binding. For example, mutations in the regions 1272-1279, 1324-1343, 1351-1365, and 1374-1377 produced 2-3 times more DNA cleavage in the presence of etoposide than wild-type TOP2A. Further, several mutants displayed changes in relaxation and/or decatenation activity. Together, these results support previous findings that the C-terminal domain of TOP2A influences catalytic activity and interacts with the substrate DNA. Furthermore, we hypothesize that it may be possible to regulate the enzyme by targeting positions in the C-terminal domain. Because the C-terminal domain differs between the two human TOP2 isoforms, this strategy may provide a means for selectively targeting TOP2A for therapeutic inhibition. Additional studies are warranted to explore these results in more detail.

A global map of associations between types of protein posttranslational modifications and human genetic diseases

There are >200 types of protein posttranslational modifications (PTMs) described in eukaryotes, each with unique proteome coverage and functions. We hypothesized that some genetic diseases may be caused by the removal of a specific type of PTMs by genomic variants and the consequent deregulation of particular functions. We collected >320,000 human PTMs representing 59 types and crossed them with >4M nonsynonymous DNA variants annotated with predicted pathogenicity and disease associations. We report >1.74M PTM-variant co-occurrences that an enrichment analysis distributed into 215 pairwise associations between 18 PTM types and 148 genetic diseases. Of them, 42% were not previously described. Removal of lysine acetylation exerts the most pronounced effect, and less studied PTM types such as S-glutathionylation or S-nitrosylation show relevance. Using pathogenicity predictions, we identified PTM sites that may produce particular diseases if prevented. Our results provide evidence of a substantial impact of PTM-specific removal on the pathogenesis of genetic diseases and phenotypes.

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There is an enrichment of disease-associated nsSNVs preventing certain types of PTMs

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We report 215 pairwise associations between 18 PTM types and 148 genetic diseases

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The removal of lysine acetylation exerts the most pronounced effect

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We report a set of PTM sites that may produce particular diseases if prevented

Physiological relevance of post-translational regulation of the spindle assembly checkpoint protein BubR1

BubR1 is an essential component of the spindle assembly checkpoint (SAC) during mitosis where it functions to prevent anaphase onset to ensure proper chromosome alignment and kinetochore-microtubule attachment. Loss or mutation of BubR1 results in aneuploidy that precedes various potential pathologies, including cancer and mosaic variegated aneuploidy (MVA). BubR1 is also progressively downregulated with age and has been shown to be directly involved in the aging process through suppression of cellular senescence. Post-translational modifications, including but not limited to phosphorylation, acetylation, and ubiquitination, play a critical role in the temporal and spatial regulation of BubR1 function. In this review, we discuss the currently characterized post-translational modifications to BubR1, the enzymes involved, and the biological consequences to BubR1 functionality and implications in diseases associated with BubR1. Understanding the molecular mechanisms promoting these modifications and their roles in regulating BubR1 is important for our current understanding and future studies of BubR1 in maintaining genomic integrity as well as in aging and cancer.

Importin α phosphorylation promotes TPX2 activation by GM130 to control astral microtubules and spindle orientation

Spindle orientation is important in multiple developmental processes as it determines cell fate and function. The orientation of the spindle depends on the assembly of a proper astral microtubule network. Here, we report that the spindle assembly factor TPX2 regulates astral microtubules. TPX2 in the spindle pole area is activated by GM130 (GOLGA2) on Golgi membranes to promote astral microtubule growth. GM130 relieves TPX2 inhibition by competing for importin α1 (KPNA2) binding. Mitotic phosphorylation of importin α at serine 62 (S62) by CDK1 switches its substrate preference from TPX2 to GM130, thereby enabling competition-based activation. Importin α S62A mutation impedes local TPX2 activation and compromises astral microtubule formation, ultimately resulting in misoriented spindles. Blocking the GM130-importin α-TPX2 pathway impairs astral microtubule growth. Our results reveal a novel role for TPX2 in the organization of astral microtubules. Furthermore, we show that the substrate preference of the important mitotic modulator importin α is regulated by CDK1-mediated phosphorylation.

Leaving no-one behind: how CENP-E facilitates chromosome alignment

Chromosome alignment and biorientation is essential for mitotic progression and genomic stability. Most chromosomes align at the spindle equator in a motor-independent manner. However, a subset of polar kinetochores fail to bi-orient and require a microtubule motor-based transport mechanism to move to the cell equator. Centromere Protein E (CENP-E/KIF10) is a kinesin motor from the Kinesin-7 family, which localizes to unattached kinetochores during mitosis and utilizes plus-end directed microtubule motility to slide mono-oriented chromosomes to the spindle equator. Recent work has revealed how CENP-E cooperates with chromokinesins and dynein to mediate chromosome congression and highlighted its role at aligned chromosomes. Additionally, we have gained new mechanistic insights into the targeting and regulation of CENP-E motor activity at the kinetochore. Here, we will review the function of CENP-E in chromosome congression, the pathways that contribute to CENP-E loading at the kinetochore, and how CENP-E activity is regulated during mitosis.

The association of Plk1 with the astrin-kinastrin complex promotes formation and maintenance of a metaphase plate

Errors in mitotic chromosome segregation can lead to DNA damage and aneuploidy, both hallmarks of cancer. To achieve synchronous error-free segregation, mitotic chromosomes must align at the metaphase plate with stable amphitelic attachments to microtubules emanating from opposing spindle poles. The astrin-kinastrin (astrin is also known as SPAG5 and kinastrin as SKAP) complex, also containing DYNLL1 and MYCBP, is a spindle and kinetochore protein complex with important roles in bipolar spindle formation, chromosome alignment and microtubule-kinetochore attachment. However, the molecular mechanisms by which astrin-kinastrin fulfils these diverse roles are not fully understood. Here, we characterise a direct interaction between astrin and the mitotic kinase Plk1. We identify the Plk1-binding site on astrin as well as four Plk1 phosphorylation sites on astrin. Regulation of astrin by Plk1 is dispensable for bipolar spindle formation and bulk chromosome congression, but promotes stable microtubule-kinetochore attachments and metaphase plate maintenance. It is known that Plk1 activity is required for effective microtubule-kinetochore attachment formation, and we suggest that astrin phosphorylation by Plk1 contributes to this process.

Neurofibromin Structure, Functions and Regulation

Neurofibromin is a large and multifunctional protein encoded by the tumor suppressor gene NF1, mutations of which cause the tumor predisposition syndrome neurofibromatosis type 1 (NF1). Over the last three decades, studies of neurofibromin structure, interacting partners, and functions have shown that it is involved in several cell signaling pathways, including the Ras/MAPK, Akt/mTOR, ROCK/LIMK/cofilin, and cAMP/PKA pathways, and regulates many fundamental cellular processes, such as proliferation and migration, cytoskeletal dynamics, neurite outgrowth, dendritic-spine density, and dopamine levels. The crystallographic structure has been resolved for two of its functional domains, GRD (GAP-related (GTPase-activating protein) domain) and SecPH, and its post-translational modifications studied, showing it to be localized to several cell compartments. These findings have been of particular interest in the identification of many therapeutic targets and in the proposal of various therapeutic strategies to treat the symptoms of NF1. In this review, we provide an overview of the literature on neurofibromin structure, function, interactions, and regulation and highlight the relationships between them.

Transgelin-2 and phosphoregulation of the LIC2 subunit of dynein govern mitotic spindle orientation

The molecular motor dynein is essential for mitotic spindle orientation, which defines the axis of cell division. The light intermediate chain subunits, LIC1 and LIC2, define biochemically and functionally distinct vertebrate dynein complexes, with LIC2-dynein playing a crucial role in ensuring spindle orientation. We reveal a novel, mitosis-specific interaction of LIC2-dynein with the cortical actin-bundling protein transgelin-2. Transgelin-2 is required for maintaining proper spindle length, equatorial metaphase chromosome alignment, spindle orientation and timely anaphase onset. We show that transgelin-2 stabilizes the cortical recruitment of LGN-NuMA, which together with dynein is required for spindle orientation. The opposing actions of transgelin-2 and LIC2-dynein maintain optimal cortical levels of LGN-NuMA. In addition, we show that the highly conserved serine 194 phosphorylation of LIC2 is required for proper spindle orientation, by maintaining mitotic centrosome integrity to ensure optimal astral microtubule nucleation. The work reveals two specific mechanisms through which LIC2-dynein regulates mitotic spindle orientation; namely, through a new interactor transgelin-2, which is required for engagement of LGN-NuMA with the actin cortex, and through mitotic phosphoregulation of LIC2 to control microtubule nucleation from the poles.This article has an associated First Person interview with the first author of the paper.

The Hec1/Ndc80 tail domain is required for force generation at kinetochores, but is dispensable for kinetochore-microtubule attachment formation and Ska complex recruitment

The conserved kinetochore-associated NDC80 complex (composed of Hec1/Ndc80, Nuf2, Spc24, and Spc25) has well-documented roles in mitosis including 1) connecting mitotic chromosomes to spindle microtubules to establish force-transducing kinetochore–microtubule attachments and 2) regulating the binding strength between kinetochores and microtubules such that correct attachments are stabilized and erroneous attachments are released. Although the NDC80 complex plays a central role in forming and regulating attachments to microtubules, additional factors support these processes as well, including the spindle and kinetochore-associated (Ska) complex. Multiple lines of evidence suggest that Ska complexes strengthen attachments by increasing the ability of NDC80 complexes to bind microtubules, especially to depolymerizing microtubule plus ends, but how this is accomplished remains unclear. Using cell-based and in vitro assays, we demonstrate that the Hec1 tail domain is dispensable for Ska complex recruitment to kinetochores and for generation of kinetochore–microtubule attachments in human cells. We further demonstrate that Hec1 tail phosphorylation regulates kinetochore–microtubule attachment stability independently of the Ska complex. Finally, we map the location of the Ska complex in cells to a region near the coiled-coil domain of the NDC80 complex and demonstrate that this region is required for Ska complex recruitment to the NDC80 complex-–microtubule interface.

Kinesin-7 CENP-E regulates chromosome alignment and genome stability of spermatogenic cells

Kinesin-7 CENP-E is an essential kinetochore motor required for chromosome alignment and congression. However, the specific functions of CENP-E in the spermatogenic cells during spermatogenesis remain unknown. In this study, we find that CENP-E proteins are expressed in the spermatogonia, spermatocytes, and the elongating spermatids. CENP-E inhibition by specific inhibitor GSK923295 results in the disruption of spermatogenesis and cell cycle arrest of spermatogenic cells. Both spermatogonia and spermatocytes are arrested in metaphase and several chromosomes are not aligned at the equatorial plate. We find that CENP-E inhibition leads to chromosome misalignment, the spindle disorganization, and the formation of the aneuploidy cells. Furthermore, the inhibition of CENP-E results in the defects in the formation of spermatids, including the sperm head condensation and the sperm tail formation. We have revealed that kinesin-7 CENP-E is essential for chromosome alignment and genome stability of the spermatogenic cells.

Hyaluronan Mediated Motility Receptor (HMMR) Encodes an Evolutionarily Conserved Homeostasis, Mitosis, and Meiosis Regulator Rather than a Hyaluronan Receptor

Hyaluronan is an extracellular matrix component that absorbs water in tissues and engages cell surface receptors, like Cluster of Differentiation 44 (CD44), to promote cellular growth and movement. Consequently, CD44 demarks stem cells in normal tissues and tumor-initiating cells isolated from neoplastic tissues. Hyaluronan mediated motility receptor (HMMR, also known as RHAMM) is another one of few defined hyaluronan receptors. HMMR is also associated with neoplastic processes and its role in cancer progression is often attributed to hyaluronan-mediated signaling. But, HMMR is an intracellular, microtubule-associated, spindle assembly factor that localizes protein complexes to augment the activities of mitotic kinases, like polo-like kinase 1 and Aurora kinase A, and control dynein and kinesin motor activities. Expression of HMMR is elevated in cells prior to and during mitosis and tissues with detectable HMMR expression tend to be highly proliferative, including neoplastic tissues. Moreover, HMMR is a breast cancer susceptibility gene product. Here, we briefly review the associations between HMMR and tumorigenesis as well as the structure and evolution of HMMR, which identifies Hmmr-like gene products in several insect species that do not produce hyaluronan. This review supports the designation of HMMR as a homeostasis, mitosis, and meiosis regulator, and clarifies how its dysfunction may promote the tumorigenic process and cancer progression.

Neurofibromin Is an Estrogen Receptor-α Transcriptional Co-repressor in Breast Cancer

We report that neurofibromin, a tumor suppressor and Ras-GAP (GTPase-activating protein), is also an estrogen receptor-α (ER) transcriptional co-repressor through leucine/isoleucine-rich motifs that are functionally independent of GAP activity. GAP activity, in turn, does not affect ER binding. Consequently, neurofibromin depletion causes estradiol hypersensitivity and tamoxifen agonism, explaining the poor prognosis associated with neurofibromin loss in endocrine therapy-treated ER⁺ breast cancer. Neurofibromin-deficient ER⁺ breast cancer cells initially retain sensitivity to selective ER degraders (SERDs). However, Ras activation does play a role in acquired SERD resistance, which can be reversed upon MEK inhibitor addition, and SERD/MEK inhibitor combinations induce tumor regression. Thus, neurofibromin is a dual repressor for both Ras and ER signaling, and co-targeting may treat neurofibromin-deficient ER⁺ breast tumors.

Hec1/Ndc80 Tail Domain Function at the Kinetochore-Microtubule Interface

Successful mitotic cell division is critically dependent on the formation of correct attachments between chromosomes and spindle microtubules. Microtubule attachments are mediated by kinetochores, which are large proteinaceous structures assembled on centromeric chromatin of mitotic chromosomes. These attachments must be sufficiently stable to transduce force; however, the strength of these attachments are also tightly regulated to ensure timely, error-free progression through mitosis. The highly conserved, kinetochore-associated NDC80 complex is a core component of the kinetochore-microtubule attachment machinery in eukaryotic cells. A small, disordered region within the Hec1 subunit of the NDC80 complex – the N-terminal “tail” domain – has been actively investigated during the last decade due to its roles in generating and regulating kinetochore-microtubule attachments. In this review, we discuss the role of the NDC80 complex, and specifically the Hec1 tail domain, at the kinetochore-microtubule interface, and how recent studies provide a more unified view of Hec1 tail domain function.

Mitotic kinase anchoring proteins: the navigators of cell division

The coordinated activities of many protein kinases, acting on multiple protein substrates, ensures the error-free progression through mitosis of eukaryotic cells. Enormous research effort has thus been devoted to studying the roles and regulation of these mitotic kinases, and to the identification of their physiological substrates. Central for the timely deployment of specific protein kinases to their appropriate substrates during the cell division cycle are the many anchoring proteins, which serve critical regulatory roles. Through direct association, anchoring proteins are capable of modulating the catalytic activity and/or sub-cellular distribution of the mitotic kinases they associate with. The key roles of some anchoring proteins in cell division are well-established, whilst others are still being unearthed. Here, we review the current knowledge on anchoring proteins for some mitotic kinases, and highlight how targeting anchoring proteins for inhibition, instead of the mitotic kinases themselves, could be advantageous for disrupting the cell division cycle.

The Tumor Suppressor Roles of MYBBP1A, a Major Contributor to Metabolism Plasticity and Stemness

The MYB binding protein 1A (MYBBP1A, also known as p160) acts as a co-repressor of multiple transcription factors involved in many physiological processes. Therefore, MYBBP1A acts as a tumor suppressor in multiple aspects related to cell physiology, most of them very relevant for tumorigenesis. We explored the different roles of MYBBP1A in different aspects of cancer, such as mitosis, cellular senescence, epigenetic regulation, cell cycle, metabolism plasticity and stemness. We especially reviewed the relationships between MYBBP1A, the inhibitory role it plays by binding and inactivating c-MYB and its regulation of PGC-1α, leading to an increase in the stemness and the tumor stem cell population. In addition, MYBBP1A causes the activation of PGC-1α directly and indirectly through c-MYB, inducing the metabolic change from glycolysis to oxidative phosphorylation (OXPHOS). Therefore, the combination of these two effects caused by the decreased expression of MYBBP1A provides a selective advantage to tumor cells. Interestingly, this only occurs in cells lacking pVHL. Finally, the loss of MYBBP1A occurs in 8%–9% of renal tumors. tumors, and this subpopulation could be studied as a possible target of therapies using inhibitors of mitochondrial respiration.

Kinesin-6 family motor KIF20A regulates central spindle assembly and acrosome biogenesis in mouse spermatogenesis

Kinesin-6 KIF20A is essential for microtubule organization and central spindle assembly during cytokinesis. However, the functions of KIF20A in meiotic division and spermatogenesis remain elusive. Here, we report that kinesin-6 KIF20A locates at the microtubules in mouse spermatogenic cells and co-localizes with the spindle midzone and midbody. We demonstrate that central spindle organization and chromosomal stability are regulated by KIF20A in male meiotic division. KIF20A inhibition leads to the defects in central spindle assembly and cytokinetic abscission, and finally results in the increase of aneuploid cells and the alteration of cell populations in the spermatogenic cells. Furthermore, we have revealed that kinesin-6 KIF20A is associated with the formation and maturation of the acrosomes during spermatogenesis. Our findings have identified the specific roles of KIF20A in central spindle organization in meiotic division.

Cell cycle-independent furrowing triggered by phosphomimetic mutations of the INCENP STD motif requires Plk1

Timely and precise control of Aurora B kinase, the chromosomal passenger complex (CPC) catalytic subunit, is essential for accurate chromosome segregation and cytokinesis. Post-translational modifications of CPC subunits are directly involved in controlling Aurora B activity. Here, we identified a highly conserved acidic STD-rich motif of INCENP that is phosphorylated during mitosis in vivo and by Plk1 in vitro and is involved in controlling Aurora B activity. By using an INCENP conditional-knockout cell line, we show that impairing the phosphorylation status of this region disrupts chromosome congression and induces cytokinesis failure. In contrast, mimicking constitutive phosphorylation not only rescues cytokinesis but also induces ectopic furrows and contractile ring formation in a Plk1- and ROCK1-dependent manner independent of cell cycle and microtubule status. Our experiments identify the phospho-regulation of the INCENP STD motif as a novel mechanism that is key for chromosome alignment and cytokinesis.This article has an associated First Person interview with the first author of the paper.

Protein Serine/Threonine Phosphatases: Keys to Unlocking Regulators and Substrates

Protein serine/threonine phosphatases (PPPs) are ancient enzymes, with distinct types conserved across eukaryotic evolution. PPPs are segregated into types primarily on the basis of the unique interactions of PPP catalytic subunits with regulatory proteins. The resulting holoenzymes dock substrates distal to the active site to enhance specificity. This review focuses on the subunit and substrate interactions for PPP that depend on short linear motifs. Insights about these motifs from structures of holoenzymes open new opportunities for computational biology approaches to elucidate PPP networks. There is an expanding knowledge base of posttranslational modifications of PPP catalytic and regulatory subunits, as well as of their substrates, including phosphorylation, acetylation, and ubiquitination. Cross talk between these posttranslational modifications creates PPP-based signaling. Knowledge of PPP complexes, signaling clusters, as well as how PPPs communicate with each other in response to cellular signals should unlock the doors to PPP networks and signaling "clouds" that orchestrate and coordinate different aspects of cell physiology.

Spindle-F-actin interactions in mitotic spindles in an intact vertebrate epithelium

Mitotic spindles are well known to be assembled from and dependent on microtubules. In contrast, whether actin filaments (F-actin) are required for or are even present in mitotic spindles has long been controversial. Here we have developed improved methods for simultaneously preserving F-actin and microtubules in fixed samples and exploited them to demonstrate that F-actin is indeed associated with mitotic spindles in intact Xenopus laevis embryonic epithelia. We also find that there is an “F-actin cycle,” in which the distribution and organization of spindle F-actin changes over the course of the cell cycle. Live imaging using a probe for F-actin reveals that at least two pools of F-actin are associated with mitotic spindles: a relatively stable internal network of cables that moves in concert with and appears to be linked to spindles, and F-actin “fingers” that rapidly extend from the cell cortex toward the spindle and make transient contact with the spindle poles. We conclude that there is a robust endoplasmic F-actin network in normal vertebrate epithelial cells and that this network is also a component of mitotic spindles. More broadly, we conclude that there is far more internal F-actin in epithelial cells than is commonly believed.

Mechanisms of kinesin-7 CENP-E in kinetochore-microtubule capture and chromosome alignment during cell division

Chromosome congression is essential for faithful chromosome segregation and genomic stability in cell division. Centromere-associated protein E (CENP-E), a plus-end-directed kinesin motor, is required for congression of pole-proximal chromosomes in metaphase. CENP-E accumulates at the outer plate of kinetochores and mediates the kinetochore-microtubule capture. CENP-E also transports the chromosomes along spindle microtubules towards the equatorial plate. CENP-E interacts with Bub1-related kinase, Aurora B and core kinetochore components during kinetochore-microtubule attachment. In this review, we introduce the structures and mechanochemistry of kinesin-7 CENP-E. We highlight the complicated interactions between CENP-E and partner proteins during chromosome congression. We summarise the detailed roles and mechanisms of CENP-E in mitosis and meiosis, including the kinetochore-microtubule capture, chromosome congression/alignment in metaphase and the regulation of spindle assembly checkpoint. We also shed a light on the roles of CENP-E in tumourigenesis and CENP-E's specific inhibitors.

The microtubule-associated protein EML3 regulates mitotic spindle assembly by recruiting the Augmin complex to spindle microtubules

In all eukaryotes, a functional mitotic spindle is essential for distributing duplicated chromosomes into daughter cells. Mitotic spindle assembly involves highly ordered arrangement of microtubules (MTs). The Augmin protein complex recruits γ-tubulin ring complex (γ-TuRC) to MTs and thereby promotes MT-based MT nucleation and mitotic spindle assembly. However, several factors that may promote Augmin recruitment to MTs remain unknown. Here, we show that echinoderm microtubule-associated protein-like 3 (EML3), an MT-associated protein, facilitates binding between MTs and Augmin/γ-TuRC and recruiting the latter to MTs for proper mitotic spindle assembly and kinetochore-MT connections. Using immunofluorescence microscopy, live-cell imaging, and immunoprecipitation assays, we found that EML3 recruits Augmin/γ-TuRC to the MTs to enhance MT-based MT nucleation in both spindle and small acentrosomal asters. We also noted that the EML3-mediated recruitment is controlled by cyclin-dependent kinase 1 (CDK1), which phosphorylated EML3 at Thr-881 and promoted its binding to Augmin/γ-TuRC. RNAi-mediated EML3 knockdown in HeLa cells reduced spindle localization of Augmin/γ-TuRC, which resulted in abnormal spindle assembly and caused kinetochore-MT misconnection. The introduction of exogenous WT or a Thr-881 phosphorylation mimic EML3 variant into the EML3 knockdown cells restored normal Augmin/γ-TuRC localization and spindle assembly. The EML3 knockdown also affected the spindle assembly checkpoint, delaying chromosome congression and cell division. Taken together, our results indicate that EML3 regulates mitotic spindle assembly and the kinetochore-MT connection by regulating MT-based MT nucleation and recruiting Augmin/γ-TuRC to MTs.

Casein kinase 1α: biological mechanisms and theranostic potential

Casein kinase 1α (CK1α) is a multifunctional protein belonging to the CK1 protein family that is conserved in eukaryotes from yeast to humans. It regulates signaling pathways related to membrane trafficking, cell cycle progression, chromosome segregation, apoptosis, autophagy, cell metabolism, and differentiation in development, circadian rhythm, and the immune response as well as neurodegeneration and cancer. Given its involvement in diverse cellular, physiological, and pathological processes, CK1α is a promising therapeutic target. In this review, we summarize what is known of the biological functions of CK1α, and provide an overview of existing challenges and potential opportunities for advancing theranostics.

Aurora B opposes PP1 function in mitosis by phosphorylating the conserved PP1-binding RVxF motif in PP1 regulatory proteins

Protein phosphatase 1 (PP1) is a highly conserved protein phosphatase that performs most of the serine- and threonine-dephosphorylation reactions in eukaryotes and opposes the actions of a diverse set of serine and threonine (Ser-Thr) protein kinases. PP1 gains substrate specificity through binding to a large number (>200) of regulatory proteins that control PP1 localization, activity, and interactions with substrates. PP1 recognizes the well-characterized RVxF binding motif that is present in many of these regulatory proteins, thus generating a multitude of distinct PP1 holoenzymes. We showed that a subset of the RVxF binding motifs, in which x is a phosphorylatable amino acid (RV[S/T]F), was phosphorylated specifically during mitosis and that this phosphorylation event abrogated the interaction of PP1 with the regulatory protein. We determined that this phosphorylation was primarily governed by the mitotic protein kinase Aurora B and that high phosphorylation site stoichiometry of these sites maintained the phosphorylation of PP1 substrates during mitosis by disrupting the assembly of PP1 holoenzymes. We generated an antibody that recognizes the phosphorylated form of the RV[S/T]F motif (RVp[S/T]F) and used it to identify known PP1 regulatory proteins (KNL1, CDCA2, and RIF1) and multiple proteins that could potentially act as PP1 binding partners (UBR5, ASPM, SEH1, and ELYS) governed by this mechanism. Together, these data suggest a general regulatory mechanism by which the coordinated activities of Aurora B and PP1 control mitotic progression.

Phosphorylation of BACH1 switches its function from transcription factor to mitotic chromosome regulator and promotes its interaction with HMMR

The transcription repressor BACH1 performs mutually independent dual roles in transcription regulation and chromosome alignment during mitosis by supporting polar ejection force of mitotic spindle. We now found that the mitotic spindles became oblique relative to the adhesion surface following endogenous BACH1 depletion in HeLa cells. This spindle orientation rearrangement was rescued by re-expression of BACH1 depending on its interactions with HMMR and CRM1, both of which are required for the positioning of mitotic spindle, but independently of its DNA-binding activity. A mass spectrometry analysis of BACH1 complexes in interphase and M phase revealed that BACH1 lost during mitosis interactions with proteins involved in chromatin and gene expression but retained interactions with HMMR and its known partners including CHICA. By analyzing BACH1 modification using stable isotope labeling with amino acids in cell culture, mitosis-specific phosphorylations of BACH1 were observed, and mutations of these residues abolished the activity of BACH1 to restore mitotic spindle orientation in knockdown cells and to interact with HMMR. Detailed histological analysis of Bach1-deficient mice revealed lengthening of the epithelial fold structures of the intestine. These observations suggest that BACH1 performs stabilization of mitotic spindle orientation together with HMMR and CRM1 in mitosis, and that the cell cycle-specific phosphorylation switches the transcriptional and mitotic functions of BACH1.

Two-step interphase microtubule disassembly aids spindle morphogenesis

BACKGROUND

Entry into mitosis triggers profound changes in cell shape and cytoskeletal organisation. Here, by studying microtubule remodelling in human flat mitotic cells, we identify a two-step process of interphase microtubule disassembly.

RESULTS

First, a microtubule-stabilising protein, Ensconsin/MAP7, is inactivated in prophase as a consequence of its phosphorylation downstream of Cdk1/cyclin B. This leads to a reduction in interphase microtubule stability that may help to fuel the growth of centrosomally nucleated microtubules. The peripheral interphase microtubules that remain are then rapidly lost as the concentration of tubulin heterodimers falls following dissolution of the nuclear compartment boundary. Finally, we show that a failure to destabilise microtubules in prophase leads to the formation of microtubule clumps, which interfere with spindle assembly.

CONCLUSIONS

This analysis highlights the importance of the step-wise remodelling of the microtubule cytoskeleton and the significance of permeabilisation of the nuclear envelope in coordinating the changes in cellular organisation and biochemistry that accompany mitotic entry.

MARK3-mediated phosphorylation of ARHGEF2 couples microtubules to the actin cytoskeleton to establish cell polarity

The PAR-1-MARK pathway controls cell polarity through the phosphorylation of microtubule-associated proteins. Rho-Rac guanine nucleotide exchange factor 2 (ARHGEF2), which activates Ras homolog family member A (RHOA), is anchored to the microtubule network and sequestered in an inhibited state through binding to dynein light chain Tctex-1 type 1 (DYNLT1). We showed in mammalian cells that liver kinase B1 (LKB1) activated the microtubule affinity-regulating kinase 3 (MARK3), which in turn phosphorylated ARHGEF2 at Ser¹⁵¹ This modification disrupted the interaction between ARHGEF2 and DYNLT1 by generating a 14-3-3 binding site in ARHGEF2, thus causing ARHGEF2 to dissociate from microtubules. Phosphorylation of ARHGEF2 by MARK3 stimulated RHOA activation and the formation of stress fibers and focal adhesions, and was required for organized cellular architecture in three-dimensional culture. Protein phosphatase 2A (PP2A) dephosphorylated Ser¹⁵¹ in ARHGEF2 to restore the inhibited state. Thus, we have identified a regulatory switch controlled by MARK3 that couples microtubules to the actin cytoskeleton to establish epithelial cell polarity through ARHGEF2.

HMMR acts in the PLK1-dependent spindle positioning pathway and supports neural development

Oriented cell division is one mechanism progenitor cells use during development and to maintain tissue homeostasis. Common to most cell types is the asymmetric establishment and regulation of cortical NuMA-dynein complexes that position the mitotic spindle. Here, we discover that HMMR acts at centrosomes in a PLK1-dependent pathway that locates active Ran and modulates the cortical localization of NuMA-dynein complexes to correct mispositioned spindles. This pathway was discovered through the creation and analysis of Hmmr-knockout mice, which suffer neonatal lethality with defective neural development and pleiotropic phenotypes in multiple tissues. HMMR over-expression in immortalized cancer cells induces phenotypes consistent with an increase in active Ran including defects in spindle orientation. These data identify an essential role for HMMR in the PLK1-dependent regulatory pathway that orients progenitor cell division and supports neural development.

FAM83D, a microtubule-associated protein, promotes tumor growth and progression of human gastric cancer

FAM83D, a microtubule-associated protein (MAP), is overexpressed in diverse types of human cancer. The expression and critical role of FAM83D in human gastric cancer (GC), however, remains largely unknown. Here, we conducted molecular, cellular and clinical analyses to evaluate the functional link of FAM83D to GC. FAM83D expression was elevated in gastric tumors, and its expression strongly correlated with lymph node metastasis and TNM stage. In addition, over-expression of FAM83D in GC cell lines enhanced cell proliferation, cycle progression, migration, invasion, as well as tumor growth and metastatic dissemination in vivo. Furthermore, FAM83D exhibited a strong cell cycle correlated expression. The knockdown of FAM83D inhibited the regrowth of microtubules in GC cells. FAM83D was co-immunoprecipitated with HMMR, TPX2, and AURKA, a set of drivers of mitosis progression. Taken together, our results demonstrate FAM83D as an important player in the development of human gastric cancer, and as a potential therapeutic target for the treatment of cancer.

Dynamic expression of PGRMC1 and SERBP1 in human endometrium: an implication in the human decidualization process

OBJECTIVE

To characterize PGRMC1 and SERBP1 in human endometrium and to investigate the putative role of PGRMC1 in endometrial decidualization.

DESIGN

The PGRMC1 and SERBP1 expression in human endometrium was determined throughout the menstrual cycle. We analyzed the colocalization of PGRMC1 and SERBP1. Then, endometrial stromal cells (ESCs) were isolated to investigate the functional effect of PGRMC1 overexpression on decidualization.

SETTING

IVI clinic.

PATIENT(S)

Endometrial biopsies were collected from fertile volunteers (n = 61) attending the clinic as ovum donors.

INTERVENTION(S)

Endometrial samples of 61 healthy fertile women.

MAIN OUTCOME MEASURE(S)

In vivo localization of PGRMC1 and SERBP1 was assessed by immunohistochemistry. The PGRMC1/SERBP1 colocalization was investigated in vitro and in vivo. Decidualization effect of PGRMC1 overexpression was evaluated in primary ESC cultures.

RESULT(S)

The PGRMC1 was detected in the endometrial stroma throughout the menstrual cycle, but decreased in the late secretory phase. The SERBP1 immunostaining was present in stroma and increased in the entire the menstrual cycle. The PGRMC1 and SERBP1 colocalized in the cytoplasmic fractions of nondecidualized and decidualized ESC. The PGRMC1 overexpression significantly inhibited in vitro decidualization.

CONCLUSION(S)

Our results suggest that classic P receptors (PRs) are not the only kind playing a role in the normal physiology of the endometrium. The human decidualization process could be altered by the overexpression or mislocalization of PGRMC1 in ESC.

Proteomics in Cell Division

Cell division requires a coordinated action of the cell cycle machinery, cytoskeletal elements, chromosomes, and membranes. Cell division studies have greatly benefitted from the mass spectrometry (MS)-based proteomic approaches for probing the biochemistry of highly dynamic complexes and their coordination with each other as a cell progresses into division. In this review, the authors first summarize a wide-range of proteomic studies that focus on the identification of sub-cellular components/protein complexes of the cell division machinery including kinetochores, mitotic spindle, midzone, and centrosomes. The authors also highlight MS-based large-scale analyses of the cellular components that are largely understudied during cell division such as the cell surface and lipids. Then, the authors focus on posttranslational modification analyses, especially phosphorylation and the resulting crosstalk with other modifications as a cell undergoes cell division. Combining proteomic approaches that probe the biochemistry of cell division components with functional genomic assays will lead to breakthroughs toward a systems-level understanding of cell division.

Nuclear survivin promoted by acetylation is associated with the aggressive phenotype of oral squamous cell carcinoma

Defects in apoptotic pathway contribute to development and progression of oral cancer. Survivin, a member of the inhibitors of apoptosis protein (IAP) family, is increased in many types of cancers. However, it is unclear whether increased survivin is associated with oral squamous cell carcinomas (OSCC), and what mechanisms may involve in. In this study, we examined survivin expression in OSCC compared with normal oral tissues via immunohistochemical staining. The results showed that, not only total survivin is increased in OSCCs, but also the subcellular location of survivin is changed in OSCCs compared with normal oral tissues. In most of normal oral tissues, survivin staining was either negative, or cytoplasmic positive/nuclear negative; whereas in most of OSCC tissues, survivin staining was nuclear positive. Statistic analysis indicates that nuclear survivin, rather than total or cytoplasmic one, correlates with tumor TNM stage and differentiation grade. Consistently, in vitro analysis showed that survivin is in cytoplasm in normal human oral kinotinocyte (HOK) cells; whereas it is in nucleus in OSCC HN6 cells. Importantly, treatment of HOK cells with HDAC inhibitor Trichostatin A (TSA) induces survivin acetylation and promotes its nuclear localization. Moreover, nuclear survivin in OSCC cells was acetylated at K129 in its C-terminal, suggesting that the acetylation is important for nuclear location of survivin. Our study demonstrates that it is nuclear survivin, rather than total or cytoplasmic one, associates with TNM stage and tumor grade of OSCC. Thus, we propose nuclear survivin as a prognostic marker for the progression of OSCC.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

Phosphoproteomic analysis of human mitotic chromosomes identified a chromokinesin KIF4A

Protein phosphorylation is the prime post-translational modification to drive cell division. Identification of phosphorylated proteins and their related kinases has uncovered molecular networks underlying mitotic processes, including chromosome assembly. Here we aimed to identify phosphoproteins from a mitotic chromosome-enriched lysate biochemically using mass spectrometry. We employed the Polo-box domain (PBD) of Polo-like kinase to tether phosphorylated proteins in the lysate. Resulting candidates included a number of chromosomal proteins that would not be identified unless using chromosome-enriched fractions. Among them, we focused to a chromokinesin KIF4A which becomes concentrated along the longitudinal axis of mitotic condensed chromatids. We found that KIF4A is phosphorylated specifically during mitosis, depending on the activity of Cdk1 and Aurora B, which turned out to be required for KIF4A to interact with condensin I. The molecular link between KIF4A and condensin raises an interesting possibility that the interaction of two distantly related ATPases is triggered by mitotic phosphorylation.

Large-Scale Mitotic Cell Synchronization

Understanding cell growth and cell division involves the study of regulatory events that occur in a cell cycle phase-dependent manner. Studies analyzing cell cycle regulatory mechanisms and cell cycle progression invariably require synchronization of cell populations at specific cell cycle stages. Several methods have been established to synchronize cells, including serum deprivation, contact inhibition, centrifugal elutriation, and drug-dependent synchronization. Despite potential adverse cellular consequences of synchronizing cells by pharmacological agents, drug-dependent methods can be advantageous when studying later cell cycle events to ensure specific enrichment at selected mitotic stages. This chapter describes protocols used in our laboratory for isolating mitotic mammalian cells in a large-scale manner. In particular, we discuss the technical aspects of adherent or suspension cell isolation, the methods necessary to enrich cells at different mitotic stages and the optimized culture conditions.

Mitotic phosphotyrosine network analysis reveals that tyrosine phosphorylation regulates Polo-like kinase 1 (PLK1)

Tyrosine phosphorylation is closely associated with cell proliferation. During the cell cycle, serine and threonine phosphorylation plays the leading role, and such phosphorylation events are most dynamic during the mitotic phase of the cell cycle. However, mitotic phosphotyrosine is not well characterized. Although a few functionally-relevant mitotic phosphotyrosine sites have been characterized, evidence suggests that this modification may be more prevalent than previously appreciated. Here, we examined tyrosine phosphorylation in mitotic human cells including those on spindle-associated proteins.? Database mining confirmed ~2000 mitotic phosphotyrosine sites, and network analysis revealed a number of subnetworks that were enriched in tyrosine-phosphorylated proteins, including components of the kinetochore or spindle and SRC family kinases. We identified Polo-like kinase 1 (PLK1), a major signaling hub in the spindle subnetwork, as phosphorylated at the conserved Tyr217 in the kinase domain. Substitution of Tyr217 with a phosphomimetic residue eliminated PLK1 activity in vitro and in cells. Further analysis showed that Tyr217 phosphorylation reduced the phosphorylation of Thr210 in the activation loop, a phosphorylation event necessary for PLK1 activity. Our data indicate that mitotic tyrosine phosphorylation regulated a key serine/threonine kinase hub in mitotic cells and suggested that spatially separating tyrosine phosphorylation events can reveal previously unrecognized regulatory events and complexes associated with specific structures of the cell cycle.

The functional role for condensin in the regulation of chromosomal organization during the cell cycle

In all organisms, the control of cell cycle progression is a fundamental process that is essential for cell growth, development, and survival. Through each cell cycle phase, the regulation of chromatin organization is essential for natural cell proliferation and maintaining cellular homeostasis. During mitosis, the chromatin morphology is dramatically changed to have a "thread-like" shape and the condensed chromosomes are segregated equally into two daughter cells. Disruption of the mitotic chromosome architecture physically impedes chromosomal behaviors, such as chromosome alignment and chromosome segregation; therefore, the proper mitotic chromosome structure is required to maintain chromosomal stability. Accumulating evidence has demonstrated that mitotic chromosome condensation is induced by condensin complexes. Moreover, recent studies have shown that condensin also modulates interphase chromatin and regulates gene expression. This review mainly focuses on the molecular mechanisms that condensin uses to exert its functions during the cell cycle progression. Moreover, we discuss the condensin-mediated chromosomal organization in cancer cells.