Stathmin regulates centrosomal nucleation of microtubules and tubulin dimer/polymer partitioning

Interplay between stochastic enzyme activity and microtubule stability drives detyrosination enrichment on microtubule subsets

Microtubules in cells consist of functionally diverse subpopulations carrying distinct post-translational modifications (PTMs). Akin to the histone code, the tubulin code regulates a myriad of microtubule functions, ranging from intracellular transport to chromosome segregation. However, how individual PTMs only occur on subsets of microtubules to contribute to microtubule specialization is not well understood. In particular, microtubule detyrosination, the removal of the C-terminal tyrosine on α-tubulin subunits, marks the stable population of microtubules and modifies how microtubules interact with other microtubule-associated proteins to regulate a wide range of cellular processes. Previously, we found that in certain cell types, only ∼30% of microtubules are highly enriched with the detyrosination mark and that detyrosination spans most of the length of a microtubule, often adjacent to a completely tyrosinated microtubule. How the activity of a cytosolic detyrosinase, vasohibin (VASH), leads to only a small subpopulation of highly detyrosinated microtubules is unclear. Here, using quantitative super-resolution microscopy, we visualized nascent microtubule detyrosination events in cells consisting of 1-3 detyrosinated α-tubulin subunits after nocodazole washout. Microtubule detyrosination accumulates slowly and in a dispersed pattern across the microtubule length. By visualizing single molecules of VASH in live cells, we found that VASH engages with microtubules stochastically on a short timescale, suggesting limited removal of tyrosine per interaction, consistent with the super-resolution results. Combining these quantitative imaging results with simulations incorporating parameters from our experiments, we provide evidence for a stochastic model for cells to establish a subset of detyrosinated microtubules via a detyrosination-stabilization feedback mechanism.

Polarized microtubule remodeling transforms the morphology of reactive microglia and drives cytokine release

Microglial reactivity is a pathological hallmark in many neurodegenerative diseases. During stimulation, microglia undergo complex morphological changes, including loss of their characteristic ramified morphology, which is routinely used to detect and quantify inflammation in the brain. However, the underlying molecular mechanisms and the relation between microglial morphology and their pathophysiological function are unknown. Here, proteomic profiling of lipopolysaccharide (LPS)-reactive microglia identifies microtubule remodeling pathways as an early factor that drives the morphological change and subsequently controls cytokine responses. We find that LPS-reactive microglia reorganize their microtubules to form a stable and centrosomally-anchored array to facilitate efficient cytokine trafficking and release. We identify cyclin-dependent kinase 1 (Cdk-1) as a critical upstream regulator of microtubule remodeling and morphological change in-vitro and in-situ. Cdk-1 inhibition also rescues tau and amyloid fibril-induced morphology changes. These results demonstrate a critical role for microtubule dynamics and reorganization in microglial reactivity and modulating cytokine-mediated inflammatory responses.

Cell cycle-related genes associate with sensitivity to hydrogen peroxide-induced toxicity

Reactive oxygen species (ROS) such as hydrogen peroxide (H2O2) are well-described agents in physiology and pathology. Chronic inflammation causes incessant H2O2 generation associated with disease occurrences such as diabetes, autoimmunity, and cancer. In cancer, conditioning of the tumor microenvironment, e.g., hypoxia and ROS generation, has been associated with disease outcomes and therapeutic efficacy. Many reports have investigated the roles of the action of H2O2 across many cell lines and disease models. The genes predisposing tumor cell lines to H2O2-mediated demise are less deciphered, however. To this end, we performed in-house transcriptional profiling of 35 cell lines and simultaneously investigated each cell line's H2O2 inhibitory concentration (IC25) based on metabolic activity. More than 100-fold differences were observed between the most resistant and sensitive cell lines. Correlation and gene ontology pathway analysis identified a rigid association with genes intertwined in cell cycle progression and proliferation, as such functional categories dominated the top ten significant processes. The ten most substantially correlating genes (Spearman r > 0.70 or < -0.70) were validated using qPCR, showing complete congruency with microarray analysis findings. Western blotting confirmed the correlation of cell cycle-related proteins negatively correlating with H2O2 IC25. Top genes related to ROS production or antioxidant defense were only modest in correlation (Spearman r > 0.40 or < -0.40). In conclusion, our in-house transcriptomic correlation analysis revealed a set of cell cycle-associated genes associated with a priori resistance or sensitivity to H2O2-induced cellular demise with the detailed and causative roles of individual genes remaining unclear.

HN1 interacts with γ-tubulin to regulate centrosomes in advanced prostate cancer cells

Prostate cancer is one of the most common cancer for men worldwide with advanced forms showing supernumerary or clustered centrosomes. Hematological and neurological expressed 1 (HN1) also known as Jupiter Microtubule Associated Homolog 1 (JPT1) belongs to a small poorly understood family of genes that are evolutionarily conserved across vertebrate species. The co-expression network of HN1 from the TCGA PRAD dataset indicates the putative role of HN1 in centrosome-related processes in the context of prostate cancer. HN1 expression is low in normal RWPE-1 cells as compared to cancerous androgen-responsive LNCaP and androgen insensitive PC-3 cells. HN1 overexpression resulted in differential response for cell proliferation and cell cycle changes in RWPE-1, LNCaP, and PC-3 cells. Since HN1 overexpression increased the proliferation rate in PC-3 cells, these cells were used for functional characterization of HN1 in advanced prostate carcinogenesis. Furthermore, alterations in HN expression led to an increase in abnormal to normal nuclei ratio and increased chromosomal aberrations in PC-3 cells. We observed the co-localization of HN1 with γ-tubulin foci in prostate cancer cells, further validated by immunoprecipitation. HN1 was observed as physically associated with γ-tubulin and its depletion led to increased γ-tubulin foci and disruption in microtubule spindle assembly. Higher HN1 expression was correlated with prostate cancer as compared to normal tissues. The restoration of HN1 expression after silencing suggested that it has a role in centrosome clustering, implicating a potential role of HN1 in cell division as well as in prostate carcinogenesis warranting further studies.

PICT1 is critical for regulating the Rps27a-Mdm2-p53 pathway by microtubule polymerization inhibitor against cervical cancer

In our previous study, it showed that P-3F, a podophyllotoxin derivative, causes the increased level of p53 expression by enhancing p53 stability, resulting from blockage of the Mdm2-p53 feedback loop via nucleolus-to-nucleoplasm translocation of Rps27a in human cervical cancer HeLa cell line. However, the mechanism of regulating Rps27a localization remains to be studied. In the current study, it has been demonstrated that the level of protein interacting with carboxyl terminus 1 (PICT1), originally identified as a tumor suppressor, was decreased in a concentration-dependent manner in response to P-3F, leading to inhibition of human cervical cancer cell lines proliferation. Also remarkably, reduction of serine phosphorylation of STMN1 at position 16 induced by P-3F was required in the downregulation of PICT1, in which p53 activity was likely to be directly involved. Note as well that, PICT1 also played an important role in p53 stability enhancement by inhibiting Mdm2-mediated p53 ubiquitination due to Rps27a translocation from the nucleolus to the nucleoplasm to interact with Mdm2 following treatment with P-3F. Collectively, these findings indicated that P-3F, a microtubule polymerization inhibitor, promotes the decreased level of PICT1 expression, which is critical for regulating the Rps27a-Mdm2-p53 pathway against cervical cancer.

TMT-Based Quantitative Proteomic Analysis Reveals the Effect of Bone Marrow Derived Mesenchymal Stem Cell on Hair Follicle Regeneration

Hair loss (HL) is a common chronic problem of poorly defined etiology. Herein, we explored the functionality of bone marrow-derived mesenchymal stem cell (BMSC) and conditioned medium (MSC-CM) as regulators of hair follicle proliferation and regeneration, and the mechanistic basis for such activity. BMSC were cultured and identified in vitro through the induction of multilineage differentiation and the use of a CCK-8 kit. The dorsal skin of mice was then injected with BMSC and MSC-CM, and the impact of these injections on hair cycle transition and hair follicle stem cell (HFSC) proliferation was then evaluated via hematoxylin and eosin (H&E) staining and immunofluorescent (IF) staining. We then conducted a tandem mass tags (TMT)-based quantitative proteomic analysis of control mice and mice treated with BMSC or MSC-CM to identify differentially expressed proteins (DEPs) associated with these treatments. Parallel reaction monitoring (PRM) was utilized as a means of verifying our proteomic analysis results. Herein, we found that BMSC and MSC-CM injection resulted in the transition of telogen hair follicles to anagen hair follicles, and we observed the enhanced proliferation of HFSCs positive for Krt15 and Sox9. Our TMT analyses identified 1,060 and 770 DEPs (fold change＞1.2 or＜0.83 and p ＜ 0.05) when comparing the BMSC vs. control and MSC-CM vs. control groups, respectively. Subsequent PRM validation of 14 selected DEPs confirmed these findings, and led to the identification of Stmn1, Ncapd2, Krt25, and Ctps1 as hub DEPs in a protein-protein interaction network. Together, these data suggest that BMSC and MSC-CM treatment can promote the proliferation of HFSCs, thereby facilitating hair follicle regeneration. Our proteomics analyses further indicate that Krt25, Cpm, Stmn1, and Mb may play central roles in hair follicle transition in this context and may represent viable clinical targets for the treatment of HL.

A Bioinformatics Analysis Identifies the Telomerase Inhibitor MST-312 for Treating High-STMN1-Expressing Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a relatively chemo-resistant tumor. Several multi-kinase inhibitors have been approved for treating advanced HCC. However, most HCC patients are highly refractory to these drugs. Therefore, the development of more effective therapies for advanced HCC patients is urgently needed. Stathmin 1 (STMN1) is an oncoprotein that destabilizes microtubules and promotes cancer cell migration and invasion. In this study, cancer genomics data mining identified STMN1 as a prognosis biomarker and a therapeutic target for HCC. Co-expressed gene analysis indicated that STMN1 expression was positively associated with cell-cycle-related gene expression. Chemical sensitivity profiling of HCC cell lines suggested that High-STMN1-expressing HCC cells were the most sensitive to MST-312 (a telomerase inhibitor). Drug-gene connectivity mapping supported that MST-312 reversed the STMN1-co-expressed gene signature (especially BUB1B, MCM2/5/6, and TTK genes). In vitro experiments validated that MST-312 inhibited HCC cell viability and related protein expression (STMN1, BUB1B, and MCM5). In addition, overexpression of STMN1 enhanced the anticancer activity of MST-312 in HCC cells. Therefore, MST-312 can be used for treating STMN1-high expression HCC.

Nuclear Isoforms of Neurofibromin Are Required for Proper Spindle Organization and Chromosome Segregation

Mitotic spindles are highly organized, microtubule (MT)-based, transient structures that serve the fundamental function of unerring chromosome segregation during cell division and thus of genomic stability during tissue morphogenesis and homeostasis. Hence, a multitude of MT-associated proteins (MAPs) regulates the dynamic assembly of MTs in preparation for mitosis. Some tumor suppressors, normally functioning to prevent tumor development, have now emerged as significant MAPs. Among those, neurofibromin, the product of the Neurofibromatosis-1 gene (NF1), a major Ras GTPase activating protein (RasGAP) in neural cells, controls also the critical function of chromosome congression in astrocytic cellular contexts. Cell type- and development-regulated splicings may lead to the inclusion or exclusion of NF1exon51, which bears a nuclear localization sequence (NLS) for nuclear import at G2; yet the functions of the produced NLS and ΔNLS neurofibromin isoforms have not been previously addressed. By using a lentiviral shRNA system, we have generated glioblastoma SF268 cell lines with conditional knockdown of NLS or ΔNLS transcripts. In dissecting the roles of NLS or ΔNLS neurofibromins, we found that NLS-neurofibromin knockdown led to increased density of cytosolic MTs but loss of MT intersections, anastral spindles featuring large hollows and abnormal chromosome positioning, and finally abnormal chromosome segregation and increased micronuclei frequency. Therefore, we propose that NLS neurofibromin isoforms exert prominent mitotic functions.

Synthetic lethality of RB1 and aurora A is driven by stathmin-mediated disruption of microtubule dynamics

RB1 mutational inactivation is a cancer driver in various types of cancer including lung cancer, making it an important target for therapeutic exploitation. We performed chemical and genetic vulnerability screens in RB1-isogenic lung cancer pair and herein report that aurora kinase A (AURKA) inhibition is synthetic lethal in RB1-deficient lung cancer. Mechanistically, RB1^−/− cells show unbalanced microtubule dynamics through E2F-mediated upregulation of the microtubule destabilizer stathmin and are hypersensitive to agents targeting microtubule stability. Inhibition of AURKA activity activates stathmin function via reduced phosphorylation and facilitates microtubule destabilization in RB1^−/− cells, heavily impacting the bipolar spindle formation and inducing mitotic cell death selectively in RB1^−/− cells. This study shows that stathmin-mediated disruption of microtubule dynamics is critical to induce synthetic lethality in RB1-deficient cancer and suggests that upstream factors regulating microtubule dynamics, such as AURKA, can be potential therapeutic targets in RB1-deficient cancer.

Retinoblastoma susceptibility gene (RB1) is frequently mutated in lung cancers. Here the authors perform chemical and genetic vulnerability screens and identify aurora A kinase (AURKA) as a synthetic lethal candidate for RB1-deficient lung cancer cells and that AURKA inhibition sensitizes these cells to mitotic cell death.

Monte Carlo simulations of microtubule arrays: The critical roles of rescue transitions, the cell boundary, and tubulin concentration in shaping microtubule distributions

Microtubules are dynamic polymers required for a number of processes, including chromosome movement in mitosis. While regulators of microtubule dynamics have been well characterized, we lack a convenient way to predict how the measured dynamic parameters shape the entire microtubule system within a cell, or how the system responds when specific parameters change in response to internal or external signals. Here we describe a Monte Carlo model to simulate an array of dynamic microtubules from parameters including the cell radius, total tubulin concentration, microtubule nucleation rate from the centrosome, and plus end dynamic instability. The algorithm also allows dynamic instability or position of the cell edge to vary during the simulation. Outputs from simulations include free tubulin concentration, average microtubule lengths, length distributions, and individual length changes over time. Using this platform and reported parameters measured in interphase LLCPK1 epithelial cells, we predict that sequestering ~ 15-20% of total tubulin results in fewer microtubules, but promotes dynamic instability of those remaining. Simulations also predict that lowering nucleation rate will increase the stability and average length of the remaining microtubules. Allowing the position of the cell's edge to vary over time changed the average length but not the number of microtubules and generated length distributions consistent with experimental measurements. Simulating the switch from interphase to prophase demonstrated that decreased rescue frequency at prophase is the critical factor needed to rapidly clear the cell of interphase microtubules prior to mitotic spindle assembly. Finally, consistent with several previous simulations, our results demonstrate that microtubule nucleation and dynamic instability in a confined space determines the partitioning of tubulin between monomer and polymer pools. The model and simulations will be useful for predicting changes to the entire microtubule array after modification to one or more parameters, including predicting the effects of tubulin-targeted chemotherapies.

Neuronal Intrinsic Regenerative Capacity: The Impact of Microtubule Organization and Axonal Transport

In the adult vertebrate central nervous system, axons generally fail to regenerate. In contrast, peripheral nervous system axons are able to form a growth cone and regenerate upon lesion. Among the multiple intrinsic mechanisms leading to the formation of a new growth cone and to successful axon regrowth, cytoskeleton organization and dynamics is central. Here we discuss how multiple pathways that define the regenerative capacity converge into the regulation of the axonal microtubule cytoskeleton and transport. We further explore the use of dorsal root ganglion neurons as a model to study the neuronal regenerative ability. Finally, we address some of the unanswered questions in the field, including the mechanisms by which axonal transport might be modulated by injury, and the relationship between microtubule organization, dynamics, and axonal transport. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 00: 000-000, 2018.

The Microtubule Network and Cell Death Are Regulated by an miR-34a/Stathmin 1/βIII-Tubulin Axis

MicroRNA-34a (miR-34a) is a master regulator of signaling networks that maintains normal physiology and disease and is currently in development as a miRNA-based therapy for cancer. Prior studies have reported low miR-34a expression in osteosarcoma; however, the molecular mechanisms underlying miR-34a activity in osteosarcoma are not well-defined. Therefore, this study evaluated the role of miR-34a in regulating signal transduction pathways that influence cell death in osteosarcoma. Levels of miR-34a were attenuated in human osteosarcoma cells and xenografts of the Pediatric Preclinical Testing Consortium (PPTC). Bioinformatics predictions identified stathmin 1 (STMN1) as a potential miR-34a target. Biotin pull-down assay and luciferase reporter analysis confirmed miR-34a target interactions within the STMN1 mRNA 3'-untranslated region. Overexpression of miR-34a in osteosarcoma cells suppressed STMN1 expression and reduced cell growth in vitro Restoration of miR-34a led to microtubule destabilization and increased βIII-tubulin expression, with corresponding G1-G2 phase cell-cycle arrest and apoptosis. Knockdown of the Sp1 transcription factor, by siRNA silencing, also upregulated βIII-tubulin expression in osteosarcoma cells, suggesting that miR-34a indirectly affects Sp1. Validating the coordinating role of miR-34a in microtubule destabilization, when miR-34a was combined with either microtubule inhibitors or chemotherapy, STMN1 phosphorylation was suppressed and there was greater cytotoxicity in osteosarcoma cells. These results demonstrate that miR-34a directly represses STMN1 gene and protein expression and upregulates βIII-tubulin, leading to disruption of the microtubule network and cell death.Implications: The miR-34a/STMN1/βIII-tubulin axis maintains the microtubule cytoskeleton in osteosarcoma, and combining miR-34a with microtubule inhibitors can be investigated as a novel therapeutic strategy. Mol Cancer Res; 15(7); 953-64. ©2017 AACR.

Polarity Reversal by Centrosome Repositioning Primes Cell Scattering during Epithelial-to-Mesenchymal Transition

During epithelial-to-mesenchymal transition (EMT), cells lining the tissue periphery break up their cohesion to migrate within the tissue. This dramatic reorganization involves a poorly characterized reorientation of the apicobasal polarity of static epithelial cells into the front-rear polarity of migrating mesenchymal cells. To investigate the spatial coordination of intracellular reorganization with morphological changes, we monitored centrosome positioning during EMT in vivo, in developing mouse embryos and mammary gland, and in vitro, in cultured 3D cell aggregates and micropatterned cell doublets. In all conditions, centrosomes moved from their off-centered position next to intercellular junctions toward extracellular matrix adhesions on the opposite side of the nucleus, resulting in an effective internal polarity reversal. This move appeared to be supported by controlled microtubule network disassembly. Sequential release of cell confinement using dynamic micropatterns, and modulation of microtubule dynamics, confirmed that centrosome repositioning was responsible for further cell disengagement and scattering.

α-Synuclein regulates the partitioning between tubulin dimers and microtubules at neuronal growth cone

The partitioning between tubulin dimers and microtubules is fundamental for the regulation of several neuronal activities, from neuronal polarization and processes extension to growth cone remodelling. This phenomenon is modulated by several proteins, including the well-known microtubule destabilizer Stathmin. We recently demonstrated that α-Synuclein, a presynaptic protein associated to Parkinson's disease, shares structural and functional properties with Stathmin, and we showed that α-Synuclein acts as a foldable dynamase. Here, we pinpoint the impact of wild type α-Synuclein on the partitioning between tubulin dimers and microtubules and show that Parkinson's disease-linked mutants lose this capability. Thus, our results indicate a new role for α-Synuclein in regulating microtubule system and support the concept that microtubules and α-Synuclein are partners in the modulation of neuronal health and degenerative processes. Furthermore, these data strengthen our hypothesis of the existence of a functional redundancy between α-Synuclein and Stathmin.

The epidermal growth factor receptor decreases Stathmin 1 and triggers catagen entry in the mouse

The epidermal growth factor receptor (EGFR) is necessary for normal involution of hair follicles after the growth phase of anagen, although the mechanisms through which it acts are not well understood. In this report, we used transcriptional profiling of microdissected hair follicles from mice with skin-targeted deletion of Egfr to investigate how EGFR activation triggers catagen. Immunofluorescence for phospho-EGFR in mouse skin revealed increased activation of EGFR in follicular keratinocytes at catagen onset. Consistent with other models of EGFR deficiency, mice with skin-targeted deletion of Egfr (Krt14-Cre(+) /Egfr(fl/fl) ) exhibited a delayed and asynchronous catagen entry. Transcriptional profiling at the time of normal catagen onset at post-natal day (P) 17 revealed increased expression of the mitotic regulator Rcc2 in hair follicles lacking EGFR. Rcc2 protein was strongly immunopositive in the nuclei of control follicular keratinocytes at P16 then rapidly decreased until it was undetectable between P18 and 21. In contrast, Rcc2 expression continued in Egfr mutant follicles throughout this period. Proliferation, measured by bromodeoxyuridine incorporation, was also significantly increased in Egfr mutant follicular keratinocytes compared to controls at P18-21. Similarly, Rcc2-regulated mitotic regulator Stathmin 1 was strikingly reduced in control but not Egfr mutant follicles between P17 and P19. Deletion of Stmn1, in turn, accelerated catagen entry associated with premature cessation of proliferation in the hair follicles. These data reveal EGFR suppression of mitotic regulators including Rcc2 and Stathmin 1 as a mechanism for catagen induction in mouse skin.

The Fanconi Anemia C Protein Binds to and Regulates Stathmin-1 Phosphorylation

The Fanconi anemia (FA) proteins are involved in a signaling network that assures the safeguard of chromosomes. To understand the function of FA proteins in cellular division events, we investigated the interaction between Stathmin-1 (STMN1) and the FA group C (FANCC) protein. STMN1 is a ubiquitous cytosolic protein that regulates microtubule dynamics. STMN1 activities are regulated through phosphorylation-dephosphorylation mechanisms that control assembly of the mitotic spindle, and dysregulation of STMN1 phosphorylation is associated with mitotic aberrancies leading to chromosome instability and cancer progression. Using different biochemical approaches, we showed that FANCC interacts and co-localizes with STMN1 at centrosomes during mitosis. We also showed that FANCC is required for STMN1 phosphorylation, as mutations in FANCC reduced serine 16- and 38-phosphorylated forms of STMN1. Phosphorylation of STMN1 at serine 16 is likely an event dependent on a functional FA pathway, as it is reduced in FANCA- and FANCD2-mutant cells. Furthermore, FA-mutant cells exhibited mitotic spindle anomalies such as supernumerary centrosomes and shorter mitotic spindles. These results suggest that FA proteins participate in the regulation of cellular division via the microtubule-associated protein STMN1.

Centrosome nucleates numerous ephemeral microtubules and only few of them participate in the radial array

It is generally accepted that long microtubules (MTs) grow from the centrosome with their minus ends anchored there and plus ends directed towards cell membrane. However, recent findings show this scheme to be an oversimplification. To further analyze the relationship between the centrosome and the MT array we undertook a detailed study on the MTs growing from the centrosome after microinjection of Cy3 labeled tubulin and transfection of cells with EB1-GFP. To evaluate MTs around the centrosome two approaches were used: path photobleaching across the centrosome area (Komarova et al., ) and sequential image subtraction analysis (Vorobjev et al., ). We show that about 50% of MTs had been nucleated at the centrosome are short-living: their mean length was 1.8 ± 0.8 μm and their life span - 7 ± 2 s. MTs initiated from the centrosome also rarely reach cell margin, since their elongation was limited and growth after shortening (rescue) was rare. After initial growth all MTs associated with the centrosome converted to pause or shortening. After pause MTs associated with the centrosome mainly depolymerized via the plus end shortening. Stability of the minus ends of cytoplasmic MTs was the same as for centrosomal ones. We conclude that in fibroblasts (1) the default behavior of free MTs in the cell interior is biased dynamic instability (i.e., random walk of the plus ends with significant positive drift); (2) MTs born at the centrosome show "dynamic instability" type behavior with no boundary; and (3) that the extended radial array is formed predominantly by MTs not associated with the centrosome.

Microtubule-associated proteins control the kinetics of microtubule nucleation

Microtubules are born and reborn continuously, even during quiescence. These polymers are nucleated from templates, namely γ-tubulin ring complexes (γ-TuRCs) and severed microtubule ends. Using single-molecule biophysics, we show that nucleation from γ-TuRCs, axonemes and seed microtubules requires tubulin concentrations that lie well above the critical concentration. We measured considerable time lags between the arrival of tubulin and the onset of steady-state elongation. Microtubule-associated proteins (MAPs) alter these time lags. Catastrophe factors (MCAK and EB1) inhibited nucleation, whereas a polymerase (XMAP215) and an anti-catastrophe factor (TPX2) promoted nucleation. We observed similar phenomena in cells. We conclude that GTP hydrolysis inhibits microtubule nucleation by destabilizing the nascent plus ends required for persistent elongation. Our results explain how MAPs establish the spatial and temporal profile of microtubule nucleation.

P18/Stathmin1 is regulated by miR-31 in ovarian cancer in response to taxane

MicroRNAs (miRNAs) have been reported to regulate the development of chemoresistance in many tumors. Stathmin 1 (STMN1) is a microtubule-depolymerizing molecule, involved in chemo-response; however, the mechanism of its regulation is unknown. Herein, the immunohistochemical study indicated significant upregulation of the STMN1 in the ovarian cancer tissues defined as resistant tumors compared with those defined as responsive tumors. STMN1 level elevated in the chemoresistant ovarian cancer cells, KF-TX, compared with the parental, KF, ones. Targeting STMN1 by siRNA restored taxane-sensitivity of KF-TX cells. Screening miRNA profiles from KF/KF-TX cellular set followed by bioinformatics-based prediction, revealed that miR-31 could be a possible regulator of STMN1. Down-modulation of miR-31 was verified by quantitative RT-PCR in the cellular set used. Overexpression of miR-31 in KF-TX cells (KF-TX-miR-31) significantly restored chemo-response and reduced STMN1 expression as well. STMN1 reduction-associated cellular characteristics such as enhanced microtubule polymerization and stability, as indicated by acetylated tubulin quantification, confocal visualization, and G2 phase delay, were observed in KF-TX-miR-31 cells, indicating the functional reduction of STMN1. miR-31 suppressed the luciferase activity in reporter construct containing the STMN1 3'-untranslated region (3'-UTR), confirming that miR-31 directly targets STMN1. miR-31 has therapeutic potency when introduced into ovarian cancer, in combination with taxane.

Sep(t)arate or not – how some cells take septin-independent routes through cytokinesis

Cytokinesis is the final step of cell division, and is a process that requires a precisely coordinated molecular machinery to fully separate the cytoplasm of the parent cell and to establish the intact outer cell barrier of the daughter cells. Among various cytoskeletal proteins involved, septins are known to be essential mediators of cytokinesis. In this Commentary, we present recent observations that specific cell divisions can proceed in the absence of the core mammalian septin SEPT7 and its Drosophila homolog Peanut (Pnut) and that thus challenge the view that septins have an essential role in cytokinesis. In the pnut mutant neuroepithelium, orthogonal cell divisions are successfully completed. Similarly, in the mouse, Sept7-null mutant early embryonic cells and, more importantly, planktonically growing adult hematopoietic cells undergo productive proliferation. Hence, as discussed here, mechanisms must exist that compensate for the lack of SEPT7 and the other core septins in a cell-type-specific manner. Despite there being crucial non-canonical immune-relevant functions of septins, septin depletion is well tolerated by the hematopoietic system. Thus differential targeting of cytokinesis could form the basis for more specific anti-proliferative therapies to combat malignancies arising from cell types that require septins for cytokinesis, such as carcinomas and sarcomas, without impairing hematopoiesis that is less dependent on septin.

Silencing stathmin-modulating efficiency of chemotherapy for esophageal squamous cell cancer with paclitaxel

Paclitaxel (PTX) is broadly considered the drug of choice for treating human esophageal squamous cell cancer (ESCC). However, PTX resistance often ultimately leads to treatment failure. stathmin, or Op18, is a ubiquitously expressed 19-kDa cytosolic phosphoprotein that can integrate various cellular regulatory signals. stathmin overexpression could lead to resistance to chemotherapeutic agents. In this study we investigated the effect of stathmin gene silencing, using small interfering RNA (stathmin siRNA), on the efficacy of PTX in ESCC. Transfection of stathmin siRNA could significantly inhibit stathmin mRNA and protein levels in ESCC cell lines EC9706 and Eca-109. The silencing of stathmin combined with PTX significantly inhibited the proliferation of EC9706 and Eca-109 cells, with a significantly higher proportion of cells at G2/M phase and this antiproliferative effect was accompanied by an increase in apoptosis rates and morphology changes of EC9706 and Eca-109. Thus, combined chemotherapeutic agent PTX and stathmin siRNA could potentially enhance the therapeutic outcomes of PTX in treating ESCC.

Unbiased proteomic and transcript analyses reveal that stathmin-1 silencing inhibits colorectal cancer metastasis and sensitizes to 5-fluorouracil treatment

Colorectal cancer metastasis is a major cause of mortality worldwide, which may only be controlled with novel methods limiting tumor dissemination and chemoresistance. High stathmin-1 (STMN1) expression was previously established as a hallmark of colorectal cancer progression and predictor of poor survival; however, the mechanism of action is less clear. This work demonstrates that STMN1 silencing arrests tumor-disseminative cascades by inhibiting multiple metastatic drivers, and repressing oncogenic and mesenchymal transcription. Using a sensitive iTRAQ labeling proteomic approach that quantified differential abundance of 4562 proteins, targeting STMN1 expression was shown to reinstate the default cellular program of metastatic inhibition, and promote cellular adhesion via amplification of hemidesmosomal junctions and intermediate filament tethering. Silencing STMN1 also significantly improved chemoresponse to the classical colorectal cancer therapeutic agent, 5FU, via a novel caspase-6 (CASP6)-dependent mechanism. Interestingly, the prometastatic function of STMN1 was independent of p53 but required phosphorylations at S25 or S38; abrogating phosphorylative events may constitute an alternative route to achieving metastatic inhibition. These findings establish STMN1 as a potential target in antimetastatic therapy, and demonstrate the power of an approach coupling proteomics and transcript analyses in the global assessment of treatment benefits and potential side-effects.

IMPLICATIONS

Stathmin-1 is a potential candidate in colorectal cancer therapy that targets simultaneously the twin problems of metastatic spread and chemoresistance.

TACC3 protein regulates microtubule nucleation by affecting γ-tubulin ring complexes

Centrosome-mediated microtubule nucleation is essential for spindle assembly during mitosis. Although γ-tubulin complexes have primarily been implicated in the nucleation process, details of the underlying mechanisms remain poorly understood. Here, we demonstrated that a member of the human transforming acidic coiled-coil (TACC) protein family, TACC3, plays a critical role in microtubule nucleation at the centrosome. In mitotic cells, TACC3 knockdown substantially affected the assembly of microtubules in the astral region and impaired microtubule nucleation at the centrosomes. The TACC3 depletion-induced mitotic phenotype was rescued by expression of the TACC3 C terminus predominantly consisting of the TACC domain, suggesting that the TACC domain plays an important role in microtubule assembly. Consistently, experiments with the recombinant TACC domain of TACC3 demonstrated that this domain possesses intrinsic microtubule nucleating activity. Co-immunoprecipitation and sedimentation experiments revealed that TACC3 mediates interactions with proteins of both the γ-tubulin ring complex (γ-TuRC) and the γ-tubulin small complex (γ-TuSC). Interestingly, TACC3 depletion resulted in reduced levels of γ-TuRC and increased levels of γ-TuSC, indicating that the assembly of γ-TuRC from γ-TuSC requires TACC3. Detailed analyses suggested that TACC3 facilitates the association of γ-TuSC-specific proteins with the proteins known to be involved in the assembly of γ-TuRC. Consistent with such a role for TACC3, the suppression of TACC3 disrupted localization of γ-TuRC proteins to the centrosome. Our findings reveal that TACC3 is involved in the regulation of microtubule nucleation at the centrosome and functions in the stabilization of the γ-tubulin ring complex assembly.

Stathmin destabilizing microtubule dynamics promotes malignant potential in cancer cells by epithelial-mesenchymal transition

BACKGROUND

Stathmin is a ubiquitous cytosolic regulatory phosphoprotein and is overexpressed in different human malignancies. The main physiological function of stathmin is to interfere with microtubule dynamics by promoting depolymerization of microtubules or by preventing polymerization of tubulin heterodimers. Stathmin plays important roles in regulating many cellular functions as a result of its microtubule-destabilizing activity. Currently, the critical roles of stathmin in cancer cells, as well as in lymphocytes have been valued. This review discusses stathmin and microtubule dynamics in cancer development, and hypothesizes their possible relationship with epithelial-mesenchymal transition (EMT).

DATA SOURCES

A PubMed search using such terms as "stathmin", "microtubule dynamics", "epithelial-mesenchymal transition", "EMT", "malignant potential" and "cancer" was performed to identify relevant studies published in English. More than 100 related articles were reviewed.

RESULTS

The literature clearly documented the relationship between stathmin and its microtubule-destabilizing activity of cancer development. However, the particular mechanism is poorly understood. Microtubule disruption is essential for EMT, which is a crucial process during cancer development. As a microtubule-destabilizing protein, stathmin may promote malignant potential in cancer cells by initiating EMT.

CONCLUSIONS

We propose that there is a stathmin-microtubule dynamics-EMT (S-M-E) axis during cancer development. By this axis, stathmin together with its microtubule-destabilizing activity contributes to EMT, which stimulates the malignant potential in cancer cells.

Clinical significance of serum Stathmin in esophageal squamous cell carcinoma

AIM: To detect serum Stathmin in patients with esophageal squamous cell carcinoma (ESCC) and evaluate its correlation with clinicopathological parameters in ESCC.

METHODS: Serum levels of Stathmin were measured by enzyme-linked immunosorbent assay (ELISA) in 74 patients with ESCC and 81 healthy individuals. Serum levels of Cyfra21-1 were measured by ELISA, and those of CEA, CA19-9 and CA72-4 were measured by chemiluminescence.

RESULTS: Serum levels of Stathmin in ESCC patients were significantly higher than those in healthy controls (P < 0.001). Serum levels of Stathmin were correlated with lymph node metastasis (P = 0.036), but not with age, gender, tumor differentiation, TNM stage or tumor size. According to the ROC curve, the sensitivity and specificity of serum Stathmin were significantly higher than those of Cyfra21-1 for diagnosis of esophageal squamous cell carcinoma. The sensitivity of serum Stathmin for diagnosis of ESCC was 94.6%.

CONCLUSION: Serum Stathmin levels increase in ESCC patients. Detection of serum Stathmin is helpful for diagnosis and prognostic evaluation of esophageal cancer. Stathmin may serve as a new potential serum tumor marker for clinical detection, therapy and prognostic evaluation of ESCC.

Stathmin and microtubules regulate mitotic entry in HeLa cells by controlling activation of both Aurora kinase A and Plk1

Depletion of stathmin, a microtubule (MT) destabilizer, delays mitotic entry by ∼4 h in HeLa cells. Stathmin depletion reduced the activity of CDC25 and its upstream activators, Aurora A and Plk1. Chemical inhibition of both Aurora A and Plk1 was sufficient to delay mitotic entry by 4 h, while inhibiting either kinase alone did not cause a delay. Aurora A and Plk1 are likely regulated downstream of stathmin, because the combination of stathmin knockdown and inhibition of Aurora A and Plk1 was not additive and again delayed mitotic entry by 4 h. Aurora A localization to the centrosome required MTs, while stathmin depletion spread its localization beyond that of γ-tubulin, indicating an MT-dependent regulation of Aurora A activation. Plk1 was inhibited by excess stathmin, detected in in vitro assays and cells overexpressing stathmin-cyan fluorescent protein. Recruitment of Plk1 to the centrosome was delayed in stathmin-depleted cells, independent of MTs. It has been shown that depolymerizing MTs with nocodazole abrogates the stathmin-depletion induced cell cycle delay; in this study, depolymerization with nocodazole restored Plk1 activity to near normal levels, demonstrating that MTs also contribute to Plk1 activation. These data demonstrate that stathmin regulates mitotic entry, partially via MTs, to control localization and activation of both Aurora A and Plk1.

Overexpression of stathmin 1 confers an independent prognostic indicator in nasopharyngeal carcinoma

Data mining on public domain identified that stathmin 1 (STMN1) transcript was significantly higher expressed in nasopharyngeal carcinoma (NPC). Also known as the oncoprotein 18, STMN1 performs an important function in regulating rapid microtubule remodeling of the cytoskeleton in response to the cellular conditions. Immunoexpression of STMN1 was retrospectively assessed in biopsies of 124 consecutive NPC patients without initial distant metastasis and treated with consistent guidelines. The outcome was correlated with clinicopathological features and patient survivals. Results indicated that high STMN1 expressions (50 %) were correlated with advanced age (p = 0.027), higher T stage (p = 0.003), and overall clinical stage (p = 0.006) by the 7th American Joint Committee of Cancer Staging. In multivariate analyses, high STMN1 expression emerged as an independent prognosticator for worse disease-specific survival (p = 0.001), distal metastasis-free survival (p = 0.003), and local recurrence-free survival (p = 0.006). Exogenous expression of E2F transcription factor 1 (E2F1) or/and its dimeric partner, transcription factor Dp-1 (TFDP1), notably induced the STMN1 protein level in a NPC-derived cell line, TW01. Accordingly, high STMN1 protein level is commonly associated with adverse prognosticators and confers tumor aggressiveness in patients with NPC, and its upregulation might be attributed to E2F1 and/or TFDP1 transactivation.

Reversible action of diaminothiazoles in cancer cells is implicated by the induction of a fast conformational change of tubulin and suppression of microtubule dynamics

Diaminothiazoles are novel cytotoxic compounds that have shown efficacy toward different cancer cell lines. They show potent antimitotic and antiangiogenic activity upon binding to the colchicine-binding site of tubulin. However, the mechanism of action of diaminothiazoles at the molecular level is not known. Here, we show a reversible binding to tubulin with a fast conformational change that allows the lead diaminothiazole DAT1 [4-amino-5-benzoyl-2-(4-methoxy phenyl amino)thiazole] to cause a reversible mitotic block. DAT1 also suppresses microtubule dynamic instability at much lower concentration than its IC(50) value in cancer cells. Both growth and shortening events were reduced by DAT1 in a concentration-dependent way. Colchicine, the long-studied tubulin-binding drug, has previously failed in the treatment of cancer due to its toxicity, even though it generates a strong apoptotic response. The toxicity is attributable to its slow removal from the cell due to irreversible tubulin binding caused by a slow conformational change. DAT1 binds to tubulin at an optimal pH lower than colchicine. Tubulin conformational studies showed that the binding environments of DAT1 and colchicine are different. Molecular dynamic simulations showed a difference in the number of H-bonding interactions that accounts for the different pH optima. This study gives an insight of the action of compounds targeting tubulin's colchicine-binding site, as many such compounds have entered into clinical trials recently.

The Microtubule Regulatory Protein Stathmin Is Required to Maintain the Integrity of Axonal Microtubules in Drosophila

Axonal transport, a form of long-distance, bi-directional intracellular transport that occurs between the cell body and synaptic terminal, is critical in maintaining the function and viability of neurons. We have identified a requirement for the stathmin (stai) gene in the maintenance of axonal microtubules and regulation of axonal transport in Drosophila. The stai gene encodes a cytosolic phosphoprotein that regulates microtubule dynamics by partitioning tubulin dimers between pools of soluble tubulin and polymerized microtubules, and by directly binding to microtubules and promoting depolymerization. Analysis of stai function in Drosophila, which has a single stai gene, circumvents potential complications with studies performed in vertebrate systems in which mutant phenotypes may be compensated by genetic redundancy of other members of the stai gene family. This has allowed us to identify an essential function for stai in the maintenance of the integrity of axonal microtubules. In addition to the severe disruption in the abundance and architecture of microtubules in the axons of stai mutant Drosophila, we also observe additional neurological phenotypes associated with loss of stai function including a posterior paralysis and tail-flip phenotype in third instar larvae, aberrant accumulation of transported membranous organelles in stai deficient axons, a progressive bang-sensitive response to mechanical stimulation reminiscent of the class of Drosophila mutants used to model human epileptic seizures, and a reduced adult lifespan. Reductions in the levels of Kinesin-1, the primary anterograde motor in axonal transport, enhance these phenotypes. Collectively, our results indicate that stai has an important role in neuronal function, likely through the maintenance of microtubule integrity in the axons of nerves of the peripheral nervous system necessary to support and sustain long-distance axonal transport.

Tubulin tyrosine ligase and stathmin compete for tubulin binding in vitro

Tubulin partition between soluble and polymeric forms is tightly regulated in cells. Stathmin and tubulin tyrosine ligase (TTL) each form stable complexes with tubulin and inhibit tubulin polymerization. Here we explore the mutual relationship between these proteins in vitro and demonstrate that full-length stathmin and TTL compete for binding to tubulin and fail to make a stable tubulin:stathmin:TTL triple complex in solution. Moreover, stathmin depresses TTL tubulin tyrosination activity in vitro. These results suggest either that TTL and stathmin have a partially overlapping footprint on the tubulin dimer or that stathmin induces a tubulin conformation incompatible with stable TTL binding.

Specific in vivo labeling of tyrosinated α-tubulin and measurement of microtubule dynamics using a GFP tagged, cytoplasmically expressed recombinant antibody

GFP-tagged proteins are used extensively as biosensors for protein localization and function, but the GFP moiety can interfere with protein properties. An alternative is to indirectly label proteins using intracellular recombinant antibodies (scFvs), but most antibody fragments are insoluble in the reducing environment of the cytosol. From a synthetic hyperstable human scFv library we isolated an anti-tubulin scFv, 2G4, which is soluble in mammalian cells when expressed as a GFP-fusion protein. Here we report the use of this GFP-tagged scFv to label microtubules in fixed and living cells. We found that 2G4-GFP localized uniformly along microtubules and did not disrupt binding of EB1, a protein that binds microtubule ends and serves as a platform for binding by a complex of proteins regulating MT polymerization. TOGp and CLIP-170 also bound microtubule ends in cells expressing 2G4-GFP. Microtubule dynamic instability, measured by tracking 2G4-GFP labeled microtubules, was nearly identical to that measured in cells expressing GFP-α-tubulin. Fluorescence recovery after photobleaching demonstrated that 2G4-GFP turns over rapidly on microtubules, similar to the turnover rates of fluorescently tagged microtubule-associated proteins. These data indicate that 2G4-GFP binds relatively weakly to microtubules, and this conclusion was confirmed in vitro. Purified 2G4 partially co-pelleted with microtubules, but a significant fraction remained in the soluble fraction, while a second anti-tubulin scFv, 2F12, was almost completely co-pelleted with microtubules. In cells, 2G4-GFP localized to most microtubules, but did not co-localize with those composed of detyrosinated α-tubulin, a post-translational modification associated with non-dynamic, more stable microtubules. Immunoblots probing bacterially expressed tubulins confirmed that 2G4 recognized α-tubulin and required tubulin’s C-terminal tyrosine residue for binding. Thus, a recombinant antibody with weak affinity for its substrate can be used as a specific intracellular biosensor that can differentiate between unmodified and post-translationally modified forms of a protein.

The E2F transcription factor 1 transactives stathmin 1 in hepatocellular carcinoma

BACKGROUND

Through data mining the Stanford Microarray Database, the stathmin 1 (STMN1) transcript was found to be frequently upregulated in the hepatocellular carcinoma (HCC) with low alpha-fetoprotein level. The molecular mechanism of STMN1 upregulation in HCCs remained unclear.

METHODS

Quantitative RT-PCR, immunoblotting, immunohistochemistry, and transfection of expression or small hairpin RNA interference plasmids, chromatin immunoprecipitation (ChIP), and quantitative ChIP assays were performed in HCC specimens or 2 distinct HCC-derived cell lines. Dual luciferase assay and site-directed mutagenesis were applied to analyze the activities of STMN1 proximal promoter region.

RESULTS

STMN1 mRNA and proteins were significantly associated with several clinicopathological features. High STMN1 or E2F1 immunoexpression was predictive of poor overall survival (OS) rate (P < .01). In HCC-derived cell lines, E2F1 was elevated before STMN1 mRNA during the cell cycle. Exogenous expression of E2F1 or both transcription factor DP-1 (TFDP1) and E2F1 genes induced E2F1 and STMN1 mRNA (P < .01). Knockdown of the E2F1 gene suppressed E2F1 and STMN1 mRNA and E2F1 and STMN1 protein levels (P < .05). The promoter activity of STMN1 gene increased with overexpression of both E2F1 and TFDP1 genes (P < .05); however, it decreased when mutations were introduced in the E2F1-binding sites (P < .05).

CONCLUSIONS

Upregulation of E2F1 and STMN1 proteins associate with worse outcomes in patients with HCC. E2F1 significantly correlates with STMN1 protein level in HCC lesions and in vitro transactivation assays, suggesting that STMN1 gene is transactivated by the E2F1 protein.

Inhibition of stathmin1 accelerates the metastatic process

The oncoprotein stathmin 1 (STMN1) is upregulated in most, if not all, cancers of epithelial cell origin; therefore STMN1 is considered a target for cancer therapy. However, its role during metastasis has not been investigated. Here, we report for the first time that STMN1 strongly inhibits metastatic behavior in both normal epithelial and cancerous epithelial cells. Initially, loss-of-STMN1 compromises cell-cell adhesion. This is followed by epithelial-to-mesenchymal transition (EMT), increased cell migration, and metastasis via cooperative activation of p38 and through TGF-β-independent and -dependent mechanisms. In contrast, expressing STMN1 restores cell-cell adhesion and reverses the metastatic cascade. Primary prostate epithelial cell cultures from benign to undifferentiated adenocarcinoma (UA) clinical biopsies show that EMT-like cells arise while the cancer is still organ-confined and that their emergence is tumor-stage specific. Furthermore, primary EMT-like cells exhibit metastatic behavior both in vitro and in vivo as compared with their non-EMT counterpart. These observations predict that using STMN1 as a generic therapeutic target might accelerate metastasis. Instead, there may be a tumor stage-specific window-of-opportunity in which conserving STMN1 expression is required to inhibit emergence of metastatic disease.

Role of Oncoprotein 18 in metastasis of hepatocellular carcinoma

AIM: To explore the role of Oncoprotein 18 (Op18) in metastasis of hepatocellular carcinoma (HCC).

METHODS: Op18 expression was repressed in HCCLM3 cells by RNA interference, and interference effect was evaluated by RT-PCR and Western blot. Cell adhesion, migration and invasion were analyzed by cell adhesion assay and Transwell assay in vitro. Op18 expression was detected by RT-PCR and immunohistochemistry in 96 HCC specimens with or without metastasis.

RESULTS: Op18 expression was effectively inhibited by RNA interference in HCCLM3 cells. Cell adhesion was significantly suppressed in the RNAi group compared to the mock group (20 min: 0.616 ± 0.057 vs 0.944 ± 0.068; 40 min: 0.740 ± 0.0713 vs 1.196 ± 0.115; 60 min: 1.001 ± 0.083 vs 1.441 ± 0.053; all P < 0.05). Transwell assay revealed that cell migration and invasion decreased in the RNAi group compared to the mock group (migration: 0.145 ± 0.011 vs 0.206 ± 0.008; invasion: 0.127 ± 0.008 vs 0.168 ± 0.012; both P < 0.01). Op18 expression was detected in 96 HCC tissues with or without metastasis. RT-PCR demonstrated that Op18 was overexpressed in HCC tissues with metastasis (n = 48) compared with HCC tissues without metastasis (n = 48, Op18/GAPDH relative ratio: 0.560 ± 0.128 vs 0.414 ± 0.086); and IHC results also indicated that Op18 expression was up-regulated in HCC tissues with metastasis in comparison with HCC tissues without metastasis (integrated density: 624.771 ± 100.032 vs 413.786 ± 71.833, P < 0.01).

CONCLUSION: Overexpression of Op18 may play an important role in HCC metastasis.

The microtubule cytoskeleton is required for a G2 cell cycle delay in cancer cells lacking stathmin and p53

In several cancer cell lines, depleting the microtubule (MT)-destabilizing protein stathmin/oncoprotein18 leads to a G2 cell cycle delay and apoptosis. These phenotypes are observed only in synergy with low levels of p53, but the pathway(s) activated by stathmin depletion to delay the cell cycle are unknown. We found that stathmin depletion caused greater MT stability in synergy with loss of p53, measured by the levels of acetylated α-tubulin and the rate of centrosomal MT nucleation. Nocodazole or vinblastine-induced MT depolymerization abrogated the stathmin-depletion induced G2 delay, measured by the percentage of cells staining positive for several markers (TPX2, CDK1 with inhibitory phosphorylation), indicating that MTs are required to lengthen G2. Live cell imaging showed that stathmin depletion increased time in G2 without an impact on the duration of mitosis, indicating that the longer interphase duration is not simply a consequence of a previous slowed mitosis. In contrast, stabilization of MTs with paclitaxel (8 nM) slowed mitosis without lengthening the duration of interphase, demonstrating that increased MT stability alone is not sufficient to delay cells in G2.

Syntaxin 1C, a soluble form of syntaxin, attenuates membrane recycling by destabilizing microtubules

Syntaxin 1C (STX1C), produced by alternative splicing of the stx1A gene, is a soluble syntaxin lacking a SNARE domain and a transmembrane domain. It is unclear how soluble syntaxin can control intracellular membrane trafficking. We found that STX1C affected microtubule (MT) dynamics through its tubulin-binding domain (TBD) and regulated recycling of intracellular vesicles carrying glucose transporter-1 (GLUT1). We demonstrated that the amino acid sequence VRSK of the TBD was important for the interaction between STX1C and tubulin and that wild-type STX1C (STX1C-WT), but not the TBD mutant, reduced the V(max) of glucose transport and GLUT1 translocation to the plasma membrane in FRSK cells. Moreover, by time-lapse analysis, we revealed that STX1C-WT suppressed MT stability and vesicle-transport motility in cells expressing GFP-α-tubulin, whereas TBD mutants had no effect. We also identified that GLUT1 was recycled in the 45 minutes after endocytosis and that GLUT1 vesicles moved along with MTs. Finally, we showed, by a recycling assay and FCM analysis, that STX1C-WT delayed the recycling phase of GLUT1 to PM, without affecting the endocytotic process of GLUT1. These data indicate that STX1C delays the GLUT1 recycling phase by suppressing MT stability and vesicle-transport motility through its TBD, providing the first insight into how soluble syntaxin controls membrane trafficking.

Characterization and detection of cellular and proteomic alterations in stable stathmin-overexpressing, taxol-resistant BT549 breast cancer cells using offgel IEF/PAGE difference gel electrophoresis

Stathmin/oncoprotein 18, a protein that regulates microtubule dynamics, is highly expressed in a number of tumors including leukemia, lymphoma, neuroblastoma, breast, ovarian, and prostate cancers. High stathmin levels have been associated with the development of resistance to the widely used anti-cancer drug taxol ((®)Taxol, paclitaxel). The mechanisms of stathmin-mediated taxol resistance are not well-understood at the molecular level. To better understand the role of stathmin in taxol resistance, we stably overexpressed stathmin twofold in BT549 human breast cancer cells and characterized several cell processes involved in the mechanism of action of taxol. After stable overexpression of stathmin, neither the cell doubling time nor the mitotic index was altered and the microtubule polymer mass was reduced only modestly (by 18%). Unexpectedly, microtubule dynamicity was reduced by 29% after stathmin overexpression, resulting primarily from reduction in the catastrophe frequency. Sensitivity to taxol was reduced significantly (by 44%) in a clonogenic assay, and stathmin appeared to protect the cells from the spindle-damaging effects of taxol. The results suggest that in the stably stathmin-overexpressing clones, compensatory gene expression occurred that resulted in normal rates of cell proliferation and prevented the increase in catastrophe frequency expected in response to stathmin. Stathmin overexpression protected the cells from taxol-induced abnormal mitoses, and thus induced taxol resistance. Using offgel IEF/PAGE difference gel electrophoresis, we identified a number of proteins whose expression is reduced in the taxol-resistant stathmin-overexpressing cell lines, including proteins involved in the cytoskeleton and cell structure, the stress response, protein folding, glycolysis, and catalysis.

Stathmin1 overexpression associated with polyploidy, tumor-cell invasion, early recurrence, and poor prognosis in human hepatoma

Frequent intrahepatic metastasis causes early tumor recurrence and dismaying prognosis of human hepatocellular carcinoma (HCC). We recently identified overexpression of stathmin1 (STMN1) in human HCC. This study was designed to elucidate the clinical and biological significance of overexpression of STMN1 in HCC. Expression of STMN1 was conducted by quantitative reverse transcription-polymerase chain reaction and immunoblotting assays on 58 pairs of HCC and para-tumor liver tissues from patients with HCC along with normal liver tissues as the controls. Association of STMN1 overexpression with tumor recurrence and prognosis was investigated by Kaplan-Meier cumulative survival and Cox Regression analyses. Roles of STMN1 in cell cycle, cell motility, and invasion were determined by in vitro assays. STMN1 overexpression in hepatoma was strongly associated with local invasion (P = 0.031), early recurrence (P = 0.002), and poor prognosis (P = 0.005), and was an independent indicator for tumor recurrence (P = 0.0045). STMN1 overexpression further identified subgroups of HCC patients with higher tumor recurrence and worse prognosis among HCC patients with early tumor stage (T1) or intermediate histological grades (G2 and G3), both of whom represent the majority of HCC patients receiving primary curative hepatectomy. Silencing STMN1 expression via RNA interference suppressed invasion activity, while ectopic expression of STMN1 enhanced cell invasion and caused polyploidy of cells. In conclusion, STMN1 overexpression could predict early tumor recurrence and poor prognosis, particularly at early stage of hepatoma. Overexpression of STMN1 promoted polyploidy formation, tumor-cell invasion, and intrahepatic metastasis, suggesting that STMN1 can be a target for anti-cancer therapy of human hepatoma.

Upregulated Op18/stathmin activity causes chromosomal instability through a mechanism that evades the spindle assembly checkpoint

Op18/stathmin (Op18) is a microtubule-destabilizing protein that is phosphorylation-inactivated during mitosis and its normal function is to govern tubulin subunit partitioning during interphase. Human tumors frequently overexpress Op18 and a tumor-associated Q18-->E mutation has been identified that confers hyperactivity, destabilizes spindle microtubules, and causes mitotic aberrancies, polyploidization, and chromosome loss in K562 leukemia cells. Here we determined whether wild-type and mutant Op18 have the potential to cause chromosomal instability by some means other than interference with spindle assembly, and thereby bypassing the spindle assembly checkpoint. Our approach was based on Op18 derivatives with distinct temporal order of activity during mitosis, conferred either by differential phosphorylation inactivation or by anaphase-specific degradation through fusion with the destruction box of cyclin B1. We present evidence that excessive Op18 activity generates chromosomal instability through interference occurring subsequent to the metaphase-to-anaphase transition, which reduces the fidelity of chromosome segregation to spindle poles during anaphase. Similar to uncorrected merotelic attachment, this mechanism evades detection by the spindle assembly checkpoint and thus provides an additional route to chromosomal instability.

Global up-regulation of microtubule dynamics and polarity reversal during regeneration of an axon from a dendrite

Axon regeneration is crucial for recovery after trauma to the nervous system. For neurons to recover from complete axon removal they must respecify a dendrite as an axon: a complete reversal of polarity. We show that Drosophila neurons in vivo can convert a dendrite to a regenerating axon and that this process involves rebuilding the entire neuronal microtubule cytoskeleton. Two major microtubule rearrangements are specifically induced by axon and not dendrite removal: 1) 10-fold up-regulation of the number of growing microtubules and 2) microtubule polarity reversal. After one dendrite reverses its microtubules, it initiates tip growth and takes on morphological and molecular characteristics of an axon. Only neurons with a single dendrite that reverses polarity are able to initiate tip growth, and normal microtubule plus-end dynamics are required to initiate this growth. In addition, we find that JNK signaling is required for both the up-regulation of microtubule dynamics and microtubule polarity reversal initiated by axon injury. We conclude that regulation of microtubule dynamics and polarity in response to JNK signaling is key to initiating regeneration of an axon from a dendrite.

Determination of microtubule dynamic instability in living cells

The precise regulation of microtubules and their dynamics is critical for cell cycle progression, cell signaling, intracellular transport, cell polarization, and organismal development. For example, mitosis, cell migration, and axonal outgrowth all involve rapid and dramatic changes in microtubule organization and dynamics. Microtubule-associated proteins (MAPs) such as MAP2 and tau (Bunker et al., 2004; Dhamodharan and Wadsworth, 1995) and microtubule-interacting proteins such as stathmin, the kinesin MCAK, and EB1 (Cassimeris, 1999; Moore and Wordeman, 2004; Ringhoff and Cassimeris, 2009; Rusan et al., 2001) as well as numerous clinically approved or experimental anti-mitotic drugs including the taxanes, vinca alkaloids, and colchicine-like compounds modulate microtubule dynamic in cells (Jordan, 2002; Jordan and Kamath, 2007). In this chapter, we describe methods to analyze the dynamic instability of microtubules in living cells by microscopy of microinjected or expressed fluorescent tubulin, time-lapse microscopy, and analysis of time-dependent microtubule length changes.

Predominant regulators of tubulin monomer-polymer partitioning and their implication for cell polarization

The microtubule-system organizes the cytoplasm during interphase and segregates condensed chromosomes during mitosis. Four unrelated conserved proteins, XMAP215/Dis1/TOGp, MCAK, MAP4 and Op18/stathmin, have all been implicated as predominant regulators of tubulin monomer-polymer partitioning in animal cells. However, while studies employing the Xenopus egg extract model system indicate that the partitioning is largely governed by the counteractive activities of XMAP215 and MCAK, studies of human cell lines indicate that MAP4 and Op18 are the predominant regulators of the interphase microtubule-array. Here, we review functional interplay of these proteins during interphase and mitosis in various cell model systems. We also review the evidence that MAP4 and Op18 have interphase-specific, counteractive and phosphorylation-inactivated activities that govern tubulin subunit partitioning in many mammalian cell types. Finally, we discuss evidence indicating that partitioning regulation by MAP4 and Op18 may be of significance to establish cell polarity.

Gene expression profiles in mouse embryo fibroblasts lacking stathmin, a microtubule regulatory protein, reveal changes in the expression of genes contributing to cell motility

BACKGROUND

Stathmin (STMN1) protein functions to regulate assembly of the microtubule cytoskeleton by destabilizing microtubule polymers. Stathmin over-expression has been correlated with cancer stage progression, while stathmin depletion leads to death of some cancer cell lines in culture. In contrast, stathmin-null mice are viable with minor axonopathies and loss of innate fear response. Several stathmin binding partners, in addition to tubulin, have been shown to affect cell motility in culture. To expand our understanding of stathmin function in normal cells, we compared gene expression profiles, measured by microarray and qRT-PCR, of mouse embryo fibroblasts isolated from STMN1+/+ and STMN1-/- mice to determine the transcriptome level changes present in the genetic knock-out of stathmin.

RESULTS

Microarray analysis of STMN1 loss at a fold change threshold of > or = 2.0 revealed expression changes for 437 genes, of which 269 were up-regulated and 168 were down-regulated. Microarray data and qRT-PCR analysis of mRNA expression demonstrated changes in the message levels for STMN4, encoding RB3, a protein related to stathmin, and in alterations to many tubulin isotype mRNAs. KEGG Pathway analysis of the microarray data indicated changes to cell motility-related genes, and qRT-PCR plates specific for focal adhesion and ECM proteins generally confirmed the microarray data. Several microtubule assembly regulators and motors were also differentially regulated in STMN1-/- cells, but these changes should not compensate for loss of stathmin.

CONCLUSION

Approximately 50% of genes up or down regulated (at a fold change of > or = 2) in STMN1-/- mouse embryo fibroblasts function broadly in cell adhesion and motility. These results support models indicating a role for stathmin in regulating cell locomotion, but also suggest that this functional activity may involve changes to the cohort of proteins expressed in the cell, rather than as a direct consequence of stathmin-dependent regulation of the microtubule cytoskeleton.