Structural basis for the interaction of tubulin with proteins and drugs that affect microtubule dynamics

CYLD coordinates with EB1 to regulate microtubule dynamics and cell migration

Cylindromatosis (CYLD), a deubiquitinase involved in inflammation and tumorigenesis via the modulation of cell signaling, has recently been identified as a critical regulator of microtubule dynamics. CYLD has also been shown to stimulate cell migration and thereby contribute to normal physiological processes. However, it remains elusive how the regulation of microtubule dynamic properties by CYLD is connected to its role in mediating cell migration. In this study, we performed yeast 2-hybrid screening with CYLD as bait and identified 7 CYLD-interacting proteins, including end-binding protein 1 (EB1). The CYLD-EB1 interaction was confirmed both in cells and in vitro, and these 2 proteins colocalized at the plus ends of microtubules. Interestingly, the association of CYLD with EB1 was significantly increased upon the stimulation of cell migration. CYLD coordinated with EB1 to orchestrate tail retraction, centrosome reorientation, and leading-edge microtubule stabilization in migratory cells. In addition, CYLD acted in concert with EB1 to regulate microtubule assembly in vitro, microtubule nucleation at the centrosome, and microtubule growth at the cell periphery. These data provide mechanistic insights into the actions of CYLD in the regulation of microtubule dynamics and cell migration. These findings also support the notion that coordinated actions of microtubule-binding proteins are critical for microtubule-mediated cellular events.

Induction of the intrinsic apoptotic pathway via a new antimitotic agent in an esophageal carcinoma cell line

Background

2-Ethyl-3-O-sulphamoyl-estra-1,3,5(10)16-tetraene (ESE-16) is a unique, in silico-designed compound with possible anticancer properties, which were identified in our laboratory. This compound is capable of interfering with microtubule dynamics and is believed to have potential carbonic anhydrase IX inhibiting activity.

In this study, it was investigated whether ESE-16 is capable of inducing apoptosis in vitro in the esophageal carcinoma SNO cell line via the intrinsic pathway at a concentration of 0.2 μM with an exposure time of 24 hours.

Results

Qualitative results were obtained via light microscopy, transmission electron microscopy and confocal microscopy. Results showed hallmarks of apoptosis in the ESE-16-treated cells. In addition, data revealed an increase in the number of ESE-16-treated cells blocked in metaphase. Cell death via apoptosis in the ESE-16-treated cells was confirmed by studying the internal ultrastructure of the cells via transmission electron microscopy, while confocal microscopy revealed abnormal spindle formation and condensed chromatin in ESE-16-treated cells, thus confirming metaphase block.

Quantitative results were obtained via flow cytometry and spectrophotometry. Cell death via apoptosis in ESE-16-treated cells was quantitatively confirmed by the Annexin V-FITC apoptosis detection assay. Flow cytometry and spectrophotometry revealed dissipation of mitochondrial membrane potential and an increase in superoxide levels in the ESE-16-treated cells when compared to the relevant controls. Both initiator caspase 9 and effector caspase 3 activities were increased, which demonstrates that ESE-16 causes cell death in a caspase-dependent manner.

Conclusions

This was the first in vitro study conducted to investigate the action mechanism of ESE-16 on an esophageal carcinoma cell line. The results provided valuable information on the action mechanism of this potential anticancer agent. It can be concluded that the novel in silico-designed compound exerts an anti-proliferative effect on the esophageal carcinoma SNO cell line by disrupting microtubule function resulting in metaphase block. This culminates in apoptotic cell death via the intrinsic apoptotic pathway. This research provided cellular targets warranting in vivo assessment of ESE-16’s potential as an anticancer agent.

Enhancement of microtubule-associated protein-1 Alpha gene expression in osteoblasts by low level laser irradiation

BACKGROUND

Low level laser irradiation (LLLI) stimulates bone regeneration. However, the molecular mechanisms leading to this is not yet understood. The stepwise subtractive cDNA cloning technology has been developed, coupled with DNA homology searched in DNA database is useful to identify the certain gene.

AIM

In order to understand the mechanism, we attempted to identify genes whose expressions are enhanced by LLLI. MC3T3-E1 osteoblastic cells were irradiated with an 830 nm Ga-Al-As diode laser, and a cDNA library was constructed using subtractive gene cloning.

MATERIAL AND METHODS

The cDNA library of osteoblasts which was treated by LLLI was constructed. Nucleotide sequences were analyzed and homology searched in a DNA database using BLASTN program to identify the gene with altered expression. Altered mRNA levels by LLLI were confirmed by reverse transcription polymerase chain reaction (RT-PCR) and real-time PCR.

RESULTS

The DNA sequence of a subtracted gene clone MCL129 indicated high homology (99%) with the microtubule-associated protein 1A (MAP1A) gene. Increase in MAP-1A mRNA level by LLLI was successfully confirmed by RT-PCR and real-time PCR.

DISCUSSION

MAP1A has been shown to promote microtubule assembly and its functional expression. Microtubules play an important role in cell division, cell shape and polarity, cell movement, intracellular transport, signal transduction, and synthesis and secretion of collagen. Thus, enhancement of MAP1A gene expression by LLLI may be one of the molecular mechanisms involved in accelerating bone formation by LLLI.

CONCLUSION

LLLI irradiation enhances MAP1A gene expression and modulates microtubule assembly and the functional structure of microtubules, in turn, stimulates osteoblastic proliferation and differentiation.

Linobiflavonoid inhibits human lung adenocarcinoma A549 cells: effect on tubulin protein

The antitumor bioactivities of linobiflavonoid were studied through evaluating its in vitro cytotoxicity against several cell lines (A549, H1975, SMMC-7721, HEP-2 and Vero cells), with the aid of 3-(4,5)-dimethylthiazoly1)-3,5-diphenytetrazolium bromide (MTT) assay. It was found that linobiflavonoid shows more notable inhibiting activity against A549 cells, with IC50 value of 4.67 μM. Furthermore, western blot analysis revealed that linobiflavonoid is able to increase the expression of β-tubulin, whereas not α-tubulin. In virtuale simulations indicated that linobiflavonoid specifically interacts with the binding pocket which is located at the top of β-tubulin, due to the presence of strong hydrophobic effects between the core templates and the hydrophobic surface of the tubulin protein (TB) binding site. The binding energy (E inter ) was calculated to be -140.47 kcal/mol. Results above suggest that linobiflavonoid possesses anti-A549 properties relating to β-tubulin depolymerization inhibition.

Cyclin-dependent kinases regulate splice-specific targeting of dynamin-related protein 1 to microtubules

The splice isoform Drp1-x01 promotes mitochondrial fission and is regulated by Cdk phosphorylation-dependent changes in microtubule association.

Fission and fusion reactions determine mitochondrial morphology and function. Dynamin-related protein 1 (Drp1) is a guanosine triphosphate–hydrolyzing mechanoenzyme important for mitochondrial fission and programmed cell death. Drp1 is subject to alternative splicing of three exons with previously unknown functional significance. Here, we report that splice variants including the third but excluding the second alternative exon (x01) localized to and copurified with microtubule bundles as dynamic polymers that resemble fission complexes on mitochondria. A major isoform in immune cells, Drp1-x01 required oligomeric assembly and Arg residues in alternative exon 3 for microtubule targeting. Drp1-x01 stabilized and bundled microtubules and attenuated staurosporine-induced mitochondrial fragmentation and apoptosis. Phosphorylation of a conserved Ser residue adjacent to the microtubule-binding exon released Drp1-x01 from microtubules and promoted mitochondrial fragmentation in a splice form–specific manner. Phosphorylation by Cdk1 contributed to dissociation of Drp1-x01 from mitotic microtubules, whereas Cdk5-mediated phosphorylation modulated Drp1-x01 targeting to interphase microtubules. Thus, alternative splicing generates a latent, cytoskeletal pool of Drp1 that is selectively mobilized by cyclin-dependent kinase signaling.

Comparative analyses of the β-tubulin gene and molecular modeling reveal molecular insight into the colchicine resistance in kinetoplastids organisms

Differential susceptibility to microtubule agents has been demonstrated between mammalian cells and kinetoplastid organisms such as Leishmania spp. and Trypanosoma spp. The aims of this study were to identify and characterize the architecture of the putative colchicine binding site of Leishmania spp. and investigate the molecular basis of colchicine resistance. We cloned and sequenced the β-tubulin gene of Leishmania (Viannia) guyanensis and established the theoretical 3D model of the protein, using the crystallographic structure of the bovine protein as template. We identified mutations on the Leishmania   β-tubulin gene sequences on regions related to the putative colchicine-binding pocket, which generate amino acid substitutions and changes in the topology of this region, blocking the access of colchicine. The same mutations were found in the β-tubulin sequence of kinetoplastid organisms such as Trypanosoma cruzi, T. brucei, and T. evansi. Using molecular modelling approaches, we demonstrated that conformational changes include an elongation and torsion of an α-helix structure and displacement to the inside of the pocket of one β-sheet that hinders access of colchicine. We propose that kinetoplastid organisms show resistance to colchicine due to amino acids substitutions that generate structural changes in the putative colchicine-binding domain, which prevent colchicine access.

Multiple tubulins: evolutionary aspects and biological implications

Plant tubulin is a dimeric protein that contributes to formation of microtubules, major intracellular structures that are involved in the control of fundamental processes such as cell division, polarity of growth, cell-wall deposition, intracellular trafficking and communications. Because it is a structural protein whose function is confined to the role of microtubule formation, tubulin may be perceived as an uninteresting gene product, but such a perception is incorrect. In fact, tubulin represents a key molecule for studying fundamental biological issues such as (i) microtubule evolution (also with reference to prokaryotic precursors and the formation of cytomotive filaments), (ii) protein structure with reference to the various biochemical features of members of the FstZ/tubulin superfamily, (iii) isoform variations contributed by the existence of multi-gene families and various kinds of post-translational modifications, (iv) anti-mitotic drug interactions and mode of action, (v) plant and cell symmetry, as determined using a series of tubulin mutants, (vi) multiple and sophisticated mechanisms of gene regulation, and (vii) intron molecular evolution. In this review, we present and discuss many of these issues, and offer an updated interpretation of the multi-tubulin hypothesis.

Unprecedented inhibition of tubulin polymerization directed by gold nanoparticles inducing cell cycle arrest and apoptosis

The effect of gold nanoparticles (AuNPs) on the polymerization of tubulin has not been examined till now. We report that interaction of weakly protected AuNPs with microtubules (MTs) could cause inhibition of polymerization and aggregation in the cell free system. We estimate that single citrate capped AuNPs could cause aggregation of ∼10(5) tubulin heterodimers. Investigation of the nature of inhibition of polymerization and aggregation by Raman and Fourier transform-infrared (FTIR) spectroscopies indicated partial conformational changes of tubulin and microtubules, thus revealing that AuNP-induced conformational change is the driving force behind the observed phenomenon. Cell culture experiments were carried out to check whether this can happen inside a cell. Dark field microscopy (DFM) combined with hyperspectral imaging (HSI) along with flow cytometric (FC) and confocal laser scanning microscopic (CLSM) analyses suggested that AuNPs entered the cell, caused aggregation of the MTs of A549 cells, leading to cell cycle arrest at the G0/G1 phase and concomitant apoptosis. Further, Western blot analysis indicated the upregulation of mitochondrial apoptosis proteins such as Bax and p53, down regulation of Bcl-2 and cleavage of poly(ADP-ribose) polymerase (PARP) confirming mitochondrial apoptosis. Western blot run after cold-depolymerization revealed an increase in the aggregated insoluble intracellular tubulin while the control and actin did not aggregate, suggesting microtubule damage induced cell cycle arrest and apoptosis. The observed polymerization inhibition and cytotoxic effects were dependent on the size and concentration of the AuNPs used and also on the incubation time. As microtubules are important cellular structures and target for anti-cancer drugs, this first observation of nanoparticles-induced protein's conformational change-based aggregation of the tubulin-MT system is of high importance, and would be useful in the understanding of cancer therapeutics and safety of nanomaterials.

Megakaryocyte polyploidy is inhibited by lysyl oxidase propeptide

Megakaryocytes (MKs), the platelet precursors, undergo an endomitotic cell cycle that leads to polyploidy. Lysyl oxidase propeptide (LOX-PP) is generated from lysyl oxidase (LOX) pro-enzyme after proteolytical cleavage. We recently reported that LOX, a known matrix cross-linking enzyme, contributes to MK lineage expansion. In addition, LOX expression levels are ploidy-dependent, with polyploidy MKs having minimal levels. This led us to test the effects of LOX-PP on the number and ploidy of primary MKs. LOX-PP significantly decreases mouse bone marrow MK ploidy coupled with a reduction in MK size. MK number is unchanged upon LOX-PP treatment. Analysis of LOX-PP- or vehicle-treated MKs by western blotting revealed a reduction in ERK1/2 phosphorylation and in the levels of its downstream targets, cyclin D3 and cyclin E, which are known to play a central role in MK endomitosis. Pull-down assays and immunochemistry staining indicated that LOX-PP interacts with α-tubulin and the mictotubules, which can contribute to decreased MK ploidy. Thus, our findings defined a role for LOX-PP in reducing MK ploidy. This suggests that high-level expression of LOX in aberrantly proliferating MKs could play a part in inhibiting their polyploidization via LOX-PP.

Altered TUBB3 expression contributes to the epothilone response of mitotic cells

Background:

Epothilones are a novel group of microtubule (mt) targeting cancer drugs that bind to the β-subunit of the αβ-tubulin dimer. Epothilones inhibit cell proliferation and induce cell death by interfering with the normal mt function. In this study, we examined the consequences of altered expression of human β-tubulin isotypes in terms of the epothilone drug response in human lung and breast cancer cell lines.

Methods:

The β-tubulin isotypes TUBB2A–C, TUBB3 and TUBB were silenced or overexpressed in A549, A549EpoB40 and MCF7 cell lines in the presence or absence of epothilones. The drug effects on cell proliferation, mitosis and mt dynamics were determined using live cell microscopy and immunofluorescence assays.

Results:

Loss of TUBB3 enhanced the action of epothilones. TUBB3 knockdown increased the severity of drug-induced mitotic defects and resulted in stabilisation of the mt dynamics in cells. Moreover, exogenous expression of TUBB3 in the epothilone resistant cell line conferred the response to drug treatments. In contrast, reduced levels of TUBB2A–C or TUBB had not apparent effect on the cells’ response to epothilones.

Conclusion:

Our results show that the expression of TUBB3 contributes to the cellular response to epothilones, putatively by having an impact on the mt dynamics.

Sodium selenite alters microtubule assembly and induces apoptosis in vitro and in vivo

Background

Previous studies demonstrated that selenite induced cancer-cell apoptosis through multiple mechanisms; however, effects of selenite on microtubules in leukemic cells have not been demonstrated.

Methods

The toxic effect of selenite on leukemic HL60 cells was performed with cell counting kit 8. Selenite effects on cell cycle distribution and apoptosis induction were determined by flow cytometry. The contents of cyclin B1, Mcl-1, AIF, cytochrome C, insoluble and soluble tubulins were detected with western blotting. Microtubules were visualized with indirect immunofluorescence microscopy. The interaction between CDK1 and Mcl-1 was assessed with immunoprecipitation. Decreasing Mcl-1 and cyclin B1 expression were carried out through siRNA interference. The alterations of Mcl-1 and cyclin B1 in animal model were detected with either immunohistochemical staining or western blotting. In situ detection of apoptotic ratio was performed with TUNEL assay.

Results

Our current results showed that selenite inhibited the growth of HL60 cells and induced mitochondrial-related apoptosis. Furthermore, we found that microtubule assembly in HL60 cells was altered, those cells were arrested at G2/M phase, and Cyclin B1 was up-regulated and interacted with CDK1, which led to down-regulation of the anti-apoptotic protein Mcl-1. Finally, in vivo experiments confirmed the in vitro microtubule disruption effect and alterations in Cyclin B1 and Mcl-1 levels by selenite.

Conclusions

Taken together, the results from our study indicate that microtubules are novel targets of selenite in leukemic HL60 cells.

An improved small-molecule inhibitor of FtsZ with superior in vitro potency, drug-like properties, and in vivo efficacy

The bacterial cell division protein FtsZ is an attractive target for small-molecule antibacterial drug discovery. Derivatives of 3-methoxybenzamide, including compound PC190723, have been reported to be potent and selective antistaphylococcal agents which exert their effects through the disruption of intracellular FtsZ function. Here, we report the further optimization of 3-methoxybenzamide derivatives towards a drug candidate. The in vitro and in vivo characterization of a more advanced lead compound, designated compound 1, is described. Compound 1 was potently antibacterial, with an average MIC of 0.12 μg/ml against all staphylococcal species, including methicillin- and multidrug-resistant Staphylococcus aureus and Staphylococcus epidermidis. Compound 1 inhibited an S. aureus strain carrying the G196A mutation in FtsZ, which confers resistance to PC190723. Like PC190723, compound 1 acted on whole bacterial cells by blocking cytokinesis. No interactions between compound 1 and a diverse panel of antibiotics were measured in checkerboard experiments. Compound 1 displayed suitable in vitro pharmaceutical properties and a favorable in vivo pharmacokinetic profile following intravenous and oral administration, with a calculated bioavailability of 82.0% in mice. Compound 1 demonstrated efficacy in a murine model of systemic S. aureus infection and caused a significant decrease in the bacterial load in the thigh infection model. A greater reduction in the number of S. aureus cells recovered from infected thighs, equivalent to 3.68 log units, than in those recovered from controls was achieved using a succinate prodrug of compound 1, which was designated compound 2. In summary, optimized derivatives of 3-methoxybenzamide may yield a first-in-class FtsZ inhibitor for the treatment of antibiotic-resistant staphylococcal infections.

Highly dynamic microtubules improve the effectiveness of early stages of human influenza A/NWS/33 virus infection in LLC-MK2 cells

BACKGROUND

This study aims to investigate the role of microtubule dynamics in the initiation of NWS/33 human influenza A (NWS) virus infection in MDCK and LLC-MK2 mammalian kidney cells. We previously demonstrated a host-dependent role of the actin cytoskeleton in inducing restriction during the early phases of NWS infection. Furthermore, we showed the differential infectious entry of NWS virus in the above mentioned cell models.

METHODOLOGY/PRINCIPAL FINDINGS

By first employing a panel of microtubule-modulators, we evidenced that microtubule-stabilization negatively interferes with NWS replication in LLC-MK2 but not in MDCK cells. Conversely, microtubule-depolymerization improves NWS growth in LLC-MK2 but not in the MDCK model. By using immunofluorescence labelling and Western blotting analyses upon NWS infection in mammalian kidney cells, it was observed that the occurrence of alpha-tubulin hyperacetylation--a post-translational modified form suggestive of stable microtubules--was significantly delayed in LLC-MK2 when compared to MDCK cells. Furthermore, mock-infected LLC-MK2 cells were shown to have higher levels of both acetylated alpha-tubulin and microtubule-associated protein 4 (MAP4), the latter being essential for the maintenance of normal microtubule polymer levels in interphase epithelial cells. Finally, to obtain highly dynamic microtubules in LLC-MK2 cells, we knocked down the expression of MAP4 by using a RNA-mediated RNA interference approach. The results evidenced that MAP4 silencing improves NWS growth in LLC-MK2 cells.

CONCLUSION

By evidencing the cell type-dependent regulatory role of microtubule dynamics on NWS replication in mammalian kidney cells, we demonstrated that microtubule-stabilization represents a restriction factor for the initiation of NWS infection in LLC-MK2 but not in MDCK cells.

Vinflunine: a new vision that may translate into antiangiogenic and antimetastatic activity

Microtubules and tubulin are major dynamic and structural cellular components that play a key role in several cell functions, including division, signalling and intracellular trafficking. Normal epithelial cells have a highly structured, rigid cytoskeletal network that is compatible with cell motility. Thus, tubulin and microtubules are compelling cellular targets for chemotherapy. In fact, among anticancer agents, those that target microtubules constitute one of the most effective classes of chemotherapeutics in cancer. The list of compounds that target either tubulin or microtubules is extensive and consists of chemically unique compounds that bind to the tubulin dimers and destabilize microtubules (Vinca alkaloids) and those that bind to the microtubule polymer and stabilize microtubules (taxanes). Tumour-induced angiogenesis, the formation of new capillaries from existing blood vessels, and epithelial-mesenchymal transition are two steps that are critical for both tumour growth and metastatic spread. Three possible mechanisms of action are described with vinflunine, the new-generation Vinca alkaloid to arrive in clinical practice are as follows: it acts against tubulin and microtubules, disrupts newly formed blood vessels and seems to be able to reduce the metastatic process as shown in preclinical studies. These findings support the hypothesis that vinflunine, by blocking microtubule functions that contribute to cell shape, polarization, migration and other processes, might be responsible not only for tumour-cytostatic but also for specific antiangiogenic or antiepithelial-mesenchymal transition effects.

Random mutagenesis of β-tubulin defines a set of dispersed mutations that confer paclitaxel resistance

PURPOSE

Previous research showed that mutations in β1-tubulin are frequently involved in paclitaxel resistance but the question of whether the mutations are restricted by cell-type specific differences remains obscure.

METHODS

To circumvent cellular constraints, we randomly mutagenized β-tubulin cDNA, transfected it into CHO cells, and selected for paclitaxel resistance.

RESULTS

A total of 26 β1-tubulin mutations scattered throughout the sequence were identified and a randomly chosen subset were confirmed to confer paclitaxel resistance using site-directed mutagenesis of β-tubulin cDNA and transfection into wild-type cells. Immunofluorescence microscopy and biochemical fractionation studies indicated that cells expressing mutant tubulin had decreased microtubule polymer and frequently suffered mitotic defects that led to the formation of large multinucleated cells, suggesting a resistance mechanism that involves destabilization of the microtubule network. Consistent with this conclusion, the mutations were predominantly located in regions that are likely to be involved in lateral or longitudinal subunit interactions. Notably, fourteen of the new mutations overlapped previously reported mutations in drug resistant cells or in patients with developmental brain abnormalities.

CONCLUSIONS

A random mutagenesis approach allowed isolation of a wider array of drug resistance mutations and demonstrated that similar mutations can cause paclitaxel resistance and human neuronal abnormalities.

Antimitotic inhibitors

Of the agents available in the treatment of both solid and hematologic cancers, microtubule-targeted agents are among the most widely used and exploiting other mechanisms involving the microtubule and its role in mitosis is an area of continued interest. This review will focus on novel microtubule-targeted agents, both recently approved (eg, ixabepilone and eribulin) and in later-stage clinical trials, and kinase inhibitors that aim to directly inhibit the mitotic spindle, such as the aurora kinase, pololike kinase, and kinsein-spindle protein inhibitors.

Cytoskeletal changes in actin and microtubules underlie the developing surface mechanical properties of sensory and supporting cells in the mouse cochlea

Correct patterning of the inner ear sensory epithelium is essential for the conversion of sound waves into auditory stimuli. Although much is known about the impact of the developing cytoskeleton on cellular growth and cell shape, considerably less is known about the role of cytoskeletal structures on cell surface mechanical properties. In this study, atomic force microscopy (AFM) was combined with fluorescence imaging to show that developing inner ear hair cells and supporting cells have different cell surface mechanical properties with different developmental time courses. We also explored the cytoskeletal organization of developing sensory and non-sensory cells, and used pharmacological modulation of cytoskeletal elements to show that the developmental increase of hair cell stiffness is a direct result of actin filaments, whereas the development of supporting cell surface mechanical properties depends on the extent of microtubule acetylation. Finally, this study found that the fibroblast growth factor signaling pathway is necessary for the developmental time course of cell surface mechanical properties, in part owing to the effects on microtubule structure.

The small molecule inhibitor PR-619 of deubiquitinating enzymes affects the microtubule network and causes protein aggregate formation in neural cells: implications for neurodegenerative diseases

A pathological hallmark of many neurodegenerative diseases is the aggregation of proteins. Protein aggregate formation may be linked to a failure of the ubiquitin proteasome system (UPS) and/or the autophagy pathway. The UPS involves the ubiquitination of proteins followed by proteasomal degradation. Deubiquitination of target proteins is performed by proteases called deubiquitinating proteins (DUBs). Inhibition of DUBs may lead to the dysregulation of homeostasis and have pathological consequences. To assess the effects of DUB-inhibition, we have used the oligodendroglial cell line, OLN-t40, stably expressing the longest human tau isoform. Cells were incubated with PR-619, a broad-range, reversible inhibitor of ubiquitin isopeptidases. Incubation with PR-619 led to morphological changes, the upregulation of heat shock proteins (HSP), including HSP70 and αB-crystallin, and to protein aggregates near the MTOC, containing ubiquitin, HSPs, and the ubiquitin binding protein p62, which may provide a link between the UPS and autophagy. Thus, inhibition of DUB activity caused stress responses and the formation of protein aggregates resembling pathological inclusions observed in aggregopathies. Furthermore, PR-619 led to the stabilization of the microtubule network, possibly through the modulation of tau phosphorylation, and small tau deposits assembled near the MTOC. Hence, organization and integrity of the cytoskeleton were affected, which is particularly important for the maintenance of the cellular architecture and intracellular transport processes, and essential for the functionality and survival of neural cells. Our data demonstrate that DUB inhibitors provide a useful tool to elucidate the manifold mechanisms of DUB functions in cells and their dysregulation in neurodegenerative diseases. This article is part of a Special Issue entitled: Ubiquitin Drug Discovery and Diagnostics.

A telomere-dependent DNA damage checkpoint induced by prolonged mitotic arrest

Telomere shortening and disruption of telomeric components are pathways that induce telomere deprotection. Here we describe another pathway, in which prolonged mitotic arrest induces damage signals at telomeres in human cells. Exposure to microtubule drugs, kinesin inhibitors, proteasome inhibitors or the disruption of proper chromosome cohesion resulted in the formation of damage foci at telomeres. Induction of mitotic telomere deprotection coincided with dissociation of TRF2 from telomeres, telomeric 3'-overhang degradation and ATM activation, and deprotection could be suppressed by TRF2 overexpression or inhibition of Aurora B kinase. Normal cells that escaped from prolonged mitotic arrest halted in the following G1 phase, whereas cells lacking p53 continued to cycle and became aneuploid. We propose a telomere-dependent mitotic-duration monitoring system that reacts to improper progression through mitosis.

The relative effect of citral on mitotic microtubules in wheat roots and BY2 cells

The plant volatile monoterpene citral is a highly active compound with suggested allelopathic traits. Seed germination and seedling development are inhibited in the presence of citral, and it disrupts microtubules in both plant and animal cells in interphase. We addressed the following additional questions: can citral interfere with cell division; what is the relative effect of citral on mitotic microtubules compared to interphase cortical microtubules; what is its effect on newly formed cell plates; and how does it affect the association of microtubules with γ-tubulin? In wheat seedlings, citral led to inhibition of root elongation, curvature of newly formed cell walls and deformation of microtubule arrays. Citral's effect on microtubules was both dose- and time-dependent, with mitotic microtubules appearing to be more sensitive to citral than cortical microtubules. Association of γ-tubulin with microtubules was more sensitive to citral than were the microtubules themselves. To reveal the role of disrupted mitotic microtubules in dictating aberrations in cell plates in the presence of citral, we used tobacco BY2 cells expressing GFP-Tua6. Citral disrupted mitotic microtubules, inhibited the cell cycle and increased the frequency of asymmetric cell plates in these cells. The time scale of citral's effect in BY2 cells suggested a direct influence on cell plates during their formation. Taken together, we suggest that at lower concentrations, citral interferes with cell division by disrupting mitotic microtubules and cell plates, and at higher concentrations it inhibits cell elongation by disrupting cortical microtubules.

Chemotaxis of cell populations through confined spaces at single-cell resolution

Cell migration is crucial for both physiological and pathological processes. Current in vitro cell motility assays suffer from various drawbacks, including insufficient temporal and/or optical resolution, or the failure to include a controlled chemotactic stimulus. Here, we address these limitations with a migration chamber that utilizes a self-sustaining chemotactic gradient to induce locomotion through confined environments that emulate physiological settings. Dynamic real-time analysis of both population-scale and single-cell movement are achieved at high resolution. Interior surfaces can be functionalized through adsorption of extracellular matrix components, and pharmacological agents can be administered to cells directly, or indirectly through the chemotactic reservoir. Direct comparison of multiple cell types can be achieved in a single enclosed system to compare inherent migratory potentials. Our novel microfluidic design is therefore a powerful tool for the study of cellular chemotaxis, and is suitable for a wide range of biological and biomedical applications.

Synthesis and evaluation of 1,5-disubstituted tetrazoles as rigid analogues of combretastatin A-4 with potent antiproliferative and antitumor activity

Tubulin, the major structural component of microtubules, is a target for the development of anticancer agents. Two series of 1,5-diaryl substituted 1,2,3,4-tetrazoles were concisely synthesized, using a palladium-catalyzed cross-coupling reaction, and identified as potent antiproliferative agents and novel tubulin polymerization inhibitors that act at the colchicine site. SAR analysis indicated that compounds with a 4-ethoxyphenyl group at the N-1 or C-5 position of the 1,2,3,4-tetrazole ring exhibited maximal activity. Several of these compounds also had potent activity in inhibiting the growth of multidrug resistant cells overexpressing P-glycoprotein. Active compounds induced apoptosis through the mitochondrial pathway with activation of caspase-9 and caspase-3. Furthermore, compound 4l significantly reduced in vivo the growth of the HT-29 xenograft in a nude mouse model, suggesting that 4l is a promising new antimitotic agent with clinical potential.

Chemical modulators working at pharmacological interface of target proteins

For last few decades, the active site cleft and substrate-binding site of enzymes as well as ligand-binding site of the receptors have served as the main pharmacological space for drug discovery. However, rapid accumulation of proteome and protein network analysis data has opened a new therapeutic space that is the interface between the interacting proteins. Due to the complexity of the interaction modes and the numbers of the participating components, it is still challenging to identify the chemicals that can accurately control the protein-protein interactions at desire. Nonetheless, the number of chemical drugs and candidates working at the interface of the interacting proteins are rapidly increasing. This review addresses the current case studies and state-of-the-arts in the development of small chemical modulators controlling the interactions of the proteins that have pathological implications in various human diseases such as cancer, immune disorders, neurodegenerative and infectious diseases.

Bacterial plasmid partition machinery: a minimalist approach to survival

The accurate segregation or partition of replicated DNA is essential for ensuring stable genome transmission. Partition of bacterial plasmids requires only three elements: a centromere-like DNA site and two proteins, a partition NTPase, and a centromere-binding protein (CBP). Because of this simplicity, partition systems have served as tractable model systems to study the fundamental molecular mechanisms required for DNA segregation at an atomic level. In the last few years, great progress has been made in this endeavor. Surprisingly, these studies have revealed that although the basic partition components are functionally conserved between three types of plasmid partition systems, these systems employ distinct mechanisms of DNA segregation. This review summarizes the molecular insights into plasmid segregation that have been achieved through these recent structural studies.

Targeting the Wolbachia cell division protein FtsZ as a new approach for antifilarial therapy

The use of antibiotics targeting the obligate bacterial endosymbiont Wolbachia of filarial parasites has been validated as an approach for controlling filarial infection in animals and humans. Availability of genomic sequences for the Wolbachia (wBm) present in the human filarial parasite Brugia malayi has enabled genome-wide searching for new potential drug targets. In the present study, we investigated the cell division machinery of wBm and determined that it possesses the essential cell division gene ftsZ which was expressed in all developmental stages of B. malayi examined. FtsZ is a GTPase thereby making the protein an attractive Wolbachia drug target. We described the molecular characterization and catalytic properties of Wolbachia FtsZ. We also demonstrated that the GTPase activity was inhibited by the natural product, berberine, and small molecule inhibitors identified from a high-throughput screen. Furthermore, berberine was also effective in reducing motility and reproduction in B. malayi parasites in vitro. Our results should facilitate the discovery of selective inhibitors of FtsZ as a novel anti-symbiotic approach for controlling filarial infection. NOTE: The nucleotide sequences reported in this paper are available in GenBank™ Data Bank under the accession number wAlB-FtsZ (JN616286).

The human Ino80 binds to microtubule via the E-hook of tubulin: implications for the role in spindle assembly

The human INO80 chromatin remodeling complex, comprising the Ino80 ATPase (hIno80) and the associated proteins such as Tip49a, has been implicated in a variety of nuclear processes other than transcription. We previously have found that hIno80 interacts with tubulin and co-localizes with the mitotic spindle and is required for spindle formation. To better understand the role of hIno80 in spindle formation, we further investigated the interaction between hIno80 and microtubule. Here, we show that the N-terminal domain, dispensable for the nucleosome remodeling activity, is important for hIno80 to interact with tubulin and co-localize with the spindle. The hIno80 N-terminal domain binds to monomeric tubulin and polymerized microtubule in vitro, and the E-hook of tubulin, involved in the polymerization of microtubule, is critical for this binding. Tip49a, which has been reported to associate with the spindle, does not bind to microtubule in vitro and dispensable for spindle formation in vivo. These results suggest that hIno80 can play a direct role in the spindle assembly independent of its chromatin remodeling activity.

Septin GTPases spatially guide microtubule organization and plus end dynamics in polarizing epithelia

Filamentous and microtubule-associated septin GTPases guide the reorganization of the microtubule network during epithelial cell polarization.

Establishment of epithelial polarity requires the reorganization of the microtubule (MT) cytoskeleton from a radial array into a network positioned along the apicobasal axis of the cell. Little is known about the mechanisms that spatially guide the remodeling of MTs during epithelial polarization. Septins are filamentous guanine triphosphatases (GTPases) that associate with MTs, but the function of septins in MT organization and dynamics is poorly understood. In this paper, we show that in polarizing epithelia, septins guide the directionality of MT plus end movement by suppressing MT catastrophe. By enabling persistent MT growth, two spatially distinct populations of septins, perinuclear and peripheral filaments, steer the growth and capture of MT plus ends. This navigation mechanism is essential for the maintenance of perinuclear MT bundles and for the orientation of peripheral MTs as well as for the apicobasal positioning of MTs. Our results suggest that septins provide the directional guidance cues necessary for polarizing the epithelial MT network.

Anticancer activity of chamaejasmine: effect on tubulin protein

In this work, the anticancer activity of chamaejasmine was studied by evaluating its in vitro cytotoxicity against several human cancer cell lines (MCF-7, A549, SGC-7901, HCT-8, HO-4980, Hela, HepG2, PC-3, LNCap, Vero and MDCK) using the MTT assay. Results indicated chamaejasmine showed more notable anticancer activity than taxol against PC-3 cells, with IC₅₀ values of 2.28 and 3.98 µM, respectively. Furthermore, Western blot analysis showed that chamaejasmine was able to increase the expression of β-tubulin, but not α-tubulin. In silico simulations indicated that chamaejasmine specifically interacts with the active site which is located at the top of β-tubulin, thanks to the presence of strong hydrophobic effects between the core templates and the hydrophobic surface of the TB active site. The binding energy (E_inter) was calculated to be −164.77 kcal·mol⁻¹. Results presented here suggest that chamaejasmine possesses anti-cancer properties relating to β-tubulin depolymerization inhibition, and therefore is a potential source of anticancer leads for the pharmaceutical industry.

Megakaryocyte lineage-specific class VI β-tubulin suppresses microtubule dynamics, fragments microtubules, and blocks cell division

Class VI β-tubulin (β6) is the most divergent tubulin produced in mammals and is found only in platelets and mature megakaryocytes. To determine how this unique tubulin isotype affects microtubule assembly and organization, we expressed the cDNA in tissue culture cells under the control of a tetracycline regulated promoter. The β6 coassembled with other endogenous β-tubulin isotypes into a normal microtubule array; but once the cells entered mitosis it caused extensive fragmentation of the microtubules, disrupted the formation of the spindle apparatus, and allowed entry into G1 phase without cytokinesis to produce large multinucleated cells. The microtubule fragments persisted into subsequent cell cycles and accumulated around the membrane in a marginal band-like appearance. The persistence of the fragments could be traced to a pronounced suppression of microtubule dynamic instability. Impairment of centrosomal nucleation also contributed to the loss of a normal microtubule cytoskeleton. Incorporation of β6 allowed microtubules to resist the effects of colcemid and maytansine, but not vinblastine or paclitaxel; however, cellular resistance to colcemid or maytansine did not occur because expression of β6 prevented cell division. The results indicate that many of the morphological features of megakaryocyte differentiation can be recapitulated in non-hematopoietic cells by β6 expression and they provide a mechanistic basis for understanding these changes.

Coordinated interactions between actin and microtubules through crosslinkers in neurite retraction induced by lysophosphatidic acid

Neurite development requires rearrangement of cytoskeletal elements, which are mechanically and functionally integrated with each other. Although the process of how an extracellular signal induces rearrangement of a single element has been closely examined, the mechanisms by which the signal regulates cytoskeletal integration during cell shape changes are poorly understood. We previously reported that lysophosphatidic acid (LPA) induces actin polymerization-dependent microtubule (MT) rearrangement, leading to neurite retraction in cultured neurons. Here we examined whether the crosslinker proteins were involved in LPA-induced neurite retraction using immortalized mouse neuroblast TR cells. When the MT-binding domains of MACF (MT actin-crosslinking factor) were exogenously expressed in TR cells, MTs were found to be stabilized and become resistant to exposure to LPA. On the other hand, expression of MT-associated protein 2c showed no effect on LPA-induced neurite retraction. These findings suggest that MACF is involved in actin-dependent MT rearrangement during LPA-induced neurite retraction.

Cerebrospinal fluid and blood biomarkers of neuroaxonal damage in multiple sclerosis

Following emerging evidence that neurodegenerative processes in multiple sclerosis (MS) are present from its early stages, an intensive scientific interest has been directed to biomarkers of neuro-axonal damage in body fluids of MS patients. Recent research has introduced new candidate biomarkers but also elucidated pathogenetic and clinical relevance of the well-known ones. This paper reviews the existing data on blood and cerebrospinal fluid biomarkers of neuroaxonal damage in MS and highlights their relation to clinical parameters, as well as their potential predictive value to estimate future disease course, disability, and treatment response. Strategies for future research in this field are suggested.

The tumor suppressor RASSF1A is a novel effector of small G protein Rap1A

Rap1A is a small G protein implicated in a spectrum of biological processes such as cell proliferation, adhesion, differentiation, and embryogenesis. The downstream effectors through which Rap1A mediates its diverse effects are largely unknown. Here we show that Rap1A, but not the related small G proteins Rap2 or Ras, binds the tumor suppressor Ras association domain family 1A (RASSF1A) in a manner that is regulated by phosphorylation of RASSF1A. Interaction with Rap1A is shown to influence the effect of RASSF1A on microtubule behavior.

Drugs that target dynamic microtubules: a new molecular perspective

Microtubules have long been considered an ideal target for anticancer drugs because of the essential role they play in mitosis, forming the dynamic spindle apparatus. As such, there is a wide variety of compounds currently in clinical use and in development that act as antimitotic agents by altering microtubule dynamics. Although these diverse molecules are known to affect microtubule dynamics upon binding to one of the three established drug domains (taxane, vinca alkaloid, or colchicine site), the exact mechanism by which each drug works is still an area of intense speculation and research. In this study, we review the effects of microtubule-binding chemotherapeutic agents from a new perspective, considering how their mode of binding induces conformational changes and alters biological function relative to the molecular vectors of microtubule assembly or disassembly. These "biological vectors" can thus be used as a spatiotemporal context to describe molecular mechanisms by which microtubule-targeting drugs work.

Identification of mouse MARVELD1 as a microtubule associated protein that inhibits cell cycle progression and migration

MARVEL domain-containing 1 (MARVELD1) is a newly identified nuclear protein; however its function has not been clear until now. Here, we report that mouse MARVELD1 (mMARVELD1), which is highly conserved between mice and humans, exhibits cell cycle-dependent cellular localization. In NIH3T3 cells, MARVELD1 was observed in the nucleus and at the perinuclear region during interphase, but was localized at the mitotic spindle and midbody at metaphase, and a significant fraction of mMARVELD1 translocated to the plasma membrane during anaphase. In addition, treatment of cells with colchicine, a microtubule-depolymerizing agent, resulted in translocation of mMARVELD1 to the plasma membrane, and association of mMARVELD1 and α-tubulin was confirmed by co-immunoprecipitation. Finally, overexpression of mMARVELD1 resulted in a remarkable inhibition of cell proliferation, G1-phase arrest, and reduced cell migration. These findings indicate that mMARVELD1 is a microtubule-associated protein that plays an important role in cell cycle progression and migration.

Microtubules as antifungal and antiparasitic drug targets

INTRODUCTION

Diseases caused by fungi and parasites are major illnesses in humans as well as in animals. Microtubule-targeted drugs are highly effective for the treatment of fungal and parasitic infections; however, several human parasitic infections such as malaria, trypanosomiasis and leishmaniasis do not have effective remedial drugs. In addition, the emergence of drug-resistant fungi and parasites makes the discovery of new drugs imperative.

AREAS COVERED

This article describes similarities and dissimilarities between parasitic, fungal and mammalian tubulins and focuses on microtubule-targeting agents and therapeutic approaches for the treatment of fungal and parasitic diseases. New microtubule-targeted antileishmanial, antimalarial and antifungal drugs, with structures, biological activities and related patents, are described. The potential of dsRNA against tubulin to inhibit proliferation of protozoan and helminthic parasites is also discussed. Patent documents up to 2010 have been searched on USPTO, Patentscope, and Espacenet resources.

EXPERT OPINION

The article suggests that vaccination with tubulin may offer novel opportunities for the antiparasitic treatment. Native or recombinant tubulin used as antigen has been shown to elicit immune response and cure infection partially or fully in animals upon challenge by protozoan parasites and helminths, thus indicating the suitability of tubulin as a vaccine against parasitic diseases.

Acetylated microtubules are required for fusion of autophagosomes with lysosomes

Background

Autophagy is a dynamic process during which isolation membranes package substrates to form autophagosomes that are fused with lysosomes to form autolysosomes for degradation. Although it is agreed that the LC3II-associated mature autophagosomes move along microtubular tracks, it is still in dispute if the conversion of LC3I to LC3II before autophagosomes are fully mature and subsequent fusion of mature autophagosomes with lysosomes require microtubules.

Results

We use biochemical markers of autophagy and a collection of microtubule interfering reagents to test the question. Results show that interruption of microtubules with either microtubule stabilizing paclitaxel or destabilizing nocodazole similarly impairs the conversion of LC3I to LC3II, but does not block the degradation of LC3II-associated autophagosomes. Acetylation of microtubules renders them resistant to nocodazole treatment. Treatment with vinblastine that causes depolymerization of both non-acetylated and acetylated microtubules results in impairment of both LC3I-LC3II conversion and LC3II-associated autophagosome fusion with lysosomes.

Conclusions

Acetylated microtubules are required for fusion of autophagosomes with lysosomes to form autolysosomes.