CKAP2 ensures chromosomal stability by maintaining the integrity of microtubule nucleation sites

Integrity of the microtubule spindle apparatus and intact cell division checkpoints are essential to ensure the fidelity of distributing chromosomes into daughter cells. Cytoskeleton-associated protein 2, CKAP2, is a microtubule-associated protein that localizes to spindle poles and aids in microtubule stabilization, but the exact function and mechanism of action are poorly understood. In the present study, we utilized RNA interference to determine the extent to which the expression of CKAP2 plays a role in chromosome segregation. CKAP2-depleted cells showed a significant increase of multipolar mitoses and other spindle pole defects. Notably, when interrogated for microtubule nucleation capacity, CKAP2-depleted cells showed a very unusual phenotype as early as two minutes after release from mitotic block, consisting of dispersal of newly polymerized microtubule filaments through the entire chromatin region, creating a cage-like structure. Nevertheless, spindle poles were formed after one hour of mitotic release suggesting that centrosome-mediated nucleation remained dominant. Finally, we showed that suppression of CKAP2 resulted in a higher incidence of merotelic attachments, anaphase lagging, and polyploidy. Based on these results, we conclude that CKAP2 is involved in the maintenance of microtubule nucleation sites, focusing microtubule minus ends to the spindle poles in early mitosis, and is implicated in maintaining genome stability.

Measurement of in vitro microtubule polymerization by turbidity and fluorescence

Tubulin polymerization may be conveniently monitored by the increase in turbidity (optical density, or OD) or by the increase in fluorescence intensity of diamidino-phenylindole. The resulting data can be a quantitative measure of microtubule (MT) assembly, but some care is needed in interpretation, especially of OD data. Buffer formulations used for the assembly reaction significantly influence the polymerization, both by altering the critical concentration for polymerization and by altering the exact polymer produced-for example, by increasing the production of sheet polymers in addition to MT. Both the turbidity and the fluorescence methods are useful for demonstrating the effect of MT-stabilizing or -destabilizing additives.

Fabrication of hydrogels with steep stiffness gradients for studying cell mechanical response

Many fundamental cell processes, such as angiogenesis, neurogenesis and cancer metastasis, are thought to be modulated by extracellular matrix stiffness. Thus, the availability of matrix substrates having well-defined stiffness profiles can be of great importance in biophysical studies of cell-substrate interaction. Here, we present a method to fabricate biocompatible hydrogels with a well defined and linear stiffness gradient. This method, involving the photopolymerization of films by progressively uncovering an acrylamide/bis-acrylamide solution initially covered with an opaque mask, can be easily implemented with common lab equipment. It produces linear stiffness gradients of at least 115 kPa/mm, extending from ∼1 kPa to 240 kPa (in units of Young's modulus). Hydrogels with less steep gradients and narrower stiffness ranges can easily be produced. The hydrogels can be covalently functionalized with uniform coatings of proteins that promote cell adhesion. Cell spreading on these hydrogels linearly correlates with hydrogel stiffness, indicating that this technique effectively modifies the mechanical environment of living cells. This technique provides a simple approach that produces steeper gradients, wider rigidity ranges, and more accurate profiles than current methods.

Quantitative scheme for full-field polarization rotating fluorescence microscopy using a liquid crystal variable retarder

We present a quantitative scheme for full-field polarization rotating fluorescence microscopy. A quarter-wave plate, in combination with a liquid crystal variable retarder, provides a tunable method to rotate polarization states of light prior to its being coupled into a fluorescence microscope. A calibration of the polarization properties of the incident light is performed in order to correct for elliptical polarization states. This calibration allows the response of the sample to linear polarization states of light to be recovered. Three known polarization states of light can be used to determine the average fluorescent dipole orientations in the presence of a spatially varying dc offset or background polarization-invariant fluorescence signal. To demonstrate the capabilities of this device, we measured a series of full-field fluorescence polarization images from fluorescent analogs incorporated in the lipid membrane of Burkitts lymphoma CA46 cells. The fluorescent lipid-like analogs used in this study are molecules that are labeled by either a DiI (1,1(')-Dioctadecyl 3,3,3('),3(')-Tetramethylindocarbocyanine) fluorophore in its head group or a Bodipy (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) molecule in its acyl chain. A spatially varying contrast in the normalized amplitude was observed on the cell surface, where the orientation of the DiI molecules is tangential to the cell membrane. The internally labeled cellular structures showed zero response to changes in linear polarization, and the net linear polarization amplitude for these regions was zero. This instrument provides a low cost calibrated method that may be coupled to existing fluorescence microscopes to perform investigations of cellular processes that involve a change in molecular orientations.

Inhibitors targeting mitosis: tales of how great drugs against a promising target were brought down by a flawed rationale

Although they have been advocated with an understandable enthusiasm, mitosis-specific agents such as inhibitors of mitotic kinases and kinesin spindle protein have not been successful clinically. These drugs were developed as agents that would build on the success of microtubule-targeting agents while avoiding the neurotoxicity that encumbers drugs such as taxanes and vinca alkaloids. The rationale for using mitosis-specific agents was based on the thesis that the clinical efficacy of microtubule-targeting agents could be ascribed to the induction of mitotic arrest. However, the latter concept, which has long been accepted as dogma, is likely important only in cell culture and rapidly growing preclinical models, and irrelevant in patient tumors, where interference with intracellular trafficking on microtubules is likely the principal mechanism of action. Here we review the preclinical and clinical data for a diverse group of inhibitors that target mitosis and identify the reasons why these highly specific, myelosuppressive compounds have failed to deliver on their promise.

Hindered diffusion in polymeric solutions studied by fluorescence correlation spectroscopy

Diffusion of molecules in the crowded and charged interior of the cell has long been of interest for understanding cellular processes. Here, we introduce a model system of hindered diffusion that includes both crowding and binding. In particular, we obtained the diffusivity of the positively charged protein, ribonuclease A (RNase), in solutions of dextrans of various charges (binding) and concentrations (crowding), as well as combinations of both, in a buffer of physiological ionic strength. Using fluorescence correlation spectroscopy, we observed that the diffusivity of RNase was unaffected by the presence of positively charged or neutral dextrans in the dilute regime but was affected by crowding at higher polymer concentrations. Conversely, protein diffusivity was significantly reduced by negatively charged dextrans, even at 0.4 μM (0.02% w/v) dextran. The diffusivity of RNase decreased with increasing concentrations of negative dextran, and the amount of bound RNase increased until it reached a plateau of ∼80% bound RNase. High salt concentrations were used to establish the electrostatic nature of the binding. Binding of RNase to the negatively charged dextrans was further confirmed by ultrafiltration.

Mutations in the β-tubulin binding site for peloruside A confer resistance by targeting a cleft significant in side chain binding

Peloruside A is a microtubule-stabilizing macrolide that binds to beta tubulin at a site distinct from the taxol site. The site was previously identified by H-D exchange mapping and molecular docking as a region close to the outer surface of the microtubule and confined in a cavity surrounded by a continuous loop of protein folded so as to center on Y340. We have isolated a series of peloruside A-resistant lines of the human ovarian carcinoma cell line A2780(1A9) to better characterize this binding site and the consequences of altering residues in it. Four resistant lines (Pel A-D) are described with single-base mutations in class I β-tubulin that result in the following substitutions: R306H, Y340S, N337D, and A296S in various combinations. The mutations are localized to peptides previously identified by Hydrogen-Deuterium exchange mapping, and center on a cleft in which the drug side chain appears to dock. The Pel lines are 10-15-fold resistant to peloruside A and show cross resistance to laulimalide but not to any other microtubule stabilizers. They show no cross-sensitivity to any microtubule destabilizers, nor to two drugs with targets unrelated to microtubules. Peloruside A induces G2/M arrest in the Pel cell lines at concentrations 10-15 times that required in the parental line. The cells show notable changes in morphology compared to the parental line.

Production of reliable MALDI spectra with quality threshold clustering of replicates

We present the first application of the quality threshold (QT) clustering algorithm to mass spectrometry (MS) data. The unique abilities of QT clustering to yield precision nodes that are commensurate with the mass measurement precision of the instrument are exploited to generate a consensus spectrum out of multiple replicate spectra. The spectral dot product and confidence intervals are used as a tool for evaluating the similarity and reproducibility between the consensus and replicates. The method is equally applicable to high and low resolution measurements. This paper demonstrates applications to linear spectra from a matrix assisted laser desorption ionization (MALDI) time of flight (TOF) instrument as well as peptide fragmentation data obtained from a TOF/TOF after unimolecular decomposition. The advantages of clustering to mitigate the inherent precision the shortcomings of MALDI data are discussed.

Alpha2B-adrenergic receptor interaction with tubulin controls its transport from the endoplasmic reticulum to the cell surface

It is well recognized that the C terminus (CT) plays a crucial role in modulating G protein-coupled receptor (GPCR) transport from the endoplasmic reticulum (ER) to the cell surface. However the molecular mechanisms that govern CT-dependent ER export remain elusive. To address this issue, we used α(2B)-adrenergic receptor (α(2B)-AR) as a model GPCR to search for proteins interacting with the CT. By using peptide-conjugated affinity matrix combined with proteomics and glutathione S-transferase fusion protein pull-down assays, we identified tubulin directly interacting with the α(2B)-AR CT. The interaction domains were mapped to the acidic CT of tubulin and the basic Arg residues in the α(2B)-AR CT, particularly Arg-437, Arg-441, and Arg-446. More importantly, mutation of these Arg residues to disrupt tubulin interaction markedly inhibited α(2B)-AR transport to the cell surface and strongly arrested the receptor in the ER. These data provide the first evidence indicating that the α(2B)-AR C-terminal Arg cluster mediates its association with tubulin to coordinate its ER-to-cell surface traffic and suggest a novel mechanism of GPCR export through physical contact with microtubules.

Site-specific orthogonal labeling of the carboxy terminus of alpha-tubulin

A fluorescent probe has been attached to the carboxy terminus of the alpha-subunit of alpha,beta-tubulin by an enzymatic reaction followed by a chemical reaction. The unnatural amino acid 3-formyltyrosine is attached to the carboxy terminus of alpha-tubulin through the use of the enzyme tubulin tyrosine ligase. The aromatic aldehyde of the unnatural amino acid serves as an orthogonal electrophile that specifically reacts with a fluorophore containing an aromatic hydrazine functional group, which in this case is 7-hydrazino-4-methyl coumarin. Conditions for covalent bond formation between the unnatural amino acid and the fluorophore are mild, allowing fluorescently labeled tubulin to retain its ability to assemble into microtubules. A key feature of the labeling reaction is that it produces a red shift in the fluorophore's absorption and emission maxima, accompanied by an increase in its quantum yield; thus, fluorescently labeled protein can be observed in the presence of unreacted fluorophore. Both the enzymatic and coupling reaction can occur in living cells. The approach presented here should be applicable to a wide variety of in vitro systems.

Microtubule-disrupting chemotherapeutics result in enhanced proteasome-mediated degradation and disappearance of tubulin in neural cells

We sought to examine the effects of microtubule-targeting agents (MTA) on neural cells to better understand the problem of neurotoxicity, their principal side effect, and to possibly develop a model of clinical toxicity. Studies showed that microtubule-depolymerizing agents (MDA) not only disassembled microtubules in neural HCN2 cells but also led to rapid disappearance of tubulin, and that this was specific for MDAs. Tubulin levels decreased to 20% as early as 8 hours after adding vincristine, and to 1% to 30% (mean, 9.8 +/- 7.6%; median of 7%) after 100 nmol/L vincristine for 24 hours. This disappearance was reversible. An increase in both glu-terminated and acetylated tubulin, markers of stable tubulin, preceded reaccumulation of soluble tubulin, suggesting a priority for stabilizing tubulin first as microtubules before replenishing the soluble pool. Similar results were shown with other MDAs. Furthermore, microtubule reassembly did not arise from a central focus but instead appeared to involve dispersed nucleation, as evidenced by the appearance of small, stable microtubule stubs throughout the cytoplasm. In contrast, experiments with four nonneural "normal" cell lines and four cancer cell lines resulted in microtubule destabilization but only modest tubulin degradation. Evidence for proteasome-mediated degradation was obtained by demonstrating that adding a proteasome inhibitor before vincristine prevented tubulin disappearance. In summary, MDAs lead to rapid disappearance of tubulin in neural but not in other normal or cancer cells. These results underscore the fine control that occurs in neural cells and may further our understanding of neurotoxicity following MDAs.

Carcinomas contain a matrix metalloproteinase-resistant isoform of type I collagen exerting selective support to invasion

Collagen fibers affect metastasis in two opposing ways, by supporting invasive cells but also by generating a barrier to invasion. We hypothesized that these functions might be performed by different isoforms of type I collagen. Carcinomas are reported to contain alpha1(I)(3) homotrimers, a type I collagen isoform normally not present in healthy tissues, but the role of the homotrimers in cancer pathophysiology is unclear. In this study, we found that these homotrimers were resistant to all collagenolytic matrix metalloproteinases (MMP). MMPs are massively produced and used by cancer cells and cancer-associated fibroblasts for degrading stromal collagen at the leading edge of tumor invasion. The MMP-resistant homotrimers were produced by all invasive cancer cell lines tested, both in culture and in tumor xenografts, but they were not produced by cancer-associated fibroblasts, thereby comprising a specialized fraction of tumor collagen. We observed the homotrimer fibers to be resistant to pericellular degradation, even upon stimulation of the cells with proinflammatory cytokines. Furthermore, we confirmed an enhanced proliferation and migration of invasive cancer cells on the surface of homotrimeric versus normal (heterotrimeric) type I collagen fibers. In summary, our findings suggest that invasive cancer cells may use homotrimers for building MMP-resistant invasion paths, supporting local proliferation and directed migration of the cells whereas surrounding normal stromal collagens are cleaved. Because the homotrimers are universally secreted by cancer cells and deposited as insoluble, MMP-resistant fibers, they offer an appealing target for cancer diagnostics and therapy.

Abstract 1219: Tubulin binding to mitochondrial VDAC: A new regulator of oxidative metabolism and apoptosis? A new role for tubulin

Tubulin binding to the outer mitochondrial membrane (OMM) was first reported many years ago but the function was unknown; we showed recently that dimeric tubulin binds to VDAC, the major metabolite channel in the OMM, and that this results in closing the channel to ATP-ADP exchange, with a consequent drop in oxidative phosphorylation. This could act as a regulator of normal oxidative metabolism, but excess tubulin binding could also lead to changes in the inner membrane potential, and possibly lead to mitochondrial swelling and triggering of apoptosis. To evaluate the plausibility of this mechanism for either regulation of normal metabolism or induction of apoptosis, the concentrations of tubulin and VDAC in the cell need to be known, as a huge discrepancy in either direction could make the mechanism unlikely. We have used semiquantitative Western blots to measure the concentrations of tubulin and VDAC in total cell extracts from a number of cancer cell lines. As expected, we found tubulin to be an abundant protein (∼0.5 % of total protein), but we also found VDAC to be more abundant than perhaps commonly thought (∼0.2 % of total protein). Thus we conclude that tubulin regulation of VDAC permeability is a plausible mechanism. Further experiments are underway to test this mechanism and its role in induction of apoptosis by chemotherapy agents, especially microtubule-directed ones.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1219.

Isolating tubulin from nonneural sources

Tubulin is a highly conserved, negatively charged protein that is found in essentially all eukaryotic cells. These properties ensure that isolation protocols successful in one system will likely work, with a few modifications, in most systems. Tubulin has been isolated most frequently from mammalian brain, and the main difference encountered in other systems versus brain is that tubulin is much less abundant in nearly all other sources than it is in brain. This means that attempting to purify tubulin by direct polymerization from a homogenate will often fail or be quite inefficient. However, the conservation of negative charge on tubulin means that an initial ion exchange step can be used to both purify and concentrate the protein from most systems. Polymerization-competent tubulin can usually be obtained by inducing polymerization in the salt eluate from the ion exchange step. We describe protocols for this procedure and describe its application to a number of vertebrate, fungal, protozoal, and plant sources.

Clinical outcome in children with recurrent neuroblastoma treated with ABT-751 and effect of ABT-751 on proliferation of neuroblastoma cell lines and on tubulin polymerization in vitro

BACKGROUND

ABT-751, an orally bioavailable sulfonamide, binds beta-tubulin to inhibit microtubule polymerization. We described response and event-free survival (EFS) in children with neuroblastoma and other solid tumors receiving ABT-751, assessed in vitro cytotoxicity of ABT-751 and evaluated the effect of ABT-751 on tubulin polymerization in peripheral blood mononuclear cells (PBMC) and pediatric tumor cell lines.

PROCEDURE

Patients with neuroblastoma (n = 50) or other solid tumors (n = 26) enrolled on the ABT-751 pediatric phase I and pilot trials were reviewed. The sulforhodamine B (SRB) and ACEA Real-Time Cell Electronic Sensing (RT-CES) assays were used to determine the in vitro cytotoxicity. Pharmacodynamic effects on tubulin polymerization/depolymerization were assessed by Western blot and confocal microscopy using antibodies specific for post-translational modifications of polymerized tubulin.

RESULTS

Forty-five patients with neuroblastoma were evaluated for anti-tumor response. No complete or partial responses were documented. The median EFS was 9.3 weeks for children with neuroblastoma and 3.3 weeks for children other solid tumors (P < 0.0001). The ABT-751 IC(50) was 0.6-2.6 mcM in neuroblastoma and 0.7-4.6 mcM in other solid tumor cell lines. Following drug exposure, polymerized tubulin decreased in a concentration- and time-dependent manner in cell lines.

CONCLUSIONS

In children treated with ABT-751, the EFS is longer in children with neuroblastoma as compared to other diagnoses. In vitro, ABT-751 was cytotoxic at concentrations tolerable in children. Effects of ABT-751 on polymerization and microtubule structure were time- and dose-dependent but not dependent on tumor type.

Measurement of ligand binding to tubulin by sulfhydryl reactivity

Ligand binding can induce shifts in protein conformation. In the case of tubulin, these drug-induced confirmational changes can prevent or stabilize microtubule polymerization. 5',5'-Dithiobis(2-nitrobenzoate) (DTNB) reacts with free and accessible sulfhydryls and stoichiometrically produces a detectable product, which allows an exact measurement of reacted thiols. Since binding of small ligands may alter conformational dynamics, it may also affect the reactivity of thiols on tubulin. Differences in DTNB reactivity with thiols upon ligand binding can therefore be used to deduce binding characteristics. We will describe two methods that use tubulin cysteine reactivity with DTNB in the presence of drug to define ligand-binding characteristics.

New sources of chemical diversity inspired by biosynthesis: rational design of a potent epothilone analogue

A concise total synthesis of (S)-14-methoxyepothilone D has been accomplished. (S)-14-Methoxyepothilone D represents a conceptually novel example of polyketide analogue design based on an alternative biogenetic pattern of extender units. The significant biological activity observed for this compound provides a foundation to support studies designed to prepare derivatives of this type through fermentation of genetically engineered organisms expressing the epothilone PKS gene cluster.

Nucleation capacity and presence of centrioles define a distinct category of centrosome abnormalities that induces multipolar mitoses in cancer cells

Analysis of centrosome number and structure has become one means of assessing the potential for aberrant chromosome segregation and aneuploidy in tumor cells. Centrosome amplification directly causes multipolar catastrophic mitoses in mouse embryonic fibroblasts (MEFs) deficient for the tumor suppressor genes Brca1 or Trp53. We observed supernumerary centrosomes in cell lines established from aneuploid, but not from diploid, colorectal carcinomas; however, multipolar mitoses were never observed. This discrepancy prompted us to thoroughly characterize the centrosome abnormalities in these and other cancer cell lines with respect to both structure and function. The most striking result was that supernumerary centrosomes in aneuploid colorectal cancer cell lines were unable to nucleate microtubules, despite the presence of gamma-tubulin, pericentrin, PLK1, and AURKA. Analysis by scanning electron microscopy revealed that these supernumerary structures are devoid of centrioles, a result significantly different from observations in aneuploid pancreatic cancer cell lines and in Trp53 or Brca1 deficient MEFs. Thus, multipolar mitoses are dependent upon the ability of extra gamma-tubulin containing structures to nucleate microtubules, and this correlated with the presence of centrioles. The assessment of centrosome function with respect to chromosome segregation must therefore take into consideration the presence of centrioles and the capacity to nucleate microtubules. The patterns and mechanisms of chromosomal aberrations in hematologic malignancies and solid tumors are fundamentally different. The former is characterized by specific chromosome translocations, whose consequence is the activation of oncogenes. Most carcinomas, however, reveal variations in the nuclear DNA content. The observed genomic imbalances and gross variations in chromosome number can result from unequal chromosome segregation during mitotic cell division. It is therefore fundamental to elucidate mechanisms involved in distribution of the genome to daughter cells. Prior to cell division, the centrosome organizes microtubules and the mitotic spindle. Deciphering the consequences of alterations in centrosome number, structure, and function is an important step towards understanding how a diploid genome is maintained. Although extra centrosomes have now been observed in carcinomas and were correlated with aneuploidy, a careful functional investigation of these structures and their role in generating chromosome imbalances may lead to the identification of distinct mechanistic pathways of genomic instability. Understanding these pathways will also be important in determining whether they are potential molecular targets of therapeutic intervention.