eIF4F is a thermo-sensing regulatory node in the translational heat shock response

Heat-shocked cells prioritize the translation of heat shock (HS) mRNAs, but the underlying mechanism is unclear. We report that HS in budding yeast induces the disassembly of the eIF4F complex, where eIF4G and eIF4E assemble into translationally arrested mRNA ribonucleoprotein particles (mRNPs) and HS granules (HSGs), whereas eIF4A promotes HS translation. Using in vitro reconstitution biochemistry, we show that a conformational rearrangement of the thermo-sensing eIF4A-binding domain of eIF4G dissociates eIF4A and promotes the assembly with mRNA into HS-mRNPs, which recruit additional translation factors, including Pab1p and eIF4E, to form multi-component condensates. Using extracts and cellular experiments, we demonstrate that HS-mRNPs and condensates repress the translation of associated mRNA and deplete translation factors that are required for housekeeping translation, whereas HS mRNAs can be efficiently translated by eIF4A. We conclude that the eIF4F complex is a thermo-sensing node that regulates translation during HS.

Integrative modeling reveals the molecular architecture of the intraflagellar transport A (IFT-A) complex

Intraflagellar transport (IFT) is a conserved process of cargo transport in cilia that is essential for development and homeostasis in organisms ranging from algae to vertebrates. In humans, variants in genes encoding subunits of the cargo-adapting IFT-A and IFT-B protein complexes are a common cause of genetic diseases known as ciliopathies. While recent progress has been made in determining the atomic structure of IFT-B, little is known of the structural biology of IFT-A. Here, we combined chemical cross-linking mass spectrometry and cryo-electron tomography with AlphaFold2-based prediction of both protein structures and interaction interfaces to model the overall architecture of the monomeric six-subunit IFT-A complex, as well as its polymeric assembly within cilia. We define monomer-monomer contacts and membrane-associated regions available for association with transported cargo, and we also use this model to provide insights into the pleiotropic nature of human ciliopathy-associated genetic variants in genes encoding IFT-A subunits. Our work demonstrates the power of integration of experimental and computational strategies both for multi-protein structure determination and for understanding the etiology of human genetic disease.

In situ architecture of the ciliary base reveals the stepwise assembly of intraflagellar transport trains

The cilium is an antenna-like organelle that performs numerous cellular functions, including motility, sensing, and signaling. The base of the cilium contains a selective barrier that regulates the entry of large intraflagellar transport (IFT) trains, which carry cargo proteins required for ciliary assembly and maintenance. However, the native architecture of the ciliary base and the process of IFT train assembly remain unresolved. In this work, we used in situ cryo-electron tomography to reveal native structures of the transition zone region and assembling IFT trains at the ciliary base in Chlamydomonas. We combined this direct cellular visualization with ultrastructure expansion microscopy to describe the front-to-back stepwise assembly of IFT trains: IFT-B forms the backbone, onto which bind IFT-A, dynein-1b, and finally kinesin-2 before entry into the cilium.

The structural basis of intraflagellar transport at a glance

The intraflagellar transport (IFT) system is a remarkable molecular machine used by cells to assemble and maintain the cilium, a long organelle extending from eukaryotic cells that gives rise to motility, sensing and signaling. IFT plays a critical role in building the cilium by shuttling structural components and signaling receptors between the ciliary base and tip. To provide effective transport, IFT-A and IFT-B adaptor protein complexes assemble into highly repetitive polymers, called IFT trains, that are powered by the motors kinesin-2 and IFT-dynein to move bidirectionally along the microtubules. This dynamic system must be precisely regulated to shuttle different cargo proteins between the ciliary tip and base. In this Cell Science at a Glance article and the accompanying poster, we discuss the current structural and mechanistic understanding of IFT trains and how they function as macromolecular machines to assemble the structure of the cilium.

In situ cryo-electron tomography and subtomogram averaging of intraflagellar transport trains

In situ cryo-electron tomography (cryo-ET) and subtomogram averaging are powerful tools, able to provide 3D structures of biological samples at sub-nanometer resolution, while preserving information about cellular context and higher-order assembly. Best results are typically achieved, when applied to highly repetitive structures, such as viruses. Other typical examples are protein complexes that decorate long stretches along ciliary microtubules at stereotypical and precise repeats, such as axonemal dyneins. For such cases, a plethora of subtomogram averaging protocols exist. In this chapter, we show how we use cryo-ET and subtomogram averaging to study the architecture of the intraflagellar transport (IFT) machinery, a more challenging target that appears only in low copy numbers per tomogram. In the IFT trains, repeating units of IFT adaptor proteins engage two oppositely directed molecular motors to quickly shuttle ciliary building blocks and other proteins to the tip of the cilium and/or back to the base. This dynamic and sporadic nature of IFT trains poses challenges for determining the localization or precise orientation of the particles to be averaged. Solutions to these problems are described in this chapter.

The cryo-EM structure of intraflagellar transport trains reveals how dynein is inactivated to ensure unidirectional anterograde movement in cilia

Movement of cargos along microtubules plays key roles in diverse cellular processes, from signalling to mitosis. In cilia, rapid movement of ciliary components along the microtubules to and from the assembly site is essential for the assembly and disassembly of the structure itself¹. This bidirectional transport, known as intraflagellar transport (IFT)², is driven by the anterograde motor kinesin-2³ and the retrograde motor dynein-1b (dynein-2 in mammals)^4,5. However, to drive retrograde transport, dynein-1b must first be delivered to the ciliary tip by anterograde IFT⁶. Although, the presence of opposing motors in bidirectional transport processes often leads to periodic stalling and slowing of cargos⁷, IFT is highly processive^1,2,8. Using cryo-electron tomography, we show that a tug-of-war between kinesin-2 and dynein-1b is prevented by loading dynein-1b onto anterograde IFT trains in an autoinhibited form and by positioning it away from the microtubule track to prevent binding. Once at the ciliary tip, dynein-1b must transition into an active form and engage microtubules to power retrograde trains. These findings provide a striking example of how coordinated structural changes mediate the behaviour of complex cellular machinery.

Differential Morphological and Biochemical Recovery from Chemotherapy-Induced Peripheral Neuropathy Following Paclitaxel, Ixabepilone, or Eribulin Treatment in Mouse Sciatic Nerves

The reversibility of chemotherapy-induced peripheral neuropathy (CIPN), a disabling and potentially permanent side effect of microtubule-targeting agents (MTAs), is becoming an increasingly important issue as treatment outcomes improve. The molecular mechanisms regulating the variability in time to onset, severity, and time to recovery from CIPN between the common MTAs paclitaxel and eribulin are unknown. Previously (Benbow et al. in Neurotox Res 29:299-313, 2016), we found that after 2 weeks of a maximum tolerated dose (MTD) in mice, paclitaxel treatment resulted in severe reductions in axon area density, higher frequency of myelin abnormalities, and increased numbers of Schwann cell nuclei in sciatic nerves. Biochemically, eribulin induced greater microtubule-stabilizing effects than paclitaxel. Here, we extended these comparative MTD studies to assess the recovery from these short-term effects of paclitaxel, eribulin, and a third MTA, ixabepilone, over the course of 6 months. Paclitaxel induced a persistent reduction in axon area density over the entire 6-month recovery period, unlike ixabepilone- or eribulin-treated animals. The abundance of myelin abnormalities rapidly declined after cessation of all drugs but recovered most slowly after paclitaxel treatment. Paclitaxel- and ixabepilone- but not eribulin-treated animals exhibited increased Schwann cell numbers during the recovery period. Tubulin composition and biochemistry rapidly returned from MTD-induced levels of α-tubulin, acetylated α-tubulin, and end-binding protein 1 to control levels following cessation of drug treatment. Taken together, sciatic nerve axons recovered more rapidly from morphological effects in eribulin- and ixabepilone-treated animals than in paclitaxel-treated animals and drug-induced increases in protein expression levels following paclitaxel and eribulin treatment were relatively transient.

Effect of hot water treatments on quality of highbush blueberries

Highbush blueberries, cv 'Burlington', were treated with 22, 45, 50, or 60 degrees C water for 15 or 30 s along with an untreated control. Fruit were then stored for 0, 1, 2, or 4 wk at 0 degrees C and 2 or 9 d at 20 degrees C prior to evaluation of microbial population and fruit quality. After 4 wk of storage, the hot water treatment at 60 degrees C resulted in 92% marketable berries, followed by 90% at 50 degrees C, 88% at 45 degrees C, and 83% at 22 degrees C compared with 76% in untreated controls. Decay incidence was reduced to 0.6%, 1.2%, 1.4%, or 2.8% with 60, 50, 45, or 22 degrees C water treatments, respectively, compared with 5.1% in controls following 4 wk at 0 degrees C and 2 d at 20 degrees C. After an additional 7 d at 20 degrees C, decay in fruit treated at 60 degrees C for 15 or 30 s remained at 1.8% and 0.4%, respectively, compared to 37.4% in controls. Weight loss of berries treated with hot water was 0.4% against 3.8% in controls, and shriveled and split berries were also reduced compared to controls (P<0.001). Aerobic plate count and yeast and mold count were reduced by 0.45 to 0.7 log at 60 degrees C for 30 s. Botrytis cinerea and Colletotrichum sp. were the dominant fungal pathogens causing decay of Burlington blueberries during storage. Hot water treatments also immediately induced an increase in ethanol and reduced fruit titratable acidity and soluble solids content, but had no significant effect on fruit firmness, pH, or most flavor volatile concentrations.

A P300-based brain-computer interface for people with amyotrophic lateral sclerosis

OBJECTIVE

The current study evaluates the efficacy of a P300-based brain-computer interface (BCI) communication device for individuals with advanced ALS.

METHODS

Participants attended to one cell of a N x N matrix while the N rows and N columns flashed randomly. Each cell of the matrix contained one character. Every flash of an attended character served as a rare event in an oddball sequence and elicited a P300 response. Classification coefficients derived using a stepwise linear discriminant function were applied to the data after each set of flashes. The character receiving the highest discriminant score was presented as feedback.

RESULTS

In Phase I, six participants used a 6 x 6 matrix on 12 separate days with a mean rate of 1.2 selections/min and mean online and offline accuracies of 62% and 82%, respectively. In Phase II, four participants used either a 6 x 6 or a 7 x 7 matrix to produce novel and spontaneous statements with a mean online rate of 2.1 selections/min and online accuracy of 79%. The amplitude and latency of the P300 remained stable over 40 weeks.

CONCLUSIONS

Participants could communicate with the P300-based BCI and performance was stable over many months.

SIGNIFICANCE

BCIs could provide an alternative communication and control technology in the daily lives of people severely disabled by ALS.

Evaluation of the in situ, time-integrated DGT technique by monitoring changes in heavy metal concentrations in estuarine waters

Various natural and anthropogenic processes influence heavy metal concentrations within estuaries. In situ, time-integrated DGT measurements made over concurrent tidal phases found significantly higher concentrations of Cu (probability p=0.017), Zn (p=0.003) and Ni (p=0.003) during the flood phase, because the incoming tide passes several point sources. DGT-reactive Cu concentrations significantly decreased with increased tidal-flushing and vice versa within a marina (correlation r=-0.788, p=0.02). DGT measurements also recorded significant increases in Cu (4 out of 4 sites, p<0.001) and Zn (3 out of 4 sites, p< or =0.015) after a 24 mm rainfall event. Finally, DGT-reactive Cu increased significantly (p<0.001) during peak boating times, due to increased numbers of Cu-antifouled boats. This study demonstrates that, with judicious selection of deployment times, DGT measurements enable changes in heavy metal concentrations to be related to various cycles and events within estuaries.

New microtubule/tubulin-targeted anticancer drugs and novel chemotherapeutic strategies

The clinical success of the taxanes and the vinca alkaloids has sparked a major search for new drugs that perturb mitotic spindle microtubule dynamics and function, yielding a large number of promising compounds. A potential valuable strategy would be to use core microtubule-targeted drugs along with novel targeted drug therapies as they are developed, where the antitumor activity of a targeted drug can be combined with the power of low dose microtubule-targeted therapy. The goal of such combination therapy is to achieve high efficacy, reduced toxicity, and reduced emergence of drug resistance.

Phenotypic divergence despite high levels of gene flow in Galápagos lava lizards (Microlophus albemarlensis)

The extent of evolutionary divergence of phenotypes between habitats is predominantly the result of the balance of differential natural selection and gene flow. Lava lizards (Microlophus albemarlensis) on the small island of Plaza Sur in the Galápagos archipelago inhabit contrasting habitats: dense vegetation on the western end of the island thins rapidly in a transitional area, before becoming absent on the eastern half. Associated with these habitats are phenotypic differences in traits linked to predator avoidance (increased wariness, sprint speed, and endurance in lizards from the sparsely vegetated habitat). This population provides an opportunity to test the hypothesis that reduced gene flow is necessary for phenotypic differentiation. There was no evidence of any differences among habitats in allele frequencies at six out of seven microsatellite loci examined, nor was there any indication of congruence between patterns of genetic variability and the change in vegetation regime. We infer that gene flow between the habitats on Plaza Sur must be sufficiently high to overcome genetic drift within habitats but that it does not preclude phenotypic differentiation.

Modulation of microtubule dynamics by drugs: a paradigm for the actions of cellular regulators

Microtubules are intrinsically dynamic polymers. Two kinds of dynamic behaviors, dynamic instability and treadmilling, are important for microtubule function in cells. Both dynamic behaviors appear to be tightly regulated, but the cellular molecules and the mechanisms responsible for the regulation remain largely unexplored. While microtubule dynamics can be modulated transiently by the interaction of regulatory molecules with soluble tubulin, the microtubule itself is likely to be the primary target of cellular molecules that regulate microtubule dynamics. The antimitotic drugs that modulate microtubule dynamics serve as excellent models for such cellular molecules. Our laboratory has been investigating the interactions of small drug molecules and stabilizing microtubule-associated proteins (MAPs) with microtubule surfaces and ends. We find that drugs such as colchicine, vinblastine, and taxol, and stabilizing MAPs such as tau, strongly modulate microtubule dynamics at extremely low concentrations under conditions in which the microtubule polymer mass is minimally affected. The powerful modulation of the dynamics is brought about by the binding of only a few drug or MAP molecules to distinct binding sites at the microtubule surface or end. Based upon our understanding of the well-studied drugs and stabilizing MAPs, it is clear that molecules that regulate dynamics such as Kin 1 and stathmin could bind to a large number of distinct tubulin sites on microtubules and employ an array of mechanisms to selectively and powerfully regulate microtubule dynamics and dynamics-dependent cellular functions.

Mechanism of action of antitumor drugs that interact with microtubules and tubulin

Microtubules, major structural components in cells, are the target of a large and diverse group of natural product anticancer drugs. Given the success of this class of drugs in cancer treatment, it can be argued that microtubules represent the single best cancer target identified to date. Microtubules are highly dynamic assemblies of the protein tubulin. They readily polymerize and depolymerize in cells, and they undergo two interesting kinds of dynamics called dynamic instability and treadmilling. These dynamic behaviors are crucial to mitosis, the process of chromosomal division to form new cells. Microtubule dynamics are highly regulated during the cell cycle by endogenous cellular regulators. In addition, many antitumor drugs and natural compounds alter the polymerization dynamics of microtubules, blocking mitosis, and consequently, inducing cell death by apoptosis. These drugs include several that inhibit microtubule polymerization at high drug concentrations, namely, the Vinca alkaloids, cryptophycins, halichondrins, estramustine, and colchicine. Another group of these compounds stimulates microtubule polymerization and stabilizes microtubules at high concentrations. These include Taxol, Taxotere, eleutherobins, epothilones, laulimalide, sarcodictyins, and discodermolide. Importantly, considerable evidence indicates that, at lower concentrations, these drugs have a common mechanism of action; they suppress the dynamics of microtubules without appreciably changing the mass of microtubules in the cell. The drugs bind to diverse sites on tubulin and at different positions within the microtubule, and they have diverse effects on microtubule dynamics. However, by their common mechanism of suppression microtubule dynamics, they all block mitosis at the metaphase/anaphase transition, and induce cell death.

Identification of the Gasa3 and Gasa4 autoimmune gastritis susceptibility genes using congenic mice and partitioned, segregative and interaction analyses

BALB/c mice thymectomized on their third day of life develop a high incidence of experimental autoimmune gastritis (EAG) which closely resembles human chronic atrophic (type A, autoimmune) gastritis. Linkage analysis of (BALB/cCrSlcxC57BL/6)F2 mice previously demonstrated that the Gasa1 and Gasa2 genes on distal Chromosome (Chr) 4 have major effects on the development of EAG in this murine model, while other loci displayed a trend towards linkage. Here, we implemented partitioned chi(2)-analysis in order to develop a better understanding of the genotypes contributing to susceptibility and resistance at each linkage region. This approach revealed that linkage of Gasa1 and Gasa2 to EAG was due to codominant and recessive BALB/cCrSlc alleles, respectively. To identify additional EAG susceptibility genes, separate linkage studies were performed on Gasa1 heterozygotes and Gasa2 C57BL/6 homozygotes plus heterozygotes so as to minimize the effects of these disease genes. The enhanced sensitivity of these analyses confirmed the existence of a third EAG susceptibility gene (designated Gasa3) on Chr 6. Epistatic interactions between the Gasa2 EAG susceptibility gene and the H2 were also identified, and the presence of an H2-linked susceptibility gene (Gasa4) confirmed by analysis of H2 congenic mice.

Resistance to Taxol in lung cancer cells associated with increased microtubule dynamics

Microtubule dynamics are crucial for mitotic spindle assembly and chromosome movement. Suppression of dynamics by Taxol appears responsible for the drug's potent ability to inhibit mitosis and cell proliferation. Although Taxol is an important chemotherapeutic agent, development of resistance limits its efficacy. To examine the role of microtubule dynamics in Taxol resistance, we measured the dynamic instability of individual rhodamine-labeled microtubules in Taxol-sensitive and -resistant living human cancer cells. Taxol-resistant A549-T12 and -T24 cell lines were selected from a human lung carcinoma cell line, A549. They are, respectively, 9- and 17-fold resistant to Taxol and require low concentrations of Taxol for proliferation. We found that microtubule dynamic instability was significantly increased in the Taxol-resistant cells. For example, with A549-T12 cells in the absence of added Taxol, microtubule dynamicity increased 57% as compared with A549 cells. The length and rate of shortening excursions increased 75 and 59%, respectively. These parameters were further increased in A549-T24 cells, with overall dynamicity increasing by 167% compared with parental cells. Thus, the decreased Taxol-sensitivity of these cells can be explained by their increased microtubule dynamics. When grown without Taxol, A549-T12 cells were blocked at the metaphase/anaphase transition and displayed abnormal mitotic spindles with uncongressed chromosomes. In the presence of 2-12 nM Taxol, the cells grew normally, suggesting that mitotic block resulted from excessive microtubule dynamics. These results indicate that microtubule dynamics play an important role in Taxol resistance, and that both excessively rapid dynamics and suppressed dynamics impair mitotic spindle function and inhibit proliferation.

Purification and characterization of native conventional kinesin, HSET, and CENP-E from mitotic hela cells

We have developed a strategy for the purification of native microtubule motor proteins from mitotic HeLa cells and describe here the purification and characterization of human conventional kinesin and two human kinesin-related proteins, HSET and CENP-E. We found that the 120-kDa HeLa cell conventional kinesin is an active motor that induces microtubule gliding at approximately 30 microm/min at room temperature. This active form of HeLa cell kinesin does not contain light chains, although light chains were detected in other fractions. HSET, a member of the C-terminal kinesin subfamily, was also purified in native form for the first time, and the protein migrates as a single band at approximately 75 kDa. The purified HSET is an active motor that induces microtubule gliding at a rate of approximately 5 microm/min, and microtubules glide for an average of 3 microm before ceasing movement. Finally, we purified native CENP-E, a kinesin-related protein that has been implicated in chromosome congression during mitosis, and we found that this form of CENP-E does not induce microtubule gliding but is able to bind to microtubules.

Mechanism of mitotic block and inhibition of cell proliferation by the semisynthetic Vinca alkaloids vinorelbine and its newer derivative vinflunine

The two second-generation Vinca alkaloids, vinorelbine and vinflunine, affect microtubule dynamics very differently from vinblastine, a first generation Vinca alkaloid. For example, vinblastine strongly suppresses the rate and extent of microtubule shortening in vitro, whereas vinorelbine and vinflunine suppress the rate and extent of microtubule growing events. We asked whether these differences result in differences in mitotic spindle organization that might be responsible for the superior antitumor activities of the two second-generation Vinca alkaloids. IC(50) values for inhibition of HeLa cell proliferation for vinflunine, vinorelbine, and vinblastine were 18, 1.25, and 0.45 nM, respectively, similar to the concentrations that induced mitotic block at the metaphase/anaphase transition (38, 3.8, and 1.1 nM, respectively), indicating that mitotic block is a major contributor to antiproliferative action for all three drugs. Mitotically blocked cells exhibited aberrant spindles, consistent with induction of block by suppression of microtubule dynamics. Despite differences in their actions on individual dynamic instability parameters, morphologically detectable differences in spindle effects among the three drugs were minimal, indicating that overall suppression of dynamics may be more important in blocking mitosis than specific effects on growth or shortening. We also found that the peak intracellular drug concentration at the mitotic IC(50) value was highest for vinflunine (4.2 +/- 0.2 microM), intermediate for vinorelbine (1.3 +/- 0.1 microM), and more than 10-fold lower for vinblastine (130 +/- 7 nM), suggesting that intracellular binding reservoir(s) may be partially responsible for vinflunine's high efficacy and minimal side effects.

Interaction of the antitumor compound cryptophycin-52 with tubulin

Cryptophycin-52 (LY355703) is currently undergoing clinical evaluation for cancer chemotherapy. It is a potent suppressor of microtubule dynamics in vitro, and low picomolar concentrations appear to inhibit cancer cell proliferation at mitosis by stabilizing spindle microtubules. In the present study, using [(3)H]cryptophycin-52, we found that the compound bound to tubulin at a single high-affinity site [apparent K(a) (3.6 +/- 1) x 10(6) L/mol, 34 degrees C]. The binding of cryptophycin-52 to tubulin was rapid, not appreciably temperature-dependent, and very poorly reversible. However, we could remove [(3)H]cryptophycin-52 from [(3)H]cryptophycin-52-tubulin complex by denaturing the complex with either urea treatment or boiling. These data suggest that the binding of cryptophycin-52 to tubulin is not covalent. A van't Hoff plot of the binding data indicated that the binding of cryptophycin-52 to tubulin is primarily entropy-driven with a minimum enthalpy contribution. In addition, cryptophycin-52 perturbed the far-ultraviolet circular dichroic spectrum of tubulin and it inhibited the colchicine-induced guanosine triphosphatase activity of tubulin, indicating that its binding to tubulin induces a conformational change in the tubulin. Competition experiments with vinblastine suggest that the binding site for crytophycin-52 may overlap with the vinblastine binding site.

Paclitaxel induces release of cytochrome c from mitochondria isolated from human neuroblastoma cells'

Paclitaxel is an antimicrotubule agent that induces mitotic block and apoptosis. We show for the first time that paclitaxel acts directly or mitochondria isolated from human cancer cells. In isolated yeast mito chondria, paclitaxel (15 microM) induced an 18% increase in the respiration rate, with no concomitant release of cytochrome c. In isolated neuroblas toma mitochondria, paclitaxel (10-100 microM) induced a 27-72% release o cytochrome c. Release was prevented by cyclosporin A, suggesting the involvement of the permeability transition pore. Doxorubicin did no induce cytochrome c release, whereas vinorelbine, another antimicrotu bule agent, did. Thus, antimicrotubule agents can directly affect mito chondria to induce apoptosis.

Novel actions of the antitumor drugs vinflunine and vinorelbine on microtubules

Vinflunine is a novel Vinca alkaloid presently in Phase I clinical trials. In preclinical studies, it exhibited superior antitumor activity to that of other Vinca alkaloids, including vinorelbine from which it was synthetically derived. Vinca alkaloids appear to inhibit cell proliferation by affecting the dynamics of spindle microtubules. Here we have analyzed the effects of vinflunine and vinorelbine on microtubule dynamic instability and treadmilling and found that these newer drugs exert effects on microtubule dynamics that differ significantly from those of the classic Vinca alkaloid, vinblastine. The major effects of vinflunine and vinorelbine on dynamic instability were a slowing of the microtubule growth rate, an increase in growth duration, and a reduction in shortening duration. In marked contrast to the action of vinblastine, they neither reduced the rate of shortening nor increased the percentage of time the microtubules spent in an attenuated state, neither growing nor shortening detectably. In addition, vinflunine and vinorelbine suppressed treadmilling, but less strongly than vinblastine. The diverse actions of these drugs on microtubules are likely to produce different effects on mitotic spindle function, leading to different effects on cell cycle progression and cell killing. Nontumor cells with normal checkpoint proteins may tolerate the relatively less powerful inhibitory effects of vinflunine and vinorelbine on microtubule dynamics better than the more powerful effects of vinblastine. Thus the unique constellation of effects of vinflunine and vinorelbine on dynamic instability and treadmilling may contribute to their superior antitumor efficacies.

Linkage analysis of systemic lupus erythematosus induced in diabetes-prone nonobese diabetic mice by Mycobacterium bovis

Systemic lupus erythematosus induced by Mycobacterium bovis in diabetes-prone nonobese diabetic mice was mapped in a backcross to the BALB/c strain. The subphenotypes-hemolytic anemia, antinuclear autoantibodies, and glomerular immune complex deposition-did not cosegregate, and linkage analysis for each trait was performed independently. Hemolytic anemia mapped to two loci: Bah1 at the MHC on chromosome 17 and Bah2 on distal chromosome 16. Antinuclear autoantibodies mapped to three loci: Bana1 at the MHC on chromosome 17, Bana2 on chromosome 10, and Bana3 on distal chromosome 1. Glomerular immune complex deposition did not show significant linkage to any genomic region. Mapping of autoantibodies (Coombs' or antinuclear autoantibodies) identified two loci: Babs1 at the MHC and Babs2 on distal chromosome 1. It has previously been reported that genes conferring susceptibility to different autoimmune diseases map nonrandomly to defined regions of the genome. One possible explanation for this clustering is that some alleles at loci within these regions confer susceptibility to multiple autoimmune diseases-the "common gene" hypothesis. With the exception of the H2, this study failed to provide direct support for the common gene hypothesis, because the loci identified as conferring susceptibility to systemic lupus erythematosus did not colocalize with those previously implicated in diabetes. However, three of the four regions identified had been previously implicated in other autoimmune diseases.

Taxol suppresses dynamics of individual microtubules in living human tumor cells

Microtubules are intrinsically dynamic polymers, and their dynamics play a crucial role in mitotic spindle assembly, the mitotic checkpoint, and chromosome movement. We hypothesized that, in living cells, suppression of microtubule dynamics is responsible for the ability of taxol to inhibit mitotic progression and cell proliferation. Using quantitative fluorescence video microscopy, we examined the effects of taxol (30-100 nM) on the dynamics of individual microtubules in two living human tumor cell lines: Caov-3 ovarian adenocarcinoma cells and A-498 kidney carcinoma cells. Taxol accumulated more in Caov-3 cells than in A-498 cells. At equivalent intracellular taxol concentrations, dynamic instability was inhibited similarly in the two cell lines. Microtubule shortening rates were inhibited in Caov-3 cells and in A-498 cells by 32 and 26%, growing rates were inhibited by 24 and 18%, and dynamicity was inhibited by 31 and 63%, respectively. All mitotic spindles were abnormal, and many interphase cells became multinucleate (Caov-3, 30%; A-498, 58%). Taxol blocked cell cycle progress at the metaphase/anaphase transition and inhibited cell proliferation. The results indicate that suppression of microtubule dynamics by taxol deleteriously affects the ability of cancer cells to properly assemble a mitotic spindle, pass the metaphase/anaphase checkpoint, and produce progeny.

In vitro pharmacology of cryptophycin 52 (LY355703) in human tumor cell lines

PURPOSE

Cryptophycin 52 (LY355703) is a new member of the cryptophycin family of antitumor agents that is currently undergoing clinical evaluation for cancer chemotherapy. The mechanism of action of the cryptophycin class of compounds is associated with an action on microtubules. This report details the pharmacological profile of this new clinical compound in a panel of human tumor cell lines.

METHODS

Antiproliferative effects of cryptophycin 52 were measured indirectly by detection of the metabolic reduction of alamarBlue. Cytoxicity was assessed by enzymatic dye activation (calcein AM) combined with dye exclusion (ethidium homodimer) and by clonogenicity assay. Cell cycle effects were evaluated using flow cytometry and fluorescence microscopy.

RESULTS

Both antiproliferative and cytotoxic effects of cryptophycin 52 were concentration- and time-dependent. IC50 values for antiproliferative activity in both solid and hematologic tumor cell lines were in the low picomolar range, and without exception, were significantly below values for the antimitotic agents paclitaxel and vinblastine. Flow cytometry and microscopic examination of tumor cells treated with cryptophycin 52 indicated that they accumulated in the mitotic phase of the cell cycle. Cryptophycin 52 was tested for its sensitivity to multidrug-resistance in several paired cell lines in which a sensitive parental line was matched with a multidrug-resistant derivative line. The resistant lines have been shown to over express Pgp and/or MRP multidrug-resistance transport factors. Compared to other antimitotic agents (paclitaxel, vinblastine, vincristine), the potency of cryptophycin 52 was shown to be minimally affected in multidrug-resistant cells compared to their sensitive parental lines.

CONCLUSION

Cryptophycin 52 has potent antimitotic, antiproliferative and cytotoxic activity in in vitro human tumor cell models. It is significantly more potent and less sensitive to multidrug resistance mechanisms than other antimitotic antitumor agents currently used in cancer therapy. These characteristics may translate into therapeutic advantages for the clinical use of cryptophycin 52 in cancer chemotherapy.

Suppression of microtubule dynamics by binding of cemadotin to tubulin: possible mechanism for its antitumor action

Cemadotin (LU103793) (NSC D-669356) is a water-soluble synthetic analogue of dolastatin 15 that inhibits cell proliferation in vitro and the growth of human tumor xenografts. Cemadotin is in phase II clinical trials as a promising cancer chemotherapeutic agent. The drug blocks cells at mitosis. Its primary mode of action has been unclear but is believed to involve an action on microtubules. We have found that cemadotin binds to tubulin and strongly suppresses microtubule dynamics. Scatchard analysis of cemadotin binding to tubulin indicated that there are two affinity classes of cemadotin-binding sites with Kd values of 19.4 microM and 136 microM. Cemadotin did not inhibit the binding of vinblastine to tubulin, and, conversely, vinblastine did not inhibit the binding of cemadotin to tubulin. By quantitative video microscopy of individual microtubules, we found that cemadotin strongly suppressed dynamic instability of microtubules assembled to steady state using bovine brain tubulin devoid of microtubule-associated proteins. It reduced the rate and extent of growing and shortening, increased the rescue frequency, and increased the percentage of time the microtubules spent in an attenuated or paused state, neither growing nor shortening detectably. At the lowest effective cemadotin concentrations, dynamics were suppressed in the absence of significant microtubule depolymerization. The results suggest that cemadotin exerts its antitumor activity by suppressing spindle microtubule dynamics through a distinct molecular mechanism by binding at a novel site in tubulin.

Antiproliferative mechanism of action of cryptophycin-52: kinetic stabilization of microtubule dynamics by high-affinity binding to microtubule ends

Cryptophycin-52 (LY355703) is a new synthetic member of the cryptophycin family of antimitotic antitumor agents that is currently undergoing clinical evaluation. At high concentrations (>/=10 times the IC50), cryptophycin-52 blocked HeLa cell proliferation at mitosis by depolymerizing spindle microtubules and disrupting chromosome organization. However, low concentrations of cryptophycin-52 inhibited cell proliferation at mitosis (IC50 = 11 pM) without significantly altering spindle microtubule mass or organization. Cryptophycin-52 appears to be the most potent suppressor of microtubule dynamics found thus far. It suppressed the dynamic instability behavior of individual microtubules in vitro (IC50 = 20 nM), reducing the rate and extent of shortening and growing without significantly reducing polymer mass or mean microtubule length. Using [3H]cryptophycin-52, we found that the compound bound to microtubule ends in vitro with high affinity (Kd, 47 nM, maximum of approximately 19.5 cryptophycin-52 molecules per microtubule). By analyzing the effects of cryptophycin-52 on dynamics in relation to its binding to microtubules, we determined that approximately 5-6 molecules of cryptophycin-52 bound to a microtubule were sufficient to decrease dynamicity by 50%. Cryptophycin-52 became concentrated in cells 730-fold, and the resulting intracellular cryptophycin-52 concentration was similar to that required to stabilize microtubule dynamics in vitro. The data suggest that cryptophycin-52 potently perturbs kinetic events at microtubule ends that are required for microtubule function during mitosis and that it acts by forming a reversible cryptophycin-52-tubulin stabilizing cap at microtubule ends.

Low potency of taxol at microtubule minus ends: implications for its antimitotic and therapeutic mechanism

In many cells, low concentrations of Taxol potently block mitosis at the transition from metaphase to anaphase, with no change in microtubule polymer mass and no microtubule bundling. Mitotic block ultimately results in apoptotic cell death and appears to be the most potent antitumor mechanism of Taxol (M. A. Jordan et al., Cancer Res. 56: 816-825, 1996). Mitotic inhibition results, at least in part, from stabilization of growing and shortening dynamics, specifically at the plus ends of microtubules, by the binding of very few Taxol molecules to the microtubule surface (M. A. Jordan et al., Proc. Natl. Acad. Sci. USA, 90: 9552-9556, 1993; W. B. Derry et al., Biochemistry, 34: 2203-2211, 1995). A number of actions of Taxol on mitotic spindle function may be due to its effects on microtubule dynamics at the minus ends of microtubules, effects that previously have not been described. Here, we determined the effects of Taxol on minus ends of purified microtubules at steady state. In contrast to the strong stabilizing effects on plus ends, substoichiometric ratios of Taxol bound to tubulin in microtubules did not affect growing, shortening, or dynamicity at minus ends. Thus, in blocked mitotic cells, Taxol can potently suppress dynamics at plus ends of spindle microtubules, whereas its impotence at minus ends permits continued microtubule depolymerization at the spindle poles. Differential effects of Taxol at opposite microtubule ends may explain Taxol's actions on spindle structure and function and its unique potent antitumor action.

Microtubules and actin filaments: dynamic targets for cancer chemotherapy

Microtubules and actin filaments play important roles in mitosis, cell signaling, and motility. Thus these cytoskeletal filaments are the targets of a growing number of anti-cancer drugs. In this review we summarize the current understanding of the mechanisms of these drugs in relation to microtubule and actin filament polymerization and dynamics. In addition, we outline how, by targeting microtubules, drugs inhibit cell proliferation by blocking mitosis at the mitotic checkpoint and inducing apoptosis. The beta-tubulin isotype specificities of new anticancer drugs and the antitumor potential of agents that act on the actin cytoskeleton are also discussed.

Trisomy 8 mosaicism: a further five cases illustrating marked clinical and cytogenetic variability

Trisomy 8 mosaicism is extremely variable in its phenotypic and cytogenic expression. We present five patients clearly demonstrating the lack of correlation between clinical and laboratory findings, and show that the aneuploid cell lines decreases with time in relation to the normal. This poses a counseling dilemma.

Mechanism of action of the unusually potent microtubule inhibitor cryptophycin 1

Cryptophycin 1 is a remarkably potent antiproliferative compound that shows excellent antitumor activity against mammary, colon, and pancreatic adenocarcinomas in mouse xenographs. At picomolar concentrations, cryptophycin 1 blocks cells in the G2/M phase of the cell cycle by an apparent action on microtubules. The compound binds to tubulin, inhibits microtubule polymerization, and depolymerizes preformed microtubules in vitro. Its exceptionally powerful antitumor activity (many-fold greater than paclitaxel or the vinca alkaloids) raises important questions about its mechanism of action. By quantitative video microscopy, we examined the effects of cryptophycin 1 on the dynamics of individual microtubules assembled to steady state from bovine brain tubulin. At low nanomolar concentrations, in the absence of net microtubule depolymerization, cryptophycin 1 potently stabilized microtubule dynamics. It reduced the rate and extent of microtubule shortening and growing and increased the frequency of rescue. The results suggest that cryptophycin 1 exerts its antiproliferative and antimitotic activity by binding reversibly and with high affinity to the ends of microtubules, perhaps in the form of a tubulin-cryptophycin 1 complex, resulting in the most potent suppression of microtubule dynamics yet described.

Inhibition of mitosis and microtubule function through direct tubulin binding by a novel antiproliferative naphthopyran LY290181

The mechanism of action of a novel antiproliferative compound LY290181 [2-amino-4-(3-pyridyl)-4H-naphtho(1,2-b)pyran-3-carbonitrile] was characterized. LY290181 is a potent inhibitor of cell proliferation, producing 50% inhibition of vascular smooth muscle, endothelial, Chinese hamster ovary, HeLa, and human erythroleukemia cells at concentrations of 8-40 nM. Cell cycle analysis showed that LY290181 caused accumulation of smooth muscle cells at the G2/M phase and induced mitotic arrest in Chinese hamster ovary cells and HeLa cells. At low concentrations (3-30 nM), LY290181 blocked transition of cells from metaphase to anaphase and disrupted mitotic spindle organization. At high concentrations (>/=100 nM), LY290181 produced a concentration-dependent loss of cytoplasmic and spindle microtubules. LY290181 inhibited the polymerization of purified bovine brain microtubule protein into microtubules, and it depolymerized preformed microtubules. Using tubulin-1-anilino-8-naphthalene sulfonate complex fluorescence, we have shown that LY290181 directly interacted with tubulin in a unique manner. These studies show that LY290181 induces cell growth arrest in prometaphase/metaphase, and tubulin appears to be its molecular target.

Taxol differentially modulates the dynamics of microtubules assembled from unfractionated and purified beta-tubulin isotypes

Substoichiometric binding of taxol to tubulin in microtubules potently suppresses microtubule dynamics, which appears to be the most sensitive antiproliferative mechanism of taxol. To determine whether the beta-tubulin isotype composition of a microtubule can modulate sensitivity to taxol, we measured the effects of substoichiometric ratios of taxol bound to tubulin in microtubules on the dynamics of microtubules composed of purified alphabeta(II)-, alphabeta(III)-, or alphabeta(IV)-tubulin isotypes and compared the results with the effects of taxol on microtubules assembled from unfractionated tubulin. Substoichiometric ratios of bound taxol in microtubules assembled from purified beta-tubulin isotypes or unfractionated tubulin potently suppressed the shortening rates and the lengths shortened per shortening event. Correlation of the suppression of the shortening rate with the stoichiometry of bound taxol revealed that microtubules composed of purified alphabeta(II)-, alphabeta(III)-, and alphabeta(IV)-tubulin were, respectively, 1.6-, 7.4-, and 7.2-fold less sensitive to the effects of bound taxol than microtubules assembled from unfractionated tubulin. These results indicate that taxol differentially modulates microtubule dynamics depending upon the beta-tubulin isotype composition. The results are consistent with recent studies correlating taxol resistance in tumor cells with increased levels of beta(III0- and beta(IV)-tubulin expression and suggest that altered cellular expression of beta-tubulin isotypes can be an important mechanism by which tumor cells develop resistance to taxol.

Differential effects of vinblastine on polymerization and dynamics at opposite microtubule ends

We have characterized the effects of vinblastine on the growing and shortening dynamics at opposite ends of individual bovine brain microtubules at steady state in vitro by video microscopy. Vinblastine exerted strikingly different effects on the dynamics and polymer mass at the plus and minus ends of microtubules. At concentrations between 0.1 and 0.4 microM, the drug strongly depolymerized microtubules at minus ends, whereas it did not significantly depolymerize microtubules at plus ends. Vinblastine stabilized plus ends by suppressing the rate and extent of growth and shortening, decreasing the catastrophe frequency, and increasing the rescue frequency. In contrast, vinblastine destabilized minus ends by increasing the catastrophe frequency and decreasing the rescue frequency, whereas it had no effect on the rate or extent of growth or shortening. Thus, vinblastine moderately increased the overall dynamicity at minus ends while strongly suppressing dynamicity at plus ends. Both the kinetic destabilization of microtubules at minus ends and the stabilization at plus ends may contribute to the altered function of mitotic spindle microtubules of cells blocked in mitosis by low concentrations of vinblastine.

Acquired spinal cord injury in human fetuses with myelomeningocele

Experimental studies have shown that there is a potential to attempt in utero repair of myelomeningocele in human fetuses. To provide a better understanding of the pathology of these lesions we prospectively studied eight stillborn human fetuses with myelomeningocele autopsied at The Johns Hopkins Hospital. The intact vertebral column with surrounding structures was removed, processed as a single block, and prepared as serial histologic sections. Study of the slides showed in all cases that in the center of the myelomeningocele the vertebral arch was open, the arrangement of meninges was such that the dura mater was open and in continuity with the deep layers of the dermis, and the pia mater was open and in continuity with a layer consisting of the superficial dermis and the epidermis. These meningeal relationships created an abnormally configured arachnoid space containing cerebrospinal fluid ventral to the spinal cord, which rested on the open pia mater and was exposed on the dorsal aspect of the sac. At the level of the myelomeningocele the naked cord had undergone varying degrees of injury up to complete loss of neural tissue. Where ventral remnants of the cord remained it was evident that a large degree of normal development of the cord had occurred. In most instances it appeared that the injury or destruction of the dorsal spinal cord was recent and consistent with occurrence during delivery. The results of this study support the concept that in utero surgery could preserve and protect the exposed spinal cord in a myelomeningocele of a human fetus and thus could reduce the severity of the neurologic deficit at birth.

Mitotic block induced in HeLa cells by low concentrations of paclitaxel (Taxol) results in abnormal mitotic exit and apoptotic cell death

Paclitaxel at low concentrations (10 nM for 20 h) induces approximately 90% mitotic block at the metaphase/anaphase transition in HeLa cells, apparently by suppressing dynamics of spindle microtubules (M. A. Jordan et al., Proc. Natl. Acad. Sci. USA, 90: 9552-9556, 1993). It is not known, however, whether inhibition of mitosis by such low paclitaxel concentrations results in cell death. In the present work, we found that after removal of paclitaxel (10 nM-1 microM), blocked cells did not resume proliferation. Instead, cells exited mitosis abnormally within 24 h. They did not progress through anaphase or cytokinesis but entered an interphase-like state (chromatin decondensed, and an interphase-like microtubule array and nuclear membranes reformed). Many cells (> or = 55%) contained multiple nuclei. Additional DNA synthesis and polyploidy did not occur. DNA degradation into nucleosome-sized fragments characteristic of apoptosis began during drug incubation and increased after drug removal. Cells died within 48-72 h. Incubation with paclitaxel (10 nM for 20 h) resulted in high intracellular drug accumulation (8.3 microM) and little efflux after paclitaxel removal; intracellular retention of paclitaxel may contribute to its efficacy. The results support the hypothesis that the most potent chemotherapeutic mechanism of paclitaxel is kinetic stabilization of spindle microtubule dynamics.

Modulation of CENP-E organization at kinetochores by spindle microtubule attachment

CENP-E is a protein of the kinesin superfamily that appears as small paired globules at kinetochores of chromosomes in mammalian cells during prometaphase and metaphase of mitosis [Yen et al., 1992: Nature 359:536-539]. In the present study we found that a significant number of chromosomes during early prometaphase in HeLa cells (approximately 30%) were stained with a CENP-E antibody in the form of large C-shaped "collars" that partially encircled the chromosomes. The C-shaped CENP-E collars were present only transiently and were completely replaced by small paired globular forms prior to metaphase. Most chromosomes had persistent CENP-E collars in cells blocked at mitosis with a vinblastine concentration sufficient to prevent all microtubule formation. Attachment of newly formed microtubules to the kinetochores after removal of vinblastine resulted in loss of the collars and replacement with small paired globules. Similarly, a higher proportion of chromosomes isolated from vinblastine-treated cells contained CENP-E collars (73%), and the "capture" (i.e., attachment) of microtubules by the chromosomes resulted in conversion of the collars into small paired globules in vitro. Thus, the CENP-E collars form prior to microtubule attachment and disappear after attachment of the chromosomes to the spindle. The CENP-E collars may facilitate capture of microtubules by chromosomes during prometaphase.

Vinblastine suppresses dynamics of individual microtubules in living interphase cells

We have characterized the effects of vinblastine on the dynamic instability behavior of individual microtubules in living BS-C-1 cells microinjected with rhodamine-labeled tubulin and have found that at low concentrations (3-64 nM), vinblastine potently suppresses dynamic instability without causing net microtubule depolymerization. Vinblastine suppressed the rates of microtubule growth and shortening, and decreased the frequency of transitions from growth or pause to shortening, also called catastrophe. In vinblastine-treated cells, both the average duration of a pause (a state of attenuated dynamics where neither growth nor shortening could be detected) and the percentage of total time spent in pause were significantly increased. Vinblastine potently decreased dynamicity, a measure of the overall dynamic activity of microtubules, reducing this parameter by 75% at 32 nM. The present work, consistent with earlier in vitro studies, demonstrates that vinblastine kinetically caps the ends of microtubules in living cells and supports the hypothesis that the potent chemotherapeutic action of vinblastine as an antitumor drug is suppression of mitotic spindle microtubule dynamics. Further, the results indicate that molecules that bind to microtubule ends can regulate microtubule dynamic behavior in living cells and suggest that endogenous regulators of microtubule dynamics that work by similar mechanisms may exist in living cells.

Microtubule dynamics: taking aim at a moving target

Antitumor drugs of the vinca alkaloid and taxane classes function by suppressing the dynamics of microtubules in spindles, blocking cell division at metaphase. The drugs bind to various sites on the tubulin dimer and at different positions within the microtubule, suggesting that there are many unexplored targets for the design of novel drugs of this type.

Kinetic stabilization of microtubule dynamics at steady state in vitro by substoichiometric concentrations of tubulin-colchicine complex

We have analyzed the effects of tubulin-colchicine (TC)-complex on the dynamic instability behavior of bovine brain microtubules at steady state in vitro using video microscopy. Incorporation of low numbers of TC-complexes per microtubule strongly suppressed dynamics at the plus ends by reducing the rate and extent of growing and shortening and by increasing the time microtubules spent in an attenuated state, neither growing nor shortening detectably. In addition, TC-complex strongly suppressed the catastrophe frequency and increased the rescue frequency. At low concentrations (0.02-0.05 microM), TC-complex suppressed dynamics without reducing the polymer mass or the mean microtubule length. Such strong suppression of microtubule dynamics by low TC-complex concentrations in the absence of polymer mass changes demonstrates that microtubule dynamics are more sensitive to the actions of TC-complex than the polymer mass. Significant reduction of polymer mass occurred at relatively high TC-complex concentration (> 0.05 microM). However, the surviving microtubules were extremely stable. Thus, TC-complex stabilizes microtubules even though the microtubules can transiently depolymerize when TC-complex is added. The data also directly establish that kinetic suppression of dynamics by colchicine at low concentrations is effected by a low number of TC-complexes at the microtubule ends.

Mitotic HeLa cells contain a CENP-E-associated minus end-directed microtubule motor

A minus end-directed microtubule motor activity from extracts of HeLa cells blocked at prometaphase/metaphase of mitosis with vinblastine has been partially purified and characterized. The motor activity was eliminated by immunodepletion of Centromere binding protein E (CENP-E). The CENP-E-associated motor activity, which was not detectable in interphase cells, moved microtubules at mean rates of 0.46 micron/s at 37 degrees C and 0.24 micron/s at 25 degrees C. The motor activity co-purified with CENP-E through several purification procedures. Motor activity was clearly not due to dynein or to kinesin. The microtubule gliding rates of the CENP-E-associated motor were different from those of dynein and kinesin. In addition, the pattern of nucleotide substrate utilization by the CENP-E-associated motor and the sensitivity to inhibitors were different from those of dynein and kinesin. The CENP-E-associated motor had an apparent native molecular weight of 874,000 Da and estimated dimensions of 2 nm x 80 nm. This is the first demonstration of motor activity associated with CENP-E, strongly supporting the hypothesis that CENP-E may act as a minus end-directed microtubule motor during mitosis.

Substoichiometric binding of taxol suppresses microtubule dynamics

We have measured the effects of taxol (10 nM to 1 microM) on the growing and shortening dynamics at the ends of individual bovine brain microtubules in vitro and have correlated the effects both with the stoichiometry of taxol binding to tubulin in microtubules and with the changes in the microtubule polymer mass. The results indicate that taxol suppresses microtubule dynamic instability differently depending upon the stoichiometry of taxol binding to the microtubules. At the lowest effective concentrations (< or = 100 nM), substoichiometric binding of taxol to tubulin in microtubules (between 0.001 and 0.01 mol of bound taxol/mol of tubulin in microtubules) potently and selectively suppresses the rate and extent of shortening at plus ends in association with some increase (28% to 60%) in the mass of microtubule polymer. At intermediate taxol concentrations (between 100 nM and 1 microM), the binding of additional taxol molecules to the microtubules (between 0.01 and 0.1 mol of taxol bound/mol of tubulin in microtubules) inhibits both growing and shortening events at both microtubule ends with no additional increase in microtubule polymer mass. At high taxol concentrations and high taxol binding stoichiometries (> or = 1 microM taxol and > or = 0.1 mol of taxol bound/mol of tubulin in microtubules), microtubule mass increases sharply and dynamics is almost completely suppressed. The data support the hypothesis that binding of a molecule of taxol to a tubulin subunit in microtubules induces a conformational change in that subunit that strongly reduces its ability to dissociate when the subunit becomes exposed at the microtubule end.

Removal of the projection domain of microtubule-associated protein 2 alters its interaction with tubulin

Microtubule-associated proteins (MAPs) can promote microtubule assembly in vitro. One of these MAPs (MAP2) consists of a short promoter domain which binds to the microtubule and promotes assembly and a long projection domain which projects out from the microtubule and may interact with other cytoskeletal elements. We have previously shown that MAP2 and another MAP, tau, differ in their interactions with tubulin in that tau, but not MAP2, promotes extensive aggregation of tubulin into spiral clusters in the presence of vinblastine and that microtuubles formed with MAP2 are more resistant than those formed with tau to the antimitotic drug maytansine [Luduena, R. F., et al. (1984), J. Biol. Chem. 259, 12890-12898; Fellous, A., et al. (1985), Cancer Res. 45, 5004-5010]. Here we have used chymotryptic digestion to remove the projection domain of MAP2 and examined the interaction of the digested MAP2 (ctMAP2) with tubulin in the presence of vinblastine and maytansine. We have found that ctMAP2 behaves very much like tau, but not like undigested MAP2, in the presence of vinblastine, in that ctMAP2 causes tubulin to polymerize into large clusters of spirals. In contrast, microtubule assembly in the presence of ctMAP2 is much more resistant to maytansine inhibition than is assembly in the presence of tau or undigested MAP2. Our results suggest that the projection domain of MAP2 may play a role in the interaction of tubulin with MAP2 during microtubule assembly.

Mechanism of mitotic block and inhibition of cell proliferation by taxol at low concentrations

Taxol inhibited HeLa cell proliferation by inducing a sustained mitotic block at the metaphase/anaphase boundary. Half-maximal inhibition of cell proliferation occurred at 8 nM taxol, and mitosis was half-maximally blocked at 8 nM taxol. Inhibition of mitosis was associated with formation of an incomplete metaphase plate of chromosomes and an altered arrangement of spindle microtubules that strongly resembled the abnormal organization that occurs with low concentrations of vinblastine and other antimitotic compounds. No increase in microtubule polymer mass occurred below 10 nM taxol. The mass of microtubules increased half-maximally at 80 nM taxol and attained maximal levels (5 times normal) at 330 nM taxol. At submicromolar concentrations, taxol suppressed growing and shortening at the ends of microtubules reassembled in vitro from bovine brain tubulin in a manner that resembled suppression by vinblastine. Taxol was concentrated in HeLa cells several hundredfold to levels that were similar to those which suppressed dynamic instability in vitro. The results indicate that taxol shares a common antiproliferative mechanism with vinblastine. At its lowest effective concentrations, taxol appears to block mitosis by kinetically stabilizing spindle microtubules and not by changing the mass of polymerized microtubules.

Kinetic stabilization of microtubule dynamic instability in vitro by vinblastine

The antiproliferative action of vinblastine at low concentrations appears to result from modulation of the polymerization dynamics of spindle microtubules rather than from depolarization of the microtubules [Jordan, M. A., Thrower, D., & Wilson, L. (1991) Cancer Res. 51, 2212-2222; (1992) J. Cell. Sci. 102, 401-416]. In the present study, we used differential interference contrast video microscopy to analyze the effects of vinblastine on the growing and shortening dynamics (dynamic instability) of individual bovine brain microtubules in vitro. With microtubules which were either depleted of microtubule-associated proteins (MAPs) or rich in MAPs, low concentrations of vinblastine (0.2 microM-1 microM) suppressed the growing and shortening rates and increased the percentage of time that the microtubules spent a state of attenuated activity, neither growing nor shortening detectably. Vinblastine also suppressed the duration of microtubule growing and shortening, and increased the duration of the attenuated state, during which the microtubules neither grew nor shortened detectably. Consistent with previous data obtained using radiolabeled nucleotide exchange in microtubule suspensions [Jordan, M. A., & Wilson, L. (1990) Biochemistry 29, 2730-2739], vinblastine suppressed growing and shortening dynamics at the kinetically more rapid plus ends. The results suggest that vinblastine kinetically stabilizes microtubule ends by modulating the gain and loss of the stabilizing GTP or GDP-Pi "cap", which is believed to be responsible for the transitions between the growing and shortening phases. The data support the hypothesis that (1) low concentrations of vinblastine inhibit mitosis by kinetically stabilizing the polymerization dynamics of spindle microtubules and that (2) the dynamics of spindle microtubules are critical for the proper progression of mitosis.

Mitotic block in HeLa cells by vinblastine: ultrastructural changes in kinetochore-microtubule attachment and in centrosomes

Previous work from this laboratory has indicated that very low concentrations of vinblastine block HeLa cells at mitosis in the presence of a full complement of microtubules and without major disruption of spindle organization. In the present study we analyzed the structural organization of mitotic spindle microtubules, chromosomes and centrosomes by electron microscopy after incubating HeLa cells for one cell cycle with 2 nM vinblastine. We found that mitotic block of HeLa cells by vinblastine was associated with alterations of the fine structure of the spindle that were subtle but profound in their apparent consequences. The cell cycle was blocked in a stage that resembled prometaphase or metaphase; chromosomes had not undergone anaphase segregation. Neither the structure of the microtubules nor the structure of the kinetochores was detectably altered by the drug. However, the number of microtubules attached to kinetochores was decreased significantly. In addition, the centrosomes were altered; the normal close association of mother and daughter centriole was lost, numerous membranous vesicles were found in the centrosomal region, and many centrioles exhibited abnormal ultrastructure and had microtubules coursing through their interiors. These findings are consistent with our previous results and indicate that inhibition of the polymerization dynamics of mitotic spindle microtubules and perhaps of centriole microtubules, rather than microtubule depolymerization, is responsible for the mitotic inhibition by vinblastine.

Temperature sensitivity of vinblastine-induced tubulin polymerization in the presence of microtubule-associated proteins

The antitumor drug vinblastine has been a useful probe for examining the interaction of tubulin with the microtubule-associated proteins (MAPs), specifically with tau and MAP 2. Although tau and MAP 2 can stimulate microtubule assembly in vitro, their specific interactions with tubulin are known to differ. For example, in the presence of vinblastine, both tau and MAP 2 cause tubulin to form spirals, but tau causes formation of clustered spirals of high turbidity, while MAP 2 causes formation of loose spirals of low turbidity [Ludueña et al., J. Biol. Chem. 259, 12890-12898 (1984)]. Although cold temperatures can inhibit microtubule assembly, cold has no effect on vinblastine-induced tubulin spiral formation. Consequently, we used the vinblastine-tubulin system to examine the interactions of tau and MAP 2 with tubulin at low temperatures. We found that tau-tubulin-vinblastine complexes form about as well at 0 degree C as at 37 degrees C. In contrast, MAP 2-tubulin-vinblastine complexes form much less well at 0 degrees C than at 37 degrees C. We find, however, that MAP 2, at 0 degrees C, will strongly inhibit, and even reverse, formation of the tau-tubulin-vinblastine complex. This suggests that the temperature-sensitive factor is the MAP 2-stimulated tubulin-tubulin interaction rather than the MAP 2-tubulin interaction per se; this raises the possibility that the tubulin-tubulin interactions stimulated by tau differ in their temperature sensitivity from those stimulated by MAP 2.

Effects of vinblastine, podophyllotoxin and nocodazole on mitotic spindles. Implications for the role of microtubule dynamics in mitosis

Inhibition of mitosis by many drugs that bind to tubulin has been attributed to depolymerization of microtubules. However, we found previously that low concentrations of vinblastine and vincristine blocked mitosis in HeLa cells with little or no depolymerization of spindle microtubules, and spindles appeared morphologically normal or nearly normal. In the present study, we characterized the effects of vinblastine, podophyllotoxin and nocodazole over broad concentration ranges on mitotic spindle organization in HeLa cells. These three drugs are known to affect the dynamics of microtubule polymerization in vitro and to depolymerize microtubules in cells. We wanted to probe further whether mitotic inhibition by these drugs is brought about by a more subtle effect on the microtubules than net microtubule depolymerization. We compared the effects of vinblastine, podophyllotoxin and nocodazole on the organization of spindle microtubules, chromosomes and centrosomes, and on the total mass of microtubules. Spindle organization was examined by immunofluorescence microscopy, and microtubule polymer mass was assayed on isolated cytoskeletons by a quantitative enzyme-linked immunoadsorbence assay for tubulin. As the drug concentration was increased, the organization of mitotic spindles changed in the same way with all three drugs. The changes were associated with mitotic arrest, but were not necessarily accompanied by net microtubule depolymerization. With podophyllotoxin, mitotic arrest was accompanied by microtubule depolymerization. In contrast, with vinblastine and nocodazole, mitotic arrest occurred in the presence of a full complement of spindle microtubules. All three drugs induced a nearly identical rearrangement of spindle microtubules, an increasingly aberrant organization of metaphase chromosomes, and fragmentation of centrosomes. The data suggest that these anti-mitotic drugs block mitosis primarily by inhibiting the dynamics of spindle microtubules rather than by simply depolymerizing the microtubules.