Fast disassembly of microtubules induced by Mg2+ or Ca2+

Electrodynamic interaction between tumor treating fields and microtubule electrophysiological activities

Tumor treating fields (TTFields) are a type of sinusoidal alternating current electric field that has proven effective in inhibiting the reproduction of dividing tumor cells. Despite their recognized impact, the precise biophysical mechanisms underlying the unique effects of TTFields remain unknown. Many of the previous studies predominantly attribute the inhibitory effects of TTFields to mitotic disruption, with intracellular microtubules identified as crucial targets. However, this conceptual framework lacks substantiation at the mesoscopic level. This study addresses the existing gap by constructing force models for tubulin and other key subcellular structures involved in microtubule electrophysiological activities under TTFields exposure. The primary objective is to explore whether the electric force or torque exerted by TTFields significantly influences the normal structure and activities of microtubules. Initially, we examine the potential effect on the dynamic stability of microtubule structures by calculating the electric field torque on the tubulin dimer orientation. Furthermore, given the importance of electrostatics in microtubule-associated activities, such as chromosome segregation and substance transport of kinesin during mitosis, we investigate the interaction between TTFields and these electrostatic processes. Our data show that the electrodynamic effects of TTFields are most likely too weak to disrupt normal microtubule electrophysiological activities significantly. Consequently, we posit that the observed cytoskeleton destruction in mitosis is more likely attributable to non-mechanical mechanisms.

Effects of exposure to alternating low-intensity, intermediate-frequency electric fields on the differentiation of human leukemic cell line U937

Studying the bioeffects of electric fields have been the subject of ongoing research which led to promising therapeutic effect, particularly in cancer treatment. Here, we investigated the impact of low-intensity, intermediate-frequency alternating electric fields on the differentiation of human myeloid leukemia cell line U937. The results showed a near twofold increase in differentiation of U937 cells treated for 24 h by alternating 600 kHz, 150 V/m electric fields, in comparison to their control groups. This measure was evaluated by latex bead phagocytosis assay, nitro blue tetrazolium test, and cell cycle analysis which revealed a significant shift in the number of cells from G2 +M to G0 +G1 phases. The simulation result for the intracellular field intensity showed around 50% attenuation with respect to the applied external field for our setup which ruled out masking of the applied field by the internal electric noise of the cell. Based on previous studies we postulate a possible calcium-related effect for the observed differentiation, yet the exact underlying mechanism requires further investigation. Finally, our results may offer a potential therapeutic method for leukemia in the future.

A review of tumor treating fields (TTFields): advancements in clinical applications and mechanistic insights

BACKGROUND

Tumor Treating Fields (TTFields) is a non-invasive modality for cancer treatment that utilizes a specific sinusoidal electric field ranging from 100 kHz to 300 kHz, with an intensity of 1 V/cm to 3 V/cm. Its purpose is to inhibit cancer cell proliferation and induce cell death. Despite promising outcomes from clinical trials, TTFields have received FDA approval for the treatment of glioblastoma multiforme (GBM) and malignant pleural mesothelioma (MPM). Nevertheless, global acceptance of TTFields remains limited. To enhance its clinical application in other types of cancer and gain a better understanding of its mechanisms of action, this review aims to summarize the current research status by examining existing literature on TTFields' clinical trials and mechanism studies.

CONCLUSIONS

Through this comprehensive review, we seek to stimulate novel ideas and provide physicians, patients, and researchers with a better comprehension of the development of TTFields and its potential applications in cancer treatment.

The Mechanisms of Action of Tumor Treating Fields

Tumor treating fields (TTFields), a new modality of cancer treatment, are electric fields transmitted transdermally to tumors. The FDA has approved TTFields for the treatment of glioblastoma multiforme and mesothelioma, and they are currently under study in many other cancer types. While antimitotic effects were the first recognized biological anticancer activity of TTFields, data have shown that tumor treating fields achieve their anticancer effects through multiple mechanisms of action. TTFields therefore have the ability to be useful for many cancer types in combination with many different treatment modalities. Here, we review the current understanding of TTFields and their mechanisms of action.

Metal cation responsive anionic microgels: behaviour towards biologically relevant divalent and trivalent ions

Anionic poly(vinylcaprolactam-co-itaconicacid-co-dimethylitaconate) microgels were synthesized via dispersion polymerization and their responsiveness towards cations, namely Mg²⁺, Sr²⁺, Cu²⁺ and Fe³⁺, was investigated. The itaconic moieties chelate the metal ions which act as a crosslinker and decrease the electrostatic repulsion within the network, leading to a decrease in the gel size. The responsiveness towards the metal ion concentration has been studied via dynamic light scattering (DLS) and the number of ions bonded within the network has been quantified with ion chromatography. Through the protonation of the carboxylate groups in the gel network, their interaction with the cations is significantly lowered, and the metals are consequently released back in solution. The number of ions released was assessed also via ion chromatography for all four ions, whilst Mg²⁺ was also used as a model ion to display the reversibility of the system. The microgels can bond and release divalent cations over multiple cycles without undergoing any loss of functionality. Moreover, these gels also selectively entrap Fe³⁺ with respect to the remaining divalent cations, opening the possibility of using the proposed gels in the digestive tract as biocompatible chelating agents to fight iron overaccumulation.

A Theoretical Study on the Biophysical Mechanisms by Which Tumor Treating Fields Affect Tumor Cells During Mitosis

OBJECTIVE

A theoretical study on the mechanisms through which Tumor Treating Fields (TTFields) affect dividing tumor cells.

METHODS

Numerical analysis was used to revisit two previously proposed mechanisms and introduce a third. We examine the previous hypotheses that: a) TTFields generate a moment that affects microtubule assembly during early mitosis, and b) dielectrophoretic (DEP) forces cause neutral particles to move toward the cleavage furrow during the telophase stage. We further introduce a new hypothesis that TTFields modify cell membrane potential in dividing tumor cells.

RESULTS

a) The Brownian energy is several orders of magnitude larger than the moment induced by TTFields on tubulin dimers. b) Adding Stokes drag forces to DEP forces shows that the motion of the particles in the cytoplasm is very slow, approximately 0.003 µm/s, and therefore, unless the duration of the telophase is long enough there will be no substantial effect from the DEP forces. c) The Schwan equation shows that electric fields at the frequencies of clinical TTFields can cause a 10%-17% change in tumor cell membrane potential.

CONCLUSION

Our studies find limited support for the previously suggested hypotheses and suggest that the TTFields affect ion channels by inducing cell membrane potential change could be a mechanism of tumor cell death.

SIGNIFICANCE

Previously suggested mechanisms of tumor cell death from TTFields are found lacking. The effect of TTFields on the tumor cell membrane potential warrants further research.

A unified model for microtubule rescue

How microtubules transition from depolymerization to polymerization, known as rescue, is poorly understood. Here we examine two models for rescue: 1) an "end-driven" model in which the depolymerizing end stochastically switches to a stable state; and 2) a "lattice-driven" model in which rescue sites are integrated into the microtubule before depolymerization. We test these models using a combination of computational simulations and in vitro experiments with purified tubulin. Our findings support the "lattice-driven" model by identifying repeated rescue sites in microtubules. In addition, we discover an important role for divalent cations in determining the frequency and location of rescue sites. We use "wash-in" experiments to show that divalent cations inhibit rescue during depolymerization, but not during polymerization. We propose a unified model in which rescues are driven by embedded rescue sites in microtubules, but the activity of these sites is influenced by changes in the depolymerizing ends.

Detailed Per-residue Energetic Analysis Explains the Driving Force for Microtubule Disassembly

Microtubules are long filamentous hollow cylinders whose surfaces form lattice structures of αβ-tubulin heterodimers. They perform multiple physiological roles in eukaryotic cells and are targets for therapeutic interventions. In our study, we carried out all-atom molecular dynamics simulations for arbitrarily long microtubules that have either GDP or GTP molecules in the E-site of β-tubulin. A detailed energy balance of the MM/GBSA inter-dimer interaction energy per residue contributing to the overall lateral and longitudinal structural stability was performed. The obtained results identified the key residues and tubulin domains according to their energetic contributions. They also identified the molecular forces that drive microtubule disassembly. At the tip of the plus end of the microtubule, the uneven distribution of longitudinal interaction energies within a protofilament generates a torque that bends tubulin outwardly with respect to the cylinder's axis causing disassembly. In the presence of GTP, this torque is opposed by lateral interactions that prevent outward curling, thus stabilizing the whole microtubule. Once GTP hydrolysis reaches the tip of the microtubule (lateral cap), lateral interactions become much weaker, allowing tubulin dimers to bend outwards, causing disassembly. The role of magnesium in the process of outward curling has also been demonstrated. This study also showed that the microtubule seam is the most energetically labile inter-dimer interface and could serve as a trigger point for disassembly. Based on a detailed balance of the energetic contributions per amino acid residue in the microtubule, numerous other analyses could be performed to give additional insights into the properties of microtubule dynamic instability.

How calcium causes microtubule depolymerization

The effects of calcium (Ca) were assessed using video-enhanced differential interference contrast light microscopy on individual microtubules in vitro. Phosphocellulose-purified (PC) and microtubule associated protein (MAP)-containing preparations of porcine brain tubulin were assembled in a flow chamber onto sperm axoneme fragments and the pattern of growth and shortening of the microtubules was observed. Tubulin plus Ca was then added to the chamber and observation continued. Ca promoted the disassembly of microtubules by specifically promoting the catastrophe reaction in both PC- and MAP-containing microtubules, without an appreciable change in elongation rate. The effect on catastrophe frequency increased very rapidly above 0.5 mM free Ca, implying a possible cooperative effect. The rescue rate remained very high after Ca addition in MAP-containing microtubules, and the shortening rate was unchanged, while in phosphocellulose-purified microtubules, rescue appeared to be decreased by Ca addition and shortening rates increased 4 to 6-fold. These results illustrate that Ca can directly destabilize growing microtubule ends without changing the effective concentration of free tubulin, and that this effect can be seen even against the background of the profound differences in dynamics conferred by the microtubule-associated proteins. Considered within models of the GTP cap, the results imply that high Ca may act to increase the rate of GTP hydrolysis within the cap.

Evidence for a direct conversion between two tubulin polymers--microtubules and helical filaments--in the foraminiferan, Allogromia laticollaris

In Allogromia, tubulin lattices transform between microtubule and helical filament states. Helical filaments are composed of approximately 10-nm-thick tubulin filaments wound into approximately 30-nm diameter coils. The transition pathway between these two lattice states was examined in vitro in detergent-lysed pseudopods. Microtubules represented the majority of the assembled tubulin polymers in the detergent extracted pseudopodia. However, microtubules transformed into helical filaments upon exposure to 10 mM Ca2+ or 50 mM Mg2+. The conversion of microtubules into helical filaments involved fragmentation of the tubulin lattice and reduction in total polymer length. Divalent cations were required for the maintenance of the helical filament state; their removal resulted in the loss of helical filaments and the re-formation of microtubules. The data support a direct transition model in which the tubulin lattice interconverts between the helical filament and microtubule states, independently of tubulin subunit concentration. We propose a structural model for the direct pathway whereby disruption of longitudinal bonds between tandem tubulin dimers in protofilaments causes the microtubule lattice to unwind into helical filaments--continuous ribbons of laterally connected tubulin dimers--from the microtubule end. Helical filaments may rewind into microtubules as longitudinal intersubunit bonds reform.

Quantitative measurement of the catastrophe rate of dynamic microtubules

Previous work has shown that catastrophe frequency is the predominant dynamic parameter of microtubules that changes dramatically during the cell cycle. As an alternative to videomicroscopy assays, we have developed a biochemical assay to measure directly the average catastrophe rate of a population of microtubules. In this assay, the growing plus end of the microtubules, polymerized off seeds, are labeled with a brief pulse of alpha-32P-GTP, followed by a cold GTP chase. The rate of loss of 32P label in microtubules measured by this method is equal to the catastrophe frequency at microtubule plus ends measured by videomicroscopy of individual microtubules. Addition of mitotic extract from Xenopus eggs increases the catastrophe rate of purified tubulin by almost 100-fold, while interphase extract alters the catastrophe rate by about 20-fold as compared to pure tubulin. Most of the catastrophe-promoting activities in both mitotic and interphase extracts is found in particulate fractions. High-speed centrifugation of extracts appears to eliminate the components required for increasing microtubule catastrophe, but does not eliminate the cell cycle difference in microtubule dynamics. This assay provides a new approach to quantitate microtubule catastrophe rates. It will be of particular interest to search for catastrophe factors associated with intracellular membranes or other insoluble components.

Characterization of neuroprotection from excitotoxicity by moderate and profound hypothermia in cultured cortical neurons unmasks a temperature-insensitive component of glutamate neurotoxicity

Although profound hypothermia has been used for decades to protect the human brain from hypoxic or ischemic insults, little is known about the underlying mechanism. We therefore report the first characterization of the effects of moderate (30 degrees C) and profound hypothermia (12 degrees to 20 degrees C) on excitotoxicity in cultured cortical neurons exposed to excitatory amino acids (EAA; glutamate, N-methyl-D-aspartate [NMDA], AMPA, or kainate) at different temperatures (12 degrees to 37 degrees C). Cooling neurons to 30 degrees C and 20 degrees C was neuroprotective, but cooling to 12 degrees C was toxic. The extent of protection depended on the temperature, the EAA receptor agonist employed, and the duration of the EAA challenge. Neurons challenged briefly (5 minutes) with all EAA were protected, as were neurons challenged for 60 minutes with NMDA, AMPA, or kainate. The protective effects of hypothermia (20 degrees and 30 degrees C) persisted after rewarming to 37 degrees C, but rewarming from 12 degrees C was deleterious. Surprisingly, however, prolonged (60 minutes) exposures to glutamate unmasked a temperature-insensitive component of glutamate neurotoxicity that was not seen with the other, synthetic EAA; this component was still mediated via NMDA receptors, not by ionotropic or metabotropic non-NMDA receptors. The temperature-insensitivity of glutamate toxicity was not explained by effects of hypothermia on EAA-evoked [Ca2+]i increases measured using high- and low-affinity Ca2+ indicators, nor by effects on mitochondrial production of reactive oxygen species. This first characterization of excitotoxicity at profoundly hypothermic temperatures reveals a previously unnoticed feature of glutamate neurotoxicity unseen with the other EAA, and also suggests that hypothermia protects the brain at the level of neurons by blocking, rather than slowing, excitotoxicity.

How tubulin subunits are lost from the shortening ends of microtubules

Microtubules exhibit dynamic instability, switching between persistent states of growth and shortening at their ends. The switch between growth and shortening has been proposed to depend on end conformation where growing ends have "straight" tubulin protofilaments stabilized by a terminal cap of GTP-tubulin, while-shortening ends have lost their GTP-tubulin cap, allowing terminal GDP-tubulin dimers to curve inside-out and peel rapidly away from the microtubule lattice. This "conformational cap" model predicts that tubulin dissociation from shortening ends is a two-step process where the average lengths of curved GDP-tubulin protofilaments at a depolymerizing end will depend on the ratio of the rate of peeling to the rate of breakage of the longitudinal bonds between adjacent curved dimers. We have tested this model for the plus and minus ends of microtubules assembled with pure porcine tubulin off the ends of axoneme fragments in standard assembly buffer. Individual microtubule ends were imaged using video-enhanced differential interference contrast light microscopy. The rate of rapid shortening was systematically increased by isothermal dilution into assembly buffer containing various concentrations of Mg2+ or Ca2+ ions. At 1 mM Mg2+ and no Ca2+, shortening occurred at 20 (plus) and 45 (minus) microns/min. The ends appeared similar in contrast to growing ends and the core of the microtubule and the ends appeared blunt or slightly frayed by negative stain electron microscopy. Above 20 mM Mg2+ or above 5 mM Ca2+, microtubule shortening occurred at 60 (plus) and 115 (minus) microns/min or faster and "knobs" were distinctly visible at depolymerizing ends, particularly at the faster minus ends, and knob contrast remained constant during many micrometers of rapid shortening. Negative stain electron microscopy revealed that these knobs were "blossoms" of inside-out curved protofilaments, some extending for several helical turns (30 to 60 dimers in length) at constant curvature from the ends. At these high shortening velocities, the peeling of curved protofilaments was confined to within several dimers of the end of the microtubule cylinder, suggesting that dimer curling and protofilament peeling is constrained to the tip by interactions between adjacent straight protofilaments. Depolymerization is produced by conformational changes in GDP-tubulin since microtubules assembled with a slowly hydrolizable analog of GTP, GMPCPP, are stable even at 20 mM Mg2+ or 5 mM Ca2+. Monte Carlo simulations show that the ratio of the peeling to breakage rate constants can control the steady-state average length of curved GDP-tubulin protofilaments at the depolymerizing end.

In vivo microtubule dynamics during experimentally induced conversions between tubulin assembly states in Allogromia laticollaris

A distinctive property of foraminiferan tubulin is that, in addition to microtubules (MTs), it exists in an alternate assembly state, helical filaments. Here, we have examined in vivo MT dynamics during experimentally induced conversions between these two assembly states in the reticulopods of the marine foraminiferan Allogromia laticollaris. Exposure to high extracellular concentrations of Mg2+ (165 mM) resulted in a complete conversion of MTs into helical filaments. However, Mg2+ treatment also induced a retrograde movement of organelles and cytoplasm, and it was necessary to inhibit this response in order to assess the effects of assembly state changes on individual MTs. This was accomplished by simultaneous treatment with high extracellular Mg2+ and 2,4-dinitrophenol (DNP). The resulting loss in MTs was detected by video enhanced DIC (VEC-DIC) microscopy as either an endwise MT shortening (at an average rate of 474 microns/min) or transformation into one or more irregularly shaped fibrils, which we termed residual fibrils. Correlative immunofluorescence and video microscopy showed residual fibrils to be composed of helical filaments. Removal of extracellular Mg2+/DNP initiated a reversal in assembly state, from helical filaments into MTs, which was completed within 5 min. VEC-DIC microscopy showed that MTs reformed by an endwise lengthening at an average rate of 216 microns/min. These results suggest that conversion between alternate tubulin assembly states provides a more rapid means to build and dismantle MTs than conventional subunit-driven pathways.

Dynamic instability of microtubules: Monte Carlo simulation and application to different types of microtubule lattice

Dynamic instability is the term used to describe the transition of an individual microtubule, apparently at random, between extended periods of slow growth and brief periods of rapid shortening. The typical sawtooth growth and shortening transition behavior has been successfully simulated numerically for the 13-protofilament microtubule A-lattice by a lateral cap model (Bayley, P. M., M. J. Schilstra, and S. R. Martin. 1990. J. Cell Sci. 95:33-48). This kinetic model is now extended systematically to other related lattice geometries, namely the 13-protofilament B-lattice and the 14-protofilament A-lattice, which contain structural "seams". The treatment requires the assignment of the free energies of specific protein-protein interactions in terms of the basic microtubule lattice. It is seen that dynamic instability is not restricted to the helically symmetric 13-protofilament A-lattice but is potentially a feature of all A- and B-lattices, irrespective of protofilament number. The advantages of this general energetic approach are that it allows a consistent treatment to be made for both ends of any microtubule lattice. Important features are the predominance of longitudinal interactions between tubulin molecules within the same protofilament and the implication of a relatively favorable interaction of tubulin-GDP with the growing microtubule end. For the three lattices specifically considered, the treatment predicts the dependence of the transition behavior upon tubulin concentration as a cooperative process, in good agreement with recent experimental observations. The model rationalizes the dynamic properties in terms of a metastable microtubule lattice of tubulin-GDP, stabilized by the kinetic process of tubulin-GTP addition. It provides a quantitative basis for the consideration of in vitro microtubule behaviour under both steady-state and non-steady-state conditions, for comparison with experimental data on the dilution-induced disassembly of microtubules. Similarly, the effects of small tubulin-binding molecules such as GDP and nonhydrolyzable GTP analogues are readily treated. An extension of the model allows a detailed quantitative examination of possible modes of substoichiometric action of a number of antimitotic drugs relevant to cancer chemotherapy.

Buffer conditions and non-tubulin factors critically affect the microtubule dynamic instability of sea urchin egg tubulin

The dynamic instability of individual microtubules (Mts) in cytoplasmic extracts or assembled from highly purified sea urchin egg tubulin was examined using video-enhanced, differential-interference contrast (VE-DIC) light microscopy. Extract Mts (endogenous tubulin = 12.1 microM) displayed only plus-ended growth. The elongation velocity was 7.8 microns/min for an average duration of 1.3 min before switching (catastrophe) to rapid shortening, which occurred at 13.0 microns/min for an average duration of 0.5 min before switching (rescue) back to the elongation phase. These parameters are typical of interphase Mt dynamic instability. Surprisingly, Mts assembled from purified urchin egg tubulin in standard buffers were less dynamic that those reported for purified brain tubulin or Mts in the extract. Buffer parameters were changed in an attempt to mimic the extract Mt results. The pH buffer itself, Hepes or Pipes, drastically altered Mt dynamics but could not achieve high elongation velocity with high catastrophe frequencies. Calcium at 1 microM had negligible effects, while increasing pH from 6.9 to 7.2 stimulated elongation velocity. Finally, Mt dynamics of purified egg tubulin (11.9 microM) were assayed in ultrafiltrates (MW cut-off less than 30 kD) of the cytoplasmic extracts. Mts elongated slowly at 1.2 microns/min for 26 min before a catastrophe and rapid shortening at 11.8 microns/min. Rescue was less frequent than unfiltered extracts, minus-ended growth was observed, and self-assembly occurred at slightly higher tubulin concentrations. Therefore, the egg extracts and cytoplasm must contain non-buffer factors which stimulate elongation velocity by 6.5-fold without self-assembly, increase catastrophe frequency by 20-fold, and block minus-ended growth.

Selective stimulation of in situ intermediary metabolism by free calcium in permeabilized rat adipocytes

The hypothesis that ionized calcium [Ca2+]i may stimulate in situ rat adipocyte intermediary metabolism distal to glucose transport was tested. A metabolically active porous adipocyte model was employed in which pathway metabolism is exclusively pore-dependent using glucose 6-phosphate (G6P) as substrate. Cellular [Ca2+]i was, furthermore, directly adjusted to between 0-2.5 microM via the membrane pores. Three metabolic fluxes were examined, (1) glycolysis-Krebs ([6-14C]G6P oxidation), (2) glycolysis to lactate ([U-14C]G6P to [14C]lactate) and (3) pentose pathway ([1-14C]G6P oxidation). Glycolysis-Krebs oxidation was was found to be selectively (33% above basal P less than 0.001) stimulated by 0.625 microM free calcium. In contrast, there was no effect of [Ca2+]i on the other, exclusively cytoplasmic, pathways. The stimulation of glycolysis-Krebs by [Ca2+]i was inhibited by a mitochondrial calcium channel blocker (Ruthenium red) and persisted over a range of ATP/ADP ratios. Separate studies demonstrated that 2-[1-14C]ketoglutarate oxidation was also calcium-stimulated in the porous adipocytes (160% over baseline at 1 microM [Ca2+]i). These studies thus demonstrate that physiologically relevant increments in porous adipocyte [Ca2+]i enhance overall in situ glycolytic-Krebs pathway oxidation by a mechanism which entails mitochondrial calcium uptake. Methodologically, this metabolically active porous adipocyte model presents a novel experimental approach to investigations regarding the effects of ionized calcium on intermediary metabolism beyond glucose transport.

Effects of antimitotic agents on tubulin-nucleotide interactions

The interaction of antimitotic drugs with guanine nucleotides in the tubulin-microtubule system is reviewed. Antimitotic agent-tubulin interactions can be covalent, entropic, allosteric or coupled to other equilibria (such as divalent cation binding, alternate polymer formation, or the stabilization of native tubulin structure). Antimitotics bind to tubulin at a few common sites and alter the ability of tubulin to form microtubules. Colchicine and podophyllotoxin compete for a common overlapping binding site but only colchicine induces GTPase activity and large conformational changes in the tubulin heterodimer. The vinca alkaloids, vinblastine and vincristine, the macrocyclic ansa macrolides, maytansine and ansamitocin P-3, and the fungal antimitotic, rhizoxin, share and compete for a different binding site near the exchangeable nucleotide binding site. The macrocyclic heptapeptide, phomopsin A, and the depsipeptide, dolastatin 10, bind to a site adjacent to the vinca alkaloid and nucleotide sites. Colchicine, vinca alkaloids, dolastatin 10 and phomopsin A induce alternate polymer formation (sheets for colchicine, spirals for vinblastine and vincristine and rings for dolastatin 10 and phomopsin A). Maytansine, ansamitocin P-3 and rhizoxin inhibit vinblastine-induced spiral formation. Taxol stoichiometrically induces microtubule formation and, in the presence of GTP, assembly-associated GTP hydrolysis. Analogs of guanine nucleotides also alter polymer morphology. Thus, sites on tubulin for drugs and nucleotides communicate allosterically with the interfaces that form longitudinal and lateral contacts within a microtubule. Microtubule associated proteins (MAPs), divalent cations, and buffer components can alter the surface interactions of tubulin and thus modulate the interactions between antimitotic drugs and guanine nucleotides.

Microtubule dynamics and microtubule caps: a time-resolved cryo-electron microscopy study

Microtubules display the unique property of dynamic instability characterized by phase changes between growth and shrinkage, even in constant environmental conditions. The phases can be synchronized, leading to bulk oscillations of microtubules. To study the structural basis of dynamic instability we have examined growing, shrinking, and oscillating microtubules by time-resolved cryo-EM. In particular we have addressed three questions which are currently a matter of debate: (a) What is the relationship between microtubules, tubulin subunits, and tubulin oligomers in microtubule dynamics?; (b) How do microtubules shrink? By release of subunits or via oligomers?; and (c) Is there a conformational change at microtubule ends during the transitions from growth to shrinkage and vice versa? The results show that (a) oscillating microtubules coexist with a substantial fraction of oligomers, even at a maximum of microtubule assembly; (b) microtubules disassemble primarily into oligomers; and (c) the ends of growing microtubules have straight protofilaments, shrinking microtubules have protofilaments coiled inside out. This is interpreted as a transition from a tense to a relaxed conformation which could be used to perform work, as suggested by some models of poleward chromosome movement during anaphase.

The effect of solution composition on microtubule dynamic instability

The exchange of tubulin dimer into steady-state microtubules was studied over a range of solution conditions, in order to assess the effects of various common buffer components on the dynamic instability of microtubules. In comparison with standard buffer conditions (100 mM-Pipes buffer, pH 6.5, containing 0.1 mM-EGTA, 1.8 mM-MgC12 and 1 M-glycerol), the rate and extent of exchange, and thus of dynamic instability, are suppressed by increasing the concentration of glycerol above 2 M. Exchange is enhanced by the addition of further Mg2+ (up to 17 mM) or by the addition of Ca2+ (up to 0.4 mM). Phosphate ion (150 mM) has relatively little effect on the dynamic behaviour of microtubules, as judged by the exchange method. The findings are interpreted within the framework of the Lateral Cap model for microtubule dynamic instability, in terms of the effects of these changes on the intrinsic rate constants of the system. By contrast, the extent of tubulin exchange depends selectively on the value of the dissociation rate constant for tubulin-GDP. A decrease in the extent of exchange, and hence in dynamic activity, is associated with a decreased value for this rate constant, and vice versa. The results also show good agreement of predictions of the model in treating the observed variations in the dynamic properties of individual microtubules, induced by different solution conditions.

Assembly of chick brain MAP2-tubulin microtubule protein. Analysis of tubulin subunit flux rates by immunofluorescence microscopy

A filter-based immunofluorescence-microscopy method for obtaining microtubule lengths has been developed and evaluated. Kinetic constants and mean lengths obtained show close agreement with those obtained by complementary methods applied to chick brain MAP2-tubulin microtubule protein in NaCl-supplemented buffer. The filter-based method has been used to estimate tubulin subunit flux (Jon) resulting from isothermal dilution of microtubule populations to various free tubulin concentrations, (c). This experimental Jon(c) plot is significantly different from that predicted by a variety of theoretical models, but is consistent with a 'lateral cap' model of dynamic instability [Bayley, Schilstra & Martin (1990) J. Cell. Sci. 95, 33-48] adapted to accommodate the observed vectorial GTP hydrolysis.

Highly variable effects of beryllium and beryllium fluoride on tubulin polymerization under different reaction conditions: comparison of assembly reactions dependent on microtubule-associated proteins, glycerol, dimethyl sulfoxide, and glutamate

Carlier et al. (1988, Biochemistry 27, 3555-3559; 1989, Biochemistry 28, 1783-1791) described enhancement of tubulin polymerization and stabilization of glycerol-induced microtubules by BeF3- (by addition of both BeSO4 and NaF to reaction mixtures). We were able to confirm the stabilization of glycerol-induced polymer reported by these workers, provided Mg2+ was also present in the reaction. When we examined polymerization dependent on microtubule-associated proteins (MAPs), however, we obtained very different results. BeF3- had no significant effect on this reaction, or the polymer formed, under any condition examined. Lower concentrations of BeSO4 alone, in contrast to a negligible effect in glycerol, enhanced polymerization with MAPs provided the concentrations of both Mg2+ and GTP were low; and Be2+ stabilized the polymer, if the GTP concentration was low, at both low and high Mg2+ concentrations. Higher concentrations of BeSO4 precipitated tubulin, an effect which was not affected by Mg2+, partially prevented but not reversed by MAPs, and prevented or reversed by either NaF or nucleotides at adequate concentrations. These results suggest that Be2+ binds at site(s) distinct from Mg2+ site(s), and that partial occupancy of these site(s) at lower Be2+ concentrations enhances tubulin polymerization and polymer stability, while extensive occupancy at higher Be2+ concentrations results in tubulin precipitation. Effects of Be2+ and BeF3- on polymerization dependent on dimethyl sulfoxide or glutamate were also evaluated. The dimethyl sulfoxide system displayed properties similar to those of the glycerol system, while the glutamate system was similar to the MAPs system.

Nucleotide hydrolysis in cytoskeletal assembly

Two major polymers of the cytoskeleton, actin filaments and microtubules, are assembled with expenditure of energy: the ATP/GTP tightly bound to actin/tubulin is irreversibly hydrolyzed to ADP/GTP during the assembly process, and liberation of Pi in the medium occurs subsequent to the incorporation of subunits in the polymer. Pi release acts as a switch, causing the destabilization of protein-protein interactions in the polymer, therefore regulating the dynamics of these fibres. An understanding of this regulation in vivo requires that progress be made in four areas: the chemistry of the NTPase reaction; the structure of the intermediates in nucleotide hydrolysis and the nature of the conformational switch; the regulation of parameters involved in dynamic instability of microtubules; and the possible involvement of nucleotide hydrolysis in the macroscopic organization of these polymers in highly concentrated solutions, compared with the simple case of a equilibrium polymers. Progress made along these lines will define trends for future investigation.

Role of GTP hydrolysis in microtubule polymerization: evidence for a coupled hydrolysis mechanism

The relationship between GTP hydrolysis and microtubule assembly has been investigated by using a rapid filtration method. Microtubules assembled from phosphocellulose-purified tubulin, double-labeled with [gamma-32P]- and [3H]GTP, were trapped and washed free of unbound nucleotide on glass fiber filters. The transient accumulation of microtubule-bound GTP predicted by uncoupled GTP hydrolysis models [Carlier & Pantaloni (1981) Biochemistry 20, 1918-1924; Carlier et al. (1987) Biochemistry 26, 4428-4437] during the rapid assembly of microtubules was not detectable under our experimental conditions. By calculating hypothetical time courses for the transient accumulation of microtubule-bound GTP, we demonstrate that microtubule-bound GTP would have been detectable even if the first-order rate constant for GTP hydrolysis were 4-5 times greater than the pseudo-first-order rate constant for tubulin subunit addition to microtubules. In a similar manner, we demonstrate that if GTP hydrolysis were uncoupled from microtubule assembly but were limited to the interface between GTP subunits and GDP subunits (uncoupled vectorial hydrolysis), then microtubule-bound GTP would have been detectable if GTP hydrolysis became uncoupled from microtubule assembly at less than 50 microM free tubulin, 5 times the steady-state tubulin concentration of our experimental conditions. In addition, during rapid microtubule assembly, we have not detected any microtubule-bound Pi, which has been proposed to form a stabilizing cap at the ends of microtubules [Carlier et al. (1988) Biochemistry 27, 3555-3559]. Also, several conditions that could be expected to increase the degree of potential uncoupling between GTP hydrolysis and microtubule assembly were examined, and no evidence of uncoupling was found. Our results are consistent with models that propose cooperative mechanisms that limit GTP hydrolysis to the terminal ring of tubulin subunits [e.g., O'Brien et al. (1987) Biochemistry 26, 4148-4156]. The results are also consistent with the hypothesis that a slow conformational change in tubulin subunits after GTP hydrolysis and Pi release occurs that results in destabilized microtubule ends when such subunits become exposed at the ends.