Characteristics of the polar assembly and disassembly of microtubules observed in vitro by darkfield light microscopy

In Vitro Microtubule Dynamics Assays Using Dark-Field Microscopy

Microtubules are dynamic non-covalent mesoscopic polymers. Their dynamic behavior is essential for cell biological processes ranging from intracellular transport to cell division and neurogenesis. Fluorescence microscopy has been the method of choice for monitoring microtubule dynamics in the last two decades. However, fluorescent microtubules are prone to photodamage that alters their dynamics, and the fluorescent label itself can affect microtubule properties. Dark-field imaging is a label-free technique that can generate high signal-to-noise, low-background images of microtubules at high acquisition rates without the photobleaching inherent to fluorescence microscopy. Here, we describe how to image in vitro microtubule dynamics using dark-field microscopy. The ability to image microtubules label-free allows the investigation of the dynamic properties of non-abundant tubulin species where fluorescent labeling is not feasible, free from the confounding effects arising from the addition of fluorescent labels.

Smartphone-based multi-contrast microscope using color-multiplexed illumination

We present a portable multi-contrast microscope capable of producing bright-field, dark-field, and differential phase contrast images of thin biological specimens on a smartphone platform. The microscopy method is based on an imaging scheme termed “color-coded light-emitting-diode (LED) microscopy (cLEDscope),” in which a specimen is illuminated with a color-coded LED array and light transmitted through the specimen is recorded by a color image sensor. Decomposition of the image into red, green, and blue colors and subsequent computation enable multi-contrast imaging in a single shot. In order to transform a smartphone into a multi-contrast imaging device, we developed an add-on module composed of a patterned color micro-LED array, specimen stage, and miniature objective. Simple installation of this module onto a smartphone enables multi-contrast imaging of transparent specimens. In addition, an Android-based app was implemented to acquire an image, perform the associated computation, and display the multi-contrast images in real time. Herein, the details of our smartphone module and experimental demonstrations with various biological specimens are presented.

Lissencephaly-1 is a context-dependent regulator of the human dynein complex

The cytoplasmic dynein-1 (dynein) motor plays a central role in microtubule organisation and cargo transport. These functions are spatially regulated by association of dynein and its accessory complex dynactin with dynamic microtubule plus ends. Here, we elucidate in vitro the roles of dynactin, end-binding protein-1 (EB1) and Lissencephaly-1 (LIS1) in the interaction of end tracking and minus end-directed human dynein complexes with these sites. LIS1 promotes dynactin-dependent tracking of dynein on both growing and shrinking plus ends. LIS1 also increases the frequency and velocity of processive dynein movements that are activated by complex formation with dynactin and a cargo adaptor. This stimulatory effect of LIS1 contrasts sharply with its documented ability to inhibit the activity of isolated dyneins. Collectively, our findings shed light on how mammalian dynein complexes associate with dynamic microtubules and help clarify how LIS1 promotes the plus-end localisation and cargo transport functions of dynein in vivo.

DOI:http://dx.doi.org/10.7554/eLife.21768.001

Reversible Morphological Control of Tubulin-Encapsulating Giant Liposomes by Hydrostatic Pressure

Liposomes encapsulating cytoskeletons have drawn much recent attention to develop an artificial cell-like chemical-machinery; however, as far as we know, there has been no report showing isothermally reversible morphological changes of liposomes containing cytoskeletons because the sets of various regulatory factors, that is, their interacting proteins, are required to control the state of every reaction system of cytoskeletons. Here we focused on hydrostatic pressure to control the polymerization state of microtubules (MTs) within cell-sized giant liposomes (diameters ∼10 μm). MT is the cytoskeleton formed by the polymerization of tubulin, and cytoskeletal systems consisting of MTs are very dynamic and play many important roles in living cells, such as the morphogenesis of nerve cells and formation of the spindle apparatus during mitosis. Using real-time imaging with a high-pressure microscope, we examined the effects of hydrostatic pressure on the morphology of tubulin-encapsulating giant liposomes. At ambient pressure (0.1 MPa), many liposomes formed protrusions due to tubulin polymerization within them. When high pressure (60 MPa) was applied, the protrusions shrank within several tens of seconds. This process was repeatedly inducible (around three times), and after the pressure was released, the protrusions regenerated within several minutes. These deformation rates of the liposomes are close to the velocities of migrating or shape-changing living cells rather than the shortening and elongation rates of the single MTs, which have been previously measured. These results demonstrate that the elongation and shortening of protrusions of giant liposomes is repeatedly controllable by regulating the polymerization state of MTs within them by applying and releasing hydrostatic pressure.

Enhanced dynamic instability of microtubules in a ROS free inert environment

Reactive oxygen species (ROS), one of the regulators in various biological processes, have recently been suspected to modulate microtubule (MT) dynamics in cells. However due to complicated cellular environment and unavailability of any in vitro investigation, no detail is understood yet. Here, by performing simple in vitro investigations, we have unveiled the effect of ROS on MT dynamics. By studying dynamic instability of MTs in a ROS free environment and comparing with that in the presence of ROS, we disclosed that MTs showed enhanced dynamics in the ROS free environment. All the parameters that define dynamic instability of MTs e.g., growth and shrinkage rates, rescue and catastrophe frequencies were significantly affected by the presence of ROS. This work clearly reveals the role of ROS in modulating MT dynamics in vitro, and would be a great help in understanding the role of ROS in regulation of MT dynamics in cells.

Color-coded LED microscopy for multi-contrast and quantitative phase-gradient imaging

We present a multi-contrast microscope based on color-coded illumination and computation. A programmable three-color light-emitting diode (LED) array illuminates a specimen, in which each color corresponds to a different illumination angle. A single color image sensor records light transmitted through the specimen, and images at each color channel are then separated and utilized to obtain bright-field, dark-field, and differential phase contrast (DPC) images simultaneously. Quantitative phase imaging is also achieved based on DPC images acquired with two different LED illumination patterns. The multi-contrast and quantitative phase imaging capabilities of our method are demonstrated by presenting images of various transparent biological samples.

Intrinsic microtubule GTP-cap dynamics in semi-confined systems: kinetochore-microtubule interface

In order to quantify the intrinsic dynamics associated with the tip of a GTP-cap under semi-confined conditions, such as those within a neuronal cone and at a kinetochore-microtubule interface, we propose a novel quantitative concept of critical nano local GTP-tubulin concentration (CNLC). A simulation of a rate constant of GTP-tubulin hydrolysis, under varying conditions based on this concept, generates results in the range of 0-420 s(-1). These results are in agreement with published experimental data, validating our model. The major outcome of this model is the prediction of 11 random and distinct outbursts of GTP hydrolysis per single layer of a GTP-cap. GTP hydrolysis is accompanied by an energy release and the formation of discrete expanding zones, built by less-stable, skewed GDP-tubulin subunits. We suggest that the front of these expanding zones within the walls of the microtubule represent soliton-like movements of local deformation triggered by energy released from an outburst of hydrolysis. We propose that these solitons might be helpful in addressing a long-standing question relating to the mechanism underlying how GTP-tubulin hydrolysis controls dynamic instability. This result strongly supports the prediction that large conformational movements in tubulin subunits, termed dynamic transitions, occur as a result of the conversion of chemical energy that is triggered by GTP hydrolysis (Satarić et al., Electromagn Biol Med 24:255-264, 2005). Although simple, the concept of CNLC enables the formulation of a rationale to explain the intrinsic nature of the "push-and-pull" mechanism associated with a kinetochore-microtubule complex. In addition, the capacity of the microtubule wall to produce and mediate localized spatio-temporal excitations, i.e., soliton-like bursts of energy coupled with an abundance of microtubules in dendritic spines supports the hypothesis that microtubule dynamics may underlie neural information processing including neurocomputation (Hameroff, J Biol Phys 36:71-93, 2010; Hameroff, Cognit Sci 31:1035-1045, 2007; Hameroff and Watt, J Theor Biol 98:549-561, 1982).

Comparative studies of microtubule mechanics with two competing models suggest functional roles of alternative tubulin lateral interactions

The dynamic assembly and disassembly of microtubules and the mechanical properties of these polymers are essential for many key cellular processes. Mathematical and computational modeling, especially coupled mechanochemical modeling, has contributed significantly to our understanding of microtubule dynamics. However, critical discrepancies exist between experimental observations and modeling results that need to be resolved before further progress toward a complete model can be made. Open sheet structures ranging in length from several hundred nanometers to one micron have often been observed at the growing ends of microtubules in in vitro studies. Existing modeling studies predict these sheet structures to be short and rare intermediates of microtubule disassembly rather than important components of the assembly process. Atomic force microscopy (AFM) studies also reveal interesting step-like gaps of the force-indentation curve that cannot yet be explained by existing theoretical models. We have carried out computational studies to compare the mechanical properties of two alternative models: a more conventional model where tubulin dimers are added directly into a microtubule lattice, and one that considers an additional type of tubulin lateral interaction proposed to exist in intermediate sheet structures during the microtubule assembly process. The first model involves a single type of lateral interactions between tubulin subunits, whereas the latter considers a second type that can convert to the canonical lateral contact during microtubule closure into a cylinder. Our analysis shows that only the second model can reproduce the AFM results over a broad parameter range. We propose additional studies using different sizes of AFM tips that would allow to unambiguously distinguish the relative validity of the two models.

Kinetochores generate microtubules with distal plus ends: their roles and limited lifetime in mitosis

In early mitosis, microtubules can be generated at kinetochores as well as at spindle poles. However, the role and regulation of kinetochore-derived microtubules have been unclear. In general, metaphase spindle microtubules are oriented such that their plus ends bind to kinetochores. However, we now have evidence that, during early mitosis in budding yeast, microtubules are generated at kinetochores with distal plus ends. These kinetochore-derived microtubules interact along their length with microtubules that extend from a spindle pole, facilitating kinetochore loading onto the lateral surface of spindle pole microtubules. Once kinetochores are loaded, microtubules are no longer generated at kinetochores, and those that remain disappear rapidly and do not contribute to the metaphase spindle. Stu2 (the ortholog of vertebrate XMAP215/ch-TOG) localizes to kinetochores and plays a central role in regulating kinetochore-derived microtubules. Our work provides insight into microtubule generation at kinetochores and the mechanisms that facilitate initial kinetochore interaction with spindle pole microtubules.

Simulations of tubulin sheet polymers as possible structural intermediates in microtubule assembly

The microtubule assembly process has been extensively studied, but the underlying molecular mechanism remains poorly understood. The structure of an artificially generated sheet polymer that alternates two types of lateral contacts and that directly converts into microtubules, has been proposed to correspond to the intermediate sheet structure observed during microtubule assembly. We have studied the self-assembly process of GMPCPP tubulins into sheet and microtubule structures using thermodynamic analysis and stochastic simulations. With the novel assumptions that tubulins can laterally interact in two different forms, and allosterically affect neighboring lateral interactions, we can explain existing experimental observations. At low temperature, the allosteric effect results in the observed sheet structure with alternating lateral interactions as the thermodynamically most stable form. At normal microtubule assembly temperature, our work indicates that a class of sheet structures resembling those observed at low temperature is transiently trapped as an intermediate during the assembly process. This work may shed light on the tubulin molecular interactions, and the role of sheet formation during microtubule assembly.

The spindle pole body of yeast

Microtubule organizing centers play an essential cellular role in nucleating microtubule assembly and establishing the microtubule array. The microtubule organizing center of yeast, the spindle pole body (SPB), shares many functions and properties with those other organisms. In recent years considerable new information has been generated concerning components associated with the SPB, and the mechanism by which it duplicates. This article reviews our current view of the cytology and molecular composition of the SPB of the budding yeast, Saccharomyces cerevisiae, and the fission yeast, Schizosaccharomyces pombe. Genetic studies in these organisms has revealed information about how the SPB duplicates and separates, and its roles during vegetative growth, mating and meiosis.

Dynamics of microtubules from erythrocyte marginal bands

Microtubules can adjust their length by the mechanism of dynamic instability, that is by switching between phases of growth and shrinkage. Thus far this phenomenon has been studied with microtubules that contain several components, that is, a mixture of tubulin isoforms, with or without a mixture of microtubule-associated proteins (MAPs), which can act as regulators of dynamic instability. Here we concentrate on the influence of the tubulin component. We have studied MAP-free microtubules from the marginal band of avian erythrocytes and compared them with mammalian brain microtubules. The erythrocyte system was selected because it represents a naturally stable aggregate of microtubules; second, the tubulin is largely homogeneous, in contrast to brain tubulin. Qualitatively, erythrocyte microtubules show similar features as brain microtubules, but they were found to be much less dynamic. The critical concentration of elongation, and the rates of association and dissociation of tubulin are all lower than with brain microtubules. Catastrophes are rare, rescues frequent, and shrinkage slow. This means that dynamic instability can be controlled by the tubulin isotype, independently of MAPs. Moreover, the extent of dynamic behavior is highly dependent on buffer conditions. In particular, dynamic instability is strongly enhanced in phosphate buffer, both for erythrocyte marginal band and brain microtubules. The lower stability in phosphate buffer argues against the hypothesis that a cap of tubulin.GDP.Pi subunits stabilizes microtubules. The difference in dynamics between tubulin isotypes and between the two ends of microtubules is preserved in the different buffer systems.

Polarity of microtubular structures in manchette-like formations: possible role of the "11th filament"

Bundles of microtubular structures appear in the cytoplasm of spermatids of the African frog Dicroglossus occipitalis. They are observed in the vicinity of axonemes. Natural tubulin polymerization leads to the formation of hooks on microtubular structures. They can be related to experimentally induced tubulin hooks. The direction of curvature of the hooks allows us to define the polarity of the bundles. This is opposite to the polarity of axonemal microtubules: Bundles and axonemes are antiparallel. Under colchicine action, arch-like microtubular structures are shown to open in the same direction as they lock. This enables us to characterize their opening and locking site: It corresponds to the place of the "11th filament" described in microtubular structures such axonemes. The "11th filament" is thus demonstrated to be the most susceptible to natural opening and to the action of colchicine in microtubular structures.

Modulation of the dynamic instability of tubulin assembly by the microtubule-associated protein tau

Microtubule-associated proteins (MAP), such as tau, modulate the extent and rate of microtubule assembly and play an essential role in morphogenetic processes, such as axonal growth. We have examined the mechanism by which tau affects microtubule polymerization by examining the kinetics of microtubule assembly and disassembly through direct observation of microtubules using dark-field microscopy. Tau increases the rate of polymerization, decreases the rate of transit into the shrinking phase (catastrophe), and inhibits the rate of depolymerization. Tau strongly suppresses the catastrophe rate, and its ability to do so is independent of its ability to increase the elongation rate. Thus, tau generates a partially stable but still dynamic state in microtubules. This state is perturbed by phosphorylation by MAP2 kinase, which affects all three activities by lowering the affinity of tau for the microtubule lattice.

Reactivation of isolated mitotic apparatus: metaphase versus anaphase spindles

Mitotic spindles isolated from sea urchin eggs can be reactivated to undergo mitotic processes in vitro. Spindles incubated in reactivation media containing sea urchin tubulin and nucleotides undergo pole-pole elongation similar to that observed in living cells during anaphase-B. The in vitro behavior of spindles isolated during metaphase and anaphase are compared. Both metaphase and anaphase spindles undergo pole-pole elongation with similar rates, but only in the presence of added tubulin. In contrast, metaphase but not anaphase spindles increase chromosome-pole distance in the presence of exogenous tubulin, suggesting that in vitro, tubulin can be incorporated at the kinetochores of metaphase but not anaphase chromosomes. The rate of spindle elongation, ultimate length achieved, and the increase in chromosome-pole distance for isolated metaphase spindles is related to the concentration of available tubulin. Pole-pole elongation and chromosome-pole elongation does not require added adenosine triphosphate (ATP). Guanosine triphosphate (GTP) will support all activities observed. Thus, the force generation mechanism for anaphase-B in isolated sea urchin spindles is independent of added ATP, but dependent on the availability of tubulin. These results support the hypothesis that the mechanism of force generation for anaphase-B is linked to the incorporation of tubulin into the mitotic apparatus. (If, in addition, a microtubule-dependent motor-protein(s) is acting to generate force, it does not appear to be dependent on ATP as the exclusive energy source.

UV microbeam irradiations of the mitotic spindle. II. Spindle fiber dynamics and force production

Metaphase and anaphase spindles in cultured newt and PtK1 cells were irradiated with a UV microbeam (285 nM), creating areas of reduced birefringence (ARBs) in 3 s that selectively either severed a few fibers or cut across the half spindle. In either case, the birefringence at the polewards edge of the ARB rapidly faded polewards, while it remained fairly constant at the other, kinetochore edge. Shorter astral fibers, however, remained present in the enlarged ARB; presumably these had not been cut by the irradiation. After this enlargement of the ARB, metaphase spindles recovered rapidly as the detached pole moved back towards the chromosomes, reestablishing spindle fibers as the ARB closed; this happened when the ARB cut a few fibers or across the entire half spindle. We never detected elongation of the cut kinetochore fibers. Rather, astral fibers growing from the pole appeared to bridge and then close the ARB, just before the movement of the pole toward the chromosomes. When a second irradiation was directed into the closing ARB, the polewards movement again stopped before it restarted. In all metaphase cells, once the pole had reestablished connection with the chromosomes, the unirradiated half spindle then also shortened to create a smaller symmetrical spindle capable of normal anaphase later. Anaphase cells did not recover this way; the severed pole remained detached but the chromosomes continued a modified form of movement, clumping into a telophase-like group. The results are discussed in terms of controls operating on spindle microtubule stability and mechanisms of mitotic force generation.

Dynamics of microtubules visualized by darkfield microscopy: treadmilling and dynamic instability

Individual microtubules undergoing treadmilling in vitro were visualized by darkfield light microscopy, and the relationship between treadmilling and dynamic instability was studied as a function of microtubule-associated proteins (MAPs). In order to demonstrate treadmilling directly by real-time observation, we constructed three-block microtubules, the center-block of which was decorated with Tetrahymena dynein. The decorated block can easily be distinguished from undecorated blocks in the darkfield microscope because the decorated one appears much thicker. At steady-state conditions, the length of an undecorated block at one end increased and that at another end decreased, while the decorated center-block did not change in its length. The results from these direct observations show that calf brain 3X-microtubules exhibit a treadmilling flux of 0.9 micron/h. Using a similar microscopy technique, we previously demonstrated that phosphocellulose PC-microtubules existed in either the growing or the shortening phase and alternated quite frequently at steady-state conditions (dynamic instability). How does treadmilling relate to dynamic instability? An image recording of individual 3X-microtubules containing MAPs revealed that the microtubules undergo treadmilling and do not exhibit any dynamic instability. This evidence shows that MAPs suppress the dynamic instability of microtubules. That is, treadmilling can take place in the steady state only after microtubules have been stabilized by MAPs.

Microscopic imaging of cells

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.

Polarity orientations of microtubules in squid and lobster axons

Polarity orientations of microtubules in periaxolemmal and internal regions of squid and lobster axons were determined in order to test the hypothesis that regional differences in particle transport are produced by differentially distributed microtubule subclasses. Over 95% of the microtubules in all regions of the axons investigated were oriented with plus ends located distally, pointing away from axonal somata, and there were no significant differences in orientation ratios in periaxolemmal and internal axoplasm. In axonal sheath glial cells of lobsters, microtubules were found to be oriented parallel to axonal microtubules and to have approximately equally mixed polarities. The results for axonal microtubules did not support the possibility of subclasses of axonal microtubules.

Microtubule polarity confers direction to pigment transport in chromatophores

The cellular mechanisms used to direct translocating organelles are poorly understood. It is believed that the intrinsic structural polarity of microtubules may play a role in this process. We have examined the effects that differently oriented microtubules have upon the direction of pigment transport in surgically severed melanophore arms. In a previous paper (McNiven, M. A., M. Wang, and K. R. Porter, 1984, Cell, 37:753-765) we reported that after isolation, arms repolarized and reoriented their microtubules outward from their centers as if to form new "microcells." Pigment aggregation in these arms was toward a new focal point located at the arm centers. In this study we monitored pigment movement in isolated arms containing taxol-stabilized microtubules to test if the reversal in direction of pigment transport is dependent upon the repolarization of microtubules. We report that taxol delays both the microtubule reorientation and reversal in transport direction in a concentration-dependent manner. These and other presented data suggest that the polarity of the microtubule population within a melanophore confers direction on pigment transport.

Visualization of the dynamic instability of individual microtubules by dark-field microscopy

It has previously been shown that two populations of microtubules coexist in a dynamically unstable manner in vitro: those in one population elongate while those in the other shorten and finally disappear. This conclusion was based on changes in the number and length distribution of microtubules after dilution of the microtubule solution. Here, we demonstrate directly that growing and shortening populations coexist in steady-state conditions, by visualization of the dynamic behaviour of individual microtubules in vitro by dark-field microscopy. Real-time video recording reveals that both ends of a microtubule exist in either the growing or the shortening phase and alternate quite frequently between the two phases in a stochastic manner. Moreover, growing and shortening ends can coexist on a single microtubule, one end continuing to grow simultaneously with shortening at the other end. We find no correlation in the phase conversion either among individual microtubules or between the two ends of a single microtubule. The two ends of any given microtubule have remarkably different characteristics; the active end grows faster, alternates in phase more frequently and fluctuates in length to a greater extent than the inactive end. Microtubule-associated proteins (MAPs) suppress the phase conversion and stabilize microtubules in the growing phase.

Properties of the kinetochore in vitro. I. Microtubule nucleation and tubulin binding

We have isolated chromosomes from Chinese hamster ovary cells arrested in mitosis with vinblastine and examined the interactions of their kinetochores with purified tubulin in vitro. The kinetochores nucleate microtubule (MT) growth with complex kinetics. After an initial lag phase, MTs are continuously nucleated with both plus and minus ends distally localized. This mixed polarity seems inconsistent with the formation of an ordered, homopolar kinetochore fiber in vivo. As isolated from vinblastine-arrested cells, kinetochores contain no bound tubulin. The kinetochores of chromosomes isolated from colcemid-arrested cells or of chromosomes incubated with tubulin in vitro are brightly stained after anti-tubulin immunofluorescence. This bound tubulin is probably not in the form of MTs. It is localized to the corona region by immunoelectron microscopy, where it may play a role in MT nucleation in vitro.

Treadmilling, diffusional exchange and cytoplasmic structures

Microfilaments and microtubules exchange monomers from solution by at least two mechanisms; treadmilling and diffusional exchange. Refined kinetic analysis of both mechanisms shows that this exchange may be nonlinear under certain conditions. The two mechanisms of exchange differ in some of their predictions for the behaviour of cytoplasmic structures. Studies of assembly of cytoplasmic structures in vivo suggest that diffusional exchange is probably predominant for steady-state structures and further suggest that additional mechanisms may be operating in the cell.

Directed elongation model for microtubule GTP hydrolysis

We propose a role for GTP hydrolysis in microtubule assembly in which the GTPase reaction serves to stabilize tubulin subunits in the microtubule. The GTPase reaction in tubulin subunits containing GTP at microtubule ends is presumed to occur predominately in subunits at one of the interfaces between a cap of GTP-containing tubulin subunit and a core of GDP-containing tubulin subunit in the microtubule, resulting in elongation of the core. The proposed model interprets the effects of GDP on microtubule assembly, using a reaction scheme in which GDP-containing tubulin subunits are able to add to microtubule ends. The model can account for the GTP requirement for microtubule assembly, the GDP inhibition of the rate for microtubule elongation, and the fact that a metastable state exists after the enzymic conversion of GTP to GDP, with microtubules which are at steady state. To account for the fact that the microtubule assembly and disassembly rates are nonlinearly dependent upon the tubulin subunit concentration and for the effects of GDP-containing tubulin subunits on the kinetic properties of microtubules, our scheme includes nonproductive as well as productive binding of GTP- and GDP-containing tubulin subunits. We compare our model with an alternative scheme [Hill, T. L. & Carlier, M. F. (1983) Proc. Natl. Acad. Sci. USA 80, 7234-7238], which interprets the effects of GDP on microtubule assembly using a reaction scheme in which GDP is able to exchange with GTP in GTP-containing tubulin subunits in the microtubule and in which the principal GTPase occurs in GTP-containing tubulin subunits at the microtubule/solution interface.

Dynamic instability of microtubule growth

We report here that microtubules in vitro coexist in growing and shrinking populations which interconvert rather infrequently. This dynamic instability is a general property of microtubules and may be fundamental in explaining cellular microtubule organization.

Rapid rate of tubulin dissociation from microtubules in the mitotic spindle in vivo measured by blocking polymerization with colchicine

At metaphase, the amount of tubulin assembled into spindle microtubules is relatively constant; the rate of tubulin association equals the rate of dissociation. To measure the intrinsic rate of dissociation, we microinjected high concentrations of colchicine, or its derivative colcemid, into sea urchin embryos at metaphase to bind the free tubulin, thereby rapidly blocking polymerization. The rate of microtubule disassembly was measured from a calibrated video signal by the change in birefringent retardation (BR). After an initial delay after injection of colchicine or colcemid at final intracellular concentrations of 0.1-3.0 mM, BR decreased rapidly and simultaneously throughout the central spindle and aster. Measured BR in the central half-spindle decreased exponentially to 10% of its initial value within a characteristic period of approximately 20 s; the rate constant, k = 0.11 +/- 0.023 s-1, and the corresponding half-time, t 1/2, of BR decay was approximately 6.5 +/- 1.1 s in this concentration range. Below 0.1 mM colchicine or colcemid, the rate at which BR decreased was concentration dependent. Electron micrographs showed that the rapid decrease in BR corresponded to the disappearance of nonkinetochore microtubules; kinetochore fiber microtubules were differentially stable. As a control, lumicolchicine, which does not bind to tubulin with high affinity, was shown to have no effect on spindle BR at intracellular concentrations of 0.5 mM. If colchicine and colcemid block only polymerization, then the initial rate of tubulin dissociation from nonkinetochore spindle microtubules is in the range of 180-992 dimers per second. This range of rates is based on k = 11% of the initial polymer per second and an estimate from electron micrographs that the average length of a half-spindle microtubule is 1-5.5 micron. Much slower rates of tubulin association are predicted from the characteristics of end-dependent microtubule assembly measured previously in vitro when the association rate constant is corrected for the lower rate of tubulin diffusion in the embryo cytoplasm. Various possibilities for this discrepancy are discussed.

Microtubule polarity and the direction of pigment transport reverse simultaneously in surgically severed melanophore arms

The transport of pigment through the long cytoplasmic extensions (arms) of teleost melanophores is a microtubule-dependent event. We have severed the arms from melanophores to test whether microtubules isolated from the centrosome maintain their original polarity and disposition. In addition, we have tested whether arms containing microtubules of mixed polarities alter the direction of pigment transport. We find that microtubules within severed arms eventually change their polarity and reorganize from the arm center as if to form a new minicell. Concomitant with this change is a reversal in the direction of pigment transport.

Calmodulin-microtubule association in cultured mammalian cells

A Triton X-100-lysed cell system has been used to identify calmodulin on the cytoskeleton of 3T3 and transformed SV3T3 cells. By indirect immunofluorescence, calmodulin was found to be associated with both the cytoplasmic microtubule complex and the centrosomes. A number of cytoplasmic microtubules more resistant to disassembly upon either cold (0-4 degrees C) or hypotonic treatment, as well as following dilution have been identified. Most of the stable microtubules appeared to be associated with the centrosome at one end and with the plasma membrane at the other end. These microtubules could be induced to depolymerize, however, by micromolar Ca++ concentrations. These data suggest that, by interacting directly with the microtubule, calmodulin may influence microtubule assembly and ensure the Ca++-sensitivity of both mitotic and cytoplasmic microtubules.

Spindle microtubule dynamics following ultraviolet-microbeam irradiations of mitotic diatoms

Lesions ("ARBs") generated in metaphase and anaphase central spindles of Hantzschia by an ultraviolet microbeam are devoid of microtubules previously present. In vivo, the poleward transverse edge of the lesion invariably loses birefringence poleward, until this segment has vanished; the loss is slow during metaphase and faster at anaphase. The other transverse edge, proximal to the overlap, remains stable until disassembly of the whole spindle. We conclude that the central spindle microtubules are not in flux during metaphase to telophase, and that depolymerization of these microtubules takes place only from the end distal to the pole, as during normal spindle disassembly. Microtubule polarity and the creation of free ends may determine which microtubules are disassembled during later mitosis and how disassembly proceeds.

Interference of GTP hydrolysis in the mechanism of microtubule assembly: an experimental study

This paper reports an experimental study of the interference of GTP hydrolysis in the mechanism of microtubule assembly, following the model and theory previously published [Hill, T. L. & Carlier, M.-F. (1983) Proc. Natl. Acad. Sci. USA 80, 7234-7238]. Results from dilution experiments show that microtubules depolymerize faster below the critical concentration than expected with a reversible polymerization model. The experimental plot of flux versus tubulin concentration exhibits a slope discontinuity at the critical concentration, in agreement with the theory. Theoretical points calculated by the Monte Carlo method can be fitted qualitatively to the data. A consequence of this peculiar dynamic behavior of microtubules is that the ratio of tubulin dissociation and association rate constants measured, respectively, below and above the critical concentration does not yield the true value of the critical concentration. It is emphasized that the presence of GTP at microtubule ends is necessary to maintain the stability of the polymer.

Tubulin hooks as probes for microtubule polarity: an analysis of the method and an evaluation of data on microtubule polarity in the mitotic spindle

The structural polarity of cellular microtubules can be visualized in situ by lysing cells in special buffers containing tubulin. Under these conditions, the tubulin polymerizes to form curved sheets which attach to the walls of the endogenous microtubules. When such decorated microtubules are cut in cross section and viewed in the electron microscope, they appear to bear hooks curving clockwise or counter-clockwise. The direction of hook curvature is defined by the orientation of the decorated microtubule and thus serves as a probe for microtubule polarity. In this paper we describe a way to analyze the relative frequencies of hooks of different curvatures so as to measure the fidelity of the relation between hook curvature and microtubule polarity. The assumptions of the method are tested and found to be valid to a reasonable accuracy. The correlation between hook curvature and microtubule orientation is shown to be at least 0.98 for the spindles of PtK cells and Haemanthus endosperm at all stages of division and at all places in the spindle. The correlation is shown to be valid for each hook that forms, so the polarity of those microtubules that bear multiple hooks is specified with even better certainty than 0.98. This property of hook decoration is used to reinvestigate the possibility that some of the microtubules of the kinetochore fiber might be oriented with their plus ends distal to the kinetochore (opposite to the direction previously shown to predominate). Close analysis fails to identify such oppositely oriented microtubules. The scoring of tubules bearing multiple hooks also shows that individual interzone fibers at anaphase are constructed from clusters of antiparallel microtubules. The method for estimating the correlation between hook decoration and microtubule polarity is shown to be applicable to many structures and circumstances, but we find that the hook decoration assay for microtubule polarity is not uniformly accurate. We suggest that future studies using hook decorations should employ the method of data analysis presented here to assess the accuracy of the results obtained.

Cytoplasmic tubulin from the unfertilized sea urchin egg: II. Variation of the intrinsic calcium sensitivity of Strongylocentrotus purpuratus egg tubulin as a function of temperature and brain microtubule-associated proteins

Cytoplasmic tubulin purified from unfertilized sea urchin eggs self-assembles in the absence of microtubule-associated proteins (MAPs) [Suprenant and Rebhun, 1983; Detrich and Wilson, 1983] with a critical concentration for polymerization of 0.8 mg/ml at 15-18 degrees C, a value well below the 3 mg/ml tubulin present in these eggs [Pfeffer et al, 1976]. Studies of the calcium sensitivity of unfertilized S. purpuratus (sea urchin) egg tubulin were initiated to help understand how this tubulin is maintained unassembled in the unfertilized egg. Egg microtubules, assembled at physiological temperatures (15-18 degrees C) were depolymerized by a 100-fold lower free calcium concentration than egg microtubules assembled at the higher temperatures (25-37 degrees C) generally used to assemble mammalian brain microtubules. The initial rate of egg microtubule assembly was much more sensitive to calcium than was microtubule depolymerization at steady state at 37 degrees C. However, both processes were sensitive to near physiological free calcium concentrations at 18 degrees C. The co-assembly of bovine brain MAPs and sea urchin egg tubulin produced microtubules that required a 1,000-fold higher concentration of free calcium for depolymerization than microtubules assembled at 18 degrees C from egg tubulin alone. While calcium regulatory MAPs have not yet been found in sea urchin eggs, the fact that brain MAPs interact with egg tubulin and regulate both its critical concentration for polymerization [Suprenant and Rebhun, 1983] and its calcium sensitivity, suggests that such regulatory molecules exist. These results suggest that sea urchin egg tubulin assembly in vivo could be controlled by variations in intracellular calcium levels acting in concert with urchin egg proteins similar in function to brain MAPs.

Possible treadmilling in zinc(II)-induced sheet polymers of bovine brain microtubule protein

We have examined the ability of zinc(II)-induced sheet polymers, formed from thrice-cycled bovine brain microtubule protein prepared in the absence of glycerol, to exchange with tubulin subunits at steady state. By a rapid filtration assay in which labeled GTP was used as a marker for tubulin addition and loss, we found that steady-state sheet polymers, formed in 0.5 mM-ZnCl2, 1 mM-dithiothreitol, and 100 mM-2-(N-morpholino)ethanesulfonic acid (pH 6.75) in the presence of a GTP-regenerating system at 37 degrees C, incorporated the label in a time-dependent manner to a maximum level. The steady-state uptake of label was inhibited by colchicine, podophyllotoxin and vinblastine. In pulse-chase experiments, we observed that label added onto sheet polymers in a short pulse was retained for a period equal to that required by the polymers to become fully labeled in a continuous pulse; thereafter, the label was lost gradually to a baseline level. An average of 82% of the label was retained in the sheet polymers after a "cold" chase of equal duration to the time of the pulse. Sheet polymers assembled from microtubule protein prepared in the presence of glycerol gave similar results. Using a double-labeling procedure to analyze tubulin addition and loss simultaneously, we found that the rates of steady-state addition and loss were similar. Sheet polymers retained their structural integrity throughout these experiments, as determined by electron microscopy. We believe that the data are consistent with a "treadmilling" mechanism of polymerization and depolymerization, analogous to that documented to occur in steady-state microtubules in vitro. Such a mechanism is discussed in the context of recent findings from structural studies, and a model consistent with established structural data is offered.

Microtubule polarity in the nutritive tubes of insect ovarioles

The enormous numbers of microtubules within the nutritive tubes of hemipteran ovarioles are amenable to the hook-decoration technique for determining microtubule structural polarity, as they can be microdissected from ovarioles intact. This has allowed the correlation between the polarity of this continuously elongating complex of microtubules within a nutritive tube and the direction of transport along its length; and has shown that the plus or fast growing ends of the microtubules are all situated at the anterior end of a nutritive tube proximal to the trophic region from which synthesised materials are passed back towards the developing oocytes.

Polarity of kinetochore microtubules in Chinese hamster ovary cells after recovery from a colcemid block

The polarity of kinetochore microtubules was determined in a system for which kinetochore-initiated microtubule assembly has been demonstrated. Chinese hamster ovary cells were treated with 0.3 micrograms/ml colcemid for 8 h and then released from the block. Prior to recovery, microtubules were completely absent from the cells. The recovery was monitored using light and electron microscopy to establish that the cells progress through anaphase and that the kinetochore fibers are fully functional. Since early stages of recovery are characterized by short microtubule segments that terminate in the kinetochore fibrous corona rather than on the outer disk, microtubule polarity was determined at later stages of recovery when longer kinetochore bundles had formed, allowing us to establish unambiguously the spatial relationship between microtubules, kinetochores, and chromosomes. The cells were lysed in a detergent mixture containing bovine brain tubulin under conditions that allowed the formation of polarity-revealing hooks. 20 kinetochore bundles were assayed for microtubule polarity in either thick or thin serial sections. We found that 95% of the decorated kinetochore microtubules had the same polarity and that, according to the hook curvature, the plus ends of the microtubules were at the kinetochores. Hence, the polarity of kinetochore microtubules in Chinese hamster ovary cells recovering from a colcemid block is the same as in normal untreated cells. This result suggests that microtubule polarity is likely to be important for spindle function since kinetochore microtubules show the same polarity, regardless of the pattern of spindle formation.

Studies on the effect of phosphorylation of the 20,000 Mr light chain of vertebrate smooth muscle myosin

Myosin was rapidly prepared from turkey gizzard muscle to a high level of purity, in high yield and in a non-phosphorylated state. It was consistently observed that the actin-activated Mg2+ ATPase activity of this myosin was dependent on the level of phosphorylation of the 20,000 Mr light chain, for example, in the non-phosphorylated state, the myosin Mg2+ ATPase activity was not activated by actin whereas, when the light chains were phosphorylated, the Mg2+ ATPase activity of the myosin was activated approximately ninefold by actin. Using the "desensitized" scallop myosin test system (Kendrick-Jones et al., 1976; Sellers et al., 1980) it was further demonstrated that phosphorylation of the 20,000 Mr gizzard light chain has a regulatory role. These results also suggest that the regulatory mechanisms mediated by smooth muscle myosin light chains and molluscan myosin regulatory light chains are similar, i.e. in the absence of Ca2+, both types of light chain inhibit myosin interaction with actin and this inhibition is relieved by either phosphorylation in smooth muscle or by direct calcium binding in molluscan myosins. The basis of regulation exerted by these light chains is therefore repression derepression. Using a variety of techniques, i.e. turbidity measurements, quantitative high speed centrifugation, electron microscopy and dark field light microscopy, it was observed that the stability of gizzard myosin filaments at approximately physiological conditions (0.15 M-NaCl, 1 mM-MgATP, pH 7.0) was dependent on the level of light chain phosphorylation. Using purified calmodulin-dependent light chain kinase and phosphatase, it was further shown that these gizzard myosin filaments can be reversibly assembled and disassembled as a result of phosphorylation-dephosphorylation of the 20,000 Mr light chain.

Many cytoskeletal proteins associate with the hela cytoskeleton during translation in vitro

Observations that cytoskeletal proteins assemble in vivo close to the time and site of synthesis have been confirmed and extended by an in vitro translation system. HeLa cytoskeletons prepared with Triton in a translation-extraction buffer without reticulocyte or wheat germ lysate efficiently incorporate 35S-methionine into polypeptides, and are stable during this translation. Cytoskeletal proteins translated in this way associate with the HeLa cytoskeleton independent of the concentration of soluble proteins. These associations are puromycin-resistant before the proteins are complete; the protein associations made in vitro show only minor differences from those made in vivo. The protein associations are not simply a consequence of protein solubility in the buffers used, as the associations require initiation in vivo. These results indicate that many cytoskeletal proteins associate with the cytoskeleton during translation.

The doublet microtubules of rods of the rabbit retina

The connecting cilium of the rabbit photoreceptor rod is composed of nine outer doublets, lacking dynein side arms. The central singlet microtubules are absent. In cross section, there is an inner dense ring situated between the doublets and the center core of the cilium. As the doublet microtubules progress from the connecting region into outer segments, the cylindrical array of the nine pairs of doublets spreads out as a brush-like arrangement into the incisure cavity of the outer segment. The microtubules continue as doublets for much of the length of the outer segment. The B-tubules terminate first; the A-tubules extend as single tubules into the apical region of the photoreceptor. Before the B-tubules end, they open up, forming hook-shaped projections from the A-tubules. The gradual reduction in length of these hook-shaped structures suggests that near their distal ends each B-tubule opens because of the separation of protofilament 1 of the B-tubule from protofilament 1 of the adjacent A-tubule. Subsequently, the B-tubule protofilaments terminate individually.