New insights into microtubule structure and function from the atomic model of tubulin

Photophysical Mechanisms of Photobiomodulation Therapy as Precision Medicine

Despite a significant focus on the photochemical and photoelectrical mechanisms underlying photobiomodulation (PBM), its complex functions are yet to be fully elucidated. To date, there has been limited attention to the photophysical aspects of PBM. One effect of photobiomodulation relates to the non-visual phototransduction pathway, which involves mechanotransduction and modulation to cytoskeletal structures, biophotonic signaling, and micro-oscillatory cellular interactions. Herein, we propose a number of mechanisms of PBM that do not depend on cytochrome c oxidase. These include the photophysical aspects of PBM and the interactions with biophotons and mechanotransductive processes. These hypotheses are contingent on the effect of light on ion channels and the cytoskeleton, the production of biophotons, and the properties of light and biological molecules. Specifically, the processes we review are supported by the resonant recognition model (RRM). This previous research demonstrated that protein micro-oscillations act as a signature of their function that can be activated by resonant wavelengths of light. We extend this work by exploring the local oscillatory interactions of proteins and light because they may affect global body circuits and could explain the observed effect of PBM on neuro-cortical electroencephalogram (EEG) oscillations. In particular, since dysrhythmic gamma oscillations are associated with neurodegenerative diseases and pain syndromes, including migraine with aura and fibromyalgia, we suggest that transcranial PBM should target diseases where patients are affected by impaired neural oscillations and aberrant brain wave patterns. This review also highlights examples of disorders potentially treatable with precise wavelengths of light by mimicking protein activity in other tissues, such as the liver, with, for example, Crigler-Najjar syndrome and conditions involving the dysregulation of the cytoskeleton. PBM as a novel therapeutic modality may thus behave as "precision medicine" for the treatment of various neurological diseases and other morbidities. The perspectives presented herein offer a new understanding of the photophysical effects of PBM, which is important when considering the relevance of PBM therapy (PBMt) in clinical applications, including the treatment of diseases and the optimization of health outcomes and performance.

Characterization of Microtubule Destabilizing Drugs: A Quantitative Cell-Based Assay That Bridges the Gap between Tubulin Based- and Cytotoxicity Assays

Simple Summary

The characterization of new microtubule depolymerizing agents relies mainly on purified tubulin assays in vitro and on cytotoxicity tests. However, the relationship between the in vitro effects of drugs and their effect on cell viability may not be direct. Here, we have systematically compared the effect of four reference drugs on tubulin polymerization in vitro and in cells, using a recently-developed quantitative assay of the cellular microtubule content. By comparing these results with cell viability assays, we found that this new cellular microtubule content test better predicts cellular drug toxicity than the in vitro tubulin polymerization assay. This test can thus be easily implemented in the process of discovery and characterization of novel microtubule poisons.

Abstract

(1) Background: Microtubule depolymerizing agents (MDAs) are commonly used for cancer treatment. However, the therapeutic use of such microtubule inhibitors is limited by their toxicity and the emergence of resistance. Thus, there is still a sustained effort to develop new MDAs. During the characterization of such agents, mainly through in vitro analyses using purified tubulin and cytotoxicity assays, quantitative comparisons are mandatory. The relationship between the effect of the drugs on purified tubulin and on cell viability are not always direct. (2) Methods: We have recently developed a cell-based assay that quantifies the cellular microtubule content. In this study, we have conducted a systematic comparative analysis of the effect of four well-characterized MDAs on the kinetics of in vitro tubulin assembly, on the cellular microtubule content (using our recently developed assay) and on cell viability. (3) Conclusions: These assays gave complementary results. Additionally, we found that the drugs’ effect on in vitro tubulin polymerization is not completely predictive of their relative cytotoxicity. Their effect on the cellular microtubule content, however, is closely related to their effect on cell viability. In conclusion, the assay we have recently developed can bridge the gap between in vitro tubulin assays and cell viability assays.

SxIP binding disrupts the constitutive homodimer interface of EB1 and stabilizes EB1 monomer

SxIP is a microtubule tip localizing signal found in many +TIP proteins that bind to the hydrophobic cavity of the C-terminal domain of end binding protein 1 (EB1) and then positively regulate the microtubule plus-end tracking of EBs. However, the exact mechanism of microtubule activation of EBs in the presence of SxIP signaling motif is not known. Here, we studied the effect of SxIP peptide on the native conformation of EB1 in solution. Using various NMR experiments, we found that SxIP peptide promoted the dissociation of natively formed EB1 dimer. We also discovered that I224A mutation of EB1 resulted in an unfolded C-terminal domain, which upon binding with the SxIP motif folded to its native structure. Molecular dynamics simulations also confirmed the relative structural stability of EB1 monomer in the SxIP bound state. Residual dipolar couplings and heteronuclear NOE analysis suggested that the binding of SxIP peptide at the C-terminal domain of EB1 decreased the dynamics and conformational flexibility of the N-terminal domain involved in EB1-microtubule interaction. The SxIP-induced disruption of the dimeric interactions in EB1, coupled with the reduction in conformational flexibility of the N-terminal domain of EB1, might facilitate the microtubule association of EB1.

Beta3-Tubulin is Critical for Microtubule Dynamics, Cell Cycle Regulation, and Spontaneous Release of Microvesicles in Human Malignant Melanoma Cells (A375)

Microtubules (MTs), microfilaments, and intermediate filaments, the main constituents of the cytoskeleton, undergo continuous structural changes (metamorphosis), which are central to cellular growth, division, and release of microvesicles (MVs). Altered MTs dynamics, uncontrolled proliferation, and increased production of MVs are hallmarks of carcinogenesis. Class III beta-tubulin (β3-tubulin), one of seven β-tubulin isotypes, is a primary component of MT, which correlates with enhanced neoplastic cell survival, metastasis and resistance to chemotherapy. We studied the effects of β3-tubulin gene silencing on MTs dynamics, cell cycle, and MVs release in human malignant melanoma cells (A375). The knockdown of β3-tubulin induced G2/M cell cycle arrest, impaired MTs dynamics, and reduced spontaneous MVs release. Additional studies are therefore required to elucidate the pathophysiologic and therapeutic role of β3-tubulin in melanoma.

Recent advances of cytotoxic chalconoids targeting tubulin polymerization: Synthesis and biological activity

Since microtubules have an important role in mitosis and other vital cellular functions, tubulin-targeting chemotherapy has been received growing attention in anticancer drug design and development. It was found that a number of naturally occurring compounds including distinct chalcones exert their effect by inhibition of tubulin polymerization. After the identification of tubulin polymerization as potential target for chalcone-type compounds, extensive researches have been made to design and synthesis of new anti-tubulin chalconoids. Although diverse chalcones have found to be potent anticancer agents but in the present review, we focused on the recently reported tubulin polymerization inhibitors from chalcone origin and related synthetic compounds, and their detailed synthetic methods and biological activities.

Early investigational tubulin inhibitors as novel cancer therapeutics

INTRODUCTION

Microtubules represent one of the most logical and strategic molecular targets amongst the current targets for chemotherapy, alongside DNA. In the past decade, tubulin inhibitors as cancer therapeutics have been an area of focus due to the improved understanding and biological relevance of microtubules in cellular functions. Fueled by the objective of developing novel chemotherapeutics and with the aim of establishing the benefits of tubulin inhibition, several clinical trials have been conducted with others ongoing.

AREA COVERED

At present, the antitubulin development pipeline contains an armful of agents under clinical investigation. This review focuses on novel tubulin inhibitors as cancer therapeutics. The article covers the agents which have completed the phase II studies along with the agents demonstrating promising results in phase I studies.

EXPERT OPINION

Countless clinical trials evaluating the efficacy, safety and pharmacokinetics of novel tubulin inhibitors highlights the scientific efforts being paid to establish their candidature as cancer therapeutics. Colchicine binding site inhibitors as vascular disrupting agents (VDAs) and new taxanes appear to be the most likely agents for future clinical interest. Numerous agents have demonstrated clinical benefits in terms of efficacy and survival in phase I and II studies. However conclusive benefits can only be ascertained on the basis of phase III studies.

Rational approaches, design strategies, structure activity relationship and mechanistic insights for anticancer hybrids

A Hybrid drug which comprises the incorporation of two drug pharmacophores in one single molecule are basically designed to interact with multiple targets or to amplify its effect through action on another bio target as one single molecule or to counterbalance the known side effects associated with the other hybrid part(.) The present review article offers a detailed account of the design strategies employed for the synthesis of anticancer agents via molecular hybridization techniques. Over the years, the researchers have employed this technique to discover some promising chemical architectures displaying significant anticancer profiles. Molecular hybridization as a tool has been particularly utilized for targeting tubulin protein as exemplified through the number of research papers. The microtubule inhibitors such as taxol, colchicine, chalcones, combretasatin, phenstatins and vinca alkaloids have been utilized as one of the functionality of the hybrids and promising results have been obtained in most of the cases with some of the tubulin based hybrids exhibiting anticancer activity at nanomolar level. Linkage with steroids as biological carrier vector for anticancer drugs and the inclusion of pyrrolo [2,1-c] [1,4]benzodiazepines (PBDs), a family of DNA interactive antitumor antibiotics derived from Streptomyces species in hybrid structure based drug design has also emerged as a potential strategy. Various heteroaryl based hybrids in particular isatin and coumarins have also been designed and reported to posses' remarkable inhibitory potential. Apart from presenting the design strategies, the article also highlights the structure activity relationship along with mechanistic insights revealed during the biological evaluation of the hybrids.

Looped host defense peptide CLP-19 binds to microtubules and inhibits surface expression of TLR4 on mouse macrophages

The looped host defense peptide CLP-19 is derived from a highly functional core region of the Limulus anti-LPS factor and exerts robust anti-LPS activity by directly interacting with LPS in the extracellular space. We previously showed that prophylactic administration of CLP-19 even 20 h prior to LPS challenge might significantly increase the survival rate in a lethal endotoxin shock mouse model. Such an effect may be associated with immune regulation of CLP-19. To investigate the underlying mechanisms, peptide affinity chromatography, immunofluorescence, and Western blotting procedures were used to identify α- and β-tubulin as direct and specific binding partners of CLP-19 in the mouse macrophage cell line RAW 264.7. Bioinformatic analysis using the AutoDock Vina molecular docking and PyMOL molecular graphics system predicted that CLP-19 would bind to the functional residues of both α- and β-tubulin and would be located within the groove of microtubules. Tubulin polymerization assay revealed that CLP-19 might induce polymerization of microtubules and prevent depolymerization. The immunoregulatory effect of CLP-19 involving microtubules was investigated by flow cytometry, immunofluorescence, and Western blotting, which showed that CLP-19 prophylactic treatment of RAW 264.7 cells significantly inhibited LPS-induced surface expression of TLR4. Taken together, these results suggest that CLP-19 binding to microtubules disrupts the dynamic equilibrium of microtubules, reducing the efficacy of microtubule-dependent vesicular transport that would otherwise translocate TLR4 from the endoplasmic reticulum to the cell surface.

Molecular mechanisms of Tau binding to microtubules and its role in microtubule dynamics in live cells

Despite extensive studies, the molecular mechanisms of Tau binding to microtubules (MTs) and its consequences on MT stability still remain unclear. It is especially true in cells where the spatiotemporal distribution of Tau-MT interactions is unknown. Using Förster resonance energy transfer (FRET), we showed that the Tau-MT interaction was distributed along MTs in periodic hotspots of high and low FRET intensities. Fluorescence recovery after photobleaching (FRAP) revealed a two-phase exchange of Tau with MTs as a rapid diffusion followed by a slower binding phase. A real-time FRET assay showed that high FRET occurred simultaneously with rescue and pause transitions at MT ends. To further explore the functional interaction of Tau with MTs, the binding of paclitaxel (PTX), tubulin acetylation induced by trichostatin A (TSA), and the expression of non-acetylatable tubulin were used. With PTX and TSA, FRAP curves best fitted a single phase with a long time constant, whereas with non-acetylatable α-tubulin, curves best fitted a two phase recovery. Upon incubation with PTX and TSA, the number of high and low FRET hotspots decreased by up to 50% and no hotspot was observed during rescue and pause transitions. In the presence of non-acetylatable α-tubulin, a 34% increase in low FRET hotspots occurred, and our real-time FRET assay revealed that low FRET hotspots appeared with MTs recovering growth. In conclusion, we have identified, by FRET and FRAP, a discrete Tau-MT interaction, in which Tau could induce conformational changes of MTs, favoring recovery of MT self-assembly.

Novel indole-bearing combretastatin analogues as tubulin polymerization inhibitors

Background

The combretastatins are a class of natural stilbenoids. These molecules generally share three common structural features: a trimethoxy "A"-ring, a "B"-ring containing substituent often at C3′ and C4′, and an ethene bridge between the two rings, which provides necessary structural rigidity. Members of the combretastatin family possess varying ability to cause vascular disruption in tumors. Combretastatin binds to the colchicine binding site of β-subunit of tubulin. Despite having a similar name, combretastatin is unrelated to statins, a family of cholesterol-lowering drugs.

Results

New combretastatin 2-(1-acetyl-1H-indole-3-yl)-3-(phenyl) propenoic analogues (2a to 2y), bearing indole moiety at the place of ring A of combretastatin (CA4), were synthesized and evaluated for anticancer activity against various cancer cell lines such as THP-1 (leukemia), A-549 (lung), IGROV-1 (ovary), HEP-2 (liver), MCF-7 (breast), and DU-145 (prostate). Compound 2d showed anti-cancer activity against THP-1 and MCF-7 with IC₅₀ 0.80 and 0.37 μM, respectively, and 2y showed against MCF-7 with IC₅₀ 3.60 μM comparable to paclitaxel.

Conclusions

The target compounds bind to the colchicine binding site which is situated at α and β interface of tubulin and prevent polymerization as it was confirmed by immunofluorescence technique. The molecular docking further confirmed the binding of the potent compound 2d to the colchicine binding site at α and β interface of tubulin.

MICROTUBULE-KINESIN MECHANICS BY MOLECULAR MODELING

The cellular cytoskeleton contains microtubules which function both as fundamental structural elements as well as motor protein tracks. While the structural property is connected to the properties of the tubulin dimer, its interactions with surrounding dimers and the geometric organization within the microtubule, the transport track properties are related to the interactions between the tubulin dimer and kinesin.

Based on the atomistic structures of kinesin and the tubulin dimer, we used molecular modeling to examine the interaction energy and force as function of a spatial distance of separation. From the results, elastic constants describing the system stiffness are obtained. By using the results related to the structure alone, a model of a 1 μm long microtubule is constructed as a network of elastic elements, and its mechanical properties were obtained via finite element method and compared to experimental results.

Concerning microtubule-kinesin complex, the interaction strength during a complete cycle of ATP hydrolysis was investigated. As expected, the affinity between the proteins is modulated by the type of nucleotide occupying the nucleotide binding pocket of the motor protein. The work underscores how molecular modeling can provide fundamental protein information in terms of the relation between mechanical properties and structural changes.

Drugs that target dynamic microtubules: a new molecular perspective

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

Coarse-Grained Representations of Large Biomolecular Complexes from Low-Resolution Structural Data

High-resolution atomistic structures of many large biomolecular complexes have not yet been solved by experiments, such as X-ray crystallography or NMR. Often however low-resolution information is obtained by alternative techniques, such as cryo-electron microscopy or small-angle X-ray scattering. Coarse-grained (CG) models are an appropriate choice to computationally study these complexes given the limited resolution experimental data. One of the important questions therefore is how to define CG representations from these low-resolution density maps. This work provides a space-based essential dynamics coarse-graining (ED-CG) method to define a CG representation from a density map without detailed knowledge of its underlying atomistic structure and primary sequence information. This method is demonstrated on G-actin (both the atomic structure and its density map). It is then applied to the density maps of the Escherichia coli 70S ribosome and the microtubule. The results indicate that the method can define highly CG models that still preserve functionally important dynamics of large biomolecular complexes.

Macromolecular interaction of halichondrin B analogues eribulin (E7389) and ER-076349 with tubulin by analytical ultracentrifugation

Halichondrin B is an antimitotic drug that inhibits microtubule assembly. To understand the molecular details of its interaction with tubulin, we investigated the binding of two halichondrin B analogues, eribulin (previously, ER-086526, E7389) and ER-076349, to tubulin by quantitative analytical ultracentrifugation. Eribulin is currently undergoing phase III clinical trials for cancer; ER-076349 is a closely related analogue with C.35 hydroxyl instead of C.35 primary amine [Towle, M. J., et al. (2001) Cancer Res. 61, 1013]. Below the critical concentration for microtubule assembly and in the presence of GDP, tubulin undergoes weak self-association into short curved oligomers. Eribulin inhibits this oligomer formation 4-6-fold, while ER-076349 slightly stimulates oligomer formation by 2-fold. This is in contrast to vinblastine which strongly stimulates large spiral polymers by 1000-fold under these same conditions. Vinblastine-induced spiral formation is strongly inhibited by both eribulin and ER-076349. Colchicine binding to the intradimer interface has no significant effect on small oligomer formation or the inhibitory activity of eribulin on this process. These results suggest that halichondrin B analogues bind to the interdimer interface or to the beta-subunit alone, disrupt polymer stability, and compete with vinblastine-induced spiral formation. Stathmin is known to form a tight 1:2 complex with tubulin. Eribulin strongly inhibits formation of the 1:2 stathmin-tubulin complex (>3.3 kcal/mol), while ER-076349 weakens formation of the 1:2 complex by approximately 1.9 kcal/mol. These results suggest that eribulin is a global inhibitor of tubulin polymer formation, disrupting tubulin-tubulin contacts at the interdimer interface. ER-076349 also perturbs tubulin-tubulin contacts, but in a more polymer specific manner, reflecting adaptability of the interdimer interface to drug and polymer polymorphism. These results suggest halichondrin B analogues exhibit unique tubulin-based activities that may underlie the clinical utility of these compounds.

Formins and microtubules

Formins have recently been recognized as prominent regulators of the microtubule (MT) cytoskeleton where they modulate the dynamics of selected MTs in interphase and mitosis. The association of formins with the MT cytoskeleton and their action on MT dynamics are relatively unexplored areas, yet growing evidence supports a direct role in their regulation of MT stability independent of their activity on actin. Formins regulate MT stability alone or in combination with accessory MT binding proteins that have previously been implicated in the stabilization of MTs downstream of polarity cues. As actin and MT arrays are typically remodeled downstream of signaling pathways that orchestrate cell shape and division, formins are emerging as excellent candidates for coordinating the responses of the cytoskeletal in diverse regulated and homeostatic processes.

Applying linear interaction energy method for binding affinity calculations of podophyllotoxin analogues with tubulin using continuum solvent model and prediction of cytotoxic activity

Podophyllotoxin and its analogues have important therapeutic value in the treatment of cancer, due to their ability to induce apoptosis in cancer cells in a proliferation-independent manner. These ligands bind to colchicine binding site of tubulin near the alpha- and beta-tubulin interface and interfere with tubulin polymerization. The binding free energies of podophyllotoxin-based inhibitors of tubulin were computed using a linear interaction energy (LIE) method with a surface generalized Born (SGB) continuum solvation model. A training set of 76 podophyllotoxin analogues was used to build a binding affinity model for estimating the free energy of binding for 36 inhibitors (test set) with diverse structural modifications. The average root mean square error (RMSE) between the experimental and predicted binding free energy values was 0.56kcal/mol which is comparable to the level of accuracy achieved by the most accurate methods, such as free energy perturbation (FEP) or thermodynamic integration (TI). The squared correlation coefficient between experimental and SGB-LIE estimates for the free energy for the test set compounds is also significant (R(2)=0.733). On the basis of the analysis of the binding energy, we propose that the three-dimensional conformation of the A, B, C and D rings is important for interaction with tubulin. On the basis of this insight, 12 analogues of varying ring modification were taken, tested with LIE methodology and then validated with their experimental potencies of tubulin polymerization inhibition. Low levels of RMSE for the majority of inhibitors establish the structure-based LIE method as an efficient tool for generating more potent and specific inhibitors of tubulin by testing rationally designed lead compounds based on podophyllotoxin derivatization.

Mechanical model of the tubulin dimer based on molecular dynamics simulations

The basic unit in microtubules is alphabeta-tubulin, a heterodimer consisting of an alpha- and a beta-tubulin monomer. The mechanical characteristics of the dimer as well as of the individual monomers may be used to obtain new insight into the microtubule tensile properties. In the present work, we evaluate the elastic constants of each monomer and the interaction force between them by means of molecular dynamics simulations. Molecular models of alpha-, beta-, and alphabeta-tubulins were developed starting from the 1TUB.pdb structure from the RCSB database. Simulations were carried out in a solvated environment by using explicit water molecules. In order to measure the monomers' elastic constants, simulations were performed by mimicking experiments carried out with atomic force microscopy. A different approach was used to determine the interaction force between the alpha- and beta-monomers by using 16 different monomer configurations based on different intermonomer distances. The obtained results show an elastic constant value for alpha-tubulin of 3.8-3.9 Nm, while for the beta-tubulin, the elastic constant was measured to be 3.3-3.6 Nm. The maximum interaction force between the monomers was estimated to be 11.9 nN. A mechanical model of the tubulin dimer was then constructed and, using the results from MD simulations, Young's modulus was estimated to be 0.6 GPa. A fine agreement with Young's modulus values from literature (0.1-2.5 GPa) is found, thus validating this approach for obtaining molecular scale mechanical characteristics. In perspective, these outcomes will allow exchanging atomic level description with key mechanical features enabling microtubule characterization by continuum mechanics approach.

Gene expression profile of mouse prostate tumors reveals dysregulations in major biological processes and identifies potential murine targets for preclinical development of human prostate cancer therapy

BACKGROUND

Translation of preclinical studies into effective human cancer therapy is hampered by the lack of defined molecular expression patterns in mouse models that correspond to the human counterpart. We sought to generate an open source TRAMP mouse microarray dataset and to use this array to identify differentially expressed genes from human prostate cancer (PCa) that have concordant expression in TRAMP tumors, and thereby represent lead targets for preclinical therapy development.

METHODS

We performed microarrays on total RNA extracted and amplified from eight TRAMP tumors and nine normal prostates. A subset of differentially expressed genes was validated by QRT-PCR. Differentially expressed TRAMP genes were analyzed for concordant expression in publicly available human prostate array datasets and a subset of resulting genes was analyzed by QRT-PCR.

RESULTS

Cross-referencing differentially expressed TRAMP genes to public human prostate array datasets revealed 66 genes with concordant expression in mouse and human PCa; 56 between metastases and normal and 10 between primary tumor and normal tissues. Of these 10 genes, two, Sox4 and Tubb2a, were validated by QRT-PCR. Our analysis also revealed various dysregulations in major biologic pathways in the TRAMP prostates.

CONCLUSIONS

We report a TRAMP microarray dataset of which a gene subset was validated by QRT-PCR with expression patterns consistent with previous gene-specific TRAMP studies. Concordance analysis between TRAMP and human PCa associated genes supports the utility of the model and suggests several novel molecular targets for preclinical therapy.

Protein-protein interactions as targets for small-molecule therapeutics in cancer

Small-molecule inhibition of the direct protein-protein interactions that mediate many important biological processes is an emerging and challenging area in drug design. Conventional drug design has mainly focused on the inhibition of a single protein, usually an enzyme or receptor, since these proteins often contain a clearly defined ligand-binding site with which a small-molecule drug can be designed to interact. Designing a small molecule to bind to a protein-protein interface and subsequently inhibit the interaction poses several challenges, including the initial identification of suitable protein-protein interactions, the surface area of the interface (it is often large), and the location of 'hot spots' (small regions suitable for drug binding). This article reviews the general approach to designing inhibitors of protein-protein interactions, and then focuses on recent advances in the use of small molecules targeted against a variety of protein-protein interactions that have therapeutic potential for cancer.

Mutations in alpha-tubulin confer dinitroaniline resistance at a cost to microtubule function

Protozoan microtubules are sensitive to disruption by dinitroanilines, compounds that kill intracellular Toxoplasma gondii parasites without affecting microtubules in vertebrate host cells. We previously isolated a number of resistant Toxoplasma lines that harbor mutations to the alpha1-tubulin gene. Some of the mutations are localized in or near the M and N loops, domains that coordinate lateral interactions between protofilaments. Other resistance mutations map to a computationally identified binding site beneath the N loop. Allelic replacement of wild-type alpha1-tubulin with the individual mutations is sufficient to confer dinitroaniline resistance. Some mutations seem to increase microtubule length, suggesting that they increase subunit affinity. All mutations are associated with replication defects that decrease parasite viability. When parasites bearing the N loop mutation Phe52Tyr are grown without dinitroaniline selection, they spontaneously acquired secondary mutations in the M loop (Ala273Val) or in an alpha-tubulin-specific insert that stabilizes the M loop (Asp367Val). Parasites with the double mutations have both reduced resistance and diminished incidence of replication defects, suggesting that the secondary mutations decrease protofilament affinity to increase parasite fitness.

Identification of tubulin as the molecular target of proapoptotic pyrrolo-1,5-benzoxazepines

We have demonstrated previously that certain members of a series of novel pyrrolo-1,5-benzoxazepine (PBOX) compounds potently induce apoptosis in a variety of human chemotherapy-resistant cancer cell lines and in primary ex vivo material derived from cancer patients. A better understanding of the molecular mechanisms underlying the apoptotic effects of these PBOX compounds is essential to their development as antineoplastic therapeutic agents. This study sought to test the hypothesis that proapoptotic PBOX compounds target the microtubules. We show that a representative proapoptotic PBOX compound, PBOX-6, induces apoptosis in both the MCF-7 and K562 cell lines. An accumulation of cells in G2/M precedes apoptosis in response to PBOX-6. PBOX-6 induces prometaphase arrest and causes an accumulation of cyclin B1 levels and activation of cyclin B1/CDK1 kinase in a manner similar to that of two representative antimicrotubule agents, nocodazole and paclitaxel. Indirect immunofluorescence demonstrates that both PBOX-6 and another pro-apoptotic PBOX compound, PBOX-15, cause microtubule depolymerization in MCF-7 cells. They also inhibit the assembly of purified tubulin in vitro, whereas a nonapoptotic PBOX compound (PBOX-21) has no effect on either the cellular microtubule network or on the assembly of purified tubulin. This suggests that the molecular target of the pro-apoptotic PBOX compounds is tubulin. PBOX-6 does not bind to either the vinblastine or the colchicine binding site on tubulin, suggesting that it binds to an as-yet-uncharacterised novel site on tubulin. The ability of PBOX-6 to bind tubulin and cause microtubule depolymerization confirms it as a novel candidate for antineoplastic therapy.

A common pharmacophore for a diverse set of colchicine site inhibitors using a structure-based approach

Modulating the structure and function of tubulin and microtubules is an important route to anticancer therapeutics, and therefore, small molecules that bind to tubulin and cause mitotic arrest are of immense interest. A large number of synthetic and natural compounds with diverse structures have been shown to bind at the colchicine site, one of the major binding sites on tubulin, and inhibit tubulin assembly. Using the recently determined X-ray structure of the tubulin:colchicinoid complex as the template, we employed docking studies to determine the binding modes of a set of structurally diverse colchicine site inhibitors. These binding models were subsequently used to construct a comprehensive, structure-based pharmacophore that in combination with molecular dynamics simulations confirms and extends our understanding of binding interactions at the colchicine site.

Optimized N-phenyl-N'-(2-chloroethyl)ureas as potential antineoplastic agents: synthesis and growth inhibition activity

In our ongoing research program aimed at the optimization of microtubule-self-assembly disrupting agents, we have prepared three series of phenylurea analogues (CEU), derived from N-(3-omega-hydroxyalkyl or 4-omega-hydroxyalkyl or 3-omega-hydroxyalkynyl)-phenyl-N'-(2-chloroethyl)ureas. Most compounds exhibit potent growth inhibitory activity on human colon carcinoma HT-29, human skin melanoma M21, and human breast carcinoma MCF-7 tumor cell lines, with a GI50 ranging from 250 nM to 8 microM. Among these new molecules, three CEUs exhibit GI50 in the nanomolar range. They are more potent by approximately an order of magnitude than previously described CEU analogues. As such, they are attractive hit compounds for the development of potent new alkylating antitubulin drugs.

Fractal intermediates in the self-assembly of silicatein filaments

Silicateins are proteins with catalytic, structure-directing activity that are responsible for silica biosynthesis in certain sponges; they are the constituents of macroscopic protein filaments that are found occluded within the silica needles made by Tethya aurantia. Self-assembly of the silicatein monomers and oligomers is shown to form fibrous structures by a mechanism that is fundamentally different from any previously described filament-assembly process. This assembly proceeds through the formation of diffusion-limited, fractally patterned aggregates on the path to filament formation. The driving force for this self-assembly is suggested to be entropic, mediated by the interaction of hydrophobic patches on the surfaces of the silicatein subunits that are not found on highly homologous congeners that do not form filaments. Our results are consistent with a model in which silicatein monomers associate into oligomers that are stabilized by intermolecular disulfide bonds. These oligomeric units assemble into a fractal network that subsequently condenses and organizes into a filamentous structure. These results represent a potentially general mechanism for protein fiber self-assembly.

Review: tubulin function, action of antitubulin drugs, and new drug development

Anticancer agents that interfere with microtubulin function are in widespread use in man and have a broad spectrum of activity against both hematological malignancies and solid tumors. The mechanisms of actions of these agents have been better defined during the past decade, indicating that there are distinct binding sites for these agents and that they interfere with microtubulin dynamics (growth and shortening of tubules) at low concentrations and only evoke microtubulin aggregation or dissociation at high concentrations. Tubulin has been recently described in the nucleus of cells and in mitochondria. Downstream events from tubulin binding are believed to be critical events for the generation of apoptosis in the malignant cell. The effects of vinca alkaloids and taxanes are distinct, suggesting that the interference with the tubulin cap by high-affinity binding of effective agents is not the only mechanism of cytotoxic effect, and the low-affinity binding of drug, which distorts microtubulin function, may also be important. The epothilones share some of the binding characteristics of the taxanes and are in clinical trials because of cytoxic activity in taxane resistant cells. Tubulin has additional target sites for anticancer drugs including interference with the binding and function of microtubule associated proteins and interference with motor proteins which are essential for the transport of substances within the cell. Because many of these microtubule associated proteins have an ATP binding site, both computer-aided design and combinatorial chemistry techniques can be used to make agents to interfere with their function analogous to imatinib mesylate (Gleevec). Agents that interfere with the motor protein kinesin are entering clinical trials.

Three- and four-repeat tau regulate the dynamic instability of two distinct microtubule subpopulations in qualitatively different manners. Implications for neurodegeneration

The microtubule-associated protein tau is implicated in the pathogenesis of many neurodegenerative diseases, including fronto-temporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), in which both RNA splicing and amino acid substitution mutations in tau cause dominantly inherited early onset dementia. RNA-splicing FTDP-17 mutations alter the wild-type approximately 50:50 3-repeat (3R) to 4-repeat (4R) tau isoform ratio, usually resulting in an excess of 4R tau. To examine further how splicing mutations might cause dysfunction by misregulation of microtubule dynamics, we used video microscopy to determine the in vitro behavior of individual microtubules stabilized by varying amounts of human 4R and 3R tau. At low tau:tubulin ratios (1:55 and 1:45), all 3R isoforms reduced microtubule growth rates relative to the no-tau control, whereas all 4R isoforms increased them; however, at a high tau:tubulin ratio (1:20), both 4R and 3R tau increased the growth rates. Further analysis revealed two distinct subpopulations of growing microtubules in the absence of tau. Increasing concentrations of both 4R and 3R tau resulted in an increase in the size of the faster growing subpopulation of microtubules; however, 4R tau caused a redistribution to the faster growing subpopulation at lower tau:tubulin ratios than 3R tau. This modulation of discrete growth rate subpopulations by tau suggests that tau causes a conformational shift in the microtubule resulting in altered dynamics. Quantitative and qualitative differences observed between 4R and 3R tau are consistent with a "microtubule misregulation" model in which abnormal tau isoform expression results in the inability to properly regulate microtubule dynamics, leading to neuronal death and dementia.

Crystallographic structure of tubulin: implications for dynamics and drug binding

The structure of tubulin, recently solved by electron crystallography, has given a first look at the molecular basis for some of the properties of tubulin and microtubules that have been observed over the last decades. We discuss how the structure relates to some of these properties, and how inferences about drug binding sites can explain some of the effects of the drugs on tubulin. Microtubules can form a highly dynamic system that requires careful tuning of the stability and properties of tubulin and its interactions with its many ligands. Understanding these interactions can provide fundamental information on the regulation of the microtubule system.

Microtubule's conformational cap

The molecular mechanisms that allow elongation of the unstable microtubule lattice remain unclear. It is usually thought that the GDP-liganded tubulin lattice is capped by a small layer of GTP- or GDP-Pi-liganded molecules, the so called "GTP-cap". Here, we point-out that the elastic properties of the microtubule lattice cause a difference in stability between the elongating tubulin sheet and the completed microtubule wall. The implications of our observations for microtubule structure and dynamics are discussed.

Mechanisms of Taxol resistance related to microtubules

Since its approval by the FDA in 1992 for the treatment of ovarian cancer, the use of Taxol has dramatically increased. Although treatment with Taxol has led to improvement in the duration and quality of life for some cancer patients, the majority eventually develop progressive disease after initially responding to Taxol treatment. Drug resistance represents a major obstacle to improving the overall response and survival of cancer patients. This review focuses on mechanisms of Taxol resistance that occur directly at the microtubule, such as mutations, tubulin isotype selection and post-translational modifications, and also at the level of regulatory proteins. A review of tubulin structure, microtubule dynamics, the mechanism of action of Taxol and its binding site on the microtubule are included, so that the reader can evaluate Taxol resistance in context.

Relating molecular flexibility to function: a case study of tubulin

Microtubules (MT), along with a variety of associated motor proteins, are involved in a range of cellular functions including vesicle movement, chromosome segregation, and cell motility. MTs are assemblies of heterodimeric proteins, alpha beta-tubulins, the structure of which has been determined by electron crystallography of zinc-induced, pacilitaxel-stabilized tubulin sheets. These data provide a basis for examining relationships between structural features and protein function. Here, we study the fluctuation dynamics of the tubulin dimer with the aim of elucidating its functional motions relevant to substrate binding, polymerization/depolymerization and MT assembly. A coarse-grained model, harmonically constrained according to the crystal structure, is used to explore the global dynamics of the dimer. Our results identify six regions of collective motion, comprised of structurally close but discontinuous sequence fragments, observed only in the dimeric form, dimerization being a prerequisite for domain identification. Boundaries between regions of collective motions appear to act as linkages, found primarily within secondary-structure elements that lack sequence conservation, but are located at minima in the fluctuation curve, at positions of hydrophobic residues. Residue fluctuations within these domains identify the most mobile regions as loops involved in recognition of the adjacent regions. The least mobile regions are associated with nucleotide binding sites where lethal mutations occur. The functional coupling of motions between and within regions identifies three global motions: torsional and wobbling movements, en bloc, between the alpha- and beta-tubulin monomers, and stretching longitudinally. Further analysis finds the antitumor drug pacilitaxel (TaxotereR) to reduce flexibility in the M loop of the beta-tubulin monomer; an effect that may contribute to tightening lateral interactions between protofilaments assembled into MTs. Our analysis provides insights into relationships between intramolecular tubulin movements of MT organization and function.

A-kinase anchor protein 84/121 are targeted to mitochondria and mitotic spindles by overlapping amino-terminal motifs

A-kinase anchor proteins (AKAPs) assemble multi-enzyme signaling complexes in proximity to substrate/effector proteins, thus directing and amplifying membrane-generated signals. S-AKAP84 and AKAP121 are alternative splicing products with identical NH(2) termini. These AKAPs bind and target protein kinase A (PKA) to the outer mitochondrial membrane. Tubulin was identified as a binding partner of S-AKAP84 in a yeast two-hybrid screen. Immunoprecipitation and co-sedimentation experiments in rat testis extracts confirmed the interaction between microtubules and S-AKAP84. In situ immunostaining of testicular germ cells (GC2) shows that AKAP121 concentrates on mitochondria in interphase and on mitotic spindles during M phase. Purified tubulin binds directly to S-AKAP84 but not to a deletion mutant lacking the mitochondrial targeting domain (MT) at residues 1-30. The MT is predicted to form a highly hydrophobic alpha-helical wheel that might also mediate interaction with tubulin. Disruption of the wheel by site-directed mutagenesis abolished tubulin binding and reduced mitochondrial attachment of an MT-GFP fusion protein. Some MT mutants retain tubulin binding but do not localize to mitochondria. Thus, the tubulin-binding motif lies within the mitochondrial attachment motif. Our findings indicate that S-AKAP84/AKAP121 use overlapping targeting motifs to localize signaling enzymes to mitochondrial and cytoskeletal compartments.

Structural insight into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of alpha/beta-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

Structural insights into microtubule function

Microtubules are polymers that are essential for, among other functions, cell transport and cell division in all eukaryotes. The regulation of the microtubule system includes transcription of different tubulin isotypes, folding of /¿-tubulin heterodimers, post-translation modification of tubulin, and nucleotide-based microtubule dynamics, as well as interaction with numerous microtubule-associated proteins that are themselves regulated. The result is the precise temporal and spatial pattern of microtubules that is observed throughout the cell cycle. The recent high-resolution analysis of the structure of tubulin and the microtubule has brought new insight to the study of microtubule function and regulation, as well as the mode of action of antimitotic drugs that disrupt normal microtubule behavior. The combination of structural, genetic, biochemical, and biophysical data should soon give us a fuller understanding of the exquisite details in the regulation of the microtubule cytoskeleton.

Mapping the binding site of colchicinoids on beta -tubulin. 2-Chloroacetyl-2-demethylthiocolchicine covalently reacts predominantly with cysteine 239 and secondarily with cysteine 354

2-Chloroacetyl-2-demethylthiocolchicine (2CTC) and 3-chloroacetyl-3-demethylthiocolchicine (3CTC) resemble colchicine in binding to tubulin and react covalently with beta-tubulin, forming adducts with cysteine residues 239 and 354. The adducts at Cys-239 are less stable than those at Cys-354 during formic acid digestion. Extrapolating to zero time, the Cys-239 to Cys-354 adduct ratio is 77:23 for 2CTC and 27:73 for 3CTC. Using energy minimization modeling to dock colchicinoids into the electron crystallographic model of beta-tubulin in protofilaments (Nogales, E. , Wolf, S. G., and Downing, K. H. (1998) Nature 391, 199-203), we found two potential binding sites. At one, entirely encompassed within beta-tubulin, the C2- and C3-oxygen atoms of 2CTC and 3CTC overlapped poorly with those of colchicine and thiocolchicine, but distances from the reactive carbon atoms of the analogs to the sulfur atoms of the cysteine residues were qualitatively consistent with reactivity. The other potential binding site was located at the alpha/beta interface. Here, the oxygen atoms of the analogs overlapped well with those of colchicine, but relative distances of the reactive carbons to the cysteine sulfur atoms did not correlate with the observed reactivity. A significant conformational change must occur in the colchicine binding site of tubulin in the transition from the unpolymerized to the polymerized state.

HLA-B2702 (77-83/83-77) peptide binds to beta-tubulin on human NK cells and blocks their cytotoxic capacity

It has been described that peptides derived from a highly conserved region of the alpha1 helix of the first domain of HLA class I Ags exhibit immunomodulatory capacity blocking both T and NK cell cytotoxicity. In vivo treatment with these peptides prolongs survival of MHC-mismatched allografts. However, the molecular bases of these effects are still unclear. In this study, we further analyze the mechanisms by which the dimeric peptide HLA-B2702 (77-83/83-77) induces suppression of NK cell cytotoxicity. This peptide inhibits natural and redirected lysis mediated by NK cells without significantly affecting effector-target cell binding. We have also isolated and sequenced a protein that binds this inhibitory peptide, which structurally corresponds to beta-tubulin. Tubulin is the major protein of microtubules and is involved in target cell killing. Furthermore, B2702 peptide promotes GTP-independent tubulin assembly, producing aggregates that cannot be depolymerized by cold. Treatment of NK cells with Taxol or demecolcine, which interfere with microtubule organization, also prevents NK cell cytotoxicity. Taken together, these results support the hypothesis that the peptide B2702 (77-83/83-77) exerts its inhibitory effect on NK cell cytotoxicity by inducing polymerization of microtubules and interfering with their normal assembly/disassembly dynamics.

The interaction of the B-ring of colchicine with alpha-tubulin: a novel footprinting approach

Tubulin, the major structural component of the microtubules, participates actively in mitotic spindle formation and chromosomal organization during cell division. Tubulin is the major target for a variety of anti-mitotic drugs. Some of the drugs, such as Vinca alkaloids and taxol, are routinely used for cancer chemotherapy. It is unfortunate that our knowledge of the binding sites on tubulin of these drugs is limited because of lack of a useful and appropriate tool. The photoaffinity labeling approach is the major technique available at present to detect the binding sites of drugs on tubulin. This method, however, has several limitations. First, only part of the binding site can be identified, namely, the residues which react with the photoaffinity label. Second, there are regions of tubulin which are not at the binding site but are affected by the binding of the drug; these regions can not be detected by the photoaffinity labeling approach. The third, and perhaps most serious, limitation is that the traditional approach can detect areas which have nothing to do with the binding of the ligand but which are within a certain distance of the binding site, that distance being less than the length of the photoreactive moiety attached to the ligand. There has been a great deal of controversy on the localization of the binding site of colchicine on tubulin, with some reports suggesting that the binding site is on alpha and some supporting a binding site on beta. Colchicine also has significant effects on tubulin conformation, but the regions which are affected have not been identified. We have attempted here to address these questions by a novel "footprinting" method by which the drug-binding sites and as well as the domain of tubulin affected by drug-induced conformational changes could be determined. Here, we report for the first time that the interaction of the B-ring of colchicine with the alpha-subunit affects a domain of tubulin which appears to be far from its binding site. This domain includes the cysteine residues at positions 295, 305, 315 and 316 on alpha-tubulin; these residues are located well away from the alpha/beta interface where colchicine appears to bind. This is correlated with the stabilizing effect of colchicine on the tubulin molecule. Furthermore, we also found that the B-ring of colchicine plays a major role in the stability of tubulin while the A and the C-rings have little effect on it. Our results therefore, support a model whereby colchicine binds at the alpha/beta interface of tubulin with the B-ring on the alpha-subunit and the A and the C-rings on the beta-subunit.

The interaction between lipid derivatives of colchicine and tubulin: consequences of the interaction of the alkaloid with lipid membranes

Colchicine is a potent antimitotic poison which is well known to prevent microtubule assembly by binding tubulin very tightly. Colchicine also possesses anti-inflammatory properties which are not well understood yet. Here we show that colchicine tightly interacts with lipid layers. The physical and biological properties of three different lipid derivatives of colchicine are investigated parallel to those of membrane lipids in the presence of colchicine. Upon insertion in the fatty alkyl chains, colchicine rigidifies the lipid monolayers in a fluid phase and fluidifies rigid monolayers. Similarly X-ray diffraction data show that lecithin-water phases are destabilized by colchicine. In addition, an unexpectedly drastic enhancement of the photoisomerization rate of colchicine into lumicolchicine in the lipid environment is observed and further supports insertion of the alkaloid in membranes. Finally the interaction of colchicine with lipids makes the drug inaccessible to tubulin. The possible in vivo significance of these results is discussed.

Synthesis of B-ring homologated estradiol analogues that modulate tubulin polymerization and microtubule stability

2-Methoxyestradiol is a cytotoxic human metabolite of estradiol with the ability to bind to the colchicine site of tubulin and inhibit its polymerization, and its 2-ethoxy analogue is even more potent. On the basis of a hypothetical relationship between the structures of colchicine and 2-methoxyestradiol, a B-ring-expanded 2-ethoxyestradiol analogue was synthesized in which the B-ring of the steroid is replaced by the B-ring of colchicine. The synthesis relied on the B-ring expansion of available 6-keto estradiol derivatives as opposed to a total synthesis of the homologated steroid framework. The relative configurations of the acetamido substituents in both epimers of the final product were determined by NOESY NMR and confirmed by X-ray crystallography. The epimer having the 6alpha-acetamido substituent was more active as an inhibitor of tubulin polymerization, and it was also more cytotoxic than the 6beta-epimer. These results are consistent with the proposed structural resemblance of 2-methoxyestradiol and colchicine. Several of the synthetic intermediates proved to be potent inhibitors of tubulin polymerization. On the other hand, a 3,17beta-diacetylated, B-ring-expanded analogue of 2-ethoxyestradiol having a ketone at C-6 resembled paclitaxel (Taxol) in its ability to enhance tubulin polymerization and stabilize microtubules. The corresponding 3-acetate and the 17beta-acetate were both synthesized, and it was determined that the 17beta-acetate, but not the 3-acetate, conferred on the steroid derivative its paclitaxel-like activity.

A steroid derivative with paclitaxel-like effects on tubulin polymerization

The endogenous estrogen metabolite 2-methoxyestradiol has modest antimitotic activity that may result from a weak interaction at the colchicine binding site of tubulin, but it nevertheless has in vivo antitumor activity. Synthetic efforts to improve activity led to compounds that increased inhibitory effects on cell growth, tubulin polymerization, and binding of colchicine to tubulin. This earlier work was directed at modifications in the steroid A ring, which is probably analogous to the colchicine tropolonic C ring. One of the most active analogs prepared was 2-ethoxyestradiol (2EE). We report here that different modifications in the steroid B ring of 2EE yield compounds with two apparently distinct modes of action. Simple expansion of the B ring to seven members resulted in a compound comparable to 2EE in its ability to inhibit tubulin polymerization and colchicine binding to tubulin. Acetylation of the hydroxyl groups in this analog and in 2EE essentially abolished these inhibitory properties. The introduction of a ketone functionality at C6, together with acetylation of the hydroxyls at positions 3 and 17, produced a compound with activity similar to that of paclitaxel, in that the agent enhanced tubulin polymerization into polymers that were partially stable at 0 degrees C. The acetyl group at C17, but not that at C3, was essential for this paclitaxel-like activity.

Conformation of microtubule-bound paclitaxel determined by fluorescence spectroscopy and REDOR NMR

The conformation of microtubule-bound paclitaxel has been examined by fluorescence and solid-state NMR spectroscopy. A fluorescent derivative of paclitaxel, 3'-N-debenzoyl-3'-N-(m-aminobenzoyl)paclitaxel (N-AB-PT), was prepared by semisynthesis. No differences in the microtubule-promoting activity between N-AB-PT and paclitaxel were observed, demonstrating that addition of the amino group did not adversely affect the ligand-receptor association. The distance between the fluorophore N-AB-PT and the colchicine binding site on tubulin polymers was determined through time-resolved measurements of fluorescence resonance energy transfer to be 29 +/- 2 A. The absorption and emission spectra of N-AB-PT bound to microtubules and in various solvents were measured. A plot of the Stokes shift as a function of solvent polarity was highly unusual. The Stokes shift increased linearly with solvent polarity in protic solvents, which is expected due to the nature of the fluorophore. In aprotic solvents, however, the Stokes shift was invariant with solvent polarity, indicating that the fluorophore was somehow shielded from the effects of the solvent. These data are best explained by considering the solution-state conformational properties of paclitaxel. It is known that paclitaxel adopts different conformations depending on the nature of the solvent, and these fluorescence data are consistent with the molecule adopting a "hydrophobic collapsed" conformation in protic solvents and an "extended" conformation in aprotic solvents. The Stokes shift of microtubule-bound N-AB-PT was within the protic solvent region, demonstrating that microtubule-bound paclitaxel is in a hydrophobic collapsed conformation. Microtubule-bound paclitaxel was also investigated by solid-state NMR. Paclitaxel was labeled with (19)F at the para position of the C-2 benzoyl substituent and with (13)C and (15)N in the side chain. Distances between the fluorine and carbon nuclei were determined by REDOR. The distance between the fluorine and the 3'-amide carbonyl carbon was 9.8 +/- 0.5 A, and the distance between the fluorine atom and the 3'-methine carbon was 10. 3 +/- 0.5 A. These spectroscopic data were used in conjunction with molecular modeling to refine the microtubule-bound conformation of paclitaxel and to suggest an alternative orientation of the ligand within the paclitaxel binding site.

A vulnerable period of colchicine toxicity during goldfish optic nerve regeneration

The effects of intraocular (i.o.) administration of the alkaloid colchicine on visual recovery following axotomy of the goldfish optic nerve were investigated. Under the experimental conditions used, control goldfish recovered vision, measured behaviorally, within 5-7 weeks of retro-orbital optic nerve crush. Fish injected i. o. with 0.1 microg of colchicine within 3 days of optic nerve crush (post-crush; PC) recovered vision after some delay relative to control fish, while injection with colchicine between 7 and 14 days PC produced a much more profound inhibition of recovery of vision, in most cases a complete block for the duration of the study (98 days). Further evidence for a delayed susceptibility of the regenerating optic nerve to colchicine following crush was reflected in a suppression of neurite outgrowth normally seen in explanted retinal tissue taken from PC goldfish. In addition, retrograde transport of the fluorescent dye 4-(4-didecylaminostyryl)-N-methylpyridinium iodide from the optic tectum to the retina as a measure of axonal continuity revealed substantially less labeling following i.o. administration of colchicine 1 week PC when compared to retinas from fish receiving colchicine at the time of optic nerve crush. Histological sections of the retina showed no evidence of residual retinal damage resulting from the colchicine injections or from interactions of axotomy and the drug administration. These results indicate a period of increased vulnerability of the regenerating visual system to the toxic effects of i.o. administered colchicine, beginning 3-5 days PC, and remaining until regenerating optic nerve fibers have begun to reach the tectum. While colchicine has many known effects on nerve function, it is proposed that the delayed susceptibility to disruption of regeneration observed in these experiments is largely, if not entirely, attributable to a colchicine-induced accumulation of tubulin heterodimers, which are known to block microtubule assembly and to participate in a feedback inhibition of tubulin synthesis. Thus, it is during the maximal induction of tubulin synthesis and of microtubule formation which normally occurs several days following axotomy that colchicine has its greatest effect. The results suggest that colchicine may be especially neurotoxic during neural development and regeneration.