Colchicine-binding activity distinguishes sea urchin egg and outer doublet tubulins

The adverse outcome pathway (AOP) for chemical binding to tubulin in oocytes leading to aneuploid offspring

The Organisation for Economic Co-operation and Development (OECD) has launched the Adverse Outcome Pathway (AOP) Programme to advance knowledge of pathways of toxicity and improve the use of mechanistic information in risk assessment. An AOP links a molecular initiating event (MIE) to an adverse outcome (AO) through intermediate key events (KE). Here, we present the scientific evidence in support of an AOP whereby chemicals that bind to tubulin cause microtubule depolymerization resulting in spindle disorganization followed by altered chromosome alignment and segregation and the generation of aneuploidy in female germ cells, ultimately leading to aneuploidy in the offspring. Aneuploidy, an abnormal number of chromosomes that is not an exact multiple of the haploid number, is a well-known cause of human disease and represents a major cause of infertility, pregnancy failure, and serious genetic disorders in the offspring. Among chemicals that induce aneuploidy in female germ cells, a large majority impairs microtubule dynamics and spindle function. Colchicine, a prototypical chemical that binds to tubulin and causes microtubule depolymerization, is used here to illustrate the AOP. This AOP is specific to female germ cells exposed during the periovulation period. Although the majority of the data come from rodent studies, the available evidence suggests that the MIE and KEs are conserved across species and would occur in human oocytes. The development of AOPs related to mutagenicity in germ cells is expected to aid the identification of potential hazards to germ cell genomic integrity and support regulatory efforts to protect population health.

Assembly and colchicine binding characteristics of tubulin with maximally tyrosinated and detyrosinated alpha-tubulins

The posttranslational removal and readdition of tyrosine at the C-terminus of alpha-tubulin is associated with generation of microtubule populations that differ in intracellular distributions, turnover rates, and sensitivities to microtubule-depolymerization agents. Here, we compared the in vitro assembly and colchicine binding characteristics of tubulin dimer preparations composed of alpha-tubulin that had been maximally tyrosinated (approximately 40% tyrosinated) by tubulin-tyrosine ligase and maximally detyrosinated (100% detyrosinated) by carboxypeptidase A. Maximally tyrosinated and detyrosinated tubulins had similar critical concentrations for polymerization and similar association constants for colchicine binding. Microtubules polymerized from the two tubulins also had similar steady-state mean lengths and length distributions. The growing and shortening dynamics (dynamic instability parameters) of individual microtubules made from maximally tyrosinated or detyrosinated alpha-tubulin as determined by video-enhanced dark-field microscopy were similar, but subtle differences in the growing and shortening rates were found. On balance, however, the dynamicity and thus the overall kinetic stability of the two microtubule populations were indistinguishable. The results support the idea that detyrosination of alpha-tubulin does not by itself generate stable microtubules.

Purification of a low molecular weight microtubule binding protein from sea urchin eggs

A low molecular weight microtubule binding protein(SU-MAP34) was purified from sea urchin eggs. This protein bound strongly to the microtubule formed from purified echinoderm tubulin but showed no cross-linking of microtubules. Monospecific antibody against SU-MAP34 was produced and an immunoblotting analysis showed that this protein was not a breakdown product of a protein of a higher molecular mass. Whole cell staining and confocal laser scanning microscope observation showed that SU-MAP34 localized on the filamentous structure of mitotic apparatus and this structure was identified as the microtubule with double staining using anti-SU-MAP34 and anti-tubulin. An immunoblotting experiment showed an enrichment of SU-MAP34 in a microtubule protein fraction prepared using taxol from a crude extract of the cell.

Colchicine-binding sites of brain tubulins from an antarctic fish and from a mammal are functionally similar, but not identical: implications for microtubule assembly at low temperature

The tubulins of Antarctic fishes possess adaptations that favor microtubule formation at low body temperatures (Detrich et al.: Biochemistry 28:10085-10093, 1989). To determine whether some of these adaptations may be present in a domain of tubulin that participates directly or indirectly in lateral contact between microtubule protofilaments, we have examined the energetics of the binding of colchicine, a drug thought to bind to such a site, to pure brain tubulins from an Antarctic fish (Notothenia gibberifrons) and from a mammal (the cow, Bos taurus). At temperatures between 0 and 20 degrees C, the affinity constants for colchicine binding to the fish tubulin were slightly smaller (1.5-2.6-fold) than those for bovine tubulin. van't Hoff analysis showed that the standard enthalpy changes for colchicine binding to the two tubulins were comparable (delta H degrees = +10.6 and +7.4 kcal mol-1 for piscine and bovine tubulins, respectively), as were the standard entropy changes (delta S degrees = +61.3 eu for N. gibberifrons tubulin, +51.2 eu for bovine tubulin). At saturating concentrations of the ligand, the maximal binding stoichiometry for each tubulin was approximately 1 mol colchicine/mol tubulin dimer. The data indicate that the colchicine-binding sites of the two tubulins are similar, but probably not identical, in structure. The apparent absence of major structural modifications at the colchicine site suggests that this region of tubulin is not involved in functional adaptation for low-temperature polymerization. Rather, the colchicine site of tubulin may have been conserved evolutionarily to serve in vivo as a receptor for endogenous molecules (i.e., "colchicine-like" molecules or MAPs) that regulate microtubule assembly.

Interactions of colchicine with tubulin

Colchicine exerts its biological effects through binding to the soluble tubulin heterodimer, the major component of the microtubule. The colchicine-binding abilities of tubulins from a variety of sources are summarized, and the mechanism of colchicine binding to brain tubulin is explored in depth. The relationship between colchicinoid structure and tubulin binding activity provides insight into the structural features of colchicine responsible for high affinity binding to tubulin and is reviewed for analogs in the colchicine series. The thermodynamic and kinetic aspects of the association are described and evaluated in terms of the binding mechanism. Colchicine binding to tubulin results in unusual alterations in the low energy electronic spectra of colchicine. The spectroscopic features of colchicine bound to tubulin are discussed in terms of the nature of the colchicine-tubulin complex. Attempts to locate the high affinity colchicine binding site on tubulin are presented.

Neurite elongation is blocked if microtubule polymerization is inhibited in PC12 cells

We have injected process-bearing PC12 cells with colchicine-tubulin mixed with either fluorescein-dextran or a rhodamine-labelled tubulin analogue to determine the role of microtubule polymerization in neurite elongation. Colchicine-tubulin is a specific, substoichiometric poison of microtubule assembly. We have shown that colchicine-tubulin does not cause existing PC12 microtubules to disassemble, and yet can inhibit the assembly of rhodamine-tubulin injected along with it. In population studies of neurite outgrowth in injected and uninjected cells, we find that colchicine-tubulin substantially inhibits neurite extension from injected cells over a wide variety of concentrations. In acute time-course studies of injected cells, we find that colchicine-tubulin does not block neurite outgrowth until the injectate reaches the neurite tip. Thereafter, however, it blocks process elongation completely. Thus we can conclude that microtubule polymerization in the region of the growth cone is an important element in neurite elongation. While polymerization at the cell body may be important in supplying subunits to the distal neurite, it does not play a direct role in process extension.

Separation of tubulin subunits by reversed-phase high-performance liquid chromatography

When properly solubilized with trifluoroacetic acid (TFA), alpha- and beta-tubulin subunits from a variety of sources may be resolved at high yield by reversed-phase high-performance liquid chromatography (HPLC), using a Waters muBondapak C18 column and simple linear aqueous acetonitrile gradients containing TFA. The tubulin subunits are typically the most non-polar proteins present, with the beta-tubulin subunit eluting before the alpha. Column temperature above ambient improve both the resolution and the yield; less polar solvent systems do not. Tubulins not freely soluble in aqueous TFA may be solubilized in 6 M guanidine-hydrochloric acid with no change in retention time. Other columns with shorter carbon chain lengths and larger pore size produce a single, unresolved tubulin peak. Reversed-phase HPLC analysis provides an independent comparative evaluation of organelle-specific tubulins, with characteristic retention time differences observed between homologous ciliary and flagellar outer doublet tubulin subunits and also between them and their cytoplasmic counterparts.

High degree of satellite association and decreased nucleolar organizer activity in cystic fibrosis subjects

The effect of colchicine at concentrations of 0.25 X 10(-6) M, 1.0 X 10(-6) M, and 2.0 X 10(-6) M on the degree of satellite association (SA) was estimated in phytohemagglutinin-stimulated lymphocytes of individuals in the following groups: cystic fibrosis (CF) children, obligatory CF heterozygotes, control children, and control adults. In all four groups increasing colchicine concentration caused a higher degree of SA. The degree of SA differed between the two control age groups (children vs adults) only at the lowest concentration. CF patients had a significantly higher degree of SA than CF heterozygotes and than control individuals at all colchicine concentrations; CF heterozygotes had a significantly higher degree of SA than control adults at the low and intermediate concentrations. There was a strong interaction between genotype and colchicine concentration: the differences between the CF patients and the control individuals were most distinct at the intermediate concentration and between the carriers and the control individuals at the low colchicine concentration. Colchicine had no effect on the activity of the nucleolar organizer regions (NORs), as measured by the frequency of the silver-stained NORs (AgNORs), while the frequency of AgNORs in CF patients was significantly lower as compared to control individuals. Yet, the increase in the degree of SA caused by the CF mutant allele involved specifically the satellited chromosomes carrying active NORs.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Sorting and transepithelial transport of adsorbed protein tracers: effects of temperature

The epithelium of the guinea pig yolk sac is involved in the selective transport of macromolecules to the fetus. We studied the compartments involved in sorting and transepithelial transport of protein tracers and the effect of lowered temperature (18 degrees C) on these events. Explants of yolk sac were incubated with a mixture of cationized ferritin (CF) and horseradish peroxidase (HRP, Sigma type VI). At 4 degrees C, both tracers were bound to the cell surface and binding of an HRP-gold complex was shown to be inhibited by mannan. At 37 degrees C and 18 degrees C, both tracers were taken up into tubules and vesicles in the apical cytoplasm. Usually the tubules contained a mixture of tracers, but they often showed a polarized distribution with CF and HRP at opposite ends. The vesicles also contained mixtures of the tracers, but some contained only one. In addition, there were some irregularly shaped vacuoles composed of saccules that contained either a mixture, HRP alone, or CF alone. These results suggest that these adsorbed ligands are binding to unique microdomains of the endocytic complex. After 20 min at 37 degrees C coated vesicles 100 nm in diameter were located in the apical cytoplasm and coated vesicles of the same size were located at the lateral cell membrane. Usually they contained only HRP or CF, although occasional mixtures were seen. At 18 degrees C, HRP was transported across the cells in 100 nm vesicles. However, transport of CF was completely inhibited at the lower temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Microtubule assembly is dependent on a cluster of basic residues in alpha-tubulin

Previous studies have shown that tubulin, a major protein component of the microtubule, is rendered assembly incompetent when a highly reactive lysine residue (HRL) in the alpha polypeptide of tubulin dimer is reductively methylated [cf. Sherman, G., Rosenberry, T. L., & Sternlicht, H. (1983) J. Biol. Chem. 258, 2148-2156]. In this study we demonstrate that the HRL in bovine brain tubulin is Lys-394, a residue proximal in the alpha-tubulin sequence to the highly negatively charged carboxy-terminus region (residues 412-450) previously implicated in assembly. pH studies were undertaken to probe the local environment of Lys-394. These studies indicated that Lys-394 reactivity toward HCHO is sensitive to the titration of a pKa 6.3 group presumed to be a histidine residue. This assignment is supported by our finding that histidine modification via diethyl pyrocarbonate strongly affects Lys-394 reactivity toward HCHO as well as microtubule assembly. We propose on the basis of secondary structure considerations and published sequence data for a variety of tubulins that Lys-394 is part of an evolutionarily conserved cluster of basic residues (effective charge: 2+ to 2.5+ at neutral pH) composed of Lys-394, His-393, and Arg-390, which is important for tubulin function and which renders Lys-394 reactive as a nucleophile.