Breakthrough in microtubule research

Abstract

A breakthrough occurred in microtubule research during 1988 by the demonstration of Koshland and co-workers using a microtubule-chromosome model system. They discovered that chromosome movement towards the cell pole during the anaphase of cell division is both powered and regulated by microtubule depolymerization at the chromosome end of the microtubule [1]. The experiment has bearings on further directions of basic research on cell division [2]. It is thus timely to summarize past and present linkages between microtubules and clinical practice. Microtubules are tube-shaped structures with an outer diameter of approximately 24 nm and a variable length, sometimes reaching 10–25 μm. The main building block is a dumb-bell shaped dimer protein composed of α-tubulin and β-tubulin. Microtubule-associated proteins (MAPs) and guanosine nucleotides, GTP and GDP, are essential for microtubule structure, stability and function [3, 4]. Microtubules are ubiquitous cell structures mainly involved in the formation and maintenance of cell shape and in cell transport phenomena such as chromosome separation in cell division, transport within the axon of the nerve cell, leucocyte chemotaxis and phagocytosis, cilial and flagellar movements [3–6]. The mitotic spindle is mainly composed of microtubules. Low concentrations of microtubule depolymerizers, such as colchicine, block cell division in prometaphase, the classical and conspicuous picture of ‘metaphase arrest’. Interestingly, the microtubule polymerizer taxol, which has an opposite mechanism of action, blocks cell division even more efficiently at the same stage of the cell cycle [7]. It should be emphasized that the effect of microtubule depolymerizers is not restricted to the mitosis stage of the cell cycle. Inhibition was demonstrated both in the G1 stage and G2 stage of the interphase between two cell divisions [3, 8]. Another unexplained effect of microtubule depolymerizers is that DNA synthesis was stimulated in several cell systems. The microtubule polymerizer taxol lacked such effects on interphase cells [3]. Much of the information available on the biological properties of microtubules has been derived from experiments with microtubule depolymerizers (colchicine, Catharantus derivatives, podophyllotoxin, griseofulvin), each with specific binding sites on the tubulin molecule [3]. These substances have played a rôle in current anti-inflammatory, antiviral, antifungal and antineoplastic chemotherapy. In this context, it is pertinent to remember that colchicine is the classical remedy for acute gout [3]. The mechanism of action is likely to be inhibition of neutrophil leucocyte chemotaxis due to suppression of the release of signal substances from phagocytosing leucocytes. The antifungal agent griseofluvin is as efficient as colchicine in the treatment of acute gout and lacks the serious side-effects of colchicine [cf. 9]. A semisynthetic podophyllin preparation (SPG) has been used in Scandinavia as an alternative to other cytotoxic agents in the treatment of severe rheumatoid arthritis. An interphase block in the G2 stage was found in bone marrow cells of patients treated with the preparation [10]. The observation does not exclude metaphase arrest at higher concentrations. Pure podophyllotoxin, which is stable in solution, has now replaced crude podophyllin preparations in the treatment of venereal warts, condyloma accuminatum [11]. An analogous development would seem desirable in anti-rheumatoid therapy [12]. In antineoplastic chemotherapy, a colchicine derivative (demecolchine, Colcemid®) was introduced for the treatment of leukaemia in the 1950s. The launch failed, but the substance was more convenient and stable than the parent compound and became popular in research laboratories and in the laboratories of clinical genetics for the accumulation of mitoses and improvement of chromosome morphology. The latter phenomenon is thought to be due to chromosome stretching by microtubule depolymerization [7], analogous to chromosome transport originally observed by Inové [13], nuclear segmentation of interphase cells [14], and chromosome transport in the new microtubule-chromosome model [1, 2]. The part of the microtubule inhibitors on the antineoplastic market was conquered by the Catharantus derivatives, formerly called Vinca alkaloids, in the 1960s [15]. Since then, modifications of the original molecules have emerged and disappeared. A fascinating metamorphosis is that oestramustine, originally thought to be an alkylating agent directed to oestrogen receptors by the hormone part of the molecule, is now considered to be a microtubule inhibitor with a specific action on microtubule-associated proteins [cf. 6]. The common denominator in microtubule depolymerizer therapy of inflammatory, viral, fungal, and neoplastic diseases is interference with growth and communication of key cells in the pathological process. The new microtubule-chromosome model [1, 2], together with other recent advances in the investigation of microtubule biology [16, 17], is expected to provide a tool for the improvement of such therapy.