Vinblastine-induced paracrystals and unusually large microtubules (macrotubules) in rat renal cells

George M, Freedman H, Barakat KH, Damaraju S, Hameroff S, Tuszynski JA. Computational predictions of volatile anesthetic interactions with the microtubule cytoskeleton: implications for side effects of general anesthesia

The cytoskeleton is essential to cell morphology, cargo trafficking, and cell division. As the neuronal cytoskeleton is extremely complex, it is no wonder that a startling number of neurodegenerative disorders (including but not limited to Alzheimer's disease, Parkinson's disease and Huntington's disease) share the common feature of a dysfunctional neuronal cytoskeleton. Recently, concern has been raised about a possible link between anesthesia, post-operative cognitive dysfunction, and the exacerbation of neurodegenerative disorders. Experimental investigations suggest that anesthetics bind to and affect cytoskeletal microtubules, and that anesthesia-related cognitive dysfunction involves microtubule instability, hyper-phosphorylation of the microtubule-associated protein tau, and tau separation from microtubules. However, exact mechanisms are yet to be identified. In this paper the interaction of anesthetics with the microtubule subunit protein tubulin is investigated using computer-modeling methods. Homology modeling, molecular dynamics simulations and surface geometry techniques were used to determine putative binding sites for volatile anesthetics on tubulin. This was followed by free energy based docking calculations for halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) on the tubulin body, and C-terminal regions for specific tubulin isotypes. Locations of the putative binding sites, halothane binding energies and the relation to cytoskeleton function are reported in this paper.

Gamma-tubulin can both nucleate microtubule assembly and self-assemble into novel tubular structures in mammalian cells

alpha-, beta-, and gamma-tubulins are evolutionarily highly conserved members of the tubulin gene superfamily. While the abundant members, alpha- and beta-tubulins, constitute the building blocks of cellular microtubule polymers, gamma-tubulin is a low abundance protein which localized to the pericentriolar material and may play a role in microtubule assembly. To test whether gamma-tubulin mediates the nucleation of microtubule assembly in vivo, and co-assembles with alpha- and beta-tubulins into microtubules or self-assembles into macro-molecular structures, we experimentally elevated the expression of gamma-tubulin in the cell cytoplasm. In most cells, overexpression of gamma-tubulin causes a dramatic reorganization of the cellular microtubule network. Furthermore, we show that when overexpressed, gamma-tubulin causes ectopic nucleation of microtubules which are not associated with the centrosome. In a fraction of cells, gamma-tubulin self-assembles into novel tubular structures with a diameter of approximately 50 nm (named gamma-tubules). Furthermore, unlike microtubules, gamma-tubules are resistant to cold or drug induced depolymerization. These data provide evidence that gamma-tubulin can cause nucleation of microtubule assembly and can self-assemble into novel tubular structures.

Effect and localization of trifluralin in Plasmodium falciparum gametocytes: an electron microscopic study

Trifluralin, a herbicide which is known to bind to plant and algal tubulin, induced ultrastructural changes in the microtubules of the mature Plasmodium falciparum gametocytes in vitro. Trifluralin treatment led to disassembly of the well ordered subpellicular microtubules, whereas it had no effect on microtubules of human platelets or of rat neuronal cells in vitro. The disassembled microtubules showed fragmented large tubular structures, which frequently were associated with the pellicular membranes. Electron microscopic autoradiography showed radioactive trifluralin associated with the microtubule fragments. These results provide evidence that trifluralin selectively binds to microtubules in malaria parasites and causes disruption of their structure.

Structural diversity and dynamics of microtubules and polymorphic tubulin assemblies

Tubulin, the main protein of microtubules (MTs), has the potency of forming a variety of other assembly products in vitro: rings, ring-crystals, C- and S-shaped ribbons, 10 nm fibres, hoops, sheets, heaped sheets, MT doublets, MT triplets, double-wall MTs, microtubules, curled ribbons, and paracrystals. The supramolecular subunits of all of them are the protofilaments which might be arranged either parallel to the axis (e.g., in MTs, ribbons) or curved (e.g., in hoops, microtubules). There is strong evidence that in the second case the protofilaments have an inside-out orientation compared to MTs. All assembly products mentioned are described structurally and their relevance to the in vivo situation is considered. Moreover, MTs and the other assemblies undergo permanent changes. These dynamics occurring in both individual assemblies and assembly populations are discussed from the structural point of view.

Influence of antimicrotubular drugs on the Golgi apparatus of stomach secretory mucoid cells and small intestine absorptive cells

The effects of vinblastine and colchicine on the Golgi apparatus of stomach surface mucoid and absorptive intestinal cells were compared by cytochemical analysis. The two epithelial cells were chosen because of their different specific functions in the formation of secretory granules, the production of lysosomes and the intensity of membrane traffic in the cytoplasm. For the analysis, adult mice were injected with 1 mg/100 g b.w. of vinblastine and 1 mg/100 g b.w. of colchicine. For the demonstration of cis and trans cisternae of the Golgi apparatus, prolonged osmification, thiamine pyrophosphatase and acid phosphatase activity identification were applied. After treatment with vinblastine or colchicine, polarity of stacks in the Golgi apparatus of surface mucoid cells is preserved although the number of cisternae with thiamine pyrophosphatase or acid phosphatase activity decreases. However, the Golgi apparatus of intestinal absorptive cells completely disintegrates and only a few separated cis or trans cisternae can be identified. The main effect seems to be a reduction of vesicles which can be cytochemically identified as parts of the Golgi apparatus and an accumulation of vesicles which probably originate from budding ER. Communication between the ER and the Golgi apparatus seems to be interrupted.

Paracrystalline inclusions in metaplastic ciliated cells of the human gastric mucosa. An ultrastructural study

Unusual electron-dense paracrystalline inclusions were found in metaplastic ciliated cells in the stomachs of three Japanese male patients with gastric carcinoma. These patients had not been given antitumour drugs before surgery and ethrane (enflurane) was used as the anaesthetic. Ciliated cells in the gastric mucosa are found not infrequently in the pyloric glands in association with intestinal metaplasia in elderly Japanese patients. Paracrystalline inclusions were found only in the ciliated cells and never in any other types of gastric mucosal cell. These inclusions were located in the apical portion of the ciliated cells in intimate association with the basal bodies. They consisted of twisted strings about 27 nm wide with a regularly repeated spacing of about 30 nm. On highly magnified electron micrographs, granules about 4 nm in diameter were detected. These paracrystalline inclusions have never been reported previously, although their location in ciliated cells and their morphological characteristics suggest an intimate relationship with the ciliogenesis of metaplastic ciliated cells in the human stomach.

Microtubules and macrotubules in fish meiosis

During meiosis in the male of a cyprinodontid fish, Aphyosemion splendopleure, and during the organization of the spindle of division, the spindle is made of two types of tubules: microtubules (20-25 nm) and macrotubules (30-50 nm). The macrotubules are associated only with the polar region of the meiotic apparatus and are located outside the spindle of microtubules. At the end of meiosis, the spindle microtubules depolymerize whereas the macrotubules remain. One can find them throughout the entire process of spermiogenesis; later, they disappear only at the end of spermatid maturation. We have studied four populations from Cameroon, three of them with macrotubules.

Arrangement of protofilaments in two forms of tubulin crystal induced by vinblastine

Tubulin crystals produced by incubation with vinblastine sulphate have been investigated by X-ray diffraction, electron microscopy and density measurement. The results show that crystals produced in vitro from bovine brain tubulin are made of packed helices, each consisting of a single continuous protofilament, whereas crystals produced in vivo in sea urchin eggs are built from pairs of such helices. The symmetry, cell dimensions, percentage protein and composition of the crystals are consistent with either 24 or 30 tubulin monomers per turn of the helix. The sea urchin egg crystals are more highly ordered and thus more suitable for further analysis.

Vinblastine-induced autophagocytosis: the effect of disorganization of microfilaments by cytochalasin B

The effects of disorganization of cellular microfilaments by cytochalasin B on vinblastine-induced autophagocytosis was studied in Ehrlich ascites tumor cells in vitro. Incubation with vinblastine induced a formation of autophagic vacuoles in the cytoplasm. The disorganization of microfilaments by cytochalasin B failed to inhibit vinblastine-induced autophagocytosis. Incubation with cytochalasin B alone induced a rapid formation of blebs on the cell surface. These contained cytoplasmic organelles and were connected by a narrow shaft to the main part of the cell. Thin subcortical microfilaments seen in the control cell cytoplasm were apparently relocated after cytochalasin B treatment and formed amorphous masses deeper in the cytoplasm. Vinblastine did not affect the formation of blebs after cytochalasin B treatment.

Purification and characterization of oocyte cytoplasmic tubulin and meiotic spindle tubulin of the surf clam Spisula solidissima

Assembly-competent tubulin was purified from the cytoplasm of unfertilized and parthogenetically activated oocytes, and from isolated meiotic spindles of the surf clam, Spisula solidissima. At 22 degrees C or 37 degrees C, Spisula tubulin assembled into 48-51-nm macrotubules during the first cycle of polymerization and 25-nm microtubules during the third and subsequent cycles of assembly. Macrotubules were formed from sheets of 26-27 protofilaments helically arranged at a 36 degree angle relative to the long axis of the polymer and were composed of alpha and beta tubulins and several other proteins ranging in molecular weight from 30,000 to 270,000. Third cycle microtubules contained 14-15 protofilaments in cross-section and were composed of greater than 95% alpha and beta tubulins. After three cycles of polymerization at 37 degrees C, unfertilized and activated oocyte tubulin self-assembled into microtubules at a critical concentration (Ccr) of 0.09 mg/ml. At the physiological temperature of 22 degrees C, unfertilized oocyte tubulin assembled into microtubules at a Ccr of 0.36 mg/ml, activated oocyte tubulin assembled at a Ccr of 0.42 mg/ml, and isolated meiotic spindle tubulin assembled at a Ccr of 0.33 mg/ml. The isoelectric points of tubulin from both unfertilized oocytes and isolated meiotic spindles were 5.8 for alpha tubulin and 5.6 for beta tubulin. In addition, one dimensional peptide maps of oocyte and spindle alpha and beta tubulins were very similar, if not identical. These results indicate that unfertilized oocyte tubulin and tubulin isolated from the first meiotic spindle are indistinguishable on the basis of assembly properties, isoelectric focusing, and one dimensional peptide mapping. These results suggest that the transition of tubulin from the quiescent oocyte state to that competent to form spindle microtubules in vivo does not require special modification of tubulin but may involve changes in the availability of microtubule organizing centers or assembly-promoting microtubule-associated proteins.

Light and electron microscopy of the effects of colchicine and vinblastine on maturing rat ameloblasts in vivo

Both agents caused loss of ruffled membrane and microtubules. Ruffled membrane decreased with time after treatment with vinblastine (4 mg/kg) and almost disappeared 8 h after treatment with either colchicine (4 mg/kg) or vinblastine (4 mg/kg). In vinblastine-treated animals, the number of microtubules were estimated in cross-sections of two regions of both smooth-ended and ruffle-ended ameloblasts; one region was distal and the other juxta-nuclear. In smooth-ended ameloblasts, microtubules of both the distal and juxta-nuclear cytoplasm almost disappeared after 2 h of treatment. In ruffle-ended ameloblasts, microtubules decreased in distal cytoplasm between 2 and 8 h; microtubules in the juxta-nuclear cytoplasm disappeared after 2 h. Both agents caused structural alterations and dislocation of cell organelles. The Golgi apparatus, sometimes separated into two parts, moved towards the proximal cytoplasm and an unusual accumulation of small vesicles was found in the proximal cytoplasm. These findings suggest that formation of ruffled membranes is dependent on microtubular function.

Appearance and disappearance of tubular paracrystalline structures in somatotrophs and lactotrophs during the annual life cycle of the bat (Myotis lucifugus)

The fine structure of the endocrine (granulated) cells of the adenohypophysis of the bat was examined at different periods of the annual life cycle. During the active homeothermic period (early spring until late summer) the endocrine cells were heavily granulated, and evidence of granule release was commonly seen. At this time, but not at other times (hibernation and arousal), prominent paracrystalline structures were numerous and transiently appeared in the cytoplasm of acidophils (apparently both lactotrophs and somatotrophs) but not in the cytoplasm of basophils. These paracrystalline bodies were composed of densely packed, parallel arrays of tubules. The paracrystalline structures in lactotrophs differed in size, shape, and the pattern of arrangement of component tubules from those of somatotrophs. During hibernation no evidence of exocytosis was seen, and secretory granules accumulated in the cytoplasm. Prior to arousal, secretory granules were removed by autophagy.

Microtubular aggregates in the rough endoplasmic reticulum of a myxoid chondrosarcoma

A tumor with the light microscopic and histochemical characteristics of myxoid chondrosarcoma is examined ultrastructurally. Instead of the usual features of chondroblasts, the tumor cells exhibited prominent collections of long straight microtubules within the endoplasmic reticulum. This finding is compared with other previously reported tubular aggregates. The differential diagnosis and ultrastructure of myxoid chondrosarcoma are also discussed.

The effects of vinblastine on acinar cells of the exorbital lacrimal gland of the rat

The effects of vinblastine treatment on acinar cells of the rat exorbital lacrimal gland were studied by electron microscopy. Experimental animals of both sexes were given single intraperitoneal injections of (1) vinblastine (4 mg/kg body weight) at 1 to 24 h before sacrifice; (2) pilocarpine (20 mg/kg b.w.) for 1 h; or (3) vinblastine for 1 h followed by pilocarpine for 1 h. Vinblastine treatment caused a number of changes including autophagocytosis, formation of intracisternal granules, and alteration of secretory granules. These changes varied in extent and onset between male and female rats. In addition, the Golgi apparatus was reduced in size and dispersed throughout the cytoplasm. Mitotic figures were commonly observed. Moreover, vinblastine inhibited the pilocarpine-stimulated degranulation of the acinar cells. In view of the known anti-microtubular action of vinblastine, these results suggest that microtubules are involved in various aspects of the transport, packaging, and secretion of exportable proteins in the lacrimal gland. Additionally, autophagocytosis and alteration of secretory granules may partially result from the interaction of vinblastine with membranes.

An association between mitochondria and microtubules in synaptosomes and axon terminals of cerebral cortex

A substantial number of synaptosomes prepared from rat cerebral cortex were found by electron microscopy to contain a horseshoe-shaped mitochondrion flanked by an arc of three to ten microtubules opposite the synaptic membrane specializations. The microtubules were replaced by characteristic paracrystals following the incubation of synaptosomes with vinblastine sulfate. A similar spatial organization of microtubules and mitochondria was also observed in some axon terminals of the cerebral cortex in situ. The significance of this novel observation is discussed with regard to the role of microtubules in axonal transport at the synaptic terminal and previous reports on the identification of tubulin in synaptosomes and subsynaptic structures.

Electron microscopic studies on embryonic chick spinal ganglion cells: in vitro effects of antimicrotubular agents on the Golgi complex

Spinal ganglia from 11 day chick embryos were incubated in media containing colchicine or vinblastine and subsequently examined by transmission electron microscopy. Both drugs caused a partial disappearance of cytoplasmic microtubules and a concomitant growth in the number of microfilaments in the neuroblasts. In ganglia treated with vinblastine for 4 h these effects were recorded morphometrically as a decrease of about 80% in the volume density of microtubules and a more than tenfold increase in the volume density of microfilaments. Furthermore, the cells displayed marked structural changes in the Golgi complex. The dictyosomes were mostly distinctly separated from each other and individual dictyosomes showed a decrease number of cisternae and an increased number of closely associated large vacuoles. The results are discussed with regard to the role of microtubules in the organization and function of the Golgi complex in nerve cells.

Light microscopical and ultrastructural observations on the effect of vinblastine on ameloblasts of rat incisors in vivo. I. Short-term effect on secretory ameloblasts

The highly polarized secretory ameloblasts in the incisors of rats fixed by perfusion with glutaraldehyde two or three hours after intravenous administration of vinblastine sulfate at a dosage of 5 mg per 100 g body weight were studied in the light microscope and the electron microscope. The following effects were observed: 1. All cytoplasmic microtubules in the ameloblasts had vanished. This was not accompanied by the appearance of paracrystals of microtubular protein or macrotubules. 2. The ameloblasts preserved their external features of polarized cells but lost their ability to maintain normal orderly segregation of the cell constituents; i.e. their normal internal compartmentalization and polarity had vanished. 3. The ameloblasts lost their capability of directional translocation of the secretory granules towards the cell apex. 4. Secretory granules already translocated to the cell apex regurgitated in the cell and a probably delayed discharge of secretory material had started in abnormal site at the surface. 5. The normal arrangement of ribosomes into polyribosomes on the membranes of the rough endoplasmic reticulum was no longer present; the ribosomes were apparently distributed at random. 6. New secretion was inhibited or brought to a standstill but secretory material already present in the Golgi complex appeared to be transported normally. 7. The centriole had started to develop into a cilium in many of the cells. 8. The number of autophagic vacuoles had increased.

Polymorphism of tubulin assembly. In vitro formation of sheet, twisted ribbon and microtubule

Tubulin assembly in equilibrium has been investigated by electron microscopy at different pH values in the absence or presence of Ca2+ at 37 degrees C. In the presence of 1 mM EGTA, tubulin assembled into microtubules in the pH range 6.0 to 7.0 and into sheets in the pH range 5.8 to 6.2. The sheet was composed of 8 to 22 protofilaments and frequently curled in a lateral direction. Two-block-polymers of sheet and microtubule were present in the boundary of two pH ranges (6.0-6.2). In the presence of 1 mM CaCl2, rings were formed in the pH range 6.0 to 7.0: macrotubules of diameter 75 nm and microtubules were both formed in the pH range 6.2 to 6.4, and two types of twisted ribbons in the pH range 5.8 to 6.2. One type of twisted ribbons had a pitch of around 150 nm and amplitude of around 40 nm, the other had 250 nm and 90 nm, respectively. Microtubules formed in the presence of CaCl2 were not distinguishable from those formed in the absence of CaCl2 in electron-micrographs, although they had different properties. The formation of all of these polymers was inhibited by 0.2 M KCl and was temperature dependent; they disappeared at 0 degrees C and reappeared at 37 degrees C. Interconversion between these polymers was also investigated. Globular particles were formed in the absence of magnesium ions. Addition of 50 muM colchicine induced formation of filamentous polymers. On the basis of these experimental results, the structural relation among different forms of tubulin assembly was discussed.

Studies on vinblastine-induced autophagocytosis in the mouse liver.I. The relation of lysosomal changes to general injurious effects

Intraperitoneal injection of two doses of vinblastine (10 mg/kg and 50 mg/kg) induced a prominent formation of autophagic vacuoles in mouse liver parenchymal cells in 4 h. Clearly recognizable organelles were seen inside these vacuoles. The amount of autophagy was dependent on doses in that autophagosomes almost disappeared within 12 h with the low dose, whereas with the high dose the -ells were filled with residual bodies. Defecation of lysosomal material was ovserved into the sinusoidal lumen 12 h after injection with high dose. The total activities of lysosomal enzymes acid phosphatase, beta-galactosidase, and beta-acetylglucosaminidase did not change in the liver after vinblastine administration. The soluble activities of beta-galactosidase and beta-glucosaminidase were elevated with the high dose 12 h after injection, indicating labilization of the lysosomal membranes. Simultaneously the activities of the three lysosomal enzymes were elevated in the serum. The injurious effect of VBL appeared in light-and electron microscopic levels indicating diffuse necrosis of the liver lobules with the high gose within 12 h, fat accumulation in the cells, accumulation of secretory vesicles containing very low density lipoprotein particles, partial dilatation, vesiculation of endoplasmic reticulum and dilatation of Golgi cisternae. The serum GOT and CPK levels were also elevated

Effect of vinblastine on the brush border of proximal tubule cells of rat kidney

A vinblastine-induced lesion of the brush border of proximal tubule cells of rat kidney was studied by scanning and transmission electron microscopy. Adult rats were given two tail-vein injections of vinblastine sulfate and then sacrificed either three or sixteen hours after administration of the first injection. The proximal tubules of animals treated with the drug for three hours differed from those of saline-treated controls in possessing (1) well defined areas of apical cell surface devoid of microvilli and (2) fewer microcraters in the brush border. In tubules of animals treated with vinblastine for sixteen hours, the areas devoid of microvilli were much more extensive, and microcraters were less frequently seen. Cilia did not appear to be affected by exposure to the drug and were observed both in regions devoid of microvilli and in areas with a well developed brush border. The mechanism of action of vinblastine in eliciting focal loss of microvilli of proximal tubule cells is not known. Several hypotheses to account for this lesion of the brush border are discussed.

Cytotoxicity of vinblastine and vincristine to pancreatic acinar cells

BALB/c mice were given vinblastine (Velbe), 40 and 200 mg/kg, or vincristine (Oncovin), 20 mg/kg, dissolved in saline as single intraperitoneal injections. Nontreated mice served as controls. Animals were killed 1, 2, 5, and 24 hours and 4 days after the drug administration and small pieces from the pancreas were processed for electron microscopy. Autophagocytotic activity increased 3 hours after the injection and at 5 and 24 hours there were large amounts of autophagic vacuoles containing altered cell organelles and membranous or amorphous debris in the acinar cells. Cell death and acinar atrophy were evident at later time intervals. It was concluded that the vinca alkaloids caused autophagy and degeneration in pancreatic acinar cells and that their inhibitory effect on pancreatic enzyme secretion might depend partly or solely on their cytotoxic action.

Vinblastine-induced microtubular paracrystals in prolactin cells of anterior pituitary gland of lactating rats

Vinblastine sulfate in physiological saline was injected directly into the pituitary glands of lactating rats. Injections were made through the ear canal using a syringe equipped with a 24-gauge needle. The animals were killed at 2, 4, or 6 hours after the injections. When the anterior pituitary glands were examined by electron microscopy, many microtubular paracrystalline deposits were seen in the prolactin and growth hormone cells. The usual cytoplasmic microtubules and microfilaments were not seen in the cytoplasm of these cells. Granular extrusion (exocytosis) was markedly depressed, and an accumulation of secretory granules was definitely observed in the prolactin cells after the administration of vinblastine.

Vinblastine-induced ultrastructural changes in perisinusoidal cells of the rat liver

Vinblastine, given intravenously to rats, leads to disappearance of microtubules, formation of paracrystalline inclusions and accumulation of fat droplets in the cytoplasm of the perisinusoidal cells of the liver suggesting that microtubules of the perisinusoidal cells play a role in lipid metabolism.

Substructural analysis of the microtubule and its polymorphic forms

Microtubules are composed of 13 protofilaments which are aligned parallel to the long axis of the tubule, as can be seen in thin sections of material fixed in the presence of tannic acid. Evidence is presented based upon Markham rotations of the isolated flagellar axoneme that the bridges (spoke, nexin, dynein) must connect to certain specified protofilaments. By means of geometric considerations it is concluded that the bridges in a Centrohelida and Raphidiophrys also connect certain specified protofilaments. Two polymorphic forms of microtubules were also examined using tannic acid in the fixative, a 320 A tubule produced in Echinosphaerium by treatment with low temperature, and a vinblastine-induced microtubule crystal. In both forms, the arrangement of the subunits is not in the form of linear protofilaments. The vinblastine crystal in transverse section is made up of hexagonally packed tubules approximately 320 A in diameter. The wall of each tubule in turn is composed of two parallel protofilaments which spiral upwards, giving the appearance of a barber pole. The center-to-center separation of adjacent protofilaments is 180 A. We conclude that the tubulin heterodimer lies parallel to the axis of the protofilaments, exposing the colchicine binding site.

Microtubules with 15 subunits in cockroach epidermal cells

Since Ledbetter and Porter (1964) described the 13 subunits which are visible in cross sections of negatively stained plant microtubules, subsequent observations have generally confirmed this number. By using Mizuhira's fixative composed of tannic acid and glutaraldehyde, it is easyto demonstrate the subunits of microtublules without optical reinforcement Cytoplasmic microtubules and sperm axonemes, fixed with Mizuhira's fixtive, similarly show 13 subunits (Mizuhira's and Futaesaku, 1971, 1972; Futaesaky et al., 1972; Tilney et al., 1973). This paper will describe a particular type of microtubule in insect epidermal cells fixed with the above fixative. The number of the subunits is found to be 15 in tranverse sections.

Microtubule-macrotubule transitions: intermediates after exposure to the mitotic inhibitor vinblastine

Vinblastine treatment of microtubule protein or intact microtubules assembled in vitro produced bifilar rings and bifilar helices. Suspentsions of rings and helices were demonstrated to bind [(3)H]colchicine, a diagnostic property of microtubule protein. Macrotubules are suggested to consist of tightly coiled helices formed by longitudinal compacting of loosely coiled protofilament pair intermediates.

Microtubule biogenesis and cell shape in Ochromonas. 3. Effects of herbicidal mitotic inhibitor isopropyl N-phenylcarbamate on shape and flagellum regeneration

The role of microtubules and microtubule nucleating sites in the unicell, Ochromonas has been examined through the use of two mitotic inhibitors, isopropyl N-phenylcarbamate (IPC) and isopropyl N-3-chlorophenyl carbamate (CIPC). Although IPC and CIPC have little or no effect on intact microtubules, the assembly of three separate sets of microtubules in Ochromonas has been found to be differentially affected by IPC and CIPC. The assembly of flagellar microtubules after mechanical deflagellation is partially inhibited; the reassembly of rhizoplast microtubules after pressure depolymerization is totally inhibited (however, macrotubules may form at the sites of microtubule initiation or elsewhere); and, the reassembly of the beak set of microtubules after pressure depolymerization may be unaffected although similar concentrations of IPC and CICP completely inhibit microtubule regeneration on the rhizoplast. These effects on microtubule assembly, either inhibitory or macrotubule inducing, are fully reversible. The kinetics of inhibition and reversal are found to be generally similar for both flagellar and cell shape regeneration. Incorporation data suggest that neither IPC nor CIPC has significant effects on protein synthesis in short term experiments. Conversely, inhibiting protein synthesis with cycloheximide has little effect on microtubule regeneration when IPC or CIPC is removed. Although the exact target for IPC and CIPC action remains uncertain, the available evidence suggests that the microtubule protein pool or the microtubule nucleating sites are specifically and reversibly affected. Comparative experiments using the mitotic inhibitor colchicine indicate some similarities and differences in its mode of action with respect to that of IPC and CIPC on assembly and disassembly of microtubules in these cells.