Microtubule polarity determination based on formation of protofilament hooks

Hooks and comets: The story of microtubule polarity orientation in the neuron

It is widely believed that signature patterns of microtubule polarity orientation within axons and dendrites underlie compositional and morphological differences that distinguish these neuronal processes from one another. Axons of vertebrate neurons display uniformly plus-end-distal microtubules, whereas their dendrites display non-uniformly oriented microtubules. Recent studies on insect neurons suggest that it is the minus-end-distal microtubules that are the critical feature of the dendritic microtubule array, whether or not they are accompanied by plus-end-distal microtubules. Discussed in this article are the history of these findings, their implications for the regulation of neuronal polarity across the animal kingdom, and potential mechanisms by which neurons establish the distinct microtubule polarity patterns that define axons and dendrites.

Basic fibroblast growth factor elicits formation of interstitial axonal branches via enhanced severing of microtubules

This article demonstrates that the augmentation of axonal branching induced by bFGF is explicable on the basis of an enhancement of microtubule-severing, and that three different proteins related to microtubule-severing are affected by treatment of neurons with bFGF.

The formation of interstitial axonal branches involves the severing of microtubules at sites where new branches form. Here we wished to ascertain whether basic fibroblast growth factor (bFGF) enhances axonal branching through alterations in proteins involved in the severing of microtubules. We found that treatment of cultured hippocampal neurons with bFGF heightens expression of both katanin and spastin, which are proteins that sever microtubules in the axon. In addition, treatment with bFGF enhances phosphorylation of tau at sites expected to cause it to dissociate from microtubules. This is important because tau regulates the access of katanin to the microtubule. In live-cell imaging experiments, axons of neurons treated with bFGF displayed greater numbers of dynamic free ends of microtubules, as well as greater numbers of short mobile microtubules. Entirely similar enhancement of axonal branching, short microtubule transport, and frequency of microtubule ends was observed when spastin was overexpressed in the neurons. Depletion of either katanin or spastin with siRNA diminished but did not eliminate the enhancement in branching elicited by bFGF. Collectively, these results indicate that bFGF enhances axonal branch formation by augmenting the severing of microtubules through both a spastin-based mode and a katanin-based mode.

The Kinesin Klp2 Mediates Polarization of Interphase Microtubules in Fission Yeast

Fission yeast (Schizosaccharomyces pombe) cells grow longitudinally in a manner dependent on a polarized distribution of their interphase microtubules. We found that this distribution required sliding of microtubules toward the cell center along preexisting microtubules. This sliding was mediated by the minus end-directed kinesin motor Klp2, which helped microtubules to become properly organized with plus ends predominantly oriented toward the cell ends and minus ends toward the cell center. Thus, interphase microtubules in the fission yeast require motor activities for their proper organization.

Higher plant cortical microtubule array analyzed in vitro in the presence of the cell wall

Plant morphogenesis depends on an array of microtubules in the cell cortex, the cortical array. Although the cortical array is known to be essential for morphogenesis, it is not known how the array becomes organized or how it functions mechanistically. Here, we report the development of an in vitro model that provides good access to the cortical array while preserving the array's organization and, importantly, its association with the cell wall. Primary roots of maize (Zea mays) are sectioned, without fixation, in a drop of buffer and then incubated as desired before eventual fixation. Sectioning removes cytoplasm except for a residuum comprising cortical microtubules, vesicles, and fragments of plasma membrane underlying the microtubules. The majority of the cortical microtubules remain in the cut-open cells for more than 1 h, fully accessible to the incubation solution. The growth zone or more mature tissue can be sectioned, providing access to cortical arrays that are oriented either transversely or obliquely to the long axis of the root. Using this assay, we report, first, that cortical microtubule stability is regulated by protein phosphorylation; second, that cortical microtubule stability is a function of orientation, with divergent microtubules within the array depolymerizing within minutes of sectioning; and third, that the polarity of microtubules in the cortical array is not uniform. These results suggest that the organization of the cortical array involves random nucleation followed by selective stabilization of microtubules formed at the appropriate orientation, and that the signal specifying alignment must treat orientations of +/- 180 degrees as equivalent.

Uniform apicobasal polarity of microtubules and apical location of gamma-tubulin in polarized intestinal epithelium in situ

Polarized differentiation of the intestinal epithelium has been previously shown to depend on an intact microtubular system that is essential for vectorial delivery of apical membrane proteins to the apical cell surfaces. Uniform alignment and polarization of microtubules have been suggested to provide the ultrastructural basis for vectorial transport between the Golgi apparatus and the apical cell surface. In the present study we applied the hook decoration technique to analyse the polarity of microtubules in the rat jejunal epithelium. By immunocytochemistry we studied the subcellular location of gamma-tubulin, an essential component of microtubule-organizing centers. Microtubules were found to be mainly aligned parallel to the apicobasal axis of the cells and to extend from the subterminal space underneath the apical terminal web down to the cellular basis. We found that 98% out of 1122 decorated microtubules displayed uniform apicobasal polarity with the growing ends (plus ends) pointing basally and the non-growing ends (minus ends) pointing towards the cellular apex. No differences were observed with respect to microtubular polarity between the apical, perinuclear and infranuclear cellular portions. Immunostaining specific for gamma-tubulin was restricted to the apical subterminal space underneath the rootlets of microvilli. These findings indicate that the apical subterminal space of enterocytes serves as the predominant if not exclusive microtubule-organizing compartment from which uniformly polarized microtubules grow out with their plus ends towards the cellular basis.

Microtubules in Xenopus oocytes are oriented with their minus-ends towards the cortex

Despite lacking centrosomes, stage VI Xenopus oocytes contain extensive networks of cytoplasmic microtubules (MTs). To gain additional insight into the factors regulating MT organization during oogenesis, we have used electron microscopy and "hook decoration" to examine the distribution and orientation of MTs in Xenopus oocytes. A limited survey of two "undecorated" stage VI oocytes revealed 218 MTs in images covering approximately 2,500 microm(2), and indicated that the MT number density of the animal cytoplasm was greater than that of the vegetal cytoplasm. Examination of five "decorated" stage VI oocytes (three animal and five vegetal hemispheres) revealed 653 MTs. Of these, 76% could be scored as having exclusively counterclockwise (CCW) or clockwise (CW) hooks. In the animal hemispheres, 93% of the scored MTs exhibited CCW hooks when viewed from the direction of the cortex, indicating that most MTs were oriented with their minus-ends out. MT orientation appeared relatively uniform throughout the animal cytoplasm: more than 90% of the scored MTs in the cortical (90%), subcortical (96%), or perinuclear (98%) cytoplasm were oriented with their minus-ends out. In the vegetal hemispheres, approximately 80% of the scored MTs exhibited CCW hooks, and thus were oriented with their minus-ends out; 96% of the scored MTs in stage III oocytes were oriented minus-end out. These observations support a model in which the cortex plays a significant role in MT nucleation and organization in Xenopus oocytes, and have significant implications for the MT-dependent transport and localization of cytoplasmic organelles and RNAs during oogenesis.

Nonuniform microtubular polarity established by CHO1/MKLP1 motor protein is necessary for process formation of podocytes

Podocytes are unique cells that are decisively involved in glomerular filtration. They are equipped with a complex process system consisting of major processes and foot processes whose function is insufficiently understood (Mundel, P., and W. Kriz. 1995. Anat. Embryol. 192:385-397). The major processes of podocytes contain a microtubular cytoskeleton. Taking advantage of a recently established cell culture system for podocytes with preserved ability to form processes (Mundel, P., J. Reiser, A. Zúñiga Mejía Borja, H. Pavenstädt, G.R. Davidson, W. Kriz, and R. Zeller. 1997b. Exp. Cell Res. 36:248-258), we studied the functional significance of the microtubular system in major processes. The following data were obtained: (a) Microtubules (MTs) in podocytes show a nonuniform polarity as revealed by hook-decoration. (b) CHO1/ MKLP1, a kinesin-like motor protein, is associated with MTs in podocytes. (c) Treatment of differentiating podocytes with CHO1/MKLP1 antisense oligonucleotides abolished the formation of processes and the nonuniform polarity of MTs. (d) During the recovery from taxol treatment, taxol-stabilized (nocodazole- resistant) MT fragments were distributed in the cell periphery along newly assembled nocodazole-sensitive MTs. A similar distribution pattern of CHO1/MKLP1 was found under these circumstances, indicating its association with MTs. (e) In the recovery phase after complete depolymerization, MTs reassembled exclusively at centrosomes. Taken together, these findings lead to the conclusion that the nonuniform MT polarity in podocytes established by CHO1/MKLP1 is necessary for process formation.

Polarity of microtubule assemblies during neuronal cell migration

The active migration of neurons from their sites of origin to their final destinations requires the unidirectional translocation of the nuclei and somatic cytoplasm within the growing leading processes. To explore the cellular machinery underlying this translocation, we determined the polarity of microtubules situated within the leading and trailing processes of migrating cerebellar granule cells in situ. Our analysis reveals that the newly assembled positive ends of the microtubules in the leading process uniformly face the growing tip, while their disintegrating negative ends face the nucleus. In the trailing process, by contrast, microtubule arrays are of mixed polarity. We suggest that the dynamics of slow polymerization in combination with fast disintegration of oriented microtubules create "push" and "pull" forces that contribute to the piston-like saltatory displacement of the nucleus and cytoplasm within the membrane cylinder of the leading process of the migrating neuron.

Polarity orientation and assembly process of microtubule bundles in nocodazole-treated, MAP2c-transfected COS cells

Microtubule bundles reminiscent of those found in neuronal processes are formed in fibroblasts and Sf9 cells that are transfected with the microtubule-associated proteins tau, MAP2, or MAP2c. To analyze the assembly process of these bundles and its relation to the microtubule polarity, we depolymerized the bundles formed in MAP2c-transfected COS cells using nocodazole, and observed the process of assembly of microtubule bundles after removal of the drug in cells microinjected with rhodamine-labeled tubulin. Within minutes of its removal, numerous short microtubule fragments were observed throughout the cytoplasm. These short fragments were randomly oriented and were already bundled. Somewhat longer, but still short bundles, were then found in the peripheral cytoplasm. These bundles became the primordium of the larger bundles, and gradually grew in length and width. The polarity orientation of microtubules in the reformed bundle as determined by "hook" procedure using electron microscope was uniform with the plus end distal to the cell nucleus. The results suggest that some mechanism(s) exists to orient the polarity of microtubules, which are not in direct continuity with the centrosome, during the formation of large bundles. The observed process presents a useful model system for studying the organization of microtubules that are not directly associated with the centrosomes, such as those observed in axons.

Microtubule polarity and dynamics in the control of organelle positioning, segregation, and cytokinesis in the trypanosome cell cycle

Trypanosoma brucei has a precisely ordered microtubule cytoskeleton whose morphogenesis is central to cell cycle events such as organelle positioning, segregation, mitosis, and cytokinesis. We have defined microtubule polarity and show the + ends of the cortical microtubules to be at the posterior end of the cell. Measurements of organelle positions through the cell cycle reveal a high degree of coordinate movement and a relationship with overall cell extension. Quantitative analysis of the segregation of the replicated mitochondrial genome (the kinetoplast) by the flagellar basal bodies identifies a new G2 cell cycle event marker. The subsequent mitosis then positions one "daughter" nucleus into the gap between the segregated basal bodies/kinetoplasts. The anterior daughter nucleus maintains its position relative to the anterior of the cell, suggesting an effective yet cryptic nuclear positioning mechanism. Inhibition of microtubule dynamics by rhizoxin results in a phenomenon whereby cells, which have segregated their kinetoplasts yet are compromised in mitosis, cleave into a nucleated portion and a flagellated, anucleate, cytoplast. We term these cytoplasts "zoids" and show that they contain the posterior (new) flagellum and associated basal-body/kinetoplast complex. Examination of zoids suggests a role for the flagellum attachment zone (FAZ) in defining the position for the axis of cleavage in trypanosomes. Progression through cytokinesis, (zoid formation) while mitosis is compromised, suggests that the dependency relationships leading to the classical cell cycle check points may be altered in trypanosomes, to take account of the need to segregate two unit genomes (nuclear and mitochondrial) in this cell.

Polarity and nucleation of microtubules in polarized epithelial cells

Microtubules oriented in the apicobasal axis of columnar epithelial cells are arranged with a uniform polarity with minus ends toward the apical surface, suggesting that these cytoskeletal filaments might serve as a substrate for polarized movement of membrane vesicles within the cell. It is not known whether hepatocytes, a cuboidal epithelium in which transcellular transport is a requisite step in normal apical membrane biogenesis, contain microtubules arranged with a similar polarity. In the present study, we explore the question of microtubule polarity and possible mechanisms for nucleation in the epithelial cell lines WIF-B (hepatocyte), Caco-2 (intestine), and Madin-Darby canine kidney (MDCK). Caco-2 microtubules in the apicobasal axis had uniform polarity with minus ends nearest the apical surface. After cold and nocodazole-induced depolymerization, microtubule regrowth initiated in the apical region in all three cell types. The apex of WIF-B and Caco-2 cells contained two pools of gamma-tubulin: one associated with centrosomes and the other delocalized under the apical membrane. Non-centrosomal gamma-tubulin was present in complexes that sedimented between 10S and 29S; both forms could bind microtubules. The presence of both centrosomal and noncentrosomal gamma-tubulin in apical cytoplasm suggest multiple mechanisms by which microtubule nucleation might occur in epithelial cells.

The transport properties of axonal microtubules establish their polarity orientation

It is well established that axonal microtubules (MTs) are uniformly oriented with their plus ends distal to the neuronal cell body (Heidemann, S. R., J. M. Landers, and M. A. Hamborg. 1981. J. Cell Biol. 91:661-665). However, the mechanisms by which these MTs achieve their uniform polarity orientation are unknown. Current models for axon growth differ with regard to the contributions of MT assembly and transport to the organization and elaboration of the axonal MT array. Do the transport properties or assembly properties of axonal MTs determine their polarity orientation? To distinguish between these possibilities, we wished to study the initiation and outgrowth of axons under conditions that would arrest MT assembly while maintaining substantial levels of preexisting polymer in the cell body that could still be transported into the axon. We found that we could accomplish this by culturing rat sympathetic neurons in the presence of nanomolar levels of vinblastine. In concentrations of the drug up to and including 100 nM, the neurons actively extend axons. The vinblastine-axons are shorter than control axons, but clearly contain MTs. To quantify the effects of the drug on MT mass, we compared the levels of polymer throughout the cell bodies and axons of neurons cultured overnight in the presence of 0, 16, and 50 nM vinblastine with the levels of MT polymer in freshly plated neurons before axon outgrowth. Without drug, the total levels of polymer increase by roughly twofold. At 16 nM vinblastine, the levels of polymer are roughly equal to the levels in freshly plated neurons, while at 50 nM, the levels of polymer are reduced by about half this amount. Thus, 16 nM vinblastine acts as a "kinetic stabilizer" of MTs, while 50 nM results in some net MT disassembly. At both drug concentrations, there is a progressive increase in the levels of MT polymer in the axons as they grow, and a corresponding depletion of polymer from the cell body. These results indicate that highly efficient mechanisms exist in the neuron to transport preassembled MTs from the cell body into the axon. These mechanisms are active even at the expense of the cell body, and even under conditions that promote some MT disassembly in the neuron. MT polarity analyses indicate that the MTs within the vinblastine-axons, like those in control axons, are uniformly plus-end-distal.(ABSTRACT TRUNCATED AT 400 WORDS)

The plus ends of stable microtubules are the exclusive nucleating structures for microtubules in the axon

Microtubules (MTs) in the axon have a uniform polarity orientation that is recapitulated during recovery from episodes of MT depolymerization (Heidemann, S. R., M. A. Hamborg, S. J. Thomas, B. Song, S. Lindley, and D. Chu. 1984. J. Cell Biol. 99:1289-1295). This tight regulation of their organization indicates that axonal MTs are spatially regulated by discrete nucleating structures comparable in function to the centrosome. Several authors have proposed that an especially stable class of MTs in the axon may serve as these nucleating structures. In a previous report (Baas, P. W., and M. M. Black. 1990. J. Cell Biol. 111:495-509), we determined that the axons of cultured sympathetic neurons contain two classes of MT polymer, stable and labile, that differ in their sensitivity to nocodazole by roughly 35-fold. The stable and labile polymer represent long-lived and recently assembled polymer, respectively. We also determined that these two classes of polymer can be visually distinguished at the immunoelectron microscopic level based on their content of tyrosinated alpha-tubulin: the labile polymer stains densely, while the stable polymer does not stain. In the present study, we have taken advantage of these observations to directly identify MT nucleating structures in the axon. Neuron cultures were treated with nocodazole for 6 h to completely depolymerize the labile polymer in the axon, and substantially shorten the stable polymer. The cultures were then rinsed free of the drug, permitted to reassemble polymer for various periods of time, and prepared for immunoelectron microscopic localization of tyrosinated alpha-tubulin. Serial reconstruction of consecutive thin sections was undertaken to determine the spatial relationship between the stable MTs and the newly assembled polymer. All of the new polymer assembled in direct continuity with the plus ends of stable MTs, indicating that these ends are assembly competent, and hence capable of acting as nucleating structures. Our results further indicate that no self-assembly of MTs occurs in the axon, nor do any MT nucleating structures exist in the axon other than the plus ends of stable MTs. Thus the plus ends of stable MTs are the exclusive nucleating structures for MTs in the axon.