A new perspective on microtubules and axon growth

The centrosome in animal cells and its functional homologs in plant and yeast cells

The centrosome is the principal microtubule-organizing center in mammalian cells. Until recently, the centrosome could only be studied at the ultrastructural level and defined as a functional entity. However, during the past decade a number of clever experimental strategies have been used to identify numerous molecular components of the centrosome. The identification of biochemical subunits of the centrosome complex has allowed the centrosome to be investigated in much more detail, resulting in important advances being made in our understanding of microtubule nucleation events, spindle formation, the assembly and replication of the centrosome, and the nature of the microtubule-organizing centers in plant cells and lower eukaryotes. The next several years should see additional rapid progress in our understanding of the microtubule cytoskeleton as investigators begin to assign functions to the centrosome proteins that have already been reported and as additional centrosome components are discovered.

Selective neuronal toxicity of cocaine in embryonic mouse brain cocultures

Cocaine exposure in utero causes severe alterations in the development of the central nervous system. To study the basis of these teratogenic effects in vitro, we have used cocultures of neurons and glial cells from mouse embryonic brain. Cocaine selectively affected embryonic neuronal cells, causing first a dramatic reduction of both number and length of neurites and then extensive neuronal death. Scanning electron microscopy demonstrated a shift from a multipolar neuronal pattern towards bi- and unipolarity prior to the rounding up and eventual disappearance of the neurons. Selective toxicity of cocaine on neurons was paralleled by a concomitant decrease of the culture content in microtubule-associated protein 2 (MAP2), a neuronal marker measured by solid-phase immunoassay. These effects on neurons were reversible when cocaine was removed from the culture medium. In contrast, cocaine did not affect astroglial cells and their glial fibrillary acidic protein (GFAP) content. Thus, in embryonic neuronal-glial cell cocultures, cocaine induces major neurite perturbations followed by neuronal death without affecting the survival of glial cells. Provided similar neuronal alterations are produced in the developing human brain, they could account for the qualitative or quantitative defects in neuronal pathways that cause a major handicap in brain function following in utero exposure to cocaine.

Production and utilization of detyrosinated tubulin in developing Artemia larvae: evidence for a tubulin-reactive carboxypeptidase

The reversible, enzymatically driven removal and readdition of its carboxy-terminal tyrosine are major posttranslational modifications of alpha-tubulin. To study these processes isoform-specific antibodies were produced and subsequently used to characterize tyrosinated and detyrosinated tubulin in the brine shrimp, Artemia. Tyrosinated tubulin existed in relatively constant amounts on western blots of cell-free protein extracts from Artemia at all developmental stages examined, whereas detyrosinated tubulin was present after 20-24 h of postgastrula growth. In agreement with the blots, the detyrosinated isoform was observed in immunofluorescently stained larvae after 24 h of incubation, appearing first in structures of a transient nature, namely spindles and midbodies. The elongated muscle cells encircling the gut and the epithelium bordering the gut lumen were stained extensively with antibody to detyrosinated tubulin. Detyrosination was accompanied by the appearance of a tubulin-reactive carboxypeptidase, which used both nonpolymerized and polymerized tubulin as substrate. The enzyme bound to microtubules very poorly, if at all, under conditions used in this work. Several inhibitors of carboxypeptidase A had no effect on the carboxypeptidase from Artemia and revealed similarities between this enzyme and others thought to be tubulin specific. The use of inhibitors also indicated that the carboxypeptidase from Artemia recognized aspects of tubulin structure in addition to the carboxy-terminal tyrosine. Our results support the idea that detyrosinated tubulin appears in microtubules of varying stability, and they demonstrate that Artemia possess a carboxypeptidase with the potential to detyrosinate tubulin during growth of larvae.

Effects of taxol on the polymerization and posttranslational modification of class III beta-tubulin in P19 embryonal carcinoma cells

Undifferentiated P19 embryonal carcinoma cells and P19 cells induced to differentiate along a neuronal pathway by 10(-6) M retinoic acid were treated with taxol to examine the effects of this microtubule-stabilizing drug on the subcellular sorting of class III beta-tubulin and on neurite outgrowth. P19 cells were grown on cover slips and then treated with taxol at concentrations of 10(-6) to 10(-9) M for 24 h. The microtubule cytoskeleton was examined after double-immunofluorescence labelling with a monoclonal antibody to alpha-tubulin (YOL 1/34) and a monoclonal neuron-specific class III beta-tubulin antibody (TuJ1). Treatment of undifferentiated P19 cells with concentrations of taxol greater than 4 x 10(-8) M caused microtubule bundling and multiple aster formation and promoted polymerization of the low levels of class III beta-tubulin found in these cells. In neurons, at 2 x 10(-8) M taxol, bundling of microtubules at the base of the neurite was apparent. At taxol concentrations greater than 1 x 10(-7) M, enhanced assembly of class III beta-tubulin was apparent, although long neurites were not observed. Using isoelectric focusing followed by western blotting, we detected an additional isoform of class III beta-tubulin after treatment with 10(-6) M taxol. The results indicate taxol treatment alters the normal subcellular sorting of tubulin isotypes, promotes the polymerization and posttranslational modification of class III beta-tubulin, and interferes with neurite outgrowth.

Microtubules released from the neuronal centrosome are transported into the axon

There is controversy concerning the source of new microtubules required for the development of neuronal axons. We have proposed that microtubules are released from the centrosome within the cell body of the neuron and are then translocated into the axon to support its growth. To investigate this possibility, we have developed an experimental regime that permits us to determine the fate of a small population of microtubules nucleated at the neuronal centrosome. Microtubules within cultured sympathetic neurons were depolymerized with the anti-microtubule drug nocodazole, after which the drug was removed. Microtubules rapidly and specifically reassembled from the centrosome within three minutes of nocodazole removal. At this point, low levels of vinblastine, another anti-microtubule drug, were added to the culture to inhibit further microtubule assembly while not substantially depolymerizing the small population of microtubules that had already assembled at the centrosome. Within minutes, released microtubules were apparent in the cytoplasm, and many of these had already translocated to the cell periphery by ten minutes. By one hour, virtually all of the microtubules had been released from the centrosome and were concentrated at the cell periphery. With increasing time, these microtubules appeared within and progressively farther down developing axons. Nonneuronal cells within the culture also reassembled microtubules at the centrosome, but only a small portion of these microtubules were released. These observations indicate that microtubules were released from the neuronal centrosome and transported into growing axons, and that microtubule release and relocation from the centrosome are especially active in neurons compared to nonneuronal cells.

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.

Transport of dendritic microtubules establishes their nonuniform polarity orientation

The immature processes that give rise to both axons and dendrites contain microtubules (MTs) that are uniformly oriented with their plus-ends distal to the cell body, and this pattern is preserved in the developing axon. In contrast, developing dendrites gradually acquire nonuniform MT polarity orientation due to the addition of a subpopulation of oppositely oriented MTs (Baas, P. W., M. M. Black, and G. A. Banker. 1989. J. Cell Biol. 109:3085-3094). In theory, these minus-end-distal MTs could be locally nucleated and assembled within the dendrite itself, or could be transported into the dendrite after their nucleation within the cell body. To distinguish between these possibilities, we exposed cultured hippocampal neurons to nanomolar levels of vinblastine after one of the immature processes had developed into the axon but before the others had become dendrites. At these levels, vinblastine acts as a kinetic stabilizer of MTs, inhibiting further assembly while not substantially depolymerizing existing MTs. This treatment did not abolish dendritic differentiation, which occurred in timely fashion over the next two to three days. The resulting dendrites were flatter and shorter than controls, but were identifiable by their ultrastructure, chemical composition, and thickened tapering morphology. The growth of these dendrites was accompanied by a diminution of MTs from the cell body, indicating a net transfer of MTs from one compartment into the other. During this time, minus-end-distal microtubules arose in the experimental dendrites, indicating that new MT assembly is not required for the acquisition of nonuniform MT polarity orientation in the dendrite. Minus-end-distal microtubules predominated in the more proximal region of experimental dendrites, indicating that most of the MTs at this stage of development are transported into the dendrite with their minus-ends leading. These observations indicate that transport of MTs from the cell body is an essential feature of dendritic development, and that this transport establishes the nonuniform polarity orientation of MTs in the dendrite.

A regulatory role for sphingolipids in neuronal growth. Inhibition of sphingolipid synthesis and degradation have opposite effects on axonal branching

Sphingolipids, particularly gangliosides, are enriched in neuronal membranes where they have been implicated as mediators of various regulatory events. We recently provided evidence that sphingolipid synthesis is necessary to maintain neuronal growth by demonstrating that in hippocampal neurons, inhibition of ceramide synthesis by Fumonisin B1 (FB1) disrupted axonal outgrowth (Harel, R. and Futerman, A. H. (1993) J. Biol. Chem. 268, 14476-14481). We now analyze further the relationship between neuronal growth and sphingolipid metabolism by examining the effect of an inhibitor of glucosylceramide synthesis, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1- propanol (PDMP) and by examining the effects of both FB1 and PDMP at various stages of neuronal development. No effects of FB1 or PDMP were observed during the first 2 days in culture, but by day 3 axonal morphology was significantly altered, irrespective of the time of addition of the inhibitors to the cultures. Cells incubated with FB1 or PDMP had a shorter axon plexus and less axonal branches. FB1 appeared to cause a retraction of axonal branches between days 2 and 3, although long term incubation had no apparent effect on neuronal morphology or on the segregation of axonal or dendritic proteins. In contrast, incubation of neurons with conduritol B-epoxide, an inhibitor of glucosylceramide degradation, caused an increase in the number of axonal branches and a corresponding increase in the length of the axon plexus. A direct correlation was observed between the number of axonal branch points per cell and the extent of inhibition of either sphingolipid synthesis or degradation. These results suggest that sphingolipids play an important role in the formation or stabilization of axonal branches.

Organization of the cytoskeleton in brine shrimp setal cells is molt-dependent

Fluorescence microscopy was used to examine the cytoskeleton in setal cells and antennae of the brine shrimp Artemia franciscana. Each setal cell has an elongated apical process that contains bundles of microtubules and microfilaments. When the organism molts, the apical process telescopes reversibly through the setal cell body into the hemocoel of the antenna. Staining of larval-stage Artemia with four monoclonal anti-tubulin antibodies (DM1 A, TAT, YL1/2, KMX) and with rhodamine–phalloidin indicated that the cytoskeletal elements were stable, remaining assembled as co-localized bundles in telescoping setal cells. Microtubule stability was suggested by previous observations of detyrosinated tubulin in setal cell extensions, but the microtubules were not completely detyrosinated, as shown by their interaction with YL1/2. Foci of tubulin staining within the antenna, enrichment of the microfilaments associated with the invaginating setal cell membrane, and the spatial distribution of other cytoskeletal elements were indicative of dynamic processes used in shape change during molting. Fluorescent labelling also revealed microtubules and microfilaments in tendinal cells, specialized epidermal cells that attach muscle in the antenna to the overlying cuticle.

Growth cone advance is inversely proportional to retrograde F-actin flow

In a previous study, F-actin appeared to play a key role in guiding microtubules during growth cone-target interactions. Here, F-actin flow patterns were assessed to investigate the relationship among F-actin flow, microtubule/organelle protrusion, and rates of outgrowth. We first demonstrated conditions in which surface markers (beads) moved at the same rate as underlying F-actin. These beads were then positioned, using laser tweezers, to assess F-actin movements during target interactions. We found retrograde F-actin flow was attenuated specifically along the target interaction axis in direct proportion to the rate of growth cone advance. Retrograde actin flow adjacent to the interaction axis was unperturbed. Our results suggest that growth cones transduce retrograde F-actin flux into forward movement by modulating F-actin-substrate coupling efficiency.

Cytoplasmic chaperonin complexes enter neurites developing in vitro and differ in subunit composition within single cells

Chaperonins containing t-complex polypeptide-1 (CCT) are cytosolic molecular chaperone particles implicated especially in the biogenesis of cytoskeletal proteins by promoting the correct folding of the major ubiquitous cytoskeletal components, tubulin and actin. We have purified cytosolic chaperonins from the ND7/23 cell line, determined their subunit composition and examined changes in the intracellular locations of their components during differentiation of ND7/23 cells to a neuronal phenotype by using immunocytochemistry and immunoblots. Chaperonins containing the CCT alpha (TCP1) subunit enter neuritic processes and are particularly noticeable at the leading edge of growth cone-like structures where they co-localise with actin. Chaperonins containing three other components (CCT beta, epsilon and gamma), however, remain predominantly restricted to perikaryal cytoplasm. These findings suggest a heterogeneous population of chaperonin particles within single differentiated ND7/23 cells and this may reflect specialisation of chaperonin function in different cytoplasmic compartments of a neurone. Further, since ribosomes do not enter neurites while CCT alpha-containing chaperonins do, the latter may play roles, subsequent to translation, which influence cytoskeletal elaboration during neuritogenesis.

Formation of two microtubule-nucleating sites which perform differently during centrosomal reorganization in a mouse cochlear epithelial cell

This report provides evidence for the formation of a cell surface-associated centrosome with two spatially discrete microtubule-nucleating sites that perform differently; the minus ends of microtubules remain anchored to one site but escape from the other. Centrosomal reorganization in the cells in question, outer pillar cells of the organ of Corti, indicates that its pericentriolar material becomes intimately associated with the plasma membrane at the two nucleating sites. Two large microtubules bundles assemble in each cell. A beam which includes about 1,300 microtubules spans most of the cell apex. It is positioned at right angles to a pillar with about 4,500 microtubules which is oriented parallel to the cell's longitudinal axis. The beam's microtubules elongate from, and remain attached to, a centrosomal region with two centrioles which acts as a microtubule-nucleating site. However, the elongating microtubules do not radiate from the immediate vicinity of the centrioles. During beam assembly, the minus ends of the microtubules are concentrated together close to the plasma membrane (less than 0.2 micron away in many cases) at a site which is located to one side of the cell apex. High concentrations of the pillar's microtubules elongating from one particular site have not been detected. Analyses of pillar assembly indicate that the following sequence of events occurs. Pillar microtubules elongate from an apical cell surface-associated nucleating site, which becomes more distantly separated from the centriolar locality as cell morphogenesis progresses. Microtubules do not accumulate at this apical nucleating site because they escape from it. They migrate down to lower levels in the cell where the mature bundle is finally situated and their plus ends are captured at the cell base.

Three-dimensional organization of stable microtubules and the Golgi apparatus in the somata of developing chick sensory neurons

Microtubules play a role important in regulating cell shape and in mediating organelle movements. These functions are especially important in elaborately branched neurons, which have many stable microtubules that are resistant to cold and to microtubule depolymerizing drugs. We examined the three-dimensional organization of microtubules in cell bodies of cultured chick embryo sensory neurons, using confocal laser scanning microscopy. Microtubules were visualized with antibodies against alpha-tubulin and post-translationally modified forms of alpha-tubulin that accumulate in older microtubules. Optical sections were collected through neuronal somata, and the images were reconstructed in three dimensions. In neuronal perikarya a dense network of older microtubules is co-localized with the Golgi apparatus. This complex of the Golgi and older microtubules usually lies beneath the cell nucleus and is oriented toward the substratum. From this region, older microtubules extend into each neurite. A cage of older microtubules extends around the nucleus to the top of the perikaryon. The stability of these microtubules was confirmed by their resistance to the depolymerizing drug, nocodazole. This arrangement of stable microtubules in a developing neuron provides a supporting cytoskeleton and a transport pathway for movement of cytoplasmic components between the Golgi apparatus, the perikaryon and developing neurites.

Three microtubule-organizing centres collaborate in a mouse cochlear epithelial cell during supracellularly coordinated control of microtubule positioning

Large cell surface-associated microtubule bundles that include about 3,000 microtubules assemble in certain epithelial cells called inner pillar cells in the mouse organ of Corti. Microtubule-organizing centres (MTOCs) at both ends and near the middle of each cell act in concert during control of microtubule positioning. In addition, the three cell surface-associated microtubule-organizing centres are involved in coordinating the connection of bundle microtubules to cytoskeletal components in neighbouring cells and to a basement membrane. The precisely defined locations of the three MTOCs specify the cell surface regions where microtubule ends will finally be anchored. The MTOCs are modified as anchorage proceeds. Substantial fibrous meshworks assemble at the surface sites occupied by the MTOCs and link microtubule ends to cell junctions. This procedure also connects the microtubule bundle to cytoskeletal arrays in neighbouring cells at two of the MTOC sites, and to the basilar membrane (a substantial basement membrane) in the case of the third site. A fourth meshwork that is not positioned at a major MTOC site is involved in connecting one side of the microtubule bundle to the cytoskeletons of two other cell neighbours. The term surfoskelosome is suggested for such concentrations of specialized cytoskeletal materials and junctions at cell surface anchorages for cytoskeletal arrays. The large microtubule bundle in each cell is composed of two closely aligned microtubule arrays. Bundle assembly begins with nucleation of microtubules by a centrosomal MTOC that is attached to the apical cell surface. These microtubules elongate downwards and the plus ends of many of them are apparently captured by a basal MTOC that is attached to the plasma membrane at the bottom of the cell. In the lower portion of the cell, the microtubule bundle also includes a basal array of microtubules but these elongate in the opposite direction. This investigation provides evidence that they extend upwards from the basal MTOC to be captured by a medial MTOC which is attached to the plasma membrane and situated near the mid-level of the cell. However, there are substantial indications that the basal array's microtubules are also nucleated by the apically situated centrosomal MTOC, but escape from it, and are translocated downwards for capture of their plus ends by the basal MTOC. If this is the case, then these microtubules continue to elongate after translocation and extend back up to the medial MTOC, which captures their minus ends.

Organelle transport and sorting in axons

Recent advances of the past year in the field of organelle transport have documented the existence of numerous kinesin-related proteins and the presence of multiple conventional kinesins within neurons. Biochemical and genetic mutant analyses indicate that different kinesin superfamily members transport different organelles. In addition to microtubule-based systems, actin filaments and myosin motors are associated with organelle transport in axons. The great diversity of motor proteins suggests that they may play a role in sorting, in addition to transport.

Cytoskeletal reorganization underlying growth cone motility

Recent studies have implicated cytoskeletal dynamics as an important component in directing neuronal outgrowth. By using modern imaging techniques to observe the kinetics of individual cytoskeletal elements in living cells, these results have converged upon a common theme: functional coupling between the intracellular cytoskeleton and extracellular substrates, and regulation thereof, appears to be crucial in controlling neuronal migration.

Isolated Plant Nuclei Nucleate Microtubule Assembly: The Nuclear Surface in Higher Plants Has Centrosome-like Activity

In most eukaryotic cells, microtubules (MTs) are assembled at identified nucleating sites, such as centrosomes or spindle pole bodies. Higher plant cells do not possess such centrosome-like structures. Thus, the fundamental issues of where and how the intracellular plant MTs are nucleated remain highly debatable. A large body of evidence indicates that plant MTs emerge from the nuclear periphery. In this study, we developed an in vitro assay in which isolated maize nuclei nucleate MT assembly at a tubulin concentration (14 [mu]M of neurotubulin) that is not efficient for spontaneous MT assembly. No MT-stabilizing agents, such as taxol or dimethyl sulfoxide, were used. Our model provides evidence that the nuclear surface functions as a MT-nucleating site in higher plant cells. A monoclonal antibody raised against a pericentriolar antigen immunostained the surface of isolated nuclei, and a 100-kD polypeptide in 4 M urea-treated nuclear extracts was detected.

High molecular weight tau distribution and microtubule stability in neuroblastoma N115 cells

The localization of high molecular weight (HMW) tau proteins in neuroblastoma N115 cells and of their transcripts was compared to that of non-tyrosinated and tyrosinated tubulin before and after treatment with depolymerizing drugs. Microtubules stained by tau antibodies were present both in a limited region of the cell center and in the cell processes, whereas tau transcripts were detected only in the cell body. The microtubules localized in the cell center and labeled by tau antibodies resisted colcemid treatment, whereas those in the neurites were completely depolymerized by the drug. Microtubules containing stable and unstable microtubule tracts were identified in the neurites after colcemid treatment. These composite microtubules were not labeled by tau antibodies. It is concluded that stable and unstable polymers--localized in the cell center and in the neurites, respectively--contain HMW tau proteins, whereas composite microtubules displayed in the cell processes do not. Microtubule stability in this cell line does not therefore seem to be related to the association of tau proteins to the polymers but, rather, to posttranslational modifications of the tubulin subunits.

Inhibition of microtubule nucleation at the neuronal centrosome compromises axon growth

We tested the dependence of axon growth on microtubule (MT) nucleation from the neuronal centrosome. Nocodazole diminished MTs in freshly plated neurons by > 99%. Within 5 min of drug removal, MTs reassembled at the centrosome. This response was inhibited in cells microinjected with gamma-tubulin antibody. Within 2 hr of drug removal, uninjected neurons grew > 500 microns of axon. In roughly half of the antibody-injected cells, axon growth was abolished and MT levels were reduced by approximately 87% compared with uninjected cells. In the other antibody-injected cells, axon growth was compromised but not abolished, and MT levels were reduced by approximately 38%. Thus inhibition of MT nucleation at the centrosome hindered MT reassembly, and depending on the severity of this response, axon growth was either compromised or abolished.

Microtubule organizing centers and gamma-tubulin

The polar assembly of cellular microtubules is organized by microtubule organizing centers (MTOCs). Eukaryotic cells across different species, and different cell types within single species, have morphologically diverse MTOCs, which have the common function of organizing microtubule arrays by initiating microtubule assembly and anchoring microtubules by their slow-growing 'minus' ends, thus ensuring that the rapidly growing 'plus' ends extend distally. The past few years have witnessed a variety of approaches aimed at defining the molecular components of the MTOC that are responsible for regulating microtubule assembly by defining molecules common to all MTOCs.

Tau confers drug stability but not cold stability to microtubules in living cells

We previously defined two classes of microtubule polymer in the axons of cultured sympathetic neurons that differ in their sensitivity to nocodazole by roughly 35-fold (Baas and Black (1990) J. Cell Biol. 111, 495–509). Here we demonstrate that virtually all of the microtubule polymer in these axons, including the drug-labile polymer, is stable to cold. What factors account for the unique stability properties of axonal microtubules? In the present study, we have focused on the role of tau, a microtubule-associated protein that is highly enriched in the axon, in determining the stability of microtubules to nocodazole and/or cold in living cells. We used a baculovirus vector to express very high levels of tau in insect ovarian Sf9 cells. The cells respond by extending processes that contain dense bundles of microtubules (Knops et al. (1991) J. Cell Biol. 114, 725–734). Cells induced to express tau were treated with either cold or 2 micrograms/ml nocodazole for times ranging from 5 minutes to 6 hours. The results with each treatment were very different from one another. Virtually all of the polymer was depolymerized within the first 30 minutes in cold, while little or no microtubule depolymerization was detected even after 6 hours in nocodazole. Based on these results, we conclude that tau is almost certainly a factor in conferring drug stability to axonal microtubules, but that factors other than or in addition to tau are required to confer cold stability.

Orientation, assembly, and stability of microtubule bundles induced by a fragment of tau protein

The neuronal microtubule-associated protein tau has been implicated in the development of axonal morphology including the organization of microtubules into a uniformly oriented array of microtubules commonly referred to as "bundle." Determination of the functional organization of tau has revealed that regions of tau protein which flank the microtubule-binding domain affect the bundling of microtubules in vitro with a microtubule-binding fragment of tau being most effective [Brandt and Lee, 1993: J. Biol. Chem. 268:3414-3419]. In order to study the relation of microtubule bundles that form in vitro to those observed in the axon, we determined the orientation of individual microtubules in bundles and the effects of bundling on microtubule assembly and stability in cell-free assembly reactions. Here we report that bundles induced by a microtubule-binding fragment of tau contain randomly oriented microtubules as determined by using the difference in growth rates at microtubule plus and minus ends. We demonstrate that in vitro bundling increases microtubule growth (about 30%), stabilizes microtubules against dilution- and cold-induced disassembly, and allows microtubule nucleation despite the absence of a tau region which has previously been shown to be required for tau-dependent microtubule nucleation. We conclude that conditions that stabilize microtubules can lead to bundle formation and allow microtubule assembly by a mechanism different from that employed by microtubule-associated proteins. The data also support the view that additional mechanisms besides the action of tau and tubulin exist in order to organize microtubules in the axon.

Microtubule transport and assembly cooperate to generate the microtubule array of growing axons

MTs are major architectural elements in growing axons. MTs overlap with each other along the axon, forming an array that is continuous from the cell body to the tip of the axon. The MT array constitutes a scaffolding that mechanically supports the elongate shape of the axon and also contributes directly to its shape. MTs also direct the transport of vesicular organelles between the cell body and the axon, and thereby determine, in part, the composition of the axon. In this article, I have discussed mechanisms involved in the elaboration of the MT array in growing axons, and I have emphasized the distinct but complementary roles of polymer transport mechanisms and local assembly dynamics. MTs for the axon originate in the cell body, and they are delivered to the axon by the polymer transport mechanisms. These mechanisms thus contribute directly to the shape of the axon by supplying it with essential architectural elements. The shape of the axon is further modulated by dynamic processes that alter cytoskeletal structure locally along its length. These dynamic processes include the assembly/disassembly mechanisms which influence polymer length and possibly number locally along the axon by subunit exchange between the monomer and polymer pools. In addition, the polymer transport mechanisms themselves are subject to modulation along the axon, as demonstrated by the observation that transport rate of MTs varies along the length of individual axons (Reinsch et al., 1991). Such local variations can, in and of themselves, change the number of MTs along the axon, and thereby focally affect axon shape. Thus, the dynamic processes of polymer transport and local assembly act cooperatively to shape the MT array of the axon, and thereby contribute directly to the elaboration of axonal morphology.

Gamma-tubulin: the hub of cellular microtubule assemblies

In eukaryotic cells a specialized organelle called the microtubule organizing center (MTOC) is responsible for disposition of microtubules in a radial, polarized array in interphase cells and in the spindle in mitotic cells. Eukaryotic cells across different species, and different cell types within single species, have morphologically diverse MTOCs, but these share a common function of organizing microtubule arrays. MTOCs effect microtubule organization by initiating microtubule assembly and anchoring microtubules by their slowly growing minus ends, thus ensuring that the rapidly growing plus ends extend distally in each microtubule array. The goal is to define molecular components of the MTOC responsible for regulating microtubule assembly. One approach to defining the molecules responsible for MTOC function is to look for molecules common to all MTOCs. A newly discovered centrosomal protein, gamma-tubulin, is found in MTOCs in cells from many different organisms, and has several properties which make it a candidate for both initiation of microtubule assembly and anchorage. The hypothesis that gamma-tubulin plays a role in MTOCs in microtubule initiation and anchorage is currently being tested by a variety of experimental approaches.

Microtubule nucleation and release from the neuronal centrosome

We have proposed that microtubules (MTs) destined for axons and dendrites are nucleated at the centrosome within the cell body of the neuron, and are then released for translocation into these neurites (Baas, P. W., and H. C. Joshi. 1992. J. Cell Biol. 119:171-178). In the present study, we have tested the capacity of the neuronal centrosome to act as a generator of MTs for relocation into other regions of the neuron. In cultured sympathetic neurons undergoing active axonal outgrowth, MTs are present throughout the cell body including the region around the centrosome, but very few (< 10) are directly attached to the centrosome. These results indicate either that the neuronal centrosome is relatively inactive with regard to MT nucleation, or that most of the MTs nucleated at the centrosome are rapidly released. Treatment for 6 h with 10 micrograms/ml nocodazole results in the depolymerization of greater than 97% of the MT polymer in the cell body. Within 5 min after removal of the drug, hundreds of MTs have assembled in the region of the centrosome, and most of these MTs are clearly attached to the centrosome. A portion of the MTs are not attached to the centrosome, but are aligned side-by-side with the attached MTs, suggesting that the unattached MTs were released from the centrosome after nucleation. In addition, unattached MTs are present in the cell body at decreasing levels with increasing distance from the centrosome. By 30 min, the MT array of the cell body is indistinguishable from that of controls. The number of MTs attached to the centrosome is once again diminished to fewer than 10, suggesting that the hundreds of MTs nucleated from the centrosome after 5 min were subsequently released and translocated away from the centrosome. These results indicate that the neuronal centrosome is a highly potent MT-nucleating structure, and provide strong indirect evidence that MTs nucleated from the centrosome are released for translocation into other regions of the neuron.