Individual microtubules in the axon consist of domains that differ in both composition and stability

Microtubule stability decreases axon elongation but not axoplasm production

Microtubules are a primary cytoskeletal constituent of axons and growth cones. In addition to serving as a scaffolding for axon assembly, they also provide a means of transport of organelles that are essential for outgrowth and maintenance of synaptic function. Pharmacological manipulations that disrupt net assembly of microtubules also interfere with growth cone advance and axon extension. Less is known after the effects of disrupting microtubule dynamics without affecting net assembly. To investigate this, we studied the effects of low doses of nocodazole on axon extension and microtubule organization in rat superior cervical ganglion neurons. We report that 165-330 nM nocodazole significantly reduces axon extension rate and increases axon diameter without affecting the rate of production of axoplasm or microtubule polymer, and without decreasing the average length or number of microtubules. Two observations suggested that microtubule dynamics were depressed by this dose of nocodazole. First, this treatment eliminated the highly divergent lengths and positions of microtubules characteristic of normal growth cones, inducing an array in which each microtubule terminated at roughly the same position in the proximal regions of the growth cone. Second, there was a decrease in the proportion of microtubule length containing mostly tyrosinated (newly assembled) alpha-tubulin and an increase in the proportion of microtubule length containing mostly acetylated (older, more stable) alpha-tubulin. Together, these data suggest that a decrease in dynamic instability of microtubules is sufficient to disrupt axon extension but does not interfere with axoplasm production.

Cytoskeletal changes during neurogenesis in cultures of avain neural crest cells

Neural crest cells are motile and mitotic, whereas their neuronal derivatives are terminally post-mitotic and consist of stationary cell body from which processes grow. The present study documents changes in the cytoskeleton that occur during neurogenesis in cultures of avain neural crest cells. The undifferentiated neural crest cells contain dense bundles of actin filaments throughout their cytoplasm, and a splayed array of microtubules attached to the centrosome. In newly differentiating neurons, the actin bundles are disrupted and most of the remaining actin filaments are reorganized into a cortical layer underlying the plasma membrane of the cell body and processes. Microtubules are more abundant in newly-differentiating neurons than in the undifferentiated cells, and individual microtubules can be seen dissociated from the centrosome. Neuron-specific beta-III tubulin appears in some crest cells prior to cessation of motility and cell division, and expression increases with total microtubule levels during neurogenesis. To investigate how these early cytoskeletal changes might contribute to alterations in morphology during neurogenesis, we have disrupted the cytoskeleton with pharmacologic agents. Microfilament disruption by cytochalasin immediately arrests the movement of neural crest cells and causes them to round-up, but does not significantly change the morphology of the immature neurons. Microtubule depolymerization by nocodazole slows the movement of undifferentiated cells and causes retraction of processes extended by the immature neurons. These results suggest that changes in the actin and microtubule arrays within neural crest cells govern distinct aspects of their morphogenesis into neurons.

Microtubule transport and assembly during axon growth

There is controversy concerning the mechanisms by which the axonal microtubule (MT) array is elaborated, with some models focusing on MT assembly and other models focusing on MT transport. We have proposed a composite model in which MT assembly and transport are both important (Joshi, H.C., and P.W. Baas. 1993. J. Cell Biol. 121:1191-1196). In the present study, we have taken a novel approach to evaluate the merits of this proposal. Biotinylated tubulin was microinjected into cultured neurons that had already grown short axons. The axons were then permitted to grow longer, after which the cells were prepared for immunoelectron microscopic analyses. We reasoned that any polymer that assembled or turned over subunits after the introduction of the probe should label for biotin, while any polymer that was already assembled but did not turnover should not label. Therefore, the presence in the newly grown region of the axon of any unlabeled MT polymer is indicative of MT transport. In sampled regions, the majority of the polymer was labeled, indicating that MT assembly events are active during axon growth. Varying amounts of unlabeled polymer were also present in the newly grown regions, indicating that MT transport also occurs. Together these findings demonstrate that MT assembly and transport both contribute to the elaboration of the axonal MT array.

Cloning, expression, and properties of the microtubule-stabilizing protein STOP

Nerve cells contain abundant subpopulations of cold-stable microtubules. We have previously isolated a calmodulin-regulated brain protein, STOP (stable tubule-only polypeptide), which reconstitutes microtubule cold stability when added to cold-labile microtubules in vitro. We have now cloned cDNA encoding STOP. We find that STOP is a 100.5-kDa protein with no homology to known proteins. The primary structure of STOP includes two distinct domains of repeated motifs. The central region of STOP contains 5 tandem repeats of 46 amino acids, 4 with 98% homology to the consensus sequence. The STOP C terminus contains 28 imperfect repeats of an 11-amino acid motif. STOP also contains a putative SH3-binding motif close to its N terminus. In vitro translated STOP binds to both microtubules and Ca2+-calmodulin. When STOP cDNA is expressed in cells that lack cold-stable microtubules, STOP associates with microtubules at 37 degrees C, and stabilizes microtubule networks, inducing cold stability, nocodazole resistance, and tubulin detyrosination on microtubules in transfected cells. We conclude that STOP must play an important role in the generation of microtubule cold stability and in the control of microtubule dynamics in brain.

Assembly of microfilaments and microtubules from axonally transported actin and tubulin after axotomy

The slow component (SC) of axonal transport conveys structural proteins, regulatory proteins, and glycolytic enzymes toward the axon tip at 1-6 mm/day. Following axon interruption (axotomy), the rate of outgrowth corresponds to the rate of SCb-the fastest subcomponent of SC. Both axonal outgrowth and SCb accelerate 20-25% after axotomy. Tubulin and actin are the major proteins being carried by SCb. To further characterize the acceleration of SCb, we measured the equilibrium between subunits and polymers for both actin and tubulin. We radiolabeled newly synthesized proteins in rat motor neurons by microinjecting [35S]methionine into the spinal cord 7 days after crushing the sciatic nerve (85 mm from the spinal cord). Nerves were removed 7 days later for homogenization in polymer-stabilizing buffer (PSB) and centrifugation, followed by SDS-PAGE of supernatants (S) and pellets (P). We removed beta-tubulin, actin, and the medium-weight neurofilament protein (NF-M) from each gel by using the fluorogram as a template. After solubilizing gel segments for liquid scintillation spectrometry, we expressed counts as a polymerization ratio: P/[S+P]. In the nerve segments that contained radiolabeled Scb proteins, located 24-36 mm from the spinal cord, axotomy increased the polymerization ratio of SCb actin from 0.23 to 0.36 (P < 0.05) but had no effect on SCb beta-tubulin. In a separate experiment, we added 12 microM taxol to PSB to stabilize newly assembled microtubules. Adding taxol did not alter the polymerization ratio for SCb beta-tubulin in sham-axotomized nerves but aid increase the ratio in axotomized nerves, from 0.44 to 0.63 (P < 0.05); polymerization ratios for SCb actin were unaffected. We conclude that the assembly of microfilaments and microtubules increases to provide cytoskeletal elements for axon sprouts. The resulting loss of actin and tubulin subunits may play a role in the acceleration of SCb.

Expression and posttranslational modification of class III beta-tubulin during neuronal differentiation of P19 embryonal carcinoma cells

We have used a combination of immunofluorescence microscopy, northern blotting, ELISA, and isoelectric focusing to characterize the expression of neuronal Class III beta-tubulin in P19 embryonal carcinoma cells induced to differentiate along a neuronal pathway by retinoic acid. Following 48 h differentiation, beta-III tubulin mRNA is evident and beta-III tubulin appears in the mitotic spindle of neuroblasts. Neurite outgrowth is obvious by day 3, and beta-III tubulin protein and mRNA levels increase concurrently until approximately day 7, when beta-III mRNA levels begin to decrease while protein levels remain high. In addition, increasingly acidic beta-III tubulin isoforms appear during neuronal differentiation. The expression of these isoelectric variants occurs concomitant with a temporal increase in the levels of beta-III tubulin present in the colchicine-stable microtubules. These results implicate posttranslational modifications of beta-III tubulin in the increased microtubule stability noted in differentiating P19 neurons.

Stimulation of microtubule dynamic turnover in living cells treated with okadaic acid

We have examined the effects of okadaic acid, an inhibitor of protein phosphatases type 1 and 2A, on the dynamic instability behavior of individual microtubules in living cells. Addition of 1 microM okadaic acid to PtK1 epithelial cells induced ruffling of lamellar regions; after 50 min in okadaic acid, many cells were observed to round up. Confocal microscopy of okadaic acid-treated cells stained with an antibody to tubulin showed that microtubules were more densely packed near the periphery of the rounded cells, and in many cells, a reduction in the density of microtubules near the microtubule-organizing center was observed. The dynamic behavior of individual microtubules in cells previously injected with rhodamine-labeled tubulin was quantified by tracking individual microtubules from image sequences. Microtubule dynamic turnover was markedly stimulated in cells treated with 1 microM okadaic acid for 50-60 min: The average rates of both microtubule growing and shortening increased, and the average duration of pause, or attenuation, a phase in which neither growth nor shortening could be detected, was significantly decreased. Further, okadaic acid induced an approximately twofold increase in the frequency of catastrophe transitions and a threefold decrease in the frequency of rescue transitions. Dynamicity, a measure of the net gain and loss of polymer at microtubule plus ends, increased nearly threefold in okadaic acid-treated cells. These results demonstrate that microtubule turnover is stimulated in okadaic acid-treated cells and suggest that phosphorylation of molecules which interact with microtubules may result in increased microtubule dynamic turnover in vivo.

Tubulin post-translational modifications in the primitive protist Trichomonas vaginalis

Using several specific monoclonal antibodies, we investigated the occurrence and distribution of different post-translationally modified tubulin during interphase and division of the primitive flagellated protist Trichomonas vaginalis. Immunoblotting and immunofluorescence experiments revealed that interphasic microtubular structures of T. vaginalis contained acetylated and glutamylated but non-tyrosinated and non-glycylated [Brugerolle and Adoutte, 1988: Bio Systems 21: 255-268] tubulin. Immunofluorescence studies performed on dividing cells showed that the extranuclear mitotic spindle (or paradesmosis) was acetylated and glutamylated, which contrast with the ephemeral nature of this structure. Newly formed short axostyles also contained acetylated and glutamylated tubulin suggesting that both post-translational modifications might take place very early after assembly of microtubular structures. Our results indicate that acetylation and glutamylation of tubulin appeared early in the history of eukaryotes and could reflect the occurrence of post-translational modifications of tubulin in the primitive eukaryotic cells. These cells probably had a highly ordered cross-linked microtubular cytoskeleton in which microtubules showed a low level of subunit exchange dynamics.

Stable, detyrosinated microtubules function to localize vimentin intermediate filaments in fibroblasts

Separate populations of microtubules (MTs) distinguishable by their level of posttranslationally modified tubulin subunits and by their stability in vivo have been described. In polarized 3T3 cells at the edge of an in vitro wound, we have found a striking preferential coalignment of vimentin intermediate filaments (IFs) with detyrosinated MTs (Glu MTs) rather than with the bulk of the MTs, which were tyrosinated MTs (Tyr MTs). Vimentin IFs were not stabilizing the Glu MTs since collapse of the IF network to a perinuclear location, induced by microinjection of monoclonal anti-IF antibody, had no noticeable effect on the array of Glu MTs. To test whether Glu MTs may affect the organization of IFs we regrew MTs in cells that had been treated with nocodazole to depolymerize all the MTs and to collapse IFs; the reextension of IFs into the lamella lagged behind the rapid regrowth of Tyr MTs, but was correlated with the slower reformation of Glu MTs. Similar realignment of IFs with newly formed Glu MTs was observed in serum-starved cells treated with either serum or taxol to induce the formation of Glu MTs. Next, we microinjected affinity purified antibodies specific for Glu tubulin (polyclonal SG and monoclonal 4B8) and specific for Tyr tubulin (polyclonal W2 and monoclonal YL1/2) into 3T3 cells. Both injected SG and 4B8 antibodies labeled the subset of endogenous Glu MTs; W2 and YL1/2 antibodies labeled virtually all of the cytoplasmic MTs. Injection of SG or 4B8 resulted in the collapse of IFs to a perinuclear region. This collapse was comparable to that observed after complete MT depolymerization by nocodazole. Injection of W2, YL1/2, or nonspecific control IgGs did not result in collapse of the IFs. Taken together, these results show that Glu MTs localize IFs in migrating 3T3 fibroblasts and suggest that detyrosination of tubulin acts as a signal for the recruitment of vimentin IFs to MTs.

Kinetic stabilization of microtubule dynamics at steady state by tau and microtubule-binding domains of tau

Tau is a neuronal microtubule-associated protein that plays an important role in stabilizing axonal microtubules and maintaining neuronal processes. To investigate the mechanisms by which tau performs these functions, we have determined the actions of full-length adult tau and tau peptides corresponding to two different microtubule-binding domains of tau (the first repeat, R1, VRSKIGSTENLKHQPGGG, and the first interrepeat, R1-R2 IR, KVQIINKK) on the growing and shortening dynamics at the plus ends of individual microtubules at steady state. Tau suppressed steady-state microtubule dynamics at very low molar ratios of tau to tubulin. At the lowest ratios examined (tau:tubulin ratios of 1:175 and 1:85), suppression of dynamics occurred in the absence of a detectable change in polymer mass. Tau reduced the mean rate and extent of shortening and, in contrast to previous work carried out under conditions of net polymer gain, tau also suppressed the mean rate and extent of growing. Tau also strongly increased the rescue frequency, it moderately suppressed the catastrophe frequency and it strongly increased the percentage of total time that the microtubules spent in an attenuated (pause) state, neither growing nor shortening detectably. In addition, both the R1 and R1-R2 IR tau peptides suppressed steady-state microtubule dynamics in a sequence-specific manner and in a manner that was qualitatively indistinguishable from full-length tau. The data provide significant support for a mechanism in which the binding of tau to individual tubulin subunits in microtubules induces a conformational change that strengthens inter-tubulin bonding.

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.

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.

Immunocytochemical comparison of posttranslationally modified forms of tubulin in the vestibular end-organs of the gerbil: tyrosinated, acetylated and polyglutamylated tubulin

Specific antibodies against alpha-tubulin, acetylated alpha-tubulin, tyrosinated alpha-tubulin and polyglutamylated alpha- and beta-tubulin were used to compare the distribution of posttranslationally modified tubulin in the vestibular end-organs of the gerbil. Antibodies to acetylated tubulin labeled a dense network of microtubules in the hair cells and bundles of microtubule in the supporting cells. Nerve fibers within and below the epithelium were weakly labeled. This localization paralleled that seen with antibodies to alpha-tubulin which labeled all microtubules present in the cells. Antibodies to tyrosinated tubulin labeled networks and bundles of microtubules in both hair cells and supporting cells and in addition gave intense, diffuse labeling in the cytoplasm of both cell types. It also labeled the nerve fibers. Antibodies to polyglutamylated tubulin were localized mainly in nerve fibers, and in the calyces the labeled microtubules were found running circumferentially around the type I sensory hair cells. Thus, tyrosinated tubulin was found in the fine networks of microtubules in both the sensory and supporting cells. Acetylated tubulin was found in the dense networks and bundles of microtubules in the sensory and supporting cells, but did not colocalize with polyglutamylated tubulin, which was found predominantly in the nerve fibers. The labeling patterns for the tyrosinated tubulin and posttranslationally modified tubulins in the sensory and supporting cells of the vestibular end organs differ from that seen in the organ of Corti and may reflect differences in the stability of the microtubules and the mechanical properties of the sensory epithelium.

Discrete and reversible vacuole-like dilations induced by osmomechanical perturbation of neurons

In cultured Lymnaea stagnalis neurons, osmolarity increases (upshocks) rapidly elicited large membranous dilations that could be dislodged and pushed around inside the cell with a microprobe. Subsequent osmolarity decreases (downshocks) caused these vacuole-like dilations (VLDs) to disappear. Additional upshock/downshock perturbations resulted in repeated appearance/disappearance (formation/reversal) of VLDs at discrete sites. Confocal microscopy indicated that VLDs formed as invaginations of the substrate-adherent surface of the neuron: extracellular rhodamine-dextran entered VLDs as they formed and was expelled during reversal. Our standard VLD-inducing perturbation was: 2-4 min downshock to distilled water, upshock to normal saline. However, a wide range of other osmotic perturbations (involving osmolarities up to 3.5 x normal, perturbations with or without Ca2+, replacement of ions by sucrose) were also used. We concluded that mechanical, not chemical, aspects of the osmo-mechanical shocks drove the VLD formation and reversal dynamics and that extracellular Ca2+ was not required. Following a standard perturbation, VLDs grew from invisible to their full diameter (> 10 microns) in just over a minute. Over the next 0.5-3 hr in normal saline, neurons recovered. Recovery eliminated any visible VLDs and was accompanied by cytoplasmic turmoil around the VLDs. Recovery was prevented by cytochalasin B, brefeldin A and N-ethylmaleimide but not by nocodazole. In striking contrast, these drugs did not prevent repeated VLD formation and reversal in response to standard osmo-mechanical perturbations; VLD disappearance during reversal and during recovery are different. The osmo-mechanical changes that elicited VLDs may, in an exaggerated fashion, mimic tension changes in extending and retracting neurites. In this context we postulate: (a) the trafficking or disposition of membrane between internal stores and plasma membrane is mechanosensitive, (b) normally, this mechanosensitivity provides an "on demand" system by which neurons can accommodate stretch/release perturbations and control cell shape but, (c) given sudden extreme mechanical stimuli, it yields VLDs.

Dividing neuron precursors express neuron-specific tubulin

Neuronal differentiation involves specific molecular and morphological changes in precursors and results in mature, postmitotic neurons. The expression of neuron-specific beta tubulin, as detected by the monoclonal antibody TuJ1, begins during the period of neurogenesis. Indeed, TuJ1 expression precedes that of the 160 kD neurofilament protein in both the central and peripheral nervous systems. In the embryonic rat spinal cord, bipolar cells and some mitotic cells in the ventricular zone were TuJ1 immunoreactive (IR). Sensory ganglia also contained cells with TuJ1-IR mitotic spindles in situ. In embryonic rat sensory and sympathetic ganglion cell cultures pulsed with the thymidine analog bromodeoxyuridine (BrdU), TuJ1 label was detected in the spindle of mitotic cells and in the midbody of cells joined at cytokinesis, indicating that neuron-specific tubulin expression was initiated during or before the final mitosis of neuronal progenitors. Dorsal root ganglion cultures included TuJ1-IR cells with several shapes that may reflect morphological transitions, from flattened stellate neural crest-like cells to differentiated bipolar neurons. Indeed, the presence of flattened TuJ1-IR cells was correlated with neurogenesis. Some sympathetic neuron precursors possessed long TuJ1-IR neurites, as well as TuJ1-IR spindle microtubules and BrdU-labeled chromosomes, indicating that these precursors can possess long processes during metaphase. These results support the hypothesis that neuron-specific tubulin expression represents an early molecular event in neuronal differentiation exhibited by a wide range of neuronal precursors. The cessation of proliferation can occur at different points during neuronal differentiation, as TuJ1-IR was detected in cells undergoing mitosis. Future studies directed toward understanding the molecules that initiate neuron-specific tubulin expression may lead to the factors that control the initial phases of neuronal differentiation.

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.

Identification of a novel Ca(2+)-regulated protein that is associated with the marginal band and centrosomes of chicken erythrocytes

We have identified a novel Ca(2+)-regulated protein, p23, that is expressed specifically in avian erythrocyte and thrombocyte lineages. Sequence analysis of this 23 kDa protein reveals that it bears no homology to any known sequence. In mature definitive erythrocytes p23 exists in equilibrium between a soluble and a cytoskeletal bound pool. The cytoskeletal fraction is associated with the marginal band of microtubules, centrosomes and nuclear membrane under conditions of low free [Ca2+]. An increase in free [Ca2+] to 10(−6) M is sufficient to induce dissociation of &gt; 95% of bound p23 from its target cytoskeletal binding sites, yet this [Ca2+] has little effect on calmodulin-mediated MB depolymerization. Analysis of p23 expression and localization during erythropoiesis together with results from heterologous p23 expression in tissue cultured cells demonstrated that this protein does not behave as a bone fide microtubule-associated protein. In addition, the developmental analysis revealed that although p23 is expressed early in definitive erythropoeisis, its association with the MB, centrosome and nuclear membrane occurs only in the final stages of differentiation. This cytoskeletal association correlates with marked p23 stabilization and accumulation at a time p23 expression is being markedly downregulated. We hypothesize that the mechanism of p23 association to the MB and centrosomes may be induced in part by a decrease in intracellular [Ca2+] during the terminal stages of definitive erythropoiesis.

Organization of microtubules in myelinating Schwann cells

Myelinating Schwann cells polarize their surface membrane into several ultrastructurally and biochemically distinct domains that constitute the myelin internode. Formation of these membrane domains depends on contact with appropriate axons and requires microtubule-based transport systems for site-specific targeting of membrane components. Because little is known about microtubules in myelinating Schwann cells, this study used confocal microscopy and the microtubule hook-labelling method to characterize microtubule distribution, the location of microtubule nucleation sites, and the polarity and composition of Schwann cell microtubules. In myelinating Schwann cells, microtubules were abundant within the Golgi-rich perinuclear cytoplasm; they were not attached to the centrosome. Three-fourths of the microtubules in the cytoplasmic channels located along the outer perimeter of the myelin internode had their (+) ends oriented away from the perinuclear region, whereas the remaining 25% had the opposite polarity. Depolymerization/repolymerization experiments detected microtubule nucleating sites in perinuclear cytoplasm but not along the myelin internode. Taken together, these results indicate that microtubule-mediated transport of myelin components along the internode could utilize both (+)- and (-)-end motors. Specialized microtubule tracks that target myelin proteins to specific sites were not identified on the basis of tubulin polarity or posttranslational modifications.

The initiation of neurite outgrowth by sympathetic neurons grown in vitro does not depend on assembly of microtubules

Neurite formation by dissociated chick sympathetic neurons in vitro begins when one of the many filopodia that emanate from the cell body of a neuron is invaded by cytoplasm containing microtubules and other components of axoplasm (Smith, 1994). This study was undertaken to determine whether this process depends on assembly of microtubules. To inhibit microtubule assembly, neurons were grown in medium containing nocodazole or colchicine. In one series of experiments, neurons first were exposed to the microtubule-stabilizing drug, taxol, so that existing microtubules would remain intact while assembly of new microtubules was inhibited. The ability of neurons to form neurites was assessed by time-lapse video microscopy. Neurons subsequently were stained with antibodies against the tyrosinated and acetylated forms of alpha-tubulin and examined by laser confocal microscopy to visualize microtubules. Neurons were able to form short processes despite inhibition of microtubule assembly and they did so in a way that closely resembled process formation in control medium. Processes formed by neurons that had not been pretreated with taxol were devoid of microtubules. However, microtubules were present in processes of taxol-pretreated neurons. These microtubules contained acetylated alpha-tubulin, as is typical of stable microtubules, but not tyrosinated alpha-tubulin, the form present in recently assembled microtubules. These findings show that the initial steps in neurite formation do not depend on microtubule assembly and suggest that microtubules assembled in the cell body can be translocated into developing neurites as they emerge. The results are compatible with models of neurite formation which postulate that cytoplasm from the cell body is transported into filopodia by actomyosin-based motility mechanisms.

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.

Accumulation of delta 2-tubulin, a major tubulin variant that cannot be tyrosinated, in neuronal tissues and in stable microtubule assemblies

Tubulin is the major protein component of brain tissue. It normally undergoes a cycle of tyrosination-detyrosination on the carboxy terminus of its alpha-subunit and this results in subpopulations of tyrosinated tubulin and detyrosinated tubulin. Brain tubulin preparations also contain a third major tubulin subpopulation, composed of a non-tyrosinatable variant of tubulin that lacks a carboxy-terminal glutamyl-tyrosine group on its alpha-subunit (delta 2-tubulin). Here, the abundance of delta 2-tubulin in brain tissues, its distribution in developing rat cerebellum and in a variety of cell types have been examined and compared with that of total alpha-tubulin and of tyrosinated and detyrosinated tubulin. Delta 2-tubulin accounts for approximately 35% of brain tubulin. In rat cerebellum, delta 2-tubulin appears early during neuronal differentiation and is detected only in neuronal cells. This apparent neuronal specificity of delta 2-tubulin is confirmed by examination of its distribution in cerebellar cells in primary cultures. In such cultures, neuronal cells are brightly stained with anti-delta 2-tubulin antibody while glial cells are not. Delta 2-tubulin is apparently present in neuronal growth cones. As delta 2-tubulin, detyrosinated tubulin is enriched in neuronal cells, but in contrast with delta 2-tubulin, detyrosinated tubulin is not detectable in Purkinje cells and is apparently excluded from neuronal growth cones. In a variety of cell types such as cultured fibroblasts of primary culture of bovine adrenal cortical cells, delta 2-tubulin is confined to very stable structures such as centrosomes and primary cilia. Treatment of such cells with high doses of taxol leads to the appearance of delta 2-tubulin in microtubule bundles. Delta 2-tubulin also occurs in the paracrystalline bundles of protofilamentous tubulin formed after vinblastine treatment. Delta 2-tubulin is present in sea urchin sperm flagella and it appears in sea urchin embryo cilia during development. Thus, delta 2-tubulin is apparently a marker of very long-lived microtubules. It might represent the final stage of alpha-tubulin maturation in long-lived polymers.

Identification of a novel microtubule-binding domain in microtubule-associated protein 1A (MAP1A)

Several microtubule-associated proteins (MAPs) have been shown to bind to microtubules via short sequences with repeated amino acids motifs. A microtubule-binding domain has hitherto not been defined for the adult brain microtubule-associated protein 1A (MAP1A). We have searched for a microtubule-binding domain by expressing different protein regions of MAP1A in cultured cell lines using cDNA constructs. One construct included an area with homology to the microtubule-binding domain of MAP1B (Noble et al. (1989) J. Cell Biol. 109, 437–448), but this did not bind to microtubules in transfected cells. Further investigation of other areas of MAP1A revealed a protein domain, capable of autonomously binding to microtubules, which bears no homology to any previously described microtubule-binding sequence. This MAP1A domain is rich in charged amino acids, as are other mammalian microtubule-binding domains, but unlike them has no identifiable sequence repeats. Whereas all previously described mammalian microtubule-binding domains are basic, this novel microtubule-binding domain of MAP1A is acidic. The expression of this polypeptide in cultured cell lines led to a rearrangement of the microtubules in a pattern distinct from that produced by MAP2 or tau, and increased their resistance to treatment with the microtubule depolymerising agent nocodazole. When the MAP1A microtubule-binding domain was co-expressed in cultured cell lines together with MAP2c, the MAP1A microtubule-binding domain was able to bind to the MAP2c-induced microtubule bundles. These results suggest that different microtubule-binding sequences have a common ability to stabilise microtubules but differ in their influence on microtubule arrangement in the cell. This may be significant in neurons, where microtubule-associated proteins with different microtubule-binding sequences are expressed in different cell compartments and at different times during development.

Induction of stable microtubules in 3T3 fibroblasts by TGF-beta and serum

Previous studies have shown that fibroblasts induced to migrate into an in vitro wound rapidly generate an array of stable, post-translationally detyrosinated microtubules (Glu MTs) oriented toward the direction of migration. To understand how cells generate a stable array of MTs at a specific location, we have analyzed the contribution of media components to the formation of oriented Glu MTs in wounded monolayers of 3T3 fibroblasts. When confluent monolayers were placed in serum-free medium (SFM) for 2 days before wounding, the cells contained virtually no Glu MTs or nocodazole-resistant MTs and were incapable of generating Glu MTs in response to wounding. Such SFM-treated monolayers were capable of generating oriented Glu MTs within 1 hour of wounding, if calf serum (CS) was added back to the medium. The Glu MTs in the CS refed cells were oriented toward the wound in cells at the wound edge, and were juxtanuclear in cells within the monolayer, demonstrating that CS restored the Glu MT array characteristic of each cell type. To determine the nature of the ‘Glu MT-inducing’ factor in CS, we subjected CS to different treatments and found that the CS factor was nondialyzable, resistant to heat, mild acid and trypsin, but inactivated by treatment with dithiothreitol. The factor was not absorbed by charcoal and was present in lipoprotein-deficient serum. These properties are consistent with the properties of a number of polypeptide growth factors, so we screened purified growth factors for their ability to induce Glu MTs in wounded SFM-treated monolayers. Of all the growth factors tested, only TGF-beta 1 and TGF-beta 2 induced a significant level (&gt; or = 70% of the CS response) of oriented Glu MTs. The SFM-treated cells were exquisitely sensitive to TGF-beta 1, with significant induction of Glu MTs observed at 0.01 ng/ml TGF-beta 1. Induction of Glu MTs observed by immunofluorescence after CS or TGF-beta treatments were paralleled by increases in Glu tubulin detected on western blots. The Glu MTs formed after either CS or TGF-beta 1 treatment showed enhanced resistance to nocodazole, confirming that both treatments increased the level of stable MTs in cells. The TGF-beta 1 induction of stable MTs was slower than that of CS (2-4 hours onset versus 1 hour onset), but by 24 hours the level of MT stabilization in TGF-beta 1 was even greater than that in CS.(ABSTRACT TRUNCATED AT 400 WORDS)

Different stability of posttranslationally modified brain microtubules isolated from cold-temperate fish

Microtubule proteins were isolated by a temperature-dependent assembly-disassembly method from brain tissue of for cold-temperature fish; one fresh water fish (Oncorhynchus mykiss), and three marine fish (Labrus berggylta, Zoarces viviparus and Gadus morhua). The alpha-tubulins from all four fish species were acetylated. The alpha-tubulins from the marine fish were composed of a mixture of tyrosinated and detyrosinated tubulin, while the fresh water fish tubulin only reacted with an antibody against detyrosinated tubulin. The isolated microtubules had a similar MAP composition. A 400 kD protein and a MAP2-like protein were found, but MAP1 was missing. All microtubules disassembled upon cooling to 0 degrees C. In spite of these common characteristics, the assembly of microtubules from Labrus berggylta was inhibited by colchicine and calcium, in contrast to the assembly of microtubules from Oncorhynchus mykiss and Zoarces viviparus. For the latter, colchicine was not completely inhibitory even at a concentration as high as 1 mM, and calcium induced the formation of both loosely and densely coiled ribbons. The effects of calcium and colchicine on microtubules from Oncorhynchus mykiss and Zoarces viviparus were modulated by either fish or cow MAPs, indicating that the effects are due to intrinsic properties of the fish tubulins and not the MAPs. In view of these findings, our results suggest that there is no correlation between colchicine sensitivity, inability of calcium to inhibit microtubule assembly, and acetylation and detyrosination.

Microtubule-associated protein function: lessons from expression in Spodoptera frugiperda cells

The phenotypes induced by the expression of neuronal microtubule-associated proteins (MAPs) in Sf9 cells have provided data on the in situ function of these proteins. Both MAP2 and tau can induce long processes in Sf9 cells, and the processes contain bundles of microtubules. In both cases the microtubules are aligned with their plus ends distal. Tau expression usually induces a single process that is unbranched and of uniform caliber. Processes can form even when the cells are grown in suspension. Microtubules do not extend all the way to the tip; instead the terminal region contains an actin-rich meshwork. Taxol treatment of Sf9 cells also induces the assembly of microtubules into bundles but does not induce process formation in Sf9 cells. Therefore the in vitro properties of tau as a molecule capable of assembling, stabilizing, and bundling microtubules do not fully account for the in vivo ability of tau alone to transduce microtubule assembly into a change in cell shape. The morphological features of the processes induced by MAP2 differ in highly informative ways.

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.

pH-dependent solubility and assembly of microtubules in bovine brain extracts

Alkaline pH favors the assembly of microtubules (MTs) in marine egg extracts [Suprenant and Marsh, 1987: J. Cell Sci. 184:167-180; Suprenant, 1989: Exp. Cell Res. 184:167-180; 1991: Cell Motil. Cytoskeleton 19:207-220] and mammalian brain extracts [Tiwari and Suprenant, 1993: Anal. Biochem. 215:96-103], even though the assembly of purified microtubule protein (MTP) from both of these sources is favored at slightly acidic pH. The present investigation examines whether alkaline pH has a direct or indirect effect on MT nucleation and growth in soluble brain extracts. Cell-free extracts were prepared from bovine cerebral cortex, and a nucleated assembly assay was used to demonstrate that MT assembly in brain extracts is favored at slightly acidic pH. The increase in MT mass found at alkaline pH is due to an increase in the solubility of tubulin not an increase in the extent of assembly. On average, 47.7 +/- 11.3% of the total tubulin is soluble at pH 7.2, while only 30.9 +/- 8.9% of the tubulin is soluble at pH 6.8. A model is proposed that indicates how microtubule proteins from both mammalian brain and marine eggs may be associated with pH-dependent factors.

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.

Glycolytic enzyme-tubulin interactions: role of tubulin carboxy terminals

Tubulin and microtubules were modified with the protease, subtilisin. The modification reduced the length of alpha- or beta-tubulin by cleaving a peptide fragment from the C-terminals. Generation of alpha'beta'-tubulin, which is cleaved at both the alpha- and beta-subunit terminals, and alpha beta'-tubulin, which is cleaved at the beta-subunit C-terminal, have already been reported. In this work an isotype, alpha'beta-tubulin, was produced. The three modified tubulin isotypes were compared for their ability to interact with glycolytic enzymes. Cleavage of alpha led to a poorer interaction when tested via affinity chromatography. Tubulin also inhibits the activity of aldolase and glyceraldehyde 3-phosphate dehydrogenase. When the alpha-subunit C-terminal was intact, inhibition was greatest. These results imply that the C-terminal of the tubulin alpha-subunit is responsible for interactions with glycolytic enzymes.

Protein phosphatase inhibitors induce the selective breakdown of stable microtubules in fibroblasts and epithelial cells

In many cell types, a small subset of microtubules (MTs) are unusually long-lived compared with the majority of the MTs. These "stable" MTs may be important mediators of differentiative events since they are usually found aligned with developing asymmetries of cells undergoing morphogenesis. In addition to their longevity, the stable MTs are more resistant to drug depolymerization and are enriched in post-translationally detyrosinated tubulin (Glu-tubulin). To determine the role of protein phosphorylation in the regulation of these stable MTs, we treated NIH 3T3 fibroblasts and TC-7 monkey kidney epithelial cells with okadaic acid (OA) and calyculin A, potent inhibitors of protein phosphatases 1 and 2A (PP1 and PP2A), and then localized dynamic MTs and stable MTs with antibodies specific for tyrosinated tubulin (Tyrtubulin) and Glu-tubulin, respectively. OA at 0.1-10 microM caused a rapid and complete breakdown of Glu-MTs (MTs enriched in Glu-tubulin) in both cell types without substantially affecting the number of Tyr-MTs. While all concentrations of OA over this range resulted in a complete loss of Glu-MTs, the onset of Glu-MT breakdown was proportional to the logarithm of the OA concentration. The inactive analog of OA, 1-norokadaone, had no effect at any concentration. Calyculin A also caused a selective loss of Glu-MTs but was effective at 10 nM, consistent with its more potent inhibition of PP1. That the loss of Glu-MTs reflected the loss of stable MTs from the cells was shown by the absence of nocodazole-resistant MTs in OA-treated cells. OA did not appear to activate a MT-severing activity, since no MT fragments were observed after OA treatment of cells pretreated with taxol. These results suggest that PP1 and perhaps PP2A are involved in the regulation of MT stability in cells and show that the dynamic and stable subsets of MTs are regulated differentially by protein phosphorylation.

Organization of microtubules in axonal growth cones: a role for microtubule-associated protein MAP 1B

Neuronal growth cones guide growing axons and dendrites (neurites) through developing embryos by detecting extrinsic guidance cues and transducing the signal into changes in motile behaviour. In this brief review, the role of the growth cone cytoskeleton in these events, in particular the microtubules, is discussed. Microtubules in the neurite are mainly bundled into fascicles whereas on entering the growth cone they diverge from each other and traverse the central (C)-domain of the growth cone. Occasionally, individual microtubules extend as far as the peripheral (P)-domain and may even enter filopodia. Microtubules in the growth cone are probably dynamically unstable, exchanging dimer with a large pool of soluble tubulin. It is proposed that the 'capture' of dynamically unstable microtubules by filopodial actin filament bundles is a crucial step underlying directed growth. Localised assembly of microtubules at the growth cone, rather than at the cell body followed by transport of polymer to the growth cone, may facilitate the delivery of material to specific regions of the growth cone and hence allow vectorial growth. Bundling of microtubules and capture of microtubules by filopodia both imply roles for microtubule-associated proteins (MAPs). Several microtubule-associated proteins are present within growth cones, including MAP 1B, MAP2 and tau. Recent experiments point toward a phosphorylated form of MAP 1B as an important component in neurite elongation and in particular in the bundling of microtubules in the growth cone.

Probing modifications of the neuronal cytoskeleton

The prominent death of central neurons in Alzheimer's and Parkinson's is reflected by changes in cell shape and by the formation of characteristic cytoskeletal inclusions (neurofibrillary tangles, Lewy bodies). This review focuses on the biology of neurofilaments and microtubule-associated proteins and identifies changes that can occur to these elements from basic and clinical research perspectives. Attention is directed at certain advances in neurobiology that have been especially integral to the identification of epitope domains, protein isoforms, and posttranslational (phosphorylation) events related to the composition, development, and structure of the common cytoskeletal modifications. Recently, a number of experimental strategies have emerged to simulate the aberrant changes in neurodegenerative disorders and gain insight into possible molecular events that contribute to alterations of the cytoskeleton. Descriptions of specific systems used to induce modifications are presented. In particular, unique neural transplantation methods in animals have been used to probe possible molecular and cellular conditions concerned with abnormal cytoskeletal changes in neurons.

Enteric nerve fibers of holothurians are recognized by an antibody to acetylated alpha-tubulin

The distribution of immunoreactivity to 6-11B-1, a monoclonal antibody that labels acetylated alpha-tubulin, was studied in the radial nerve and intestinal system of holothurians. As shown previously for other species, this antibody recognizes cilia and nerve fibers in Holothuria glaberrima and Holothuria mexicana. Thus, anti-acetylated alpha-tubulin can be used as a marker for nerve fibers in the enteric nervous system.

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.

Microtubule-associated protein MAP1B expression precedes the morphological differentiation of oligodendrocytes

The microtubule-associated protein MAP1B is believed to play an important role in the outgrowth of neurites from neurons (Tucker and Matus, Dev Biol 130: 423-434, 1988). We have investigated the possibility that MAP1B might participate in the formation of processes in cultured oligodendrocytes by an analysis of the expression of MAP1B during oligodendrocyte progenitor development. The appearance of the antigens recognized by the monoclonal antibodies A2B5, O4, and O1 which define distinct stages in the maturation of progenitors, was compared with the developmental expression of MAP1B. MAP1B is first detectable in O4+ preoligodendrocytes prior to the acquisition of galactocerebroside and immediately before they develop the complex process-bearing morphology characteristic of terminally differentiated myelin-forming oligodendrocytes in the CNS. In contrast, astrocytes have negligible amounts of MAP1B. These results demonstrate that the expression of MAP1B precedes the development of the mature oligodendrocyte phenotype and suggest that interactions between microtubules and MAP1B might have a role in the formation and stabilization of myelin-forming processes.

Phosphorylation of neuronal kinesin heavy and light chains in vivo

The microtubule-based motor protein kinesin is thought to drive anterograde organelle transport in axons, but nothing is known about how its force-generating activity or organelle-binding properties are regulated. Studies in other motility systems suggest that protein phosphorylation is a reasonable candidate for this function. I report here that the kinesin heavy chain (HC) and light chain (LC), as well as the 160-kDa kinesin-associated protein kinectin, are phosphorylated in vivo in cultures of chick sympathetic neurons and PC12 cells labeled metabolically with 32P. In neurons, both kinesin chains are phosphorylated exclusively on serine residues, and limiting tryptic digestion demonstrated that the phosphorylation sites are clustered in a region of < or = 5 kDa for the HC and < or = 14 kDa for the LC. Partial tryptic digestion of 32P-labeled HC followed by immunoblotting with SUK4 monoclonal anti-HC and fluorography showed that the sites of HC phosphorylation are outside the globular N-terminal head region where kinesin's microtubule-binding and mechanochemical activities reside. Treatment of metabolically labeled neurons with forskolin, phorbol esters, or calcium ionophore did not alter the extent of phosphorylation, the phosphoamino acid composition, or the V8 protease phosphopeptide maps of the HC, LC, and 160-kDa protein, with one exception: treatment with calcium ionophore reduced the specific activity of the LC. In addition, when kinesin from PC12 cells was compared with that from PC12-derived cell lines lacking protein kinase A activity, neither the extent of phosphorylation nor the phosphopeptide maps were altered for either chain. Phosphopeptide mapping experiments also showed that postlysis kinase activity can phosphorylate both the neuronal HC and LC at sites not phosphorylated in vivo.

Reversible polyglutamylation of alpha- and beta-tubulin and microtubule dynamics in mouse brain neurons

The relationship between microtubule dynamics and polyglutamylation of tubulin was investigated in young differentiating mouse brain neurons. Selective posttranslational labeling with [3H]glutamate and immunoblotting with a specific monoclonal antibody (GT335) enabled us to analyze polyglutamylation of both alpha and beta subunits. Nocodazole markedly inhibited incorporation of [3H]glutamate into alpha- and beta-tubulin, whereas taxol had no effect for alpha-tubulin and a stimulating effect for beta-tubulin. These results strongly suggest that microtubule polymers are the preferred substrate for polyglutamylation. Chase experiments revealed the existence of a reversal reaction that, in the case of alpha-tubulin, was not affected by microtubule drugs, suggesting that deglutamylation of this subunit can occur on both polymers and soluble tubulin. Evidence was obtained that deglutamylation of alpha-tubulin operates following two distinct rates depending on the length of the polyglutamyl chain, the distal units (4th-6th) being removed rapidly whereas the proximal ones (1st-3rd) appearing much more resistant to deglutamylation. Partition of glutamylated alpha-tubulin isoforms was also correlated with the length of the polyglutamyl chain. Forms bearing four to six units were recovered specifically in the polymeric fraction, whereas those bearing one to three units were distributed evenly between polymeric and soluble fractions. It thus appears that the slow rate component of the deglutamylation reaction offers to neurons the possibility to maintain a basal level of glutamylated alpha-tubulin in the soluble pool independently of microtubule dynamics. Finally, some differences observed in the glutamylation of alpha- and beta-tubulin suggest that distinct enzymes are involved.

Investigation of microtubule assembly and organization accompanying tension-induced neurite initiation

Pulling on the margin of embryonic chick sensory neurons induces neurite formation de novo. We find that these neurites contain microtubules within minutes after the application of tension and apparently normal microtubule arrays within 10–20 min. We wished to determine whether these microtubules reflected existing microtubules that were reorganized, e.g. pulled into the neurite by the applied forces, or whether they reflected primarily new assembly of tubulin. We investigated tension-induced neurite initiation in the presence of 4 nM vinblastine, a concentration that poisons net microtubule assembly but does not depolymerize extant polymers, thus separating new assembly from movements of existing microtubules. We find that vinblastine seriously compromises the ability of chick sensory neurons to initiate neurites in response to tension. The few poisoned neurites that did form were abnormal in several respects. In contrast to unpoisoned cells, poisoned neurites were prone to stretching and breaking while pulling, as though they lacked normal structural support. Indeed, poisoned neurites possessed only short microtubule fragments. We conclude that the microtubule array seen in tension-induced neurites reflects primarily new microtubule assembly, rather than existing microtubules that were reorganized to invade the neurite. This implies that tension applied to unpoisoned chick sensory neurons rapidly stimulates new microtubule assembly concomitant with neurite initiation. Examination of the tension-induced microtubules shows that both their spatial pattern and their acetylation are similar to that reported for normal growth cone-mediated neurites.

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)

Rapid and reversible tubulin tyrosination in human neutrophils stimulated by the chemotactic peptide, fMet-Leu-Phe

Neutrophil activation by specific stimuli, such as the oligopeptide chemotactic factor fMet-Leu-Phe (fMLF), is associated with an increased enzymatic addition of tyrosine to tubulin alpha-subunits, as measured by 14C tyrosine uptake. In studies using immunoblots we have found that this increased tyrosine uptake into tubulin in activated neutrophils reflects an increase in the proportion of cellular tubulin that is tyrosinated rather than simply an increase in the turnover of tyrosinated subunits. However, the increased accumulation of tyrosinated tubulin was also found to follow an initial depletion of tyrosinated tubulin and concomitant increase in detyrosinated tubulin between 0 and 60 sec following stimulation of neutrophils with fMLF. Immunogold electron microscopy studies of intact microtubules recovered from activated neutrophils demonstrated that these rapid changes in the relative content of tubulin isoforms in the cells were not associated with the formation or disappearance of microtubule microdomains composed of only one form of tubulin. Previously, we have shown that under conditions of fMLF-stimulated exocytosis there is an increased binding of neutrophil granules to endogenous microtubules. Since neutrophil activation by fMLF is associated with increased tyrosination of alpha-tubulin subunits, we speculated that rapid changes in the levels of tyrosinated tubulin in the microtubules of activated neutrophils might have a role in the regulation of granule-microtubule interactions. When the binding of purified neutrophil granules to reconstituted rat brain microtubules containing approximately 50% tyrosinated tubulin was measured by electron microscopy and compared with granule binding to microtubules that contained no detectable tyrosinated tubulin, granule-microtubule associations were found to be significantly favored by detyrosinated vs. tyrosinated tubulin. These findings indicate that interactions between cytoplasmic granules and microtubules in activated neutrophils may be modulated by rapid changes in the relative content of detyrosinated and tyrosinated tubulin in the microtubule network of the cells.

Actin depolymerisation induces process formation on MAP2-transfected non-neuronal cells

We have previously shown that microtubules in nonneuronal cells form long, stable bundles after transfection with the embryonic neuronal microtubule-associated protein MAP2c. In this study, we found that treating MAP2c-transfected cells with the actin depolymerising drug cytochalasin B led to the outgrowth of microtubule-containing processes from the cell surface. This effect was specific to MAP2c and did not occur in untransfected cells whose microtubules had been stabilised by treatment with taxol. The outgrowth and retraction of these processes during repeated cycles of cytochalasin addition and removal was followed by video time-lapse microscopy and was suggestive of a physical interaction between compressive forces exerted by the MAP2c-stabilised microtubule bundles and tensile forces originating in the cortical actin network. We suggest that MAP2c confers three properties on cellular microtubules that are essential for process outgrowth: stability, bundling and stiffness. The latter probably arises from the linking together of neighbouring tubulin subunits by three closely spaced tubulin-binding motifs in the MAP2 molecule that limits their motion relative to one another and thus reduces the flexibility of the polymer. Similar multimeric tubulin-binding domains in other proteins of the MAP2 class, including tau in axons and MAP4 in glial cells, may play the same role in the development and support of asymmetric cell morphology. Axial bundles of microtubules are found in growing neurites but not in growth cones, suggesting that the regulated expression of these MAP-induced properties makes an important contribution to the establishment of a stable process behind the advancing growth cone.

Composite microtubules of the axon: quantitative analysis of tyrosinated and acetylated tubulin along individual axonal microtubules

We have shown previously, using immunoelectron microscopy, that axonal microtubules (MTs) are composite, consisting of distinct domains that differ in their content of tyrosinated alpha-tubulin (tyr-tubulin). Here, we extend these studies using a novel preparation that permits visualization of individual axonal MTs over distances of several tens of micrometers using conventional immunofluorescence procedures. Neurons are cultured on a substratum of poly-lysine and laminin and then extracted with a MT stabilizing solution containing Triton X-100 and NaCl. These extraction conditions cause a loosening of the axonal MT array so that individual MTs separate from each other for variable distances along their length. We call this phenomenon fraying. Within the axon shaft, individual MTs can often be traced for several tens of micrometers, but fraying is most extensive in the distal 100–200 microns of the axon, where individual MTs can frequently be traced for distances of 50 to 100 microns or more to their plus ends. In some cases MTs separate completely from the axon, permitting visualization of both of their ends. Double-staining of frayed preparations with various combinations of antibodies against tyr-tubulin, acetylated alpha-tubulin (Ac-tubulin) or beta-tubulin, clearly revealed the composite nature of axonal MTs. Composite MTs consisted of two distinct domains, one that was relatively rich in tyr-tubulin and poor in Ac-tubulin, and the other that was relatively poor in tyr-tubulin and rich in Ac-tubulin. The transition between these domains was relatively abrupt, with the tyr-tubulin-rich domain extending from the transition to the plus-end of the MT. Quantitative analyses of fluorescence intensity along individual MTs using digital image processing revealed that the relative amount of tyr-tubulin increased by approximately 800% across the transition, whereas the relative amount of Ac-tubulin decreased by approximately 60%. Within the tyr-tubulin-rich domains, the relative amount of tyr-tubulin was generally not constant, but increased from the transition to the plus-end of the MT in a nonlinear manner. We propose that the specific pattern of variation in the extent of post-translational modification along an individual MT represents a snapshot of that polymer's growth history.

Cytoplasmic segregation and cytoskeletal organization in the electric catfish giant electromotoneuron with special reference to the axon hillock region

The cytoplasm of the highly polarized nerve cell is permanently segregated into domains with differing organellar composition. The mechanisms maintaining this segregation are largely unknown. In order to elucidate the potential role of cytoskeletal elements in this process we compared the cytoplasmic segregation within the giant electromotoneuron of the electric catfish (Malapterurus electricus) with the distribution of binding sites for antibodies against elements of the cytoskeleton. Most prominent cytoplasmic segregations include the formation of a subplasmalemmal cortical structure free of Nissl bodies and Golgi cisternae, the separation within the soma of domains containing rough endoplasmic reticulum and filament-rich domains, and the soma-axon transition. The cytoplasmic transition at the axon hillock forms a distinct borderline where Nissl bodies, Golgi cisternae and the bulk of lysosomes abruptly terminate and are excluded from the axoplasm. Synaptic vesicles and mitochondria are free to pass compartmental borders. Tropomyosin, spectrin, and alpha-actinin reveal a rather homogeneous immunofluorescence throughout the neuron. In contrast, neurofilament protein and tubulin display a distinctly increased immunofluorescence in the subplasmalemmal cortical layer, in dendrites as well as in the axon. The increase in immunofluorescence at the axon hillock exactly depicts the small transition zone from the somatic cytoplasm rich in Nissl bodies, Golgi cisternae and lysosomes to the differently structured axoplasm. The picture is similar for beta-tubulin, tyrosinylated and detyrosinylated alpha-tubulin. Detyrosinylated tubulin (glu-tubulin, which is contained in microtubules of increased stability) shows the most prominent enrichment in the axon. The distribution of myosin is comparable to that of neurofilament protein but there is less difference in immunofluorescence between the domains. Our results would be compatible with a role of microtubules together with (the closely associated) neurofilaments in the segregation of neuronal cytoplasmic domains. Active transport as well as stable binding to the somatic cytoskeleton might counteract a homogeneous cytoplasmic distribution of the various classes of organelles by diffusion.

FRAP analysis of the stability of the microtubule population along the neurites of chick sensory neurons

In order to study microtubule turnover in elongating neurites, chick embryo sensory neurons were microinjected with x-rhodamine tubulin, and after 6-12 hours, short segments along chosen neurites were photobleached at multiple sites. Previous studies [Lim et al., 1989; 1990] indicated that recovery of fluorescence (FRAP) in neurites occurs by the dynamic turnover of stationary microtubules. In all cases, distal bleached zones recovered fluorescence faster than bleached zones more proximally located along the same neurites. Bleached zones at growth cones completely recovered in 30-40 minutes, while bleached zones located more proximally usually recovered in 50-120 minutes. In the most proximal regions of long neurites, recovery of fluorescence was often incomplete, indicating that a significant fraction of the microtubules in these regions were very stable. These studies indicate that there are differences in microtubule stability along the length of growing neurites. These differences may arise from the combined effects of 1) modifications that stabilize and lengthen microtubules in maturing neurites and 2) the dynamic instability of the distally oriented microtubule plus ends.

Microtubule behavior in PC12 neurites: variable results obtained with photobleach technology

We have examined the effects of various means of photobleaching on the recovery of fluorescence, movement, and morphology of the microtubules in the neurites of rhodamine-tubulin-injected PC12 cells. We find that, depending on power of and time of exposure to the bleaching beam, we can generate at least three different patterns of fluorescence recovery in regenerating PC12 neurites. If bleaching is performed with a relatively low-power beam for an extended period, fluorescence in polymer recovers very little after 1 hour. Under these conditions, however, tubulin immunostaining is seen extending through the bleach zone, and microtubules are present through the bleached zone in thin section electron micrographs. If bleaching is performed with a high-power laser, for 0.5-5 seconds, fluorescence recovery also is quite slow, but electron microscopic observations reveal that no microtubules extend through the bleached region of the neurite, and the uranyl acetate-stained cytoplasm appears more electron lucent than in the unbleached neurite. Finally, if bleaching is performed by very brief exposure to a high-intensity laser beam, resulting in an incomplete reduction of fluorescence intensity through the bleach zone, fluorescence recovery occurs within 20-30 minutes, and immunostained microtubules appear intact through the bleach zone; electron microscopy confirms that microtubules extend through the bleached zone of such neurites. In all three cases, movement of the bleach zone is observed in approximately half of the experimental neurites. These results indicate that highly variable microtubule behaviors can be obtained with photobleach technology, presumably due to different levels and pathways of photodamage generated by different bleach protocols. Nevertheless, it is clear that both turnover and movement of microtubules occur in PC12 neurites, and both are likely to be involved in neurite maintenance and growth.