Microtubule reassembly from nucleating fragments during the regrowth of amputated neurites

An experimental platform for stochastic analyses of single serotonergic fibers in the mouse brain

The self-organization of the serotonergic matrix, a massive axon meshwork in all vertebrate brains, is driven by the structural and dynamical properties of its constitutive elements. Each of these elements, a single serotonergic axon (fiber), has a unique trajectory and can be supported by a soma that executes one of the many available transcriptional programs. This "individuality" of serotonergic neurons necessitates the development of specialized methods for single-fiber analyses, both at the experimental and theoretical levels. We developed an integrated platform that facilitates experimental isolation of single serotonergic fibers in brain tissue, including regions with high fiber densities, and demonstrated the potential of their quantitative analyses based on stochastic modeling. Single fibers were visualized using two transgenic mouse models, one of which is the first implementation of the Brainbow toolbox in this system. The trajectories of serotonergic fibers were automatically traced in the three spatial dimensions with a novel algorithm, and their properties were captured with a single parameter associated with the directional von Mises-Fisher probability distribution. The system represents an end-to-end workflow that can be imported into various studies, including those investigating serotonergic dysfunction in brain disorders. It also supports new research directions inspired by single-fiber analyses in the serotonergic matrix, including supercomputing simulations and modeling in physics.

Regeneration in Stentor coeruleus

We often think about regeneration in terms of replacing missing structures, such as organs or tissues, with new structures generated via cell proliferation and differentiation. But at a smaller scale, single cells, themselves, are capable of regenerating when part of the cell has been removed. A classic model organism that facilitates the study of cellular regeneration in the giant ciliate Stentor coeruleus. These cells, which can grow to more than a millimeter in size, have the ability to survive after extensive wounding of their surface, and are able to regenerate missing structures. Even a small piece of a cell can regenerate a whole cell with normal geometry, in a matter of hours. Such regeneration requires cells to be able to trigger organelle biogenesis in response to loss of structures. But subcellular regeneration also relies on intracellular mechanisms to create and maintain global patterning within the cell. These mechanisms are not understood, but at a conceptual level they involve processes that resemble those seen in animal development and regeneration. Here we discuss single-celled regeneration in Stentor from the viewpoint of standard regeneration paradigms in animals. For example, there is evidence that regeneration of the oral apparatus in Stentor follows a sender-receiver model similar to crustacean eyestalk regeneration. By drawing these analogies, we find that many of the concepts already known from the study of animal-scale regeneration and development can be applied to the study of regeneration at the cellular level, such as the concepts of determination, induction, mosaic vs. regulative development, and epimorphosis vs. morphallaxis. We propose that the similarities may go beyond analogy, and that some aspects of animal development and regeneration may have evolved by exploiting pre-existing subcellular developmental strategies from unicellular ancestors.

MAP6 is an intraluminal protein that induces neuronal microtubules to coil

Neuronal activities depend heavily on microtubules, which shape neuronal processes and transport myriad molecules within them. Although constantly remodeled through growth and shrinkage events, neuronal microtubules must be sufficiently stable to maintain nervous system wiring. This stability is somehow maintained by various microtubule-associated proteins (MAPs), but little is known about how these proteins work. Here, we show that MAP6, previously known to confer cold stability to microtubules, promotes growth. More unexpectedly, MAP6 localizes in the lumen of microtubules, induces the microtubules to coil into a left-handed helix, and forms apertures in the lattice, likely to relieve mechanical stress. These features have not been seen in microtubules before and could play roles in maintaining axonal width or providing flexibility in the face of compressive forces during development.

Mitotic Motor KIFC1 Is an Organizer of Microtubules in the Axon

KIFC1 (also called HSET or kinesin-14a) is best known as a multifunctional motor protein essential for mitosis. The present studies are the first to explore KIFC1 in terminally postmitotic neurons. Using RNA interference to partially deplete KIFC1 from rat neurons (from animals of either gender) in culture, pharmacologic agents that inhibit KIFC1, and expression of mutant KIFC1 constructs, we demonstrate critical roles for KIFC1 in regulating axonal growth and retraction as well as growth cone morphology. Experimental manipulations of KIFC1 elicit morphological changes in the axon as well as changes in the organization, distribution, and polarity orientation of its microtubules. Together, the results indicate a mechanism by which KIFC1 binds to microtubules in the axon and slides them into alignment in an ATP-dependent fashion and then cross-links them in an ATP-independent fashion to oppose their subsequent sliding by other motors.SIGNIFICANCE STATEMENT Here, we establish that KIFC1, a molecular motor well characterized in mitosis, is robustly expressed in neurons, where it has profound influence on the organization of microtubules in a number of different functional contexts. KIFC1 may help answer long-standing questions in cellular neuroscience such as, mechanistically, how growth cones stall and how axonal microtubules resist forces that would otherwise cause the axon to retract. Knowledge about KIFC1 may help researchers to devise strategies for treating disorders of the nervous system involving axonal retraction given that KIFC1 is expressed in adult neurons as well as developing neurons.

Stability properties of neuronal microtubules

Neurons are terminally differentiated cells that use their microtubule arrays not for cell division but rather as architectural elements required for the elaboration of elongated axons and dendrites. In addition to acting as compression-bearing struts that provide for the shape of the neuron, microtubules also act as directional railways for organelle transport. The stability properties of neuronal microtubules are commonly discussed in the biomedical literature as crucial to the development and maintenance of the nervous system, and have recently gained attention as central to the etiology of neurodegenerative diseases. Drugs that affect microtubule stability are currently under investigation as potential therapies for disease and injury of the nervous system. There is often a lack of consistency, however, in how the issue of microtubule stability is discussed in the literature, and this can affect the design and interpretation of experiments as well as potential therapeutic regimens. Neuronal microtubules are considered to be more stable than microtubules in dividing cells. On average, this is true, but in addition to an abundant stable microtubule fraction in neurons, there is also an abundant labile microtubule fraction. Both are functionally important. Individual microtubules consist of domains that differ in their stability properties, and these domains can also differ markedly in their composition as well as how they interact with various microtubule-related proteins in the neuron. Myriad proteins and pathways have been discussed as potential contributors to microtubule stability in neurons. © 2016 Wiley Periodicals, Inc.

Hippocampal plasticity during the progression of Alzheimer's disease

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimer's disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aβ) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.

Histone acetylation inhibitors promote axon growth in adult dorsal root ganglia neurons

Intrinsic mechanisms that guide damaged axons to regenerate following spinal cord injury remain poorly understood. Manipulation of posttranslational modifications of key proteins in mature neurons could reinvigorate growth machinery after injury. One such modification is acetylation, a reversible process controlled by two enzyme families, the histone deacetylases (HDACs) and the histone acetyl transferases (HATs), acting in opposition. Whereas acetylated histones in the nucleus are associated with upregulation of growth-promoting genes, deacetylated tubulin in the axoplasm is associated with more labile microtubules, conducive to axon growth. This study investigates the effects of HAT and HDAC inhibitors on cultured adult dorsal root ganglia (DRG) neurons and shows that inhibition of HATs by anacardic acid or CPTH2 improves axon outgrowth, whereas inhibition of HDACs by TSA or tubacin inhibits axon growth. Anacardic acid increased the number of axons able to cross an inhibitory chondroitin sulfate proteoglycan border. Histone acetylation but not tubulin acetylation level was affected by HAT inhibitors, whereas tubulin acetylation levels were increased in the presence of the HDAC inhibitor tubacin. Although the microtubule-stabilizing drug taxol did not have an effect on the lengths of DRG axons, nocodazole decreased axon lengths. Determining the mechanistic basis will require future studies, but this study shows that inhibitors of HAT can augment axon growth in adult DRG neurons, with the potential of aiding axon growth over inhibitory substrates produced by the glial scar.

Structural basis for the association of MAP6 protein with microtubules and its regulation by calmodulin

Microtubules are highly dynamic αβ-tubulin polymers. In vitro and in living cells, microtubules are most often cold- and nocodazole-sensitive. When present, the MAP6/STOP family of proteins protects microtubules from cold- and nocodazole-induced depolymerization but the molecular and structure determinants by which these proteins stabilize microtubules remain under debate. We show here that a short protein fragment from MAP6-N, which encompasses its Mn1 and Mn2 modules (MAP6(90-177)), recapitulates the function of the full-length MAP6-N protein toward microtubules, i.e. its ability to stabilize microtubules in vitro and in cultured cells in ice-cold conditions or in the presence of nocodazole. We further show for the first time, using biochemical assays and NMR spectroscopy, that these effects result from the binding of MAP6(90-177) to microtubules with a 1:1 MAP6(90-177):tubulin heterodimer stoichiometry. NMR data demonstrate that the binding of MAP6(90-177) to microtubules involve its two Mn modules but that a single one is also able to interact with microtubules in a closely similar manner. This suggests that the Mn modules represent each a full microtubule binding domain and that MAP6 proteins may stabilize microtubules by bridging tubulin heterodimers from adjacent protofilaments or within a protofilament. Finally, we demonstrate that Ca(2+)-calmodulin competes with microtubules for MAP6(90-177) binding and that the binding mode of MAP6(90-177) to microtubules and Ca(2+)-calmodulin involves a common stretch of amino acid residues on the MAP6(90-177) side. This result accounts for the regulation of microtubule stability in cold condition by Ca(2+)-calmodulin.

Dab2ip regulates neuronal migration and neurite outgrowth in the developing neocortex

Dab2ip (DOC-2/DAB2 interacting protein) is a member of the Ras GTPase-activating protein (GAP) family that has been previously shown to function as a tumor suppressor in several systems. Dab2ip is also highly expressed in the brain where it interacts with Dab1, a key mediator of the Reelin pathway that controls several aspects of brain development and function. We found that Dab2ip is highly expressed in the developing cerebral cortex, but that mutations in the Reelin signaling pathway do not affect its expression. To determine whether Dab2ip plays a role in brain development, we knocked down or over expressed it in neuronal progenitor cells of the embryonic mouse neocortex using in utero electroporation. Dab2ip down-regulation severely disrupts neuronal migration, affecting preferentially late-born principal cortical neurons. Dab2ip overexpression also leads to migration defects. Structure-function experiments in vivo further show that both PH and GRD domains of Dab2ip are important for neuronal migration. A detailed analysis of transfected neurons reveals that Dab2ip down- or up-regulation disrupts the transition from a multipolar to a bipolar neuronal morphology in the intermediate zone. Knock down of Dab2ip in neurons ex-vivo indicates that this protein is necessary for proper neurite development and for the expression of several major neuronal microtubule associated proteins (MAPs), which are important for neurite growth and stabilization. Thus, our study identifies, for the first time, a critical role for Dab2ip in mammalian cortical development and begins to reveal molecular mechanisms that underlie this function.

Assembly of a new growth cone after axotomy: the precursor to axon regeneration

The assembly of a new growth cone is a prerequisite for axon regeneration after injury. Creation of a new growth cone involves multiple processes, including calcium signalling, restructuring of the cytoskeleton, transport of materials, local translation of messenger RNAs and the insertion of new membrane and cell surface molecules. In axons that have an intrinsic ability to regenerate, these processes are executed in a timely fashion. However, in axons that lack regenerative capacity, such as those of the mammalian CNS, several of the steps that are required for regeneration fail, and these axons do not begin the growth process. Identification of the points of failure can suggest targets for promoting regeneration.

A novel role for retrograde transport of microtubules in the axon

Short microtubules move within the axon in both directions. In the past, it had been assumed that all of the short moving microtubules are oriented with their plus-ends distal to the cell body, regardless of their direction of movement. The anterogradely moving microtubules were posited to play critical roles in the establishment, expansion, and maintenance of the axonal microtubule array. There was no known function for the retrogradely moving microtubules. In considering the mechanism of their transport, we had assumed that all of the short microtubules have a plus-end-distal polarity orientation, as is characteristic of the long microtubules that dominate the axon. Here we discuss an alternative hypothesis, namely that the short microtubules moving retrogradely have the opposite polarity orientation of those moving anterogradely. Those that move anterogradely have their plus-ends distal to the cell body while those that move retrogradely have their minus ends distal to the cell body. In this view, retrograde transport is a means for clearing the axon of incorrectly oriented microtubules. This new model, if correct, has profound implications for the manner by which healthy axons preserve their characteristic pattern of microtubule polarity orientation. We speculate that pathological flaws in this mechanism may be a critical factor in the degeneration of axons during disease and injury, as well as in neuropathy caused by microtubule-active drugs.

The formation of actin waves during regeneration after axonal lesion is enhanced by BDNF

During development, axons of neurons in the mammalian central nervous system lose their ability to regenerate. To study the regeneration process, axons of mouse hippocampal neurons were partially damaged by an UVA laser dissector system. The possibility to deliver very low average power to the sample reduced the collateral thermal damage and allowed studying axonal regeneration of mouse neurons during early days in vitro. Force spectroscopy measurements were performed during and after axon ablation with a bead attached to the axonal membrane and held in an optical trap. With this approach, we quantified the adhesion of the axon to the substrate and the viscoelastic properties of the membrane during regeneration. The reorganization and regeneration of the axon was documented by long-term live imaging. Here we demonstrate that BDNF regulates neuronal adhesion and favors the formation of actin waves during regeneration after axonal lesion.

Abnormal nociception and opiate sensitivity of STOP null mice exhibiting elevated levels of the endogenous alkaloid morphine

BACKGROUND

Mice deficient for the stable tubule only peptide (STOP) display altered dopaminergic neurotransmission associated with severe behavioural defects including disorganized locomotor activity. Endogenous morphine, which is present in nervous tissues and synthesized from dopamine, may contribute to these behavioral alterations since it is thought to play a role in normal and pathological neurotransmission.

RESULTS

In this study, we showed that STOP null brain structures, including cortex, hippocampus, cerebellum and spinal cord, contain high endogenous morphine amounts. The presence of elevated levels of morphine was associated with the presence of a higher density of mu opioid receptor with a higher affinity for morphine in STOP null brains. Interestingly, STOP null mice exhibited significantly lower nociceptive thresholds to thermal and mechanical stimulations. They also had abnormal behavioural responses to the administration of exogenous morphine and naloxone. Low dose of morphine (1 mg/kg, i.p.) produced a significant mechanical antinociception in STOP null mice whereas it has no effect on wild-type mice. High concentration of naloxone (1 mg/kg) was pronociceptive for both mice strain, a lower concentration (0.1 mg/kg) was found to increase the mean mechanical nociceptive threshold only in the case of STOP null mice.

CONCLUSIONS

Together, our data show that STOP null mice displayed elevated levels of endogenous morphine, as well as an increase of morphine receptor affinity and density in brain. This was correlated with hypernociception and impaired pharmacological sensitivity to mu opioid receptor ligands.

Altered expression of synaptic protein mRNAs in STOP (MAP6) mutant mice

Stable tubule-only polypeptide (STOP) proteins are a family of microtubule associated proteins (MAPs) important in microtubule stabilization. Data indicating a role for microtubules in synaptic function has come from studies of the STOP null mouse, which exhibits synaptic deficits, in association with behavioural changes that are alleviated by antipsychotic treatment. These findings suggested that STOP mutant mice may be useful in studies of synaptic function, and could be especially relevant to schizophrenia, postulated to be a disorder of the synapse. Moreover, a genetic association between STOP and schizophrenia has been reported. This study aimed to further characterize synaptic alterations in STOP null and heterozygous mice. Using in situ hybridization histochemistry, the mRNA expression of three pre-synaptic (synaptophysin; growth associated protein-43 (GAP-43); vesicular glutamate transporter-1 (VGlut1)) and two post-synaptic (spinophilin; MAP2) proteins, was quantified in female STOP null (n = 7), heterozygous (n = 5) and wild type (n = 6) mice. For STOP null and heterozygous mice, synaptophysin, VGlut1, GAP-43 and spinophilin mRNAs were decreased in the hippocampus, whilst in addition in the null mice, synaptophysin, VGlut1 and spinophilin mRNAs were decreased in the cerebellum. Alterations in synaptic protein mRNA expression were also detected in the frontal and occipital cortex. MAP2 mRNA expression was unchanged in all brain regions. The profile of mRNA changes is broadly similar to that observed in schizophrenia. Together the data provide supporting evidence for a role for microtubules in synaptic function, and suggest that STOP, or other microtubule proteins, may contribute to the synaptic pathology of schizophrenia.

Role of MAP1B in axonal retrograde transport of mitochondria

The MAPs (microtubule-associated proteins) MAP1B and tau are well known for binding to microtubules and stabilizing these structures. An additional role for MAPs has emerged recently where they appear to participate in the regulation of transport of cargos on the microtubules found in axons. In this role, tau has been associated with the regulation of anterograde axonal transport. We now report that MAP1B is associated with the regulation of retrograde axonal transport of mitochondria. This finding potentially provides precise control of axonal transport by MAPs at several levels: controlling the anterograde or retrograde direction of transport depending on the type of MAP involved, controlling the speed of transport and controlling the stability of the microtubule tracks upon which transport occurs.

STOP knockout and NMDA NR1 hypomorphic mice exhibit deficits in sensorimotor gating

Schizophrenia is a chronic and debilitating disease which is thought to arise from a neuro-developmental disorder. Both the stable tubule-only polypeptide (STOP) protein and the N-methyl-D-aspartate (NMDA) NR1 subunit are involved in neuronal development and physiology. It has therefore been postulated that transgenic mice lacking either the STOP or the NMDAR1 gene would show a 'schizophrenic-like' phenotype. Here, STOP knockout and NMDA NR1 hypomorphic mice were assessed in a behavioural measure that can be used to detect schizophrenic-like phenotypes: a change in sensorimotor gating, measured through prepulse inhibition (PPI). STOP knockout mice were further assessed in another measure of 'schizophrenic-like behaviour': hyperlocomotion. The PPI deficit exhibited by both the STOP knockout and NMDA knockdown mice could not be reversed by acute treatment with the atyptical antipsychotic, clozapine (1 mg/kg, i.p.) but the hyperlocomotion shown by the STOP knockout mice was reversed with the same acute dose of clozapine.

Astrocytes and oligodendrocytes express different STOP protein isoforms

Many cell types contain subpopulations of microtubules that resist depolymerizing conditions, such as exposure to cold or to the drug nocodazole. This stabilization is due mainly to polymer association with STOP proteins. In mouse, neurons express two major variants of these proteins, N-STOP and E-STOP (120 kDa and 79 kDa, respectively), whereas fibroblasts express F-STOP (42 kDa) and two minor variants of 48 and 89 kDa. N- and E-STOP induce microtubule resistance to both cold and nocodazole exposure, whereas F-STOP confers microtubule stability only to the cold. Here, we investigated the expression of STOP proteins in oligodendrocytes and astrocytes in culture. We found that STOP proteins were expressed in precursor cells, in immature and mature oligodendrocytes, and in astrocytes. We found that oligodendrocytes express a major STOP variant of 89 kDa, which we called O-STOP, and two minor variants of 42 and 48 kDa. The STOP variants expressed by oligodendrocytes induce microtubule resistance to the cold and to nocodazole. For astrocytes, we found the expression of two STOP variants of 42 and 48 kDa and a new STOP isoform of 60 kDa, which we called A-STOP. The STOP variants expressed by astrocytes induce microtubule resistance to the cold but not to nocodazole, as fibroblast variants. In conclusion, astrocytes and oligodendrocytes express different isoforms of STOP protein, which show different microtubule-stabilizing capacities.

Timing of neuronal and glial ultrastructure disruption during brain slice preparation and recovery in vitro

Hippocampal slices often have more synapses than perfusion-fixed hippocampus, but the cause of this synaptogenesis is unclear. Ultrastructural evidence for synaptogenic triggers during slice preparation was investigated in 21-day-old rats. Slices chopped under warm or chilled conditions and fixed after 0, 5, 25, 60, or 180 minutes of incubation in an interface chamber were compared with hippocampi fixed by perfusion or by immersion of the whole hippocampus. There was no significant synaptogenesis in these slices compared with perfusion-fixed hippocampus, but there were other structural changes during slice preparation and recovery in vitro. Whole hippocampus and slices prepared under warm conditions exhibited an increase in axonal coated vesicles, suggesting widespread neurotransmitter release. Glycogen granules were depleted from astrocytes and neurons in 0-min slices, began to reappear by 1 hour, and had fully recovered by 3 hours. Dendritic microtubules were initially disassembled in slices, but reassembled into normal axial arrays after 5 minutes. Microtubules were short at 5 minutes (12.3 +/- 1.1 microm) but had recovered normal lengths by 3 hours (84.6 +/- 20.0 microm) compared with perfusion-fixed hippocampus (91 +/- 22 microm). Microtubules appeared transiently in 15 +/- 3% and 9 +/- 4% of dendritic spines 5 and 25 minutes after incubation, respectively. Spine microtubules were absent from perfusion-fixed hippocampus and 3-hour slices. Ice-cold dissection and vibratomy in media that blocked activity initially produced less glycogen loss, coated vesicles, and microtubule disassembly. Submersing these slices in normal oxygenated media at 34 degrees C led to glycogen depletion, as well as increased coated vesicles and microtubule disassembly within 1 minute.

Overlap of promoter and coding sequences in the mouse STOP gene (Mtap6)

The microtubule-associated proteins STOP are essential for synaptic plasticity and integrated brain function. The STOP gene (Mtap6) is composed of 4 exons and presents different developmental and tissue specific alternative transcripts resulting in the neuronal isoforms E- and N-STOP, and in the fibroblastic F-STOP isoform. We now characterize the transcription initiation sites for neuronal and non neuronal STOP transcripts. Our results show that there is a single neuronal specific promoter for transcription of E- and N-STOP mRNAs. F-STOP mRNA transcription is regulated by a distinct promoter. A remarkable feature of this promoter is that it lies within coding sequences expressed in neuronal E- and N-STOP isoforms.

Critical calpain-dependent ultrastructural alterations underlie the transformation of an axonal segment into a growth cone after axotomy of cultured Aplysia neurons

The transformation of a stable axonal segment into a motile growth cone is a critical step in the regeneration of amputated axons. In earlier studies we found that axotomy of cultured Aplysia neurons leads to a transient and local elevation of the free intracellular Ca2+ concentration, resulting in calpain activation, localized proteolysis of submembranal spectrin, and, eventually, growth cone formation. Moreover, inhibition of calpain by calpeptin prior to axotomy inhibits growth cone formation. Here we investigated the mechanisms by which calpain activation participates in the transformation of an axonal segment into a growth cone. To that end we compared the ultrastructural alterations induced by axotomy performed under control conditions with those caused by axotomy performed in the presence of calpeptin, using cultured Aplysia neurons as a model. We identified the critical calpain-dependent cytoarchitectural alterations that underlie the formation of a growth cone after axotomy. Calpain-dependent processes lead to restructuring of the neurofilaments and microtubules to form an altered cytoskeletal region 50-150 microm proximal to the tip of the transected axon in which vesicles accumulate. The dense pool of vesicles forms in close proximity to a segment of the plasma membrane along which the spectrin membrane skeleton has been proteolyzed by calpain. We suggest that the rearrangement of the cytoskeleton forms a transient cellular compartment that traps transported vesicles and serves as a locus for microtubule polymerization. We propose that this cytoskeletal configuration facilitates the fusion of vesicles with the plasma membrane, promoting the extension of the growth cone's lamellipodium. The growth process is further supported by the radial polymerization of microtubules from the growth cone's center.

The suppression of brain cold-stable microtubules in mice induces synaptic defects associated with neuroleptic-sensitive behavioral disorders

Neurons contain abundant subsets of highly stable microtubules that resist depolymerizing conditions such as exposure to the cold. Stable microtubules are thought to be essential for neuronal development, maintenance, and function. Previous work has indicated an important role of the microtubule-associated protein STOP in the induction of microtubule cold stability. Here, we developed STOP null mice. These mice were devoid of cold-stable microtubules. In contrast to our expectations, STOP-/- mice had no detectable defects in brain anatomy but showed synaptic defects, with depleted synaptic vesicle pools and impaired synaptic plasticity, associated with severe behavioral disorders. A survey of the effects of psychotropic drugs on STOP-/- mice behavior showed a remarkable and specific effect of long-term administration of neuroleptics in alleviating these disorders. This study demonstrates that STOP is a major factor responsible for the intriguing stability properties of neuronal microtubules and is important for synaptic plasticity. Additionally, STOP-/- mice may yield a pertinent model for study of neuroleptics in illnesses such as schizophrenia, currently thought to result from synaptic defects.

Identification of novel bifunctional calmodulin-binding and microtubule-stabilizing motifs in STOP proteins

Although microtubules are intrinsically labile tubulin assemblies, many cell types contain stable polymers, resisting depolymerizing conditions such as exposure to the cold or the drug nocodazole. This microtubule stabilization is largely due to polymer association with STOP proteins. There are several STOP variants, some with capacity to induce microtubule resistance to both the cold and nocodazole, others with microtubule cold stabilizing activity only. These microtubule-stabilizing effects of STOP proteins are inhibited by calmodulin and we now demonstrate that they are determined by two distinct kinds of repeated modular sequences (Mn and Mc), both containing a calmodulin-binding peptide, but displaying different microtubule stabilizing activities. Mn modules induce microtubule resistance to both the cold and nocodazole when expressed in cells. Mc modules, which correspond to the STOP central repeats, have microtubule cold stabilizing activity only. Mouse neuronal STOPs, which induce both cold and drug resistance in cellular microtubules, contain three Mn modules and four Mc modules. Compared with neuronal STOPs, the non-neuronal F-STOP lacks multiple Mn modules and this corresponds with an inability to induce nocodazole resistance. STOP modules represent novel bifunctional calmodulin-binding and microtubule-stabilizing sequences that may be essential for the generation of the different patterns of microtubule stabilization observed in cells.

Motor proteins regulate force interactions between microtubules and microfilaments in the axon

It has long been known that microtubule depletion causes axons to retract in a microfilament-dependent manner, although it was not known whether these effects are the result of motor-generated forces on these cytoskeletal elements. Here we show that inhibition of the motor activity of cytoplasmic dynein causes the axon to retract in the presence of microtubules. This response is obliterated if microfilaments are depleted or if myosin motors are inhibited. We conclude that axonal retraction results from myosin-mediated forces on the microfilament array, and that these forces are counterbalanced or attenuated by dynein-mediated forces between the microfilament and microtubule arrays.

Slow transport of unpolymerized tubulin and polymerized neurofilament in the squid giant axon

A major issue in the slow transport of cytoskeletal proteins is the form in which they are transported. We have investigated the possibility that unpolymerized as well as polymerized cytoskeletal proteins can be actively transported in axons. We report the active transport of highly diffusible tubulin oligomers, as well as transport of the less diffusible neurofilament polymers. After injection into the squid giant axon, tubulin was transported in an anterograde direction at an average rate of 2.3 mm/day, whereas neurofilament was moved at 1.1 mm/day. Addition of the metabolic poisons cyanide or dinitrophenol reduced the active transport of both proteins to less than 10% of control values, whereas disruption of microtubules by treatment of the axon with cold in the presence of nocodazole reduced transport of both proteins to approximately 20% of control levels. Passive diffusion of these proteins occurred in parallel with transport. The diffusion coefficient of the moving tubulin in axoplasm was 8.6 micrometer(2)/s compared with only 0.43 micrometer(2)/s for neurofilament. These results suggest that the tubulin was transported in the unpolymerized state and that the neurofilament was transported in the polymerized state by an energy-dependent nocodazole/cold-sensitive transport mechanism.

Different contributions of microtubule dynamics and transport to the growth of axons and collateral sprouts

Axonal growth is believed to depend on microtubule transport and microtubule dynamic instability. We now report that the growth of axon collateral branches can occur independent of microtubule dynamic instability and can rely mostly on the transport of preassembled polymer. Raising embryonic sensory neurons in concentrations of either taxol or nocodazole (NOC) that largely inhibit microtubule dynamics significantly inhibited growth of main axonal shafts but had only minor effects on collateral branch growth. The collaterals of axons raised in taxol or nocodazole often contained single microtubules with both ends clearly visible within the collateral branch ("floating" microtubules), which we interpret as microtubules undergoing transport. Furthermore, in these collaterals there was a distoproximal gradient in microtubule mass, indicating the distal accumulation of transported polymer. Treatment of cultures with a high dose of nocodazole to deplete microtubules from collaterals, followed by treatment with 4-20 nM vinblastine to inhibit microtubule repolymerization, resulted in the time-dependent reappearance and subsequent distal accumulation of floating microtubules in collaterals, providing further evidence for microtubule transport into collateral branches. Our data show that, surprisingly, the contribution of microtubule dynamics to collateral branch growth is minor compared with the important role of microtubule dynamics in growth cone migration, and they indicate that the transport of microtubules may provide sufficient cytoskeletal material for the initial growth of collateral branches.

Different contributions of microtubule dynamics and transport to the growth of axons and collateral sprouts

Axonal growth is believed to depend on microtubule transport and microtubule dynamic instability. We now report that the growth of axon collateral branches can occur independent of microtubule dynamic instability and can rely mostly on the transport of preassembled polymer. Raising embryonic sensory neurons in concentrations of either taxol or nocodazole (NOC) that largely inhibit microtubule dynamics significantly inhibited growth of main axonal shafts but had only minor effects on collateral branch growth. The collaterals of axons raised in taxol or nocodazole often contained single microtubules with both ends clearly visible within the collateral branch ("floating" microtubules), which we interpret as microtubules undergoing transport. Furthermore, in these collaterals there was a distoproximal gradient in microtubule mass, indicating the distal accumulation of transported polymer. Treatment of cultures with a high dose of nocodazole to deplete microtubules from collaterals, followed by treatment with 4-20 nM vinblastine to inhibit microtubule repolymerization, resulted in the time-dependent reappearance and subsequent distal accumulation of floating microtubules in collaterals, providing further evidence for microtubule transport into collateral branches. Our data show that, surprisingly, the contribution of microtubule dynamics to collateral branch growth is minor compared with the important role of microtubule dynamics in growth cone migration, and they indicate that the transport of microtubules may provide sufficient cytoskeletal material for the initial growth of collateral branches.

STOP Proteins

Microtubules assembled from pure tubulin in vitro are labile, rapidly depolymerized upon exposure to the cold. In contrast, in a number of cell types, cytoplasmic microtubules are stable, resistant to prolonged cold exposure. During the past years, the molecular basis of this microtubule stabilization in cells has been elucidated. Cold stability is due to polymer association with different variants of a calmodulin-regulated protein, STOP protein. The dynamic and hence the physiological consequences of STOP association with microtubules vary in different tissues. In neurons, STOP seems almost permanently associated with microtubules. STOP is apparently a major determinant of microtubule turnover in such cells and is required for normal neuronal differentiation. In cycling cells, only minor amounts of STOP are associated with interphase microtubules and STOP does not measurably affects microtubule dynamics. However, STOP is associated with mitotic microtubules in the spindle. Recent results indicate that such an association could be vital for meiosis and for the long-term fidelity of the mitotic process.

STOP proteins are responsible for the high degree of microtubule stabilization observed in neuronal cells

Neuronal differentiation and function require extensive stabilization of the microtubule cytoskeleton. Neurons contain a large proportion of microtubules that resist the cold and depolymerizing drugs and exhibit slow subunit turnover. The origin of this stabilization is unclear. Here we have examined the role of STOP, a calmodulin-regulated protein previously isolated from cold-stable brain microtubules. We find that neuronal cells express increasing levels of STOP and of STOP variants during differentiation. These STOP proteins are associated with a large proportion of microtubules in neuronal cells, and are concentrated on cold-stable, drug-resistant, and long-lived polymers. STOP inhibition abolishes microtubule cold and drug stability in established neurites and impairs neurite formation. Thus, STOP proteins are responsible for microtubule stabilization in neurons, and are apparently required for normal neurite formation.

Induction of growth cone formation by transient and localized increases of intracellular proteolytic activity

The formation of a growth cone at the tip of a transected axon is a crucial step in the subsequent regeneration of the amputated axon. During this process, the transected axon is transformed from a static segment into a motile growth cone. Despite the importance of this process for regeneration of the severed axon, little is known about the mechanisms underlying this transformation. Recent studies have suggested that Ca2+-activated proteinases underlay the morphological remodeling of neurons after injury. However, this hypothesis was never tested directly. Here we tested the ability of transient and localized increases in intracellular proteolytic activity to induce growth cone formation and neuritogenesis. Minute amounts of the proteinase trypsin were microinjected into intact axonal segments or somata of cultured Aplysia neurons, transiently elevating the intracellular protease concentration to 13-130 nM in the vicinity of the injection site. Such microinjections were followed by the formation of ectopic growth cones and irreversible neuritogenesis. Growth cones were not formed after external application of trypsin, microinjection of the carrier solution, or inactivated trypsin. Growth cone formation was not preceded by increases in free intracellular Ca2+ or changes in passive membrane properties, and was blocked by inhibitors of actin and tubulin polymerization. Trypsin-induced neuritogenesis was associated with ultrastructural alterations similar to those observed by us after axotomy. We conclude that local and transient elevations of cytoplasmic proteolytic activity can induce growth cone formation and neuritogenesis, and suggest that localized proteolytic activity plays a role in growth cone formation after axotomy.

Microtubule transport from the cell body into the axons of growing neurons

The present studies test the hypothesis that microtubules (MTs) are transported from the cell body into the axons of growing neurons. Dissociated sympathetic neurons were cultured using conditions that allow us to control the initiation of axon outgrowth. Neurons were injected with biotin-labeled tubulin (Bt-tub) and then stimulated to extend axons. The newly formed axons were then examined using immunofluorescence procedures for MTs with or without Bt-tub. Because the Bt-tub is fully assembly competent, all MTs that assemble after injection will contain Bt-tub. However, MTs that exist in the neuron at the time of injection and persist during the subsequent incubation will not contain Bt-tub. Because the neurons were injected before extending axons, MTs without Bt-tub are initially localized to the cell body. We specifically determined whether these MTs appeared in the newly formed axon. Such a result can only be explained by the transport of these MTs from their initial location in the cell body into the axon. The newly formed axons of many neurons contained MTs both with and without Bt-tub. MTs without Bt-tub were detected all along the axon and in some neurons represented a substantial portion of the total polymer in the proximal and middle regions of the axon. These results show that MTs are transported from the cell body into growing axons and that this transport is robust, delivering MTs to all regions of the newly formed axon.

Microtubule transport from the cell body into the axons of growing neurons

The present studies test the hypothesis that microtubules (MTs) are transported from the cell body into the axons of growing neurons. Dissociated sympathetic neurons were cultured using conditions that allow us to control the initiation of axon outgrowth. Neurons were injected with biotin-labeled tubulin (Bt-tub) and then stimulated to extend axons. The newly formed axons were then examined using immunofluorescence procedures for MTs with or without Bt-tub. Because the Bt-tub is fully assembly competent, all MTs that assemble after injection will contain Bt-tub. However, MTs that exist in the neuron at the time of injection and persist during the subsequent incubation will not contain Bt-tub. Because the neurons were injected before extending axons, MTs without Bt-tub are initially localized to the cell body. We specifically determined whether these MTs appeared in the newly formed axon. Such a result can only be explained by the transport of these MTs from their initial location in the cell body into the axon. The newly formed axons of many neurons contained MTs both with and without Bt-tub. MTs without Bt-tub were detected all along the axon and in some neurons represented a substantial portion of the total polymer in the proximal and middle regions of the axon. These results show that MTs are transported from the cell body into growing axons and that this transport is robust, delivering MTs to all regions of the newly formed axon.

Localized and transient elevations of intracellular Ca2+ induce the dedifferentiation of axonal segments into growth cones

The formation of a growth cone at the tip of a severed axon is a key step in its successful regeneration. This process involves major structural and functional alterations in the formerly differentiated axonal segment. Here we examined the hypothesis that the large, localized, and transient elevation in the free intracellular calcium concentration ([Ca2+]i) that follows axotomy provides a signal sufficient to trigger the dedifferentiation of the axonal segment into a growth cone. Ratiometric fluorescence microscopy and electron microscopy were used to study the relations among spatiotemporal changes in [Ca2+]i, growth cone formation, and ultrastructural alterations in axotomized and intact Aplysia californica neurons in culture. We report that, in neurons primed to grow, a growth cone forms within 10 min of axotomy near the tip of the transected axon. The nascent growth cone extends initially from a region in which peak intracellular Ca2+ concentrations of 300-500 microM are recorded after axotomy. Similar [Ca2+]i transients, produced in intact axons by focal applications of ionomycin, induce the formation of ectopic growth cones and subsequent neuritogenesis. Electron microscopy analysis reveals that the ultrastructural alterations associated with axotomy and ionomycin-induced growth cone formation are practically identical. In both cases, growth cones extend from regions in which sharp transitions are observed between axoplasm with major ultrastructural alterations and axoplasm in which the ultrastructure is unaltered. These findings suggest that transient elevations of [Ca2+]i to 300-500 microM, such as those caused by mechanical injury, may be sufficient to induce the transformation of differentiated axonal segments into growth cones.

Localized and transient elevations of intracellular Ca2+ induce the dedifferentiation of axonal segments into growth cones

The formation of a growth cone at the tip of a severed axon is a key step in its successful regeneration. This process involves major structural and functional alterations in the formerly differentiated axonal segment. Here we examined the hypothesis that the large, localized, and transient elevation in the free intracellular calcium concentration ([Ca2+]i) that follows axotomy provides a signal sufficient to trigger the dedifferentiation of the axonal segment into a growth cone. Ratiometric fluorescence microscopy and electron microscopy were used to study the relations among spatiotemporal changes in [Ca2+]i, growth cone formation, and ultrastructural alterations in axotomized and intact Aplysia californica neurons in culture. We report that, in neurons primed to grow, a growth cone forms within 10 min of axotomy near the tip of the transected axon. The nascent growth cone extends initially from a region in which peak intracellular Ca2+ concentrations of 300-500 microM are recorded after axotomy. Similar [Ca2+]i transients, produced in intact axons by focal applications of ionomycin, induce the formation of ectopic growth cones and subsequent neuritogenesis. Electron microscopy analysis reveals that the ultrastructural alterations associated with axotomy and ionomycin-induced growth cone formation are practically identical. In both cases, growth cones extend from regions in which sharp transitions are observed between axoplasm with major ultrastructural alterations and axoplasm in which the ultrastructure is unaltered. These findings suggest that transient elevations of [Ca2+]i to 300-500 microM, such as those caused by mechanical injury, may be sufficient to induce the transformation of differentiated axonal segments into growth cones.

Mitotic neuroblasts determine neuritic patterning of progeny

Neuronal precursor proliferation and axodendritic outgrowth have been regarded as strictly sequential, with process formation presumably beginning after mitotic activity ceases. We now report that sympathetic precursors in vitro often elaborate long neurites before dividing. Of 437 neuroblasts observed in 48 time-lapse recordings, 42 neuroblasts divided. Thirty (71%) of these mitotic neuroblasts had neurites prior to cytokinesis. "Paramitotic" neurites were found to contain microtubules (MTs), indicating that precursors elaborate neuritic cytoskeleton during proliferation. Remarkably, the precise neuritic pattern exhibited by parental neuroblasts was consistently reproduced by daughter cell pairs. Preservation of neuritic morphology occurred through asymmetric division, with individual neurites allocated to specific daughter cells. Paramitotic neurites either remained intact throughout mitosis (12 of 65), or "retracted" into the soma during prophase and then "regrew" within minutes after cytokinesis (53 of 65). "Retraction" and "regrowth" involved resorption of cytoplasm into the soma, then refilling of residual cell membrane, resulting in recapitulation of the parental neurite pattern. Paramitotic neuritogenesis appears to be intrinsically driven, but is responsive to environmental signals. The culture substrate influenced neurite length, but not the response of paramitotic neurites during mitosis or the preservation of neuritic morphology. However, the incidence of neurite-bearing neuroblasts increased from 38 +/- 1.3% to 94 +/- 1.1% with growth factor treatment. The surprisingly high incidence of paramitotic neurites and the fidelity with which patterning was conserved across cell generations raise the possibility that mitotic precursors engage in pathfinding. Our studies suggest a novel link between neurogenesis and cytoarchitectonic patterning.

A composite model for establishing the microtubule arrays of the neuron

Neurons generate two distinct types of processes, termed axons and dendrites, both of which rely on a highly organized array of microtubules for their growth and maintenance. Axonal microtubules are uniformly oriented with their plus ends distal to the cell body, whereas dendritic microtubules are nonuniformly oriented. In neither case are the microtubules attached to the centrosome or any detectable structure that could establish their distinct patterns of polarity orientation. Studies from our laboratory over the past few years have led us to propose the following model for the establishment of the axonal and dendritic microtubule arrays. Microtubules destined for these processes are nucleated at the centrosome within the cell body of the neuron and rapidly released. The released microtubules are then transported into developing axons and dendrites to support their growth. Early in neuronal development, the microtubules are transported with their plus ends leading into immature processes that are the common progenitors of both axons and dendrites. This sets up a uniformly plus-end distal pattern of polarity orientation, which is preserved in the developing axon. In the case of the dendrite, the plus-end-distal microtubules are joined by another population of microtubules that are transported into these processes with their minus-ends leading. Implicit in this model is that neurons have specialized machinery for regulating the release of microtubules from the centrosome and for transporting them with great specificity.

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.

Acceleration of membrane recycling by axotomy of cultured aplysia neurons

The rapid transition of a stationary axon into a motile growth cone requires the recruitment of membrane and its strategic insertion into the neurolemma. The source of membrane to support the initial rapid growth postaxotomy is not known. Using membrane capacitance measurements, we examined quantitative aspects of membrane dynamics following axotomy of cultured Aplysia neurons. Axotomy activates two processes in parallel: membrane retrieval and exocytosis. Unexpectedly, membrane retrieval is the dominant process in the majority of the experiments. Thus, while a growth cone is vigorously extending, the total neuronal surface area decreases. We suggest that the initial rapid extension phase of the newly formed growth cone postaxotomy is supported by a pool of intracellular membrane that is rapidly retrieved from the neurolemma.

Cytoplasmic mechanisms of axonal and dendritic growth in neurons

The structural mechanisms responsible for the gradual elaboration of the cytoplasmic elongation of neurons are reviewed. In addition to discussing recent work, important older work is included to inform newcomers to the field how the current perspective arose. The highly specialized axon and the less exaggerated dendrite both result from the advance of the motile growth cone. In the area of physiology, studies in the last decade have directly confirmed the classic model of the growth cone pulling forward and the axon elongating from this tension. Particularly in the case of the axon, cytoplasmic elongation is closely linked to the formation of an axial microtubule bundle from behind the advancing growth cone. Substantial progress has been made in understanding the expression of microtubule-associated proteins during neuronal differentiation to stiffen and stabilize axonal microtubules, providing specialized structural support. Studies of membrane organelle transport along the axonal microtubules produced an explosion of knowledge about ATPase molecules serving as motors driving material along microtubule rails. However, most aspects of the cytoplasmic mechanisms responsible for neurogenesis remain poorly understood. There is little agreement on mechanisms for the addition of new plasma membrane or the addition of new cytoskeletal filaments in the growing axon. Also poorly understood are the mechanisms that couple the promiscuous motility of the growth cone to the addition of cytoplasmic elements.

Osmotic dilution stimulates axonal outgrowth by making axons more sensitive to tension

Mechanical tension is a potent stimulator of axonal growth rate, which is also stimulated by osmotic dilution. We wished to determine the relationship, if any, between osmotic stimulation and tensile regulation of axonal growth. We used calibrated glass needles to apply constant force to elongate axons of cultured chick sensory neurons. We find that a neurite being pulled at a constant force will grow 50-300% faster following a 50% dilution of inorganic ions in the culture medium. That is, osmotic dilution appears to cause axons to increase their sensitivity to applied tensions. Experimental interventions suggest that this effect is not mediated by dilution of extracellular calcium, or to osmotic stimulation of adenylate cyclase, or to osmotic stimulation of mechanosensitive ion channels. Rather, experiments measuring the static tension normally borne by neurites suggest a direct mechanical effect on the cytoskeletal proteins of the neurite shaft. Our results are consistent with a formal thermodynamic model for axonal growth in which removing a compressive load on axonal microtubules promotes their assembly, thus promoting axonal elongation.

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.

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.

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.

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.