Expression of a kinesin-related motor protein induces Sf9 cells to form dendrite-like processes with nonuniform microtubule polarity orientation

Establishing neuronal polarity: microtubule regulation during neurite initiation

The initiation of nascent projections, or neurites, from the neuronal cell body is the first stage in the formation of axons and dendrites, and thus a critical step in the establishment of neuronal architecture and nervous system development. Neurite formation relies on the polarized remodelling of microtubules, which dynamically direct and reinforce cell shape, and provide tracks for cargo transport and force generation. Within neurons, microtubule behaviour and structure are tightly controlled by an array of regulatory factors. Although microtubule regulation in the later stages of axon development is relatively well understood, how microtubules are regulated during neurite initiation is rarely examined. Here, we discuss how factors that direct microtubule growth, remodelling, stability and positioning influence neurite formation. In addition, we consider microtubule organization by the centrosome and modulation by the actin and intermediate filament networks to provide an up-to-date picture of this vital stage in neuronal development.

Arf GAPs and molecular motors

Arf GTPase-activating proteins (Arf GAPs) were first identified as regulators of the small GTP-binding proteins ADP-ribosylation factors (Arfs). The Arf GAPs are a large family of proteins in metazoans, outnumbering the Arfs that they regulate. The members of the Arf GAP family have complex domain structures and some have been implicated in particular cellular functions, such as cell migration, or with particular pathologies, such as tumor invasion and metastasis. The specific effects of Arfs sometimes depend on the Arf GAP involved in their regulation. These observations have led to speculation that the Arf GAPs themselves may affect cellular activities in capacities beyond the regulation of Arfs. Recently, 2 Arf GAPs, ASAP1 and AGAP1, have been found to bind directly to and influence the activity of myosins and kinesins, motor proteins associated with filamentous actin and microtubules, respectively. The Arf GAP-motor protein interaction is critical for cellular behaviors involving the actin cytoskeleton and microtubules, such as cell migration and other cell movements. Arfs, then, may function with molecular motors through Arf GAPs to regulate microtubule and actin remodeling.

Nanoparticulate TiO2 -mediated inhibition of the Wnt signaling pathway causes dendritic development disorder in cultured rat hippocampal neurons

Titanium dioxide nanoparticles (TiO₂ NPs) are increasingly used in daily life, in industry, and in environmental clearing, but their potential neurodevelopmental toxicity has been highly debated. In this study, we explored whether TiO₂ NPs inhibited development of dendritic morphology and identified possible molecular mechanisms associated with this inhibition in primary cultured rat hippocampal neurons. Results showed that TiO₂ NPs decreased neurite length, the number of branches and the spine density, and impaired mitochondrial function in the developing neurons. Furthermore, TiO₂ NPs significantly reduced the expression of several proteins involved in canonical Wnt3a/β-catenin signaling including Wnt3a, β-catenin, p-GSK-3β, and CyclinD1 and conversely, elevated GSK-3β expression. In addition to altering expression of proteins involved in canonical Wnt3a/β-catenin signaling, TiO₂ NPs decreased expression of proteins invovled in non-canonical Wnt signaling, including, MKLP1, CRMP3, ErbB4, and KIF17. Taken together, these results indicate that suppression of dendritic development caused by TiO₂ NPs is associated with inhibition of activation of the Wnt/β-catenin pathway or non-canonical Wnt pathway-induced expression of microtubule cytoskeletal components in the developing neurons. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2139-2149, 2017.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

Cytoskeletal and signaling mechanisms of neurite formation

The formation of a neurite, the basis for axons and dendrites, begins with the concerted accumulation and organization of actin and microtubules. Whereas much is known about the proteins that play a role in these processes, because they perform similar functions in axon branching and filopodia formation, much remains to be discovered concerning the interaction of these individual cytoskeletal regulators during neurite formation. Here, we review the literature regarding various models of filopodial formation and the way in which proteins that control actin organization and polymerization induce neurite formation. Although several different regulators of actin polymerization are involved in neurite initiation, redundancy occurs between these regulators, as the effects of the loss of a single regulator can be mitigated by the addition of neurite-promoting substrates and proteins. Similar to actin dynamics, both microtubule stabilizing and destabilizing proteins play a role in neurite initiation. Furthermore, interactions between the actin and microtubule cytoskeleton are required for neurite formation. Several lines of evidence indicate that the interactions between these two components of the cytoskeleton are needed for force generation and for the localization of microtubules at sites of nascent neurites. The general theme that emerges is the existence of several central regulatory pathways on which extracellular cues converge to control and organize both actin and microtubules to induce the formation of neurites.

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

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

The nuclear localization signal of mitotic kinesin-like protein Mklp-1: effect on Mklp-1 function during cytokinesis

The mitotic kinesin-like protein (Mklp-1) localizes in the nucleus during interphase due to the presence of nuclear localization signal(s) [NLS(s)] within its sequence. Here, we mapped two NLSs to be 899SRKRRSST906 and 949KRKKP953 in the tail domain of Mklp-1, and showed that ectopic expression of a mutant Mklp-1 without the NLSs leads to cell cycle arrest at cytokinesis, indicating that the NLSs are necessary for Mklp-1 to execute its normal function during cell division. Furthermore, mutation of two serine residues in the first NLS to aspartic acid, which mimics phosphorylation, attenuated its nuclear localization function, suggesting that the function of this NLS might be regulated by phosphorylation.

Rearrangement of microtubule polarity orientation during conversion of dendrites to axons in cultured pyramidal neurons

Axons and dendrites of neurons differ in the polarity orientation of their microtubules. Whereas the polarity orientation of microtubules in axons is uniform, with all plus ends distal, that in dendrites is nonuniform. The mechanisms responsible for establishment and maintenance of microtubule polarity orientation in neuronal processes remain unclear, however. We previously described a culture system in which dendrites of rat cortical neurons convert to axons. In the present study, we examined changes in microtubule polarity orientation in such dendrites. With the use of the hooking procedure and electron microscopy, we found that microtubule polarity orientation changed from nonuniform to uniform, with a plus end-distal arrangement, in dendrites that gave rise to axons during culture of neurons for 24 h. Microtubule polarity orientation remained nonuniform in dendrites that did not elongate. Axon regeneration at the dendritic tip thus triggered the disappearance of minus end-distal microtubules from dendrites. These minus end-distal microtubules also disappeared from dendrites during axon regeneration in the presence of inhibitors of actin polymerization, suggesting that actin-dependent transport of microtubules is not required for this process and implicating a previously unidentified mechanism in the establishment and maintenance of microtubule polarity orientation in neuronal processes.

MKLP1 requires specific domains for its dendritic targeting

The mitotic kinesin-like protein 1 (MKLP1) is specifically localized to the dendrite of the developed neuron, but its targeting mechanism is still unclear. In this study, the role of distinct domains of MKLP1 in dendritic targeting was investigated by producing a series of enhanced green fluorescent protein (eGFP)-tagged MKLP1 and its variant mutations, and studying the distribution of these molecules in cultured primary hippocampal neurons using fluorescence microscopy. We have found that: (a) full-length MKLP1(1-856)-eGFP was distributed to the dendrite of hippocampal neurons, not the axon; (b) deletion of the ;motor' domain prevented the dendritic distribution; (c) deletion of the ;tail' domain caused axonal appearance; (d) mutants devoid of the ;stalk' domain were still specifically distributed to the dendrite. The results indicate that the motor and tail regions of MKLP1 are important and significant for its localization to the dendrites. We also discuss the difference between the targeting of membrane-anchoring proteins and the kinesin-like protein.

Are dendrites in Drosophila homologous to vertebrate dendrites?

Dendrites represent arborising neurites in both vertebrates and invertebrates. However, in vertebrates, dendrites develop on neuronal cell bodies, whereas in higher invertebrates, they arise from very different neuronal structures, the primary neurites, which also form the axons. Is this anatomical difference paralleled by principal developmental and/or physiological differences? We address this question by focussing on one cellular model, motorneurons of Drosophila and characterise the compartmentalisation of these cells. We find that motorneuronal dendrites of Drosophila share with typical vertebrate dendrites that they lack presynaptic but harbour postsynaptic proteins, display calcium elevation upon excitation, have distinct cytoskeletal features, develop later than axons and are preceded by restricted localisation of Par6-complex proteins. Furthermore, we demonstrate in situ and culture that Drosophila dendrites can be shifted from the primary neurite to their soma, i.e. into vertebrate-like positions. Integrating these different lines of argumentation, we propose that dendrites in vertebrates and higher invertebrates have a common origin, and differences in dendrite location can be explained through translocation of neuronal cell bodies introduced during the evolutionary process by which arthropods and vertebrates diverged from a common urbilaterian ancestor. Implications of these findings for studies of dendrite development, neuronal polarity, transport and evolution are discussed.

ZEN-4/MKLP1 is required to polarize the foregut epithelium

BACKGROUND

Epithelial tubes are a key component of organs and are generated from cells with distinct apico-basolateral polarity. Here, we describe a novel function during tubulogenesis for ZEN-4, the Caenorhabditis elegans ortholog of mitotic kinesin-like protein 1 (MKLP1), and CYK-4, which contains a RhoGAP (GTPase-activating protein) domain. Previous studies revealed that these proteins comprise centralspindlin (a complex that functions during mitosis to bundle microtubules), construct the spindle midzone, and complete cytokinesis.

RESULTS

Our analyses demonstrate that ZEN-4/MKLP1 functions postmitotically to establish the foregut epithelium. Mutants that lack ZEN-4/MKLP1 express polarity markers but fail to target these proteins appropriately to the cell cortex. Affected proteins include PAR-3/Bazooka and PKC-3/atypical protein kinase C at the apical membrane domain, and HMR-1/cadherin and AJM-1 within C. elegans apical junctions (CeAJ). Microtubules and actin are disorganized in zen-4 mutants compared to the wild-type.

CONCLUSION

We suggest that ZEN-4/MKLP1 and CYK-4/RhoGAP regulate an early step in epithelial polarization that is required to establish the apical domain and CeAJ.

Slow axonal transport and the genesis of neuronal morphology

The classic view of slow axonal transport maintains that microtubules, neurofilaments, and actin filaments move down the axon relatively coherently at rates significantly slower than those characteristic of known motor proteins. Recent studies indicate that the movement of these cytoskeletal polymers is actually rapid, asynchronous, intermittent, and most probably fueled by familiar motors such as kinesins, myosins, and cytoplasmic dynein. This new view, which is supported by both live-cell imaging and mechanistic analyses, suggests that slow axonal transport is both rapid and plastic, and hence could underlie transformations in neuronal morphology.

Central spindle assembly and cytokinesis require a kinesin-like protein/RhoGAP complex with microtubule bundling activity

A late step in cytokinesis requires the central spindle, which forms during anaphase by the bundling of antiparallel nonkinetochore microtubules. Microtubule bundling and completion of cytokinesis require ZEN-4/CeMKLP-1, a kinesin-like protein, and CYK-4, which contains a RhoGAP domain. We show that CYK-4 and ZEN-4 exist in a complex in vivo that can be reconstituted in vitro. The N terminus of CYK-4 binds the central region of ZEN-4, including the neck linker. Genetic suppression data prove the functional significance of this interaction. An analogous complex, containing equimolar amounts of a CYK-4 ortholog and MKLP-1, was purified from mammalian cells. Biochemical studies indicate that this complex, named centralspindlin, is a heterotetramer. Centralspindlin, but not its individual components, strongly promotes microtubule bundling in vitro.

Differential localization of the neurofibromatosis 1 (NF1) gene product, neurofibromin, with the F-actin or microtubule cytoskeleton during differentiation of telencephalic neurons

The protein product of the neurofibromatosis 1 gene, neurofibromin, is abundantly expressed in the cerebral cortex during development, but its physiological role remains unknown. To gain insights into the functions of neurofibromin in neurons, we examined patterns of expression and subcellular localization of neurofibromin during neuronal differentiation. Western blot analysis of telencephali homogenates throughout chick embryogenesis revealed that neurofibromin expression increased during embryonic development. Further analysis showed that telencephalic neurons were also enriched in neurofibromin in culture and that a biphasic gain in expression correlated well with both phases of differentiation in culture, first with a massive outgrowth of processes and gains in neurotransmitter phenotype differentiation, and then with synapse formation. Compared to proteins associated with distinct cytoskeleton systems, the pattern of neurofibromin expression correlated closely with that of the cortical cytoskeleton protein paxillin. Moreover, analysis of immunofluorescence staining of neurofibromin showed that in the presence of a protein crosslinker which preserves both soluble and filamentous cytoskeleton proteins after extraction with Triton X-100, neurofibromin colocalized with F-actin only during the first differentiation phase. This colocalization persisted when the actin cytoskeleton was collapsed with cytochalasin D treatment. In contrast, during the second phase of differentiation neurofibromin colocalized with microtubules, but not F-actin, and the staining pattern was disrupted with nocodazole, but not cytochalasin. A constant finding under all conditions was the presence of neurofibromin in the nucleus, which supports the idea that the bipartite nuclear targeting sequence between residues 2555 and 2572 of neurofibromin may be functional. In summary, we have shown that telencephalic neurons and astroblasts are enriched in neurofibromin and that the subcellular targeting of neurofibromin toward the actin or the microtubule cytoskeleton is developmentally regulated.

Transient suppression of cortactin ectopically induces large telencephalic neurons towards a GABAergic phenotype

Excitatory and inhibitory neuronal cell fates require specific expression of both neurotransmitter and morphological phenotypes. The role of the F-actin cytoskeleton in morphological phenotypes has been well documented, but its role in neurotransmitter phenotype expression remains unknown. Here we present evidence that the F-actin binding protein cortactin participates in determining both aspects of cell fate in large telencephalic neurons. We show that the expression of cortactin was upregulated early in development just prior to appearance of GABAergic neurons in the chick telencephalon at embryonic day 6. This program was faithfully maintained in primary neuronal cultures derived from E6 telencephalon, where immature neurons differentiate either to large pyramidal and large stellate excitatory neurons or to small inhibitory GABAergic neurons. Immunostaining revealed that cortactin was enriched in areas of membrane budding, growth cones, and in the cell cortex of immature neurons. With differentiation, intense punctate staining was also observed in an extraction-resistant cytosolic compartment of the soma and processes. More importantly, suppression of cortactin by inhibition of cortactin mRNA translation with antisense oligonucleotides caused permanent phenotypic changes. Specifically, a transient suppression of cortactin was achieved in immature neurons with a single exposure to antisense oligonucleotides. This inhibition first induced both the expression of mRNA and the enzymatic activity of GAD significantly earlier than in control neurons. Second, cortactin-suppressed large projectional neurons exhibited significantly shorter processes and growth cones with protrusive filopodia and an enlarged lamellipodia veil. Most importantly, this remodeling of neuritic outgrowth in projectional somata was accompanied by the ectopic induction of GABA (*-aminobutyric acid) expression. Considering this data altogether, it appears that cortactin may function to suppress concurrently several parameters of the GABAergic program in large developing neurons.

Depletion of a microtubule-associated motor protein induces the loss of dendritic identity

Dendrites are short stout tapering processes that are rich in ribosomes and Golgi elements, whereas axons are long thin processes of uniform diameter that are deficient in these organelles. It has been hypothesized that the unique morphological and compositional features of axons and dendrites result from their distinct patterns of microtubule polarity orientation. The microtubules within axons are uniformly oriented with their plus ends distal to the cell body, whereas microtubules within dendrites are nonuniformly oriented. The minus-end-distal microtubules are thought to arise via their specific transport into dendrites by the motor protein known as CHO1/MKLP1. According to this model, CHO1/MKLP1 transports microtubules with their minus ends leading into dendrites by generating forces against the plus-end-distal microtubules, thus creating drag on the plus-end-distal microtubules. Here we show that depletion of CHO1/MKLP1 from cultured neurons causes a rapid redistribution of microtubules within dendrites such that minus-end-distal microtubules are chased back to the cell body while plus-end-distal microtubules are redistributed forward. The dendrite grows significantly longer and thinner, loses its taper, and acquires a progressively more axon-like organelle composition. These results suggest that the forces generated by CHO1/MKLP1 are necessary for maintaining the minus-end-distal microtubules in the dendrite, for antagonizing the anterograde transport of the plus-end-distal microtubules, and for sustaining a pattern of microtubule organization necessary for the maintenance of dendritic morphology and composition. Thus, we would conclude that dendritic identity is dependent on forces generated by CHO1/MKLP1.

Overexpression of the 77-kD echinoderm microtubule-associated protein (EMAP), a WD-40 repeat protein, in baculovirus-infected Sf9 cells

The purpose of this study was to test whether any assembly-promoting microtubule-associated protein (MAP) would bundle microtubules and induce process formation in recombinant baculovirus-infected Sf9 cells, in particular, whether a non-neural MAP from a normally rounded cell would produce cellular asymmetries. To carry out these experiments, we constructed a recombinant baculovirus that expressed the full-length 77-kD EMAP, an abundant MAP that localizes to the mitotic spindle of cleavage-stage sea urchin embryos and to the interphase array of microtubules in adult coelomocytes. Expression of EMAP in Sf9 cells had no detectable effect on cellular morphology, microtubule organization, or stability. These results indicate that process formation in Sf9 cells is MAP specific.

Melatonin induction of filamentous structures in non-neuronal cells that is dependent on expression of the human mt1 melatonin receptor

Melatonin has gained recent popularity as a treatment for insomnia and other sleep disorders; however, its cellular effects are unknown. We report the effects of melatonin on the cellular morphology of Chinese hamster ovary (CHO) cells transformed to express the human melatonin receptors, mt1 and MT2. Our results show that melatonin exerts a strong influence on cellular shape and cytoskeletal organization in a receptor-dependent and possibly subtype-selective manner. The cell shape change that we see after a 5-h treatment of these non-neuronal cells with a pharmacological concentration of melatonin consists of the formation of long filamentous outgrowths that are reminiscent of the neurite processes produced by differentiating nerve cells. This morphological change occurs exclusively in cells expressing the mt1 receptor. We find that the microtubule and microfilament organization within these outgrowths is similar to that of neurites. Microtubules are required for the shape change to occur as Colcemid added in combination with melatonin completely blocks outgrowth formation. We demonstrate that the number of cells showing the altered cell shape is dependent on melatonin concentration, constant exposure to melatonin and that outgrowth frequencies increase when protein kinase A (PKA) is inhibited. Concomitant melatonin-dependent increases in MEK 1/2 and ERK 1/2 phosphorylation are noted in mt1-CHO cells only. The production of filamentous outgrowths is dependent on the translation of new protein but not the transcription of new mRNA. Outgrowth number is not controlled by centrosomes but is instead controlled by the polymerization state of the actin cytoskeleton. The results of this work show that the organization of the cytoskeleton is affected by processes specifically mediated or regulated by the mt1 receptor and may represent a novel alternative mechanism for the stimulation of process formation.

The road less traveled: emerging principles of kinesin motor utilization

Proteins of the kinesin superfamily utilize a conserved catalytic motor domain to generate movements in a wide variety of cellular processes. In this review, we discuss the rapid expansion in our understanding of how eukaryotic cells take advantage of these proteins to generate force and movement in diverse functional contexts. We summarize several recent examples revealing that the simplest view of a kinesin motor protein binding to and translocating a cargo along a microtubule track is inadequate. In fact, this paradigm captures only a small subset of the many ways in which cells harness force production of the generation of intracellular movements and functions. We also highlight several situations where the catalytic kinesin motor domain may not be used to generate movement, but instead may be used in other biochemical and functional contexts. Finally, we review some recent ideas about kinesin motor regulation, redundancy, and cargo attachment strategies.

The kinetochore of higher eucaryotes: a molecular view

This review summarizes results concerning the molecular nature of the higher eucaryotic kinetochore. The first major section of this review includes kinetochore proteins whose general functions remain to be determined, precluding their entry into a discrete functional category. Many of the proteins in this section, however, are likely to be involved in kinetochore formation or structure. The second major section is concerned with how microtubule motor proteins function to cause chromosome movement. The microtubule motors dynein, CENP-E, and MCAK have all been observed at the kinetochore. While their precise functions are not well understood, all three are implicated in chromosome movement during mitosis. Finally, the last section deals with kinetochore components that play a role in the spindle checkpoint; a checkpoint that delays mitosis until all kinetochores have attached to the mitotic spindle. Brief reviews of kinetochore morphology and of an important technical breakthrough that enabled the molecular dissection of the kinetochore are also included.

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

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

Mechanisms of trafficking in axons and dendrites: implications for development and neurodegeneration

In the area of routing and sorting of dendritic traffic, the current phenomenological data beg questions about the cellular mechanisms utilized not only to transport material but also to modulate activity in a process, even apoptosis. To aid in formulating testable hypotheses, many plausible models are developed here and linked with some of the preliminary data that supports them. We first assume that in long dendrites the sorting of membranous proteins into transport vesicles also involves the linkage of motor proteins to the vesicles. Second, we assume that the cytoskeleton in dendrites is altered from the cytoskeleton in axons and the cell body. Viral glycoproteins, MAP2 and specific mRNA sorting into dendrites provide the simplest models for analyzing vesicular, cytoskeletal and RNA sorting. In the case of viral glycoproteins, initial sorting appears to occur at the Golgi but additional routing steps involve further complexities that could best be served by an additional sorting step at the junction of the cell body and the process. Transport of the specialized cytoskeletal proteins and specific mRNAs as well as vesicular material could be controlled by a similar gatekeeper at the mouth of a process. Studies of the microtubule-organelle motor complex, regulation of microtubule-based motility by microtubule-associated proteins, and slow axonal transport all provide insights into important aspects of the routing and sorting. These processes are in turn controlled by extracellular signals such as those generated by matrix molecules or their hydrolysis products in the case of amyloid precursor protein (APP). Routing and sorting mechanisms may be central to the development of Alzheimer's disease in view of evidence that APP processing is affected, transport is disturbed, and intracellular vesicles (early endosomes) hypertrophied. Further it is possible that routing mechanisms play a role in cell-cell interactions as, for example, the possibility that pathogenic/cellular stress signals may be passed along circuits transsynaptically.

The role of motor proteins in establishing the microtubule arrays of axons and dendrites

Neurons generate two distinct types of processes called axons and dendrites, both of which rely on highly organized arrays of microtubules for their growth and maintenance. Axonal microtubules are uniformly oriented with their plus-ends distal to the cell body, while 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. Here I describe several lines of evidence from my laboratory that support a model for the establishment of these microtubule arrays based on microtubule transport by motor proteins. Microtubules destined for axons and dendrites are nucleated at the centrosome within the cell body of the neuron, and rapidly released. The released microtubules are then transported into the developing processes. Early in neuronal development, the microtubules are transported with their plus-ends-leading into the developing axon and into the immature processes that will develop into dendrites. In the case of the developing dendrites, the plus-end-distal microtubules are later joined by a population of microtubules that are transported into these processes with their minus-ends-leading. Implicit in this model is that there are molecular motor proteins that transport microtubules with the appropriate orientation into each type of process. There is precedent for molecular motor proteins transporting microtubules during mitosis, and our results suggest that the same or similar motors are utilized during the development of axons and dendrites after a neuroblast becomes terminally postmitotic.

Expression of the mitotic motor protein CHO1/MKLP1 in postmitotic neurons

The kinesin-related motor protein CHO1/MKLP1 was initially thought to be expressed only in mitotic cells, where it presumably transports oppositely oriented microtubules relative to one another in the spindle mid-zone. We have recently shown that CHO1/MKLP1 is also expressed in cultured neuronal cells, where it is enriched in developing dendrites [Sharp et al. (1997a) J. Cell Biol., 138, 833-843]. The putative function of CHO1/MKLP1 in these postmitotic cells is to intercalate minus-end-distal microtubules among oppositely oriented microtubules within developing dendrites, thereby establishing their non-uniform microtubule polarity pattern. Here we used in situ hybridization to determine whether CHO1/MKLP1 is expressed in a variety of rodent neurons both in vivo and in vitro. These analyses revealed that CHO1/MKLP1 is expressed within various neuronal populations of the brain including those in the cerebral cortex, hippocampus, olfactory bulb and cerebellum. The messenger ribonucleic acid (mRNA) levels are high within these neurons well after the completion of their terminal mitotic division and throughout the development of their dendrites. After this, the levels decrease and are relatively low within the adult brain. Parallel analyses on developing hippocampal neurons in culture indicate that the levels of expression increase dramatically just prior to dendritic development, and then decrease somewhat after the dendrites have differentiated. Dorsal root ganglion neurons, which generate axons but not dendrites, express significantly lower levels of mRNA for CHO1/MKLP1 than hippocampal or sympathetic neurons. These results are consistent with the proposed role of CHO1/MKLP1 in establishing the dendritic microtubule array.

Microtubule dynamics in living cells

Most people think of a skeleton as a solid and static framework upon which complex structures are elaborated. From what we have learned in the past decade about the cytoskeleton, it seems certain that the 'skeleton' part of the term is a bit misleading. It is clear now that the polymers that constitute the cytoskeleton, actin filaments, microtubules, and intermediate filaments, are all in fact ever-changing dynamic infrastructures of cells. Recently, advances have been made in the study of the cellular dynamics of one of the prominent components of the cytoskeleton, the microtubules. Observations in the past year have revealed some fundamental in vivo behaviors of these polymers, during interphase, during mitosis, and during the elaboration of postmitotic axonal microtubule arrays. These observations are important for the understanding of cytoplasmic organization in many types of cells.

Identification of a microtubule-associated motor protein essential for dendritic differentiation

The quintessential feature of the dendritic microtubule array is its nonuniform pattern of polarity orientation. During the development of the dendrite, a population of plus end-distal microtubules first appears, and these microtubules are subsequently joined by a population of oppositely oriented microtubules. Studies from our laboratory indicate that the latter microtubules are intercalated within the microtubule array by their specific transport from the cell body of the neuron during a critical stage in development (Sharp, D.J., W. Yu, and P.W. Baas. 1995. J. Cell Biol. 130:93- 104). In addition, we have established that the mitotic motor protein termed CHO1/MKLP1 has the appropriate properties to transport microtubules in this manner (Sharp, D.J., R. Kuriyama, and P.W. Baas. 1996. J. Neurosci. 16:4370-4375). In the present study we have sought to determine whether CHO1/MKLP1 continues to be expressed in terminally postmitotic neurons and whether it is required for the establishment of the dendritic microtubule array. In situ hybridization analyses reveal that CHO1/MKLP1 is expressed in postmitotic cultured rat sympathetic and hippocampal neurons. Immunofluorescence analyses indicate that the motor is absent from axons but is enriched in developing dendrites, where it appears as discrete patches associated with the microtubule array. Treatment of the neurons with antisense oligonucleotides to CHO1/MKLP1 suppresses dendritic differentiation, presumably by inhibiting the establishment of their nonuniform microtubule polarity pattern. We conclude that CHO1/MKLP1 transports microtubules from the cell body into the developing dendrite with their minus ends leading, thereby establishing the nonuniform microtubule polarity pattern of the dendrite.

Inhibition of a mitotic motor compromises the formation of dendrite-like processes from neuroblastoma cells

Microtubules in the axon are uniformly oriented, while microtubules in the dendrite are nonuniformly oriented. We have proposed that these distinct microtubule polarity patterns may arise from a redistribution of molecular motor proteins previously used for mitosis of the developing neuroblast. To address this issue, we performed studies on neuroblastoma cells that undergo mitosis but also generate short processes during interphase. Some of these processes are similar to axons with regard to their morphology and microtubule polarity pattern, while others are similar to dendrites. Treatment with cAMP or retinoic acid inhibits cell division, with the former promoting the development of the axon-like processes and the latter promoting the development of the dendrite-like processes. During mitosis, the kinesin-related motor termed CHO1/MKLP1 is localized within the spindle midzone where it is thought to transport microtubules of opposite orientation relative to one another. During process formation, CHO1/ MKLP1 becomes concentrated within the dendrite-like processes but is excluded from the axon-like processes. The levels of CHO1/MKLP1 increase in the presence of retinoic acid but decrease in the presence of cAMP, consistent with a role for the protein in dendritic differentiation. Moreover, treatment of the cultures with antisense oligonucleotides to CHO1/MKLP1 compromises the formation of the dendrite-like processes. We speculate that a redistribution of CHO1/MKLP1 is required for the formation of dendrite-like processes, presumably by establishing their characteristic nonuniform microtubule polarity pattern.

Cytoskeletal plasticity in cells expressing neuronal microtubule-associated proteins

MAP2 and tau are the two most prominent neuron-specific microtubule-associated proteins. They have been implicated in the stabilization of microtubules and consequently of neurite morphology. To investigate their influence on microtubule dynamics, we have tagged both proteins with green fluorescent protein and expressed them in non-neuronal cells. Time-lapse recordings of living cells showed that MAP2 and tau did not significantly affect the rates of microtubule growth and shrinkage. Longer recordings revealed the growth and disappearance of MAP-induced microtubule bundles coinciding with changes in cell shape. This supports the idea that microtubule dynamics are influenced by the cortical cytoskeleton. The dynamics-preserving stabilization of microtubules by MAP2 and tau thus provides a molecular basis for the morphological plasticity reported to exist in established neurites.

Microtubule assembly in growing dendrites

Dendritic microtubules (MTs) are nonuniform with respect to polarity orientation, with roughly equal proportions having a plus-end-distal or minus-end-distal orientation. In the present studies, we have microinjected biotin-labeled tubulin (Bt-tub) into cultured sympathetic neurons extending dendrites to explore the contribution of MT assembly to the elaboration and maintenance of the dendritic MT array. Within minutes of injecting Bt-tub, an enormous number of MTs were seen emanating from a point source in the cell body. Over time, this pattern changed such that by 120 min after injection, biotinylated MTs no longer emanated from a discrete site, but were distributed over a broad region that extended from the cell body into the dendrites. The observation that biotinylated MTs emanate from a point source in the soma at relatively short times after injection, but not at longer times, suggests that they undergo a redistribution subsequent to their initial nucleation rather than a simple radial expansion from the somal nucleation site. Bt-tub assembly also occurred in dendrites but, unlike in the cell body, assembly was dispersed throughout the dendrite rather than emanating from a discrete site. Immunoelectron microscopic analyses revealed that assembly in dendrites reflected the addition of Bt-tub onto the ends of both plus-end-distal and minus-end-distal MTs that existed in the cell at the time of injection. The time course of Bt-tub appearance in dendritic MTs suggested an average half-life of approximately 76 min for these MTs. We discuss these observations in the context of a model for generating the MT array of dendrites that combines both MT transport and MT assembly.