Cell and Molecular Biology of Microtubule Plus End Tracking Proteins

Neuron navigators: A novel frontier with physiological and pathological implications

Neuron navigators are microtubule plus-end tracking proteins containing basic and serine rich regions which are encoded by neuron navigator genes (NAVs). Neuron navigator proteins are essential for neurite outgrowth, neuronal migration, and overall neurodevelopment along with some other functions as well. The navigator proteins are substantially expressed in the developing brain and have been reported to be differentially expressed in various tissues at different ages. Over the years, the research has found neuron navigators to be implicated in a spectrum of pathological conditions such as developmental anomalies, neurodegenerative disorders, neuropathic pain, anxiety, cancers, and certain inflammatory conditions. The existing knowledge about neuron navigators remains sparse owing to their differential functions, undiscovered modulators, and unknown molecular mechanisms. Investigating the possible role of neuron navigators in various physiological processes and pathological conditions pose as a novel field that requires extensive research and might provide novel mechanistic insights and understanding of these aspects.

Septins guide noncentrosomal microtubules to promote focal adhesion disassembly in migrating cells

Endothelial cell migration is critical for vascular angiogenesis and is compromised to facilitate tumor metastasis. The migratory process requires the coordinated assembly and disassembly of focal adhesions (FA), actin, and microtubules (MT). MT dynamics at FAs deliver vesicular cargoes and enhance actomyosin contractility to promote FA turnover and facilitate cell advance. Noncentrosomal (NC) MTs regulate FA dynamics and are sufficient to drive cell polarity, but how NC MTs target FAs to control FA turnover is not understood. Here, we show that Rac1 induces the assembly of FA-proximal septin filaments that promote NC MT growth into FAs and inhibit mitotic centromere-associated kinesin (MCAK)-associated MT disassembly, thereby maintaining intact MT plus ends proximal to FAs. Septin-associated MT rescue is coupled with accumulation of Aurora-A kinase and cytoplasmic linker-associated protein (CLASP) localization to the MT between septin and FAs. In this way, NC MTs are strategically positioned to undergo MCAK- and CLASP-regulated bouts of assembly and disassembly into FAs, thereby regulating FA turnover and cell migration.

α-tubulin tail modifications regulate microtubule stability through selective effector recruitment, not changes in intrinsic polymer dynamics

Microtubules are non-covalent polymers of αβ-tubulin dimers. Posttranslational processing of the intrinsically disordered C-terminal α-tubulin tail produces detyrosinated and Δ2-tubulin. Although these are widely employed as proxies for stable cellular microtubules, their effect (and of the α-tail) on microtubule dynamics remains uncharacterized. Using recombinant, engineered human tubulins, we now find that neither detyrosinated nor Δ2-tubulin affect microtubule dynamics, while the α-tubulin tail is an inhibitor of microtubule growth. Consistent with the latter, molecular dynamics simulations show the α-tubulin tail transiently occluding the longitudinal microtubule polymerization interface. The marked differential in vivo stabilities of the modified microtubule subpopulations, therefore, must result exclusively from selective effector recruitment. We find that tyrosination quantitatively tunes CLIP-170 density at the growing plus end and that CLIP170 and EB1 synergize to selectively upregulate the dynamicity of tyrosinated microtubules. Modification-dependent recruitment of regulators thereby results in microtubule subpopulations with distinct dynamics, a tenet of the tubulin code hypothesis.

The Role of Capsid in the Early Steps of HIV-1 Infection: New Insights into the Core of the Matter

In recent years, major advances in research and experimental approaches have significantly increased our knowledge on the role of the HIV-1 capsid in the virus life cycle, from reverse transcription to integration and gene expression. This makes the capsid protein a good pharmacological target to inhibit HIV-1 replication. This review covers our current understanding of the role of the viral capsid in the HIV-1 life cycle and its interaction with different host factors that enable reverse transcription, trafficking towards the nucleus, nuclear import and integration into host chromosomes. It also describes different promising small molecules, some of them in clinical trials, as potential targets for HIV-1 therapy.

Theileria's Strategies and Effector Mechanisms for Host Cell Transformation: From Invasion to Immortalization

One of the first events that follows invasion of leukocytes by Theileria sporozoites is the destruction of the surrounding host cell membrane and the rapid association of the intracellular parasite with host microtubules. This is essential for the parasite to establish its niche within the cytoplasm of the invaded leukocyte and sets Theileria spp. apart from other members of the apicomplexan phylum such as Toxoplasma gondii and Plasmodium spp., which reside within the confines of a host-derived parasitophorous vacuole. After establishing infection, transforming Theileria species (T. annulata, T. parva) significantly rewire the signaling pathways of their bovine host cell, causing continual proliferation and resistance to ligand-induced apoptosis, and conferring invasive properties on the parasitized cell. Having transformed its target cell, Theileria hijacks the mitotic machinery to ensure its persistence in the cytoplasm of the dividing cell. Some of the parasite and bovine proteins involved in parasite-microtubule interactions have been fairly well characterized, and the schizont expresses at least two proteins on its membrane that contain conserved microtubule binding motifs. Theileria-encoded proteins have been shown to be translocated to the host cell cytoplasm and nucleus where they have the potential to directly modify signaling pathways and host gene expression. However, little is known about their mode of action, and even less about how these proteins are secreted by the parasite and trafficked to their target location. In this review we explore the strategies employed by Theileria to transform leukocytes, from sporozoite invasion until immortalization of the host cell has been established. We discuss the recent description of nuclear pore-like complexes that accumulate on membranes close to the schizont surface. Finally, we consider putative mechanisms of protein and nutrient exchange that might occur between the parasite and the host. We focus in particular on differences and similarities with recent discoveries in T. gondii and Plasmodium species.

HIV-1 capsids mimic a microtubule regulator to coordinate early stages of infection

While the microtubule end-binding protein, EB1 facilitates early stages of HIV-1 infection, how it does so remains unclear. Here, we show that beyond its effects on microtubule acetylation, EB1 also indirectly contributes to infection by delivering the plus-end tracking protein (+TIP), cytoplasmic linker protein 170 (CLIP170) to the cell periphery. CLIP170 bound to intact HIV-1 cores or in vitro assembled capsid-nucleocapsid complexes, while EB1 did not. Moreover, unlike EB1 and several other +TIPs, CLIP170 enhanced infection independently of effects on microtubule acetylation. Capsid mutants and imaging revealed that CLIP170 bound HIV-1 cores in a manner distinct from currently known capsid cofactors, influenced by pentamer composition or curvature. Structural analyses revealed an EB-like +TIP-binding motif within the capsid major homology region (MHR) that binds SxIP motifs found in several +TIPs, and variability across this MHR sequence correlated with the extent to which different retroviruses engage CLIP170 to facilitate infection. Our findings provide mechanistic insights into the complex roles of +TIPs in mediating early stages of retroviral infection, and reveal divergent capsid-based EB1 mimicry across retroviral species.

Activity-dependent neuroprotective protein (ADNP)-end-binding protein (EB) interactions regulate microtubule dynamics toward protection against tauopathy

The 1102-amino-acid activity-dependent neuroprotective protein (ADNP) was originally discovered by expression cloning through the immunological identification of its 8-amino-acid sequence NAPVSIPQ (NAP), constituting the smallest active neuroprotective fragment of the protein. ADNP expression is essential for brain formation and cognitive function and is dysregulated in a variety of neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, and schizophrenia). ADNP has been found to be mutated in autism, with an estimated prevalence of 0.17% (together, these autism cases now constitute ADNP syndrome cases) and our recent results showed somatic mutations in ADNP in Alzheimer's disease brains correlating with tauopathy. Furthermore, Adnp haploinsufficiency in mice causes an age-dependent reduction in cognitive functions coupled with tauopathy-like features such as an increased formation of tangle-like structures, defective axonal transport, and Tau hyperphosphorylation. ADNP and its derived peptides, NAP and SKIP, directly interact with end-binding proteins (EBs), which decorate plus-tips of the growing axonal cytoskeleton-microtubules (MTs). Functionally, NAP and SKIP are neuroprotective and stimulate axonal transport. Clinical trials have suggested the potential efficacy of NAP (davunetide, CP201) for improving cognitive performance/functional activities of daily living in amnestic mild cognitive impairment (aMCI) and schizophrenia patients, respectively. However, NAP was not found to be an effective treatment (though well-tolerated) for progressive supranuclear palsy (PSP) patients. Here we review the molecular mechanism of NAP activity on MTs and how NAP modulates the MT-Tau-EBs crosstalk. We offer a molecular explanation for the different protective potency of NAP in selected tauopathies (aMCI vs. PSP) expressing different ratios/pathologies of the alternatively spliced Tau mRNA and its resulting protein (aMCI expressing similar quantities of the dynamic Tau 3-MT binding isoform (Tau3R) and the Tau 4-MT binding isoform (Tau4R) and PSP enriched in Tau4R pathology). We reveal the direct effect of truncated ADNPs (resulting from de novo autism and newly discovered Alzheimer's disease-related somatic mutations) on MT dynamics. We show that the peptide SKIP affects MT dynamics and MT-Tau association. Since MT impairment is linked with neurodegenerative and neurodevelopmental conditions, the current study implicates a paucity/dysregulation of MT-interacting endogenous proteins, like ADNP, as a contributing mechanism and provides hope for NAP and SKIP as MT-modulating drug candidates.

PTPN22 interacts with EB1 to regulate T-cell receptor signaling

The PTPN22 gene encoding the Lyp/Pep protein tyrosine phosphatase is a negative regulator of T-cell receptor (TCR) signaling. Recent studies have shown that phosphorylation of end-binding protein 1 (EB1) is associated with the TCR activation. In this study, using 2-hybrid and mass spectrometry analyses, we identified EB1 as a protein associated with PTPN22. Furthermore, we discovered that EB1 specifically bound to the P1 domain of PTPN22 by competing with CSK, and the variant PTPN22-R620W does not affect the association with EB1, which is instrumental with respect to the regulation of TCR signaling. In addition, PTPN22 dephosphorylates EB1 at tyrosine-247 (Y247), which decreases the expression of the T-cell activation markers CD25 and CD69 and the phosphorylation levels of the TCR molecules ZAP-70, LAT, and Erk, leading to the eventual downregulation of the transcription factor NFAT and reduced the levels of secreted IL-2. The findings of this study provide new insights into the TCR signaling and the T-cell immune response, which are important for clarifying the mechanism of PTPN22-related autoimmune diseases.

13 Plus 1: A 30-Year Perspective on Microtubule-Based Motility in Dictyostelium

Individual gene analyses of microtubule-based motor proteins in Dictyostelium discoideum have provided a rough draft of its machinery for cytoplasmic organization and division. This review collates their activities and looks forward to what is next. A comprehensive approach that considers the collective actions of motors, how they balance rates and directions, and how they integrate with the actin cytoskeleton will be necessary for a complete understanding of cellular dynamics.

The microtubule skeleton and the evolution of neuronal complexity in vertebrates

The evolution of a highly developed nervous system is mirrored by the ability of individual neurons to develop increased morphological complexity. As microtubules (MTs) are crucially involved in neuronal development, we tested the hypothesis that the evolution of complexity is driven by an increasing capacity of the MT system for regulated molecular interactions as it may be implemented by a higher number of molecular players and a greater ability of the individual molecules to interact. We performed bioinformatics analysis on different classes of components of the vertebrate neuronal MT cytoskeleton. We show that the number of orthologs of tubulin structure proteins, MT-binding proteins and tubulin-sequestering proteins expanded during vertebrate evolution. We observed that protein diversity of MT-binding and tubulin-sequestering proteins increased by alternative splicing. In addition, we found that regions of the MT-binding protein tau and MAP6 displayed a clear increase in disorder extent during evolution. The data provide evidence that vertebrate evolution is paralleled by gene expansions, changes in alternative splicing and evolution of coding sequences of components of the MT system. The results suggest that in particular evolutionary changes in tubulin-structure proteins, MT-binding proteins and tubulin-sequestering proteins were prominent drivers for the development of increased neuronal complexity.

Microtubules at focal adhesions – a double-edged sword

Cell adhesion to the extracellular matrix is essential for cellular processes, such as migration and invasion. In response to cues from the microenvironment, integrin-mediated adhesions alter cellular behaviour through cytoskeletal rearrangements. The tight association of the actin cytoskeleton with adhesive structures has been extensively studied, whereas the microtubule network in this context has gathered far less attention. In recent years, however, microtubules have emerged as key regulators of cell adhesion and migration through their participation in adhesion turnover and cellular signalling. In this Review, we focus on the interactions between microtubules and integrin-mediated adhesions, in particular, focal adhesions and podosomes. Starting with the association of microtubules with these adhesive structures, we describe the classical role of microtubules in vesicular trafficking, which is involved in the turnover of cell adhesions, before discussing how microtubules can also influence the actin–focal adhesion interplay through RhoGTPase signalling, thereby orchestrating a very crucial crosstalk between the cytoskeletal networks and adhesions.

GAS2-like 1 coordinates cell division through its association with end-binding proteins

Cell division involves the tightly coordinated rearrangement of actin and microtubules (MTs). We have previously shown that a member of the family of growth arrest-specific 2-like proteins, GAS2-like 1 (G2L1) regulates actin-MT crosstalk through its associations with plus-end microtubule tip-binding (EB) proteins. Here we show that G2L1 is involved in the regulation of cell division. We show that the depletion of G2L1 results in a reduction in the number of cells undergoing cell division and a significant proportion of those cells that do divide are either multinucleated, display deformed nuclei, or undergo cell division at a much slower rate. Exogenous expression of G2L1 mutants revealed that the association of G2L1 with EB1 is critical for regulated cell division and blocking this interaction inhibits cell division as observed in cells lacking G2L1. Taken together, our data suggest that G2L1 controls the precise regulation and successful progression of cell division through its binding to EB-proteins.

The Rice SPOTTED LEAF4 (SPL4) Encodes a Plant Spastin That Inhibits ROS Accumulation in Leaf Development and Functions in Leaf Senescence

Lesion mimic mutants (LMMs) are usually controlled by single recessive mutations that cause the formation of necrotic lesions without pathogen invasion. These genetic defects are useful to reveal the regulatory mechanisms of defense-related programmed cell death in plants. Molecular evidence has been suggested that some of LMMs are closely associated with the regulation of leaf senescence in rice (Oryza sativa). Here, we characterized the mutation underlying spotted leaf4 (spl4), which results in lesion formation and also affects leaf senescence in rice. Map-based cloning revealed that the γ ray-induced spl4-1 mutant has a single base substitution in the splicing site of the SPL4 locus, resulting in a 13-bp deletion within the encoded microtubule-interacting-and-transport (MIT) spastin protein containing an AAA-type ATPase domain. The T-DNA insertion spl4-2 mutant exhibited spontaneous lesions similar to those of the spl4-1 mutant, confirming that SPL4 is responsible for the LMM phenotype. In addition, both spl4 mutants exhibited delayed leaf yellowing during dark-induced or natural senescence. Western blot analysis of spl4 mutant leaves suggested possible roles for SPL4 in the degradation of photosynthetic proteins. Punctate signals of SPL4-fused fluorescent proteins were detected in the cytoplasm, similar to the cellular localization of animal spastin. Based on these findings, we propose that SPL4 is a plant spastin that is involved in multiple aspects of leaf development, including senescence.

Kymograph analysis with high temporal resolution reveals new features of neurofilament transport kinetics

We have used kymograph analysis combined with edge detection and an automated computational algorithm to analyze the axonal transport kinetics of neurofilament polymers in cultured neurons at 30 ms temporal resolution. We generated 301 kymographs from 136 movies and analyzed 726 filaments ranging from 0.6 to 42 µm in length, representing ∼37,000 distinct moving and pausing events. We found that the movement is even more intermittent than previously reported and that the filaments undergo frequent, often transient, reversals which suggest that they can engage simultaneously with both anterograde and retrograde motors. Average anterograde and retrograde bout velocities (0.9 and 1.2 µm s^-1 , respectively) were faster than previously reported, with maximum sustained bout velocities of up to 6.6 and 7.8 µm s^-1 , respectively. Average run lengths (∼1.1 µm) and run times (∼1.4 s) were in the range reported for molecular motor processivity in vitro, suggesting that the runs could represent the individual processive bouts of the neurofilament motors. Notably, we found no decrease in run velocity, run length or run time with increasing filament length, which suggests that either the drag on the moving filaments is negligible or that longer filaments recruit more motors.

Microtubule Organization Determines Axonal Transport Dynamics

Axonal microtubule (MT) arrays are the major cytoskeleton substrate for cargo transport. How MT organization, i.e., polymer length, number, and minus-end spacing, is regulated and how it impinges on axonal transport are unclear. We describe a method for analyzing neuronal MT organization using light microscopy. This method circumvents the need for electron microscopy reconstructions and is compatible with live imaging of cargo transport and MT dynamics. Examination of a C. elegans motor neuron revealed how age, MT-associated proteins, and signaling pathways control MT length, minus-end spacing, and coverage. In turn, MT organization determines axonal transport progression: cargoes pause at polymer termini, suggesting that switching MT tracks is rate limiting for efficient transport. Cargo run length is set by MT length, and higher MT coverage correlates with shorter pauses. These results uncover the principles and mechanisms of neuronal MT organization and its regulation of axonal cargo transport.

Proto-Oncogenic Src Phosphorylates EB1 to Regulate the Microtubule-Focal Adhesion Crosstalk and Stimulate Cell Migration

Cell migration, a complex process critical for tumor progression and metastasis, requires a dynamic crosstalk between microtubules (MTs) and focal adhesions (FAs). However, the molecular mechanisms underlying this event remain elusive. Herein we identify the proto-oncogenic protein Src as an important player in the regulation of the MT-FA crosstalk. Src interacts with and phosphorylates end-binding protein 1 (EB1), a member of MT plus end-tracking proteins (+TIPs), both in cells and in vitro. Systematic mutagenesis reveals that tyrosine-247 (Y247) is the primary residue of EB1 phosphorylated by Src. Interestingly, both constitutively activated Src and Y247-phosphorylated EB1 localize to the centrosome and FAs. Src-mediated EB1 phosphorylation diminishes its interactions with other +TIPs, including adenomatous polyposis coli (APC) and mitotic centromere associated kinesin (MCAK). In addition, EB1 phosphorylation at Y247 enhances the rate of MT catastrophe and significantly stimulates cell migration. These findings thus demonstrate that the Src-EB1 axis plays a crucial role in regulating the crosstalk between MTs and FAs to promote cell migration.

Podosome dynamics and location in vascular smooth muscle cells require CLASP-dependent microtubule bending

Extracellular matrix (ECM) remodeling during physiological processes is mediated by invasive protrusions called podosomes. Positioning and dynamics of podosomes define the extent of ECM degradation. Microtubules are known to be involved in podosome regulation, but the role of microtubule (MT) network configuration in podosome dynamics and positioning is not well understood. Here, we show that the arrangement of the microtubule network defines the pattern of podosome formation and relocation in vascular smooth muscle cells (VSMCs). We show that microtubule plus-end targeting facilitates de novo formation of podosomes, in addition to podosome remodeling. Moreover, specialized bent microtubules with plus ends reversed towards the cell center promote relocation of podosomes from the cell edge to the cell center, resulting in an evenly distributed podosome pattern. Microtubule bending is induced downstream of protein kinase C (PKC) activation and requires microtubule-stabilizing proteins known as cytoplasmic linker associated proteins (CLASPs) and retrograde actin flow. Similar to microtubule depolymerization, CLASP depletion by siRNA blocks microtubule bending and eliminates centripetal relocation of podosomes. Podosome relocation also coincides with translocation of podosome-stimulating kinesin KIF1C, which is known to move preferentially along CLASP-associated microtubules. These findings indicate that CLASP-dependent microtubule network configuration is critical to the cellular location and distribution of KIF1C-dependent podosomes. © 2016 Wiley Periodicals, Inc.

CYLD - a deubiquitylase that acts to fine-tune microtubule properties and functions

Microtubules are dynamic structures that are crucially involved in a variety of cellular activities. The dynamic properties and functions of microtubules are regulated by various factors, such as tubulin isotype composition and microtubule-binding proteins. Initially identified as a deubiquitylase with tumor-suppressing functions, the protein cylindromatosis (CYLD) has recently been revealed to interact with microtubules, modulate microtubule dynamics, and participate in the regulation of cell migration, cell cycle progression, chemotherapeutic drug sensitivity and ciliogenesis. These findings have greatly enriched our understanding of the roles of CYLD in physiological and pathological conditions. Here, we focus on recent literature that shows how CYLD impacts on microtubule properties and functions in various biological processes, and discuss the challenges we face when interpreting results obtained from different experimental systems.

The HIV-1 protein Vpr impairs phagosome maturation by controlling microtubule-dependent trafficking

The HIV protein Vpr interacts with EB1, p150^Glued, and dynein heavy chain and perturbs the centripetal movement of phagosomes and their maturation, resulting in impaired phagolysosome biogenesis, which is important for bacterial clearance and cytokine production.

Human immunodeficiency virus type 1 (HIV-1) impairs major functions of macrophages but the molecular basis for this defect remains poorly characterized. Here, we show that macrophages infected with HIV-1 were unable to respond efficiently to phagocytic triggers and to clear bacteria. The maturation of phagosomes, defined by the presence of late endocytic markers, hydrolases, and reactive oxygen species, was perturbed in HIV-1–infected macrophages. We showed that maturation arrest occurred at the level of the EHD3/MICAL-L1 endosomal sorting machinery. Unexpectedly, we found that the regulatory viral protein (Vpr) was crucial to perturb phagosome maturation. Our data reveal that Vpr interacted with EB1, p150^Glued, and dynein heavy chain and was sufficient to critically alter the microtubule plus end localization of EB1 and p150^Glued, hence altering the centripetal movement of phagosomes and their maturation. Thus, we identify Vpr as a modulator of the microtubule-dependent endocytic trafficking in HIV-1–infected macrophages, leading to strong alterations in phagolysosome biogenesis.

Graded Control of Microtubule Severing by Tubulin Glutamylation

Microtubule-severing enzymes are critical for the biogenesis and maintenance of complex microtubule arrays in axons, spindles, and cilia where tubulin detyrosination, acetylation, and glutamylation are abundant. These modifications exhibit stereotyped patterns suggesting spatial and temporal control of microtubule functions. Using human-engineered and differentially modified microtubules we find that glutamylation is the main regulator of the hereditary spastic paraplegia microtubule severing enzyme spastin. Glutamylation acts as a rheostat and tunes microtubule severing as a function of glutamate number added per tubulin. Unexpectedly, glutamylation is a non-linear biphasic tuner and becomes inhibitory beyond a threshold. Furthermore, the inhibitory effect of localized glutamylation propagates across neighboring microtubules, modulating severing in trans. Our work provides the first quantitative evidence for a graded response to a tubulin posttranslational modification and a biochemical link between tubulin glutamylation and complex architectures of microtubule arrays such as those in neurons where spastin deficiency causes disease.

Identification of novel microtubule-binding proteins by taxol-mediated microtubule stabilization and mass spectrometry analysis

Microtubule-binding proteins (MBPs) are structurally and functionally diverse regulators of microtubule-mediated cellular processes. Alteration of MBPs has been implicated in the pathogenesis of human diseases, including cancer. MBPs can stabilize or destabilize microtubules or move along microtubules to transport various cargoes. In addition, MBPs can control microtubule dynamics through direct interaction with microtubules or coordination with other proteins. To better understand microtubule structure and function, it is necessary to identify additional MBPs. In this study, we isolated microtubules and MBPs from mammalian cells by a taxol-based method and then profiled a panel of MBPs by mass spectrometry. We discovered a number of previously uncharacterized MBPs, including several membrane-associated proteins and proteins involved in post-translational modifications, in addition to several structural components. These results support the notion that microtubules have a wide range of functions and may undergo more exquisite regulation than previously recognized.

Building the Microtubule Cytoskeleton Piece by Piece

The microtubule (MT) cytoskeleton gives cells their shape, organizes the cellular interior, and segregates chromosomes. These functions rely on the precise arrangement of MTs, which is achieved by the coordinated action of MT-associated proteins (MAPs). We highlight the first and most important examples of how different MAP activities are combined in vitro to create an ensemble function that exceeds the simple addition of their individual activities, and how the Xenopus laevis egg extract system has been utilized as a powerful intermediate between cellular and purified systems to uncover the design principles of self-organized MT networks in the cell.

Microtubule plus end-tracking proteins play critical roles in directional growth of hyphae by regulating the dynamics of cytoplasmic microtubules in Aspergillus nidulans

Cytoplasmic microtubules (MTs) serve as a rate-limiting factor for hyphal tip growth in the filamentous fungus Aspergillus nidulans. We hypothesized that this function depended on the MT plus end-tracking proteins (+TIPs) including the EB1 family protein EBA that decorated the MT plus ends undergoing polymerization. The ebAΔ mutation reduced colony growth and the mutant hyphae appeared in an undulating pattern instead of exhibiting unidirectional growth in the control. These phenotypes were enhanced by a mutation in another +TIP gene clipA. EBA was required for plus end-tracking of CLIPA, the Kinesin-7 motor KipA, and the XMAP215 homologue AlpA. In addition, cytoplasmic dynein also depended on EBA to track on most polymerizing MT plus ends, but not for its conspicuous appearance at the MT ends near the hyphal apex. The loss of EBA reduced the number of cytoplasmic MTs and prolonged dwelling times for MTs after reaching the hyphal apex. Finally, we found that colonies were formed in the absence of EBA, CLIPA, and NUDA together, suggesting that they were dispensable for fundamental functions of MTs. This study provided a comprehensive delineation of the relationship among different +TIPs and their contributions to MT dynamics and unidirectional hyphal expansion in filamentous fungi.

Mutant huntingtin affects cortical progenitor cell division and development of the mouse neocortex

A polyglutamine expansion in huntingtin (HTT) causes the specific death of adult neurons in Huntington's disease (HD). Most studies have thus focused on mutant HTT (mHTT) toxicity in adulthood, and its developmental effects have been largely overlooked. We found that mHTT caused mitotic spindle misorientation in cultured cells by altering the localization of dynein, NuMA, and the p150(Glued) subunit of dynactin to the spindle pole and cell cortex and of CLIP170 and p150(Glued) to microtubule plus-ends. mHTT also affected spindle orientation in dividing mouse cortical progenitors, altering the thickness of the developing cortex. The serine/threonine kinase Akt, which regulates HTT function, rescued the spindle misorientation caused by the mHTT, by serine 421 (S421) phosphorylation, in cultured cells and in mice. Thus, cortical development is affected in HD, and this early defect can be rescued by HTT phosphorylation at S421.

The Ca2+-activated Cl- channel ANO1/TMEM16A regulates primary ciliogenesis

The Ca²⁺-activated Cl⁻ channel ANO1/TMEM16A is located in the primary cilium, and blocking it pharmacologically or knocking it down with shRNA interferes with ciliogenesis. Before ciliogenesis, the channel is organized into a torus-shaped structure (the “nimbus”) enriched in proteins required for ciliogenesis.

Many cells possess a single, nonmotile, primary cilium highly enriched in receptors and sensory transduction machinery that plays crucial roles in cellular morphogenesis. Although sensory transduction requires ion channels, relatively little is known about ion channels in the primary cilium (with the exception of TRPP2). Here we show that the Ca²⁺-activated Cl⁻ channel anoctamin-1 (ANO1/TMEM16A) is located in the primary cilium and that blocking its channel function pharmacologically or knocking it down with short hairpin RNA interferes with ciliogenesis. Before ciliogenesis, the channel becomes organized into a torus-shaped structure (“the nimbus”) enriched in proteins required for ciliogenesis, including the small GTPases Cdc42 and Arl13b and the exocyst complex component Sec6. The nimbus excludes F-actin and coincides with a ring of acetylated microtubules. The nimbus appears to form before, or independent of, apical docking of the mother centriole. Our data support a model in which the nimbus provides a scaffold for staging of ciliary components for assembly very early in ciliogenesis and chloride transport by ANO1/TMEM16A is required for the genesis or maintenance of primary cilia.

Microtubules CLASP to Adherens Junctions in epidermal progenitor cells

Cadherin-mediated cell adhesion at Adherens Junctions (AJs) and its dynamic connections with the microtubule (MT) cytoskeleton are important regulators of cellular architecture. However, the functional relevance of these interactions and the molecular players involved in different cellular contexts and cellular compartments are still not completely understood. Here, we comment on our recent findings showing that the MT plus-end binding protein CLASP2 interacts with the AJ component p120-catenin (p120) specifically in progenitor epidermal cells. Absence of either protein leads to alterations in MT dynamics and AJ functionality. These findings represent a novel mechanism of MT targeting to AJs that may be relevant for the maintenance of proper epidermal progenitor cell homeostasis. We also discuss the potential implication of other MT binding proteins previously associated to AJs in the wider context of epithelial tissues. We hypothesize the existence of adaptation mechanisms that regulate the formation and stability of AJs in different cellular contexts to allow the dynamic behavior of these complexes during tissue homeostasis and remodeling.

Plk1 phosphorylates CLIP-170 and regulates its binding to microtubules for chromosome alignment

The microtubule (MT) cytoskeleton is essential for cellular morphogenesis, cell migration, and cell division. MT organization is primarily mediated by a variety of MT-associated proteins. Among these proteins, plus-end-tracking proteins (+TIPs) are evolutionarily conserved factors that selectively accumulate at growing MT plus ends. Cytoplasmic linker protein (CLIP)-170 is a +TIP that associates with diverse proteins to determine the behavior of MT ends and their linkage to intracellular structures, including mitotic chromosomes. However, how CLIP-170 activity is spatially and temporally controlled is largely unknown. Here, we show that phosphorylation at Ser312 in the third serine-rich region of CLIP-170 is increased during mitosis. Polo-like kinase 1 (Plk1) is responsible for this phosphorylation during the mitotic phase of dividing cells. In vitro analysis using a purified CLIP-170 N-terminal fragment showed that phosphorylation by Plk1 diminishes CLIP-170 binding to the MT ends and lattice without affecting binding to EB3. Furthermore, we demonstrate that during mitosis, stable kinetochore/MT attachment and subsequent chromosome alignment require CLIP-170 and a proper phosphorylation/dephosphorylation cycle at Ser312. We propose that CLIP-170 phosphorylation by Plk1 regulates proper chromosome alignment by modulating the interaction between CLIP-170 and MTs in mitotic cells and that CLIP-170 activity is stringently controlled by its phosphorylation state, which depends on the cellular context.

Regulation of EB1/3 proteins by classical MAPs in neurons

Microtubules (MTs) are key cytoskeletal elements in developing and mature neurons. MT reorganization underlies the morphological changes that occur during neuronal development. Furthermore, MTs contribute to the maintenance of neuronal architecture, enable intracellular transport and act as scaffolds for signaling molecules. Thus, a fine-tuned regulation of MT dynamics and stability is crucial for the correct differentiation and functioning of neurons. Different types of proteins contribute to the regulation of the MT state, such as plus-end tracking proteins (+TIPs), which interact with the plus-ends of growing microtubules, and classical microtubule-associated proteins (MAPs), which bind along the microtubule lattice. Recent evidence indicates that MAPs interplay with End Binding Proteins (EBs), the core +TIPs, in neuronal cells. This might contribute to the orchestrated regulation of MT dynamics in neurons. In this mini-review article, we address recent research on the neuronal crosstalk between EBs and classical MAPs and speculate on its possible functional relevance.

Microtubule organization and microtubule-associated proteins in plant cells

Plants have unique microtubule (MT) arrays, cortical MTs, preprophase band, mitotic spindle, and phragmoplast, in the processes of evolution. These MT arrays control the directions of cell division and expansion especially in plants and are essential for plant morphogenesis and developments. Organizations and functions of these MT arrays are accomplished by diverse MT-associated proteins (MAPs). This review introduces 10 of conserved MAPs in eukaryote such as γ-TuC, augmin, katanin, kinesin, EB1, CLASP, MOR1/MAP215, MAP65, TPX2, formin, and several plant-specific MAPs such as CSI1, SPR2, MAP70, WVD2/WDL, RIP/MIDD, SPR1, MAP18/PCaP, EDE1, and MAP190. Most of the studies cited in this review have been analyzed in the particular model plant, Arabidopsis thaliana. The significant knowledge of A. thaliana is the important established base to understand MT organizations and functions in plants.

Notch signalling in adult neurons: a potential target for microtubule stabilization

Cytoskeletal dysfunction has been proposed during the last decade as one of the main mechanisms involved in the aetiology of several neurodegenerative diseases. Microtubules are basic elements of the cytoskeleton and the dysregulation of microtubule stability has been demonstrated to be causative for axonal transport impairment, synaptic contact degeneration, impaired neuronal function leading finally to neuronal loss. Several pathways are implicated in the microtubule assembly/disassembly process. Emerging evidence is focusing on Notch as a microtubule dynamics regulator. We demonstrated that activation of Notch signalling results in increased microtubule stability and changes in axonal morphology and branching. By contrast, Notch inhibition leads to an increase in cytoskeleton plasticity with intense neurite remodelling. Until now, several microtubule-binding compounds have been tested and the results have provided proof of concept that microtubule-binding agents or compounds with the ability to stabilize microtubules may have therapeutic potential for the treatment of Alzheimer's disease and other neurodegenerative diseases. In this review, based on its key role in cytoskeletal dynamics modulation, we propose Notch as a new potential target for microtubule stabilization.

HIV-1 induces the formation of stable microtubules to enhance early infection

Stable microtubule (MT) subsets form distinct networks from dynamic MTs and acquire distinguishing posttranslational modifications, notably detyrosination and acetylation. Acting as specialized tracks for vesicle and macromolecular transport, their formation is regulated by the end-binding protein EB1, which recruits proteins that stabilize MTs. We show that HIV-1 induces the formation of acetylated and detyrosinated stable MTs early in infection. Although the MT depolymerizing agent nocodazole affected dynamic MTs, HIV-1 particles localized to nocodazole-resistant stable MTs, and infection was minimally affected. EB1 depletion or expression of an EB1 carboxy-terminal fragment that acts as a dominant-negative inhibitor of MT stabilization prevented HIV-1-induced stable MT formation and suppressed early viral infection. Furthermore, we show that the HIV-1 matrix protein targets the EB1-binding protein Kif4 to induce MT stabilization. Our findings illustrate how specialized MT-binding proteins mediate MT stabilization by HIV-1 and the importance of stable MT subsets in viral infection.

Plus-end tracking proteins, CLASPs, and a viral Akt mimic regulate herpesvirus-induced stable microtubule formation and virus spread

Although microtubules (MTs) frequently form highly dynamic networks, subsets of MTs become stabilized in response to environmental cues and function as specialized tracks for vesicle and macromolecular trafficking. MT stabilization is controlled by specialized plus-end tracking proteins (+TIPs) whose accumulation at the MT ends is facilitated by the end-binding protein, EB1, and regulated by various signaling pathways. As cargoes themselves, viruses are dependent on MTs for their intracellular movement. Although many viruses affect MT organization, the potential contribution of MT stabilization by +TIPs to infection remains unknown. Here we show that early in infection of primary human fibroblasts, herpes simplex virus type 1 (HSV-1) disrupts the centrosome, the primary MT organizing center in many cell types. As infection progresses HSV-1 induces the formation of stable MT subsets through inactivation of glycogen synthase kinase 3beta by the viral Ser/Thr kinase, Us3. Stable MT formation is reduced in cells infected with Us3 mutants and those stable MTs that form cluster around the trans-Golgi network. Downstream of glycogen synthase kinase 3beta, cytoplasmic linker-associated proteins (CLASPs), specialized host +TIPs that control MT formation at the trans-Golgi network and cortical capture, are specifically required for virus-induced MT stabilization and HSV-1 spread. Our findings demonstrate the biological importance of +TIPs to viral infection and suggest that HSV-1 has evolved to exploit the trans-Golgi network as an alternate MT organizing center to facilitate virus spread.

Aurora B spatially regulates EB3 phosphorylation to coordinate daughter cell adhesion with cytokinesis

During mitosis, human cells round up, decreasing their adhesion to extracellular substrates. This must be quickly reestablished by poorly understood cytoskeleton remodeling mechanisms that prevent detachment from epithelia, while ensuring the successful completion of cytokinesis. Here we show that the microtubule end-binding (EB) proteins EB1 and EB3 play temporally distinct roles throughout cell division. Whereas EB1 was involved in spindle orientation before anaphase, EB3 was required for stabilization of focal adhesions and coordinated daughter cell spreading during mitotic exit. Additionally, EB3 promoted midbody microtubule stability and, consequently, midbody stabilization necessary for efficient cytokinesis. Importantly, daughter cell adhesion and cytokinesis completion were spatially regulated by distinct states of EB3 phosphorylation on serine 176 by Aurora B. This EB3 phosphorylation was enriched at the midbody and shown to control cortical microtubule growth. These findings uncover differential roles of EB proteins and explain the importance of an Aurora B phosphorylation gradient for the spatiotemporal regulation of microtubule function during mitotic exit and cytokinesis.

Anti-migratory effect of vinflunine in endothelial and glioblastoma cells is associated with changes in EB1 C-terminal detyrosinated/tyrosinated status

We previously showed that vinflunine, a microtubule-targeting drug of the Vinca-alkaloid family exerted its anti-angiogenic/anti-migratory activities through an increase in microtubule dynamics and an inhibition of microtubule targeting to adhesion sites. Such effect was associated with a reduction of EB1 comet length at microtubule (+) ends. In this work we first showed that the pro-angiogenic vascular endothelial growth factor VEGF suppressed microtubule dynamics in living Human Umbilical Vein Endothelial Cells (HUVECs), increased EB1 comet length by 40%, and induced EB1 to bind all along the microtubules, without modifying its expression level. Such microtubule (+) end stabilization occurred close to the plasma membrane in the vicinity of focal adhesion as shown by TIRF microscopy experiments. Vinflunine completely abolished the effect of VEGF on EB1 comets. Interestingly, we found a correlation between the reduction of EB1 comet length by vinflunine and the inhibition of cell migration. By using 2D gel electrophoresis we demonstrated for the first time that EB1 underwent several post-translational modifications in endothelial and tumor cells. Particularly, the C-terminal EEY sequence was poorly detectable in control and VEGF-treated HUVECs suggesting the existence of a non-tyrosinated form of EB1. By using specific antibodies that specifically recognized and discriminated the native tyrosinated form of EB1 and a putative C-terminal detyrosinated form, we showed that a detyrosinated form of EB1 exists in HUVECs and tumor cells. Interestingly, vinflunine decreased the level of the detyrosinated form and increased the native tyrosinated form of EB1. Using 3-L-Nitrotyrosine incorporation experiments, we concluded that the EB1 C-terminal modifications result from a detyrosination/retyrosination cycle as described for tubulin.

Altogether, our results show that vinflunine inhibits endothelial cell migration through an alteration of EB1 comet length and EB1 detyrosination/retyrosination cycle.

Regulation of tumor angiogenesis by the microtubule-binding protein CLIP-170

Angiogenesis, the expansion of preexisting blood vessels, is a complex process required for tumor growth and metastasis. Although current antiangiogenic strategies have shown promising results in several cancer types, identification of additional antiangiogenic targets is required to improve the therapeutic response. Herein, we show that the microtubule-binding protein CLIP-170 (cytoplasmic linker protein of 170 kDa) is highly expressed in breast tumor samples and correlates positively with blood vessel density. Depletion of CLIP-170 significantly impaired vascular endothelial tube formation and sprouting in vitro and inhibited breast tumor growth in mice by decreasing tumor vascularization. Our data further show that CLIP-170 is important for the migration but not the proliferation of vascular endothelial cells. In addition, CLIP-170 promotes the polarization of endothelial cells in response to the angiogenic stimulus. These findings thus demonstrate a critical role for CLIP-170 in tumor angiogenesis and suggest its potential as a novel antiangiogenic target.

Nek7 kinase accelerates microtubule dynamic instability

The NIMA-related kinases (NRK or Nek) are emerging as conserved and crucial regulators of mitosis and cilia formation. The microtubule (MT) network has long been suspected as a major target of the Neks. However, the underlying mechanism remains unclear. Using the PlusTipTracker software, recently developed by the Danuser group, we followed the consequences of alterations in Nek7 levels on MT dynamic instability. siRNA-mediated downregulation of Nek7 in HeLa cells resulted in lower speeds of MT growth and catastrophe, reduction of the relative time spent in catastrophe, and considerably lowered the overall MT dynamicity. Co-expression of Nek7 with the siRNA treatment rescued the MT phenotypes, while ectopic overexpression of Nek7 yielded inverse characteristics compared to Nek7 downregulation. MT dynamics in mouse embryonic fibroblasts derived from targeted null mutants for Nek7 recapitulated the siRNA downregulation phenotypes. Precise MT dynamic instability is critical for accurate shaping of the mitotic spindle and for cilium formation, and higher MT dynamicity is associated with tumorigenicity. Thus, our results can supply a mechanistic explanation for Nek involvement in these processes.

Spectraplakins promote microtubule-mediated axonal growth by functioning as structural microtubule-associated proteins and EB1-dependent +TIPs (tip interacting proteins)

The correct outgrowth of axons is essential for the development and regeneration of nervous systems. Axon growth is primarily driven by microtubules. Key regulators of microtubules in this context are the spectraplakins, a family of evolutionarily conserved actin-microtubule linkers. Loss of function of the mouse spectraplakin ACF7 or of its close Drosophila homolog Short stop/Shot similarly cause severe axon shortening and microtubule disorganization. How spectraplakins perform these functions is not known. Here we show that axonal growth-promoting roles of Shot require interaction with EB1 (End binding protein) at polymerizing plus ends of microtubules. We show that binding of Shot to EB1 requires SxIP motifs in Shot's C-terminal tail (Ctail), mutations of these motifs abolish Shot functions in axonal growth, loss of EB1 function phenocopies Shot loss, and genetic interaction studies reveal strong functional links between Shot and EB1 in axonal growth and microtubule organization. In addition, we report that Shot localizes along microtubule shafts and stabilizes them against pharmacologically induced depolymerization. This function is EB1-independent but requires net positive charges within Ctail which essentially contribute to the microtubule shaft association of Shot. Therefore, spectraplakins are true members of two important classes of neuronal microtubule regulating proteins: +TIPs (tip interacting proteins; plus end regulators) and structural MAPs (microtubule-associated proteins). From our data we deduce a model that relates the different features of the spectraplakin C terminus to the two functions of Shot during axonal growth.

Move in for the kill: motile microtubule regulators

The stereotypical function of kinesin superfamily motors is to transport cargo along microtubules. However, some kinesins also shape the microtubule track by regulating microtubule assembly and disassembly. Recent work has shown that the kinesin-8 family of motors emerge as key regulators of cellular microtubule length. The studied kinesin-8s are highly processive motors that walk towards the microtubule plus-end. Once at plus-ends, they have complex effects on polymer dynamics; kinesin-8s either destabilize or stabilize microtubules, depending on the context. This review focuses on the mechanisms underlying kinesin-8-microtubule interactions and microtubule length control. We compare and contrast kinesin-8s with the other major microtubule-regulating kinesins (kinesin-4 and kinesin-13), to survey the current understanding of the diverse ways that kinesins control microtubule dynamics.

Phosphoregulation of STIM1 leads to exclusion of the endoplasmic reticulum from the mitotic spindle

The endoplasmic reticulum (ER) undergoes significant reorganization between interphase and mitosis, but the underlying mechanisms are unknown. Stromal interaction molecule 1 (STIM1) is an ER Ca(2+) sensor that activates store-operated Ca(2+) entry (SOCE) and also functions in ER morphogenesis through its interaction with the microtubule +TIP protein end binding 1 (EB1). We previously demonstrated that phosphorylation of STIM1 during mitosis suppresses SOCE. We now show that STIM1 phosphorylation is a major regulatory mechanism that excludes ER from the mitotic spindle. In mitotic HeLa cells, the ER forms concentric sheets largely excluded from the mitotic spindle. We show that STIM1 dissociates from EB1 in mitosis and localizes to the concentric ER sheets. However, a nonphosphorylatable STIM1 mutant (STIM1(10A)) colocalized extensively with EB1 and drove ER mislocalization by pulling ER tubules into the spindle. This effect was rescued by mutating the EB1 interaction site of STIM1(10A), demonstrating that aberrant association of STIM1(10A) with EB1 is responsible for the ER mislocalization. A STIM1 phosphomimetic exhibited significantly impaired +TIP tracking in interphase but was ineffective at inhibiting SOCE, suggesting different mechanisms of regulation of these two STIM1 functions by phosphorylation. Thus, ER spindle exclusion and ER-dependent Ca(2+) signaling during mitosis require multimodal STIM1 regulation by phosphorylation.

Neurofilaments are flexible polymers that often fold and unfold, but they move in a fully extended configuration

Time-lapse imaging of neurofilaments in axons of cultured nerve cells has demonstrated that these cytoskeletal polymers move along microtubule tracks in both anterograde and retrograde directions, powered by microtubule motors. The filaments exhibit short bouts of rapid intermittent movement interrupted by prolonged pauses, and the average velocity is slow because they spend most of their time pausing. Here, we show that axonal neurofilaments are also very flexible and frequently exhibit complex and dynamic folding and unfolding behaviors while they are pausing. Remarkably, however, when the filaments move in a sustained manner, we find that they always adopt an unfolded, that is, fully extended configuration, and this applies to movement in both anterograde and retrograde directions. Given the flexibility of neurofilament polymers and the apparent ease with which they can fold back on themselves, the fact that they move in a fully extended configuration suggests that moving neurofilaments may be pulled from their leading end. Thus, we speculate that motors may bind to the leading ends of neurofilaments polymers during both anterograde and retrograde motion.

Analyses of dynein heavy chain mutations reveal complex interactions between dynein motor domains and cellular dynein functions

Cytoplasmic dynein transports cargoes for a variety of crucial cellular functions. However, since dynein is essential in most eukaryotic organisms, the in-depth study of the cellular function of dynein via genetic analysis of dynein mutations has not been practical. Here, we identify and characterize 34 different dynein heavy chain mutations using a genetic screen of the ascomycete fungus Neurospora crassa, in which dynein is nonessential. Interestingly, our studies show that these mutations segregate into five different classes based on the in vivo localization of the mutated dynein motors. Furthermore, we have determined that the different classes of dynein mutations alter vesicle trafficking, microtubule organization, and nuclear distribution in distinct ways and require dynactin to different extents. In addition, biochemical analyses of dynein from one mutant strain show a strong correlation between its in vitro biochemical properties and the aberrant intracellular function of that altered dynein. When the mutations were mapped to the published dynein crystal structure, we found that the three-dimensional structural locations of the heavy chain mutations were linked to particular classes of altered dynein functions observed in cells. Together, our data indicate that the five classes of dynein mutations represent the entrapment of dynein at five separate points in the dynein mechanochemical and transport cycles. We have developed N. crassa as a model system where we can dissect the complexities of dynein structure, function, and interaction with other proteins with genetic, biochemical, and cell biological studies.

Microtubule-severing enzymes at the cutting edge

ATP-dependent severing of microtubules was first reported in Xenopus laevis egg extracts in 1991. Two years later this observation led to the purification of the first known microtubule-severing enzyme, katanin. Katanin homologs have now been identified throughout the animal kingdom and in plants. Moreover, members of two closely related enzyme subfamilies, spastin and fidgetin, have been found to sever microtubules and might act alongside katanins in some contexts (Roll-Mecak and McNally, 2010; Yu et al., 2008; Zhang et al., 2007). Over the past few years, it has become clear that microtubule-severing enzymes contribute to a wide range of cellular activities including mitosis and meiosis, morphogenesis, cilia biogenesis and disassembly, and migration. Thus, this group of enzymes is revealing itself to be among the most important of the microtubule regulators. This Commentary focuses on our growing understanding of how microtubule-severing enzymes contribute to the organization and dynamics of diverse microtubule arrays, as well as the structural and biophysical characteristics that afford them the unique capacity to catalyze the removal of tubulin from the interior microtubule lattice. Our goal is to provide a broader perspective, focusing on a limited number of particularly informative, representative and/or timely findings.

Microtubules cut loose at the cell cortex

The ability of the microtubule cytoskeleton to rapidly and locally reorganize itself in response to intra- and extracellular signals is essential to its wide range of functions. A site of tightly regulated microtubule dynamics--and the major interface between the microtubule cytoskeleton and the extracellular environment--is the cell cortex, where the selective stabilization and destabilization of microtubule plus-ends is required for normal cell division, morphogenesis and migration. In a recent study, we found that the cortex of Drosophila S2 and D17 cells is coated with the microtubule severing enzyme and plus-end depolymerase, Kat-60, which actively suppresses microtubule growth and stability along the cell edge. We have proposed that cortical Kat-60 functions by uncapping plus-ends, thereby activating another microtubule depolymerase, KLP10A, preloaded onto the end. The localized destruction of microtubule plus-ends at a specific cortical could feed into larger regulatory pathways, such as those in control of the actin cytoskeleton, to influence cell polarization and motility.

Molecular motors in cargo trafficking and synapse assembly

Every production process, be it cellular or industrial, depends on a constant supply of energy and resources. Synapses, specialized junctions in the central nervous system through which neurons signal to each other, are no exception to this rule. In order to form new synapses and alter the strength of synaptic transmission, neurons need a regulatory mechanism to deliver and remove synaptic proteins at synaptic sites. Neurons make use of active transport driven by molecular motor proteins to move synaptic cargo over either microtubules (kinesin, dynein) or actin filaments (myosin) to their specific site of action. These mechanisms are crucial for the initial establishment of synaptic specializations during synaptogenesis and for activity-dependent changes in synaptic strength during plasticity. In this chapter, we address the organization of the neuronal cytoskeleton, focus on synaptic cargo transport activities that operate in axons and dendrites, and discuss the spatial and temporal regulation of motor protein-based transport.

Mechanisms underlying the dual-mode regulation of microtubule dynamics by Kip3/kinesin-8

The kinesin-8 family of microtubule motors plays a critical role in microtubule length control in cells. These motors have complex effects on microtubule dynamics: they destabilize growing microtubules yet stabilize shrinking microtubules. The budding yeast kinesin-8, Kip3, accumulates on plus ends of growing but not shrinking microtubules. Here we identify an essential role of the tail domain of Kip3 in mediating both its destabilizing and its stabilizing activities. The Kip3 tail promotes Kip3's accumulation at the plus ends and facilitates the destabilizing effect of Kip3. However, the Kip3 tail also inhibits microtubule shrinkage and is required for promoting microtubule rescue by Kip3. These effects of the tail domain are likely to be mediated by the tubulin- and microtubule-binding activities that we describe. We propose a concentration-dependent model for the coordination of the destabilizing and stabilizing activities of Kip3 and discuss its relevance to cellular microtubule organization.

Regulation of microtubule dynamics by kinesins

The simple mechanistic and functional division of the kinesin family into either active translocators or non-motile microtubule depolymerases was initially appropriate but is now proving increasingly unhelpful, given evidence that several translocase kinesins can affect microtubule dynamics, whilst non-translocase kinesins can promote microtubule assembly and depolymerisation. Such multi-role kinesins act either directly on microtubule dynamics, by interaction with microtubules and tubulin, or indirectly, through the transport of other factors along the lattice to the microtubule tip. Here I review recent progress on the mechanisms and roles of these translocase kinesins.

Regulation of cell migration by dynamic microtubules

Microtubules define the architecture and internal organization of cells by positioning organelles and activities, as well as by supporting cell shape and mechanics. One of the major functions of microtubules is the control of polarized cell motility. In order to support the asymmetry of polarized cells, microtubules have to be organized asymmetrically themselves. Asymmetry in microtubule distribution and stability is regulated by multiple molecular factors, most of which are microtubule-associated proteins that locally control microtubule nucleation and dynamics. At the same time, the dynamic state of microtubules is key to the regulatory mechanisms by which microtubules regulate cell polarity, modulate cell adhesion and control force-production by the actin cytoskeleton. Here, we propose that even small alterations in microtubule dynamics can influence cell migration via several different microtubule-dependent pathways. We discuss regulatory factors, potential feedback mechanisms due to functional microtubule-actin crosstalk and implications for cancer cell motility.