Generation of noncentrosomal microtubule arrays

CAMSAPs add to the growing microtubule minus-end story

Free microtubule minus ends, found in many differentiated cells, contribute to polarized motility. Work from Jiang et al. (2014) in this issue of Developmental Cell shows how mammalian CAMSAP proteins stabilize minus ends, providing a key piece to the puzzle of how these minus ends are formed and stabilized.

An EB1-kinesin complex is sufficient to steer microtubule growth in vitro

Proper microtubule polarity underlies overall neuronal polarity, but mechanisms for maintaining microtubule polarity are not well understood. Previous live imaging in Drosophila dendritic arborization neurons showed that while microtubules are uniformly plus-end out in axons, dendrites possess uniformly minus-end-out microtubules [1]. Thus, maintaining uniform microtubule polarity in dendrites requires that growing microtubule plus ends entering branch points be actively directed toward the cell body. A model was proposed in which EB1 tracks the plus ends of microtubules growing into a branch and an associated kinesin-2 motor walks along a static microtubule to steer the plus end toward the cell body. However, the fast plus-end binding dynamics of EB1 [2-5] appear to be at odds with this proposed mechanical function. To test this model in vitro, we reconstituted the system by artificially dimerizing EB1 to kinesin, growing microtubules from immobilized seeds, and imaging encounters between growing microtubule plus ends and static microtubules. Consistent with in vivo observations, the EB1-kinesin complex actively steered growing microtubules. Thus, EB1 kinetics and mechanics are sufficient to bend microtubules for several seconds. Other kinesins also demonstrated this activity, suggesting this is a general mechanism for organizing and maintaining proper microtubule polarity in cells.

KIFC3 promotes mitotic progression and integrity of the central spindle in cytokinesis

Kinesin-14 motor proteins play a variety of roles during metaphase and anaphase. However, it is not known whether members of this family of motors also participate in the dramatic changes in mitotic spindle organization during the transition from telophase to cytokinesis. We have identified the minus-end-directed motor, KIFC3, as an important contributor to central bridge morphology at this stage. KIFC3's unique motor-dependent localization at the central bridge allows it to congress microtubules, promoting efficient progress through cytokinesis. Conversely, when KIFC3 function is perturbed, abscission is delayed, and the central bridge is both widened and extended. Examination of KIFC3 on growing microtubules in interphase indicates that it caps microtubules released from the centrosome, both in the region of the centrosome and in the cell periphery. In line with other kinesin-14 family members, KIFC3 may guide free microtubules to their destination at the bridge and/or may slide and crosslink central bridge microtubules in order to stage the cells for abscission.

An atypical tubulin kinase mediates stress-induced microtubule depolymerization in Arabidopsis

BACKGROUND

As sessile organisms, plants adapt to adverse environmental conditions by quickly adjusting cell physiology and metabolism. Transient depolymerization of interphase microtubules is triggered by various acute stresses and biotic interactions with pathogenic organisms. Although rapid remodeling of plant microtubule arrays in response to external stresses is an intriguing phenomenon, the underlying molecular mechanisms and the advantages of this response to plant performance are poorly understood.

RESULTS

A domain with weak homology to the slime mold actin-fragmin kinase in the Arabidopsis mitogen-activated protein kinase phosphatase PROPYZAMIDE-HYPERSENSITIVE 1 (PHS1) is a Mn2+-dependent kinase. This atypical kinase domain phosphorylates Thr349 of α-tubulin at the longitudinal interdimer interface, thereby generating a polymerization-incompetent isoform, and effectively depolymerizes microtubule arrays when ectopically expressed in plant or animal cells. The intrinsic tubulin kinase activity is normally suppressed by the phosphatase activity of PHS1 but is unmasked immediately after osmotic stress. In the phs1 null mutant, stress-induced microtubule depolymerization does not occur.

CONCLUSIONS

The rapid and reversible modification of tubulin subunits by PHS1-mediated phosphorylation enables dynamic remodeling of the plant microtubule cytoskeleton in response to external stimuli. Suppression of the potent tubulin kinase activity by the juxtaposed phosphatase domain tightly controls this stress-activated microtubule regulator.

Dystonin/BPAG1 promotes plus-end-directed transport of herpes simplex virus 1 capsids on microtubules during entry

During infection by herpes simplex virus 1 (HSV-1), the viral capsid is transported around the cytoplasm along the microtubule (MT) network. Although molecular motors have been implicated in this process, the composition of the molecular machinery required for efficient directional transport is unknown. We previously showed that dystonin (BPAG1) is recruited to HSV-1 capsids by the capsid-bound tegument protein pUL37 to promote efficient cytoplasmic transport of capsids during egress. Dystonin is a cytoskeleton cross-linker which localizes at MT plus ends and has roles in retrograde and anterograde transport in neurons. In this study, we investigated the role of dystonin during the entry stages of HSV-1 infection. Because of the way in which the MT network is organized, capsids are required to change their direction of motion along the MTs as they travel from the point of entry to the nucleus, where replication takes place. Thus, capsids first travel to the centrosome (the principal microtubule organizing center) by minus-end-directed transport and then switch polarity and travel to the nucleus by plus-end-directed transport. We observed that transport of capsids toward the centrosome was slowed, but not blocked, by dystonin depletion. However, transport of capsids away from the centrosome was significantly impaired, causing them to accumulate in the vicinity of the centrosome and reducing the numbers reaching the nucleus. We conclude that, during entry of HSV-1, dystonin has a specific role in plus-ended transport of capsids from the centrosome to the nucleus.

HSPB1 facilitates the formation of non-centrosomal microtubules

The remodeling capacity of microtubules (MT) is essential for their proper function. In mammals, MTs are predominantly formed at the centrosome, but can also originate from non-centrosomal sites, a process that is still poorly understood. We here show that the small heat shock protein HSPB1 plays a role in the control of non-centrosomal MT formation. The HSPB1 expression level regulates the balance between centrosomal and non-centrosomal MTs. The HSPB1 protein can be detected specifically at sites of de novo forming non-centrosomal MTs, while it is absent from the centrosomes. In addition, we show that HSPB1 binds preferentially to the lattice of newly formed MTs in vitro, suggesting that its function occurs by stabilizing MT seeds. Our findings open new avenues for the understanding of the role of HSPB1 in the development, maintenance and protection of cells with specialized non-centrosomal MT arrays.

MAP65/Ase1 promote microtubule flexibility

Microtubules (MTs) are dynamic cytoskeletal elements involved in numerous cellular processes. Although they are highly rigid polymers with a persistence length of 1-8 mm, they may exhibit a curved shape at a scale of few micrometers within cells, depending on their biological functions. However, how MT flexural rigidity in cells is regulated remains poorly understood. Here we ask whether MT-associated proteins (MAPs) could locally control the mechanical properties of MTs. We show that two major cross-linkers of the conserved MAP65/PRC1/Ase1 family drastically decrease MT rigidity. Their MT-binding domain mediates this effect. Remarkably, the softening effect of MAP65 observed on single MTs is maintained when MTs are cross-linked. By reconstituting physical collisions between growing MTs/MT bundles, we further show that the decrease in MT stiffness induced by MAP65 proteins is responsible for the sharp bending deformations observed in cells when they coalign at a steep angle to create bundles. Taken together, these data provide new insights into how MAP65, by modifying MT mechanical properties, may regulate the formation of complex MT arrays.

A non-canonical mode of microtubule organization operates throughout pre-implantation development in mouse

In dividing animal cells, the centrosome, comprising centrioles and surrounding pericentriolar-material (PCM), is the major interphase microtubule-organizing center (MTOC), arranging a polarized array of microtubules (MTs) that controls cellular architecture. The mouse embryo is a unique setting for investigating the role of centrosomes in MT organization, since the early embryo is acentrosomal, and centrosomes emerge de novo during early cleavages. Here we use embryos from a GFP::CETN2 transgenic mouse to observe the emergence of centrosomes and centrioles in embryos, and show that unfocused acentriolar centrosomes first form in morulae (~16-32-cell stage) and become focused at the blastocyst stage (~64-128 cells) concomitant with the emergence of centrioles. We then used high-resolution microscopy and dynamic tracking of MT growth events in live embryos to examine the impact of centrosome emergence upon interphase MT dynamics. We report that pre-implantation mouse embryos of all stages employ a non-canonical mode of MT organization that generates a complex array of randomly oriented MTs that are preferentially nucleated adjacent to nuclear and plasmalemmal membranes and cell-cell interfaces. Surprisingly, however, cells of the early embryo continue to employ this mode of interphase MT organization even after the emergence of centrosomes. Centrosomes are found at MT-sparse sites and have no detectable impact upon interphase MT dynamics. To our knowledge, the early embryo is unique among proliferating cells in adopting an acentrosomal mode of MT organization despite the presence of centrosomes, revealing that the transition to a canonical mode of interphase MT organization remains incomplete prior to implantation.

Differing effects of herpes simplex virus 1 and pseudorabies virus infections on centrosomal function

Efficient intracellular transport of the capsid of alphaherpesviruses, such as herpes simplex virus 1 (HSV-1), is known to be dependent upon the microtubule (MT) network. Typically, the MT network radiates from an MT-organizing center (MTOC), which is, in most cases, the centrosome. During herpesvirus egress, it has been assumed that capsids travel first from the nucleus to the centrosome and then from the centrosome to the site of envelopment. Here we report that the centrosome is no longer a primary MTOC in HSV-1-infected cells, but it retains this function in cells infected by another alphaherpesvirus, pseudorabies virus (PrV). As a result, MTs formed at late times after infection with PrV grow from a major, centralized MTOC, while those formed after HSV-1 infection arise from dispersed locations in the cytoplasm, indicating the presence of alternative and minor MTOCs. Thus, loss of the principal MT nucleating center in cells following HSV-1 infection raises questions about the mechanism of HSV-1 capsid egress. It is possible that, rather than passing via the centrosome, capsids may travel directly to the site of envelopment after exiting the nucleus. We suggest that, in HSV-1-infected cells, the disruption of centrosomal functions triggers reorganization of the MT network to favor noncentrosomal MTs and promote efficient viral spread.

Jun N-terminal kinase maintains tissue integrity during cell rearrangement in the gut

Tissue elongation is a fundamental morphogenetic process that generates the proper anatomical topology of the body plan and vital organs. In many elongating embryonic structures, tissue lengthening is driven by Rho family GTPase-mediated cell rearrangement. During this dynamic process, the mechanisms that modulate intercellular adhesion to allow individual cells to change position without compromising structural integrity are not well understood. In vertebrates, Jun N-terminal kinase (JNK) is also required for tissue elongation, but the precise cellular role of JNK in this context has remained elusive. Here, we show that JNK activity is indispensable for the rearrangement of endoderm cells that underlies the elongation of the Xenopus gut tube. Whereas Rho kinase is necessary to induce cell intercalation and remodel adhesive contacts, we have found that JNK is required to maintain cell-cell adhesion and establish parallel microtubule arrays; without JNK activity, the reorganizing endoderm dissociates. Depleting polymerized microtubules phenocopies this effect of JNK inhibition on endoderm morphogenesis, consistent with a model in which JNK regulates microtubule architecture to preserve adhesive contacts between rearranging gut cells. Thus, in contrast to Rho kinase, which generates actomyosin-based tension and cell movement, JNK signaling is required to establish microtubule stability and maintain tissue cohesion; both factors are required to achieve proper cell rearrangement and gut extension. This model of gut elongation has implications not only for the etiology of digestive tract defects, but sheds new light on the means by which intra- and intercellular forces are balanced to promote topological change, while preserving structural integrity, in numerous morphogenetic contexts.

Cortical microtubule arrays are initiated from a nonrandom prepattern driven by atypical microtubule initiation

The ordered arrangement of cortical microtubules in growing plant cells is essential for anisotropic cell expansion and, hence, for plant morphogenesis. These arrays are dismantled when the microtubule cytoskeleton is rearranged during mitosis and reassembled following completion of cytokinesis. The reassembly of the cortical array has often been considered as initiating from a state of randomness, from which order arises at least partly through self-organizing mechanisms. However, some studies have shown evidence for ordering at early stages of array assembly. To investigate how cortical arrays are initiated in higher plant cells, we performed live-cell imaging studies of cortical array assembly in tobacco (Nicotiana tabacum) Bright Yellow-2 cells after cytokinesis and drug-induced disassembly. We found that cortical arrays in both cases did not initiate randomly but with a significant overrepresentation of microtubules at diagonal angles with respect to the cell axis, which coincides with the predominant orientation of the microtubules before their disappearance from the cell cortex in preprophase. In Arabidopsis (Arabidopsis thaliana) root cells, recovery from drug-induced disassembly was also nonrandom and correlated with the organization of the previous array, although no diagonal bias was observed in these cells. Surprisingly, during initiation, only about one-half of the new microtubules were nucleated from locations marked by green fluorescent protein-γ-tubulin complex protein2-tagged γ-nucleation complexes (γ-tubulin ring complex), therefore indicating that a large proportion of early polymers was initiated by a noncanonical mechanism not involving γ-tubulin ring complex. Simulation studies indicate that the high rate of noncanonical initiation of new microtubules has the potential to accelerate the rate of array repopulation.

Non-centrosomal microtubules regulate F-actin organization through the suppression of GEF-H1 activity

Animal cells contain two populations of microtubules: one radiating from the centrosome and the other growing from non-centrosomal sites. Whether or not they have differing roles in cellular architecture and function remains not fully understood. The cytoplasmic protein Nezha (also known as CAMSAP3) stabilizes non-centrosomal microtubules by attaching to their minus ends. Here, we found that depletion of CAMSAP3 in HeLa cells resulted in a relative increase in centrosomal microtubules, and this change was accompanied by accelerated actin stress fiber formation. In these cells, RhoA activity was upregulated, and the soluble fraction of GEF-H1, a RhoGEF whose activity is inhibited by binding to microtubules, increased, explaining why stress fiber formation was promoted. We further found that CAMSAP3 depletion led to an increase in detyrosinated microtubules, and these microtubules did not interact with GEF-H1. These findings suggest that CAMSAP3-anchored non-centrosomal microtubules capture GEF-H1 more efficiently than other microtubules do and that a balance between these microtubules is important to maintain proper actin organization.

MAP65 coordinate microtubule growth during bundle formation

Microtubules (MTs) are highly dynamical structures that play a crucial role in cell physiology. In cooperation with microtubule-associated proteins (MAPs), MTs form bundles endowing cells with specific mechanisms to control their shape or generate forces. Whether the dynamics of MTs is affected by the lateral connections that MAPs make between MTs during bundle formation is still under debate. Using in vitro reconstitution of MT bundling, we analyzed the dynamics of MT bundles generated by two plant MAP65 (MAP65-1/4), MAP65-1 being the plant ortholog of vertebrate PRC1 and yeast Ase1. MAP65-1/4 limit the amplitude of MT bundle depolymerization and increase the elongation phases. The subsequent sustained elongation of bundles is governed by the coordination of MT growth, so that MT ends come in close vicinity. We develop a model based on the assumption that both MAP65-1/4 block MT depolymerization. Model simulations reveal that rescue frequencies are higher between parallel than between anti-parallel MTs. In consequence the polarity of bundled MTs by MAP65 controls the amplitude of bundle's growth. Our results illustrate how MAP-induced MT-bundling, which is finely tuned by MT polarity, robustly coordinates MT elongation within bundles.

Golgi outposts shape dendrite morphology by functioning as sites of acentrosomal microtubule nucleation in neurons

Microtubule nucleation is essential for proper establishment and maintenance of axons and dendrites. Centrosomes, the primary site of nucleation in most cells, lose their function as microtubule organizing centers during neuronal development. How neurons generate acentrosomal microtubules remains unclear. Drosophila dendritic arborization (da) neurons lack centrosomes and therefore provide a model system to study acentrosomal microtubule nucleation. Here, we investigate the origin of microtubules within the elaborate dendritic arbor of class IV da neurons. Using a combination of in vivo and in vitro techniques, we find that Golgi outposts can directly nucleate microtubules throughout the arbor. This acentrosomal nucleation requires gamma-tubulin and CP309, the Drosophila homolog of AKAP450, and contributes to the complex microtubule organization within the arbor and dendrite branch growth and stability. Together, these results identify a direct mechanism for acentrosomal microtubule nucleation within neurons and reveal a function for Golgi outposts in this process.

A novel PAR-1-binding protein, MTCL1, plays critical roles in organizing microtubules in polarizing epithelial cells

The establishment of epithelial polarity is tightly linked to the dramatic reorganization of microtubules (MTs) from a radial array to a vertical alignment of non-centrosomal MT bundles along the lateral membrane and a meshwork under the apical and basal membranes. However, little is known about the underlying molecular mechanism of this polarity-dependent MT remodeling. The evolutionarily conserved cell polarity-regulating kinase PAR-1, whose activity is essential for maintaining the dynamic state of MTs, plays indispensable roles to promote this process. Here, we identify a novel PAR-1-binding protein, named MTCL1 (Microtubule crosslinking factor 1), which crosslinks MTs through its N-terminal MT-binding region and subsequent coiled-coil motifs. MTCL1 colocalized with the apicobasal MT bundles in epithelial cells, and its knockdown impaired the development of these MT bundles and the epithelial cell specific columnar shape. Rescue experiments revealed that the N-terminal MT-binding region was indispensable for restoring these defects of the knockdown cells. MT regrowth assays indicated that MTCL1 was not required for the initial radial growth of MTs from the apical centrosome, but was essential for the accumulation of non-centrosomal MTs to the sublateral regions. Interestingly, MTCL1 recruited a subpopulation of PAR-1b to the apicobasal MT bundles, and its interaction with PAR-1b was required for MTCL1-dependent development of the apicobasal MT bundles. These results suggest that MTCL1 mediates the epithelial cell-specific reorganization of non-centrosomal MTs through its MT-crosslinking activity, and cooperates with PAR-1b to maintain the correct temporal balance between dynamic and stable MTs within the apicobasal MT bundles.

Herpesvirus tegument protein pUL37 interacts with dystonin/BPAG1 to promote capsid transport on microtubules during egress

Herpes simplex virus 1 (HSV-1) is a neurotropic virus that travels long distances through cells using the microtubule network. Its 125-nm-diameter capsid is a large cargo which efficiently recruits molecular motors for movement. Upon entry, capsids reach the centrosome by minus-end-directed transport. From there, they are believed to reach the nucleus by plus-end-directed transport. Plus-end-directed transport is also important during egress, when capsids leave the nucleus to reach the site of envelopment in the cytoplasm. Although capsid interactions with dynein and kinesins have been described in vitro, the actual composition of the cellular machinery recruited by herpesviruses for capsid transport in infected cells remains unknown. Here, we identify the spectraplakin protein, dystonin/BPAG1, an important cytoskeleton cross-linker involved in microtubule-based transport, as a binding partner of the HSV-1 protein pUL37, which has been implicated in capsid transport. Viral replication is delayed in dystonin-depleted cells, and, using video microscopy of living infected cells, we show that dystonin depletion strongly inhibits capsid movement in the cytoplasm during egress. This study provides new insights into the cellular requirements for HSV-1 capsid transport and identifies dystonin as a nonmotor protein part of the transport machinery.

Myomegalin is necessary for the formation of centrosomal and Golgi-derived microtubules

The generation of cellular microtubules is initiated at specific sites such as the centrosome and the Golgi apparatus that contain nucleation complexes rich in γ-tubulin. The microtubule growing plus-ends are stabilized by plus-end tracking proteins (+TIPs), mainly EB1 and associated proteins. Myomegalin was identified as a centrosome/Golgi protein associated with cyclic nucleotide phosphodiesterase. We show here that Myomegalin exists as several isoforms. We characterize two of them. One isoform, CM-MMG, harbors a conserved domain (CM1), recently described as a nucleation activator, and is related to a family of γ-tubulin binding proteins, which includes Drosophila centrosomin. It localizes at the centrosome and at the cis-Golgi in an AKAP450-dependent manner. It recruits γ-tubulin nucleating complexes and promotes microtubule nucleation. The second isoform, EB-MMG, is devoid of CM1 domain and has a unique N-terminus with potential EB1-binding sites. It localizes at the cis-Golgi and can localize to microtubule plus-ends. EB-MMG binds EB1 and affects its loading on microtubules and microtubule growth. Depletion of Myomegalin by small interfering RNA delays microtubule growth from the centrosome and Golgi apparatus, and decreases directional migration of RPE1 cells. In conclusion, the Myomegalin gene encodes different isoforms that regulate microtubules. At least two of these have different roles, demonstrating a previously unknown mechanism to control microtubules in vertebrate cells.

Rotational motion during three-dimensional morphogenesis of mammary epithelial acini relates to laminin matrix assembly

Our understanding of the mechanisms by which ducts and lobules develop is derived from model organisms and three-dimensional (3D) cell culture models wherein mammalian epithelial cells undergo morphogenesis to form multicellular spheres with a hollow central lumen. However, the mechanophysical properties associated with epithelial morphogenesis are poorly understood. We performed multidimensional live-cell imaging analysis to track the morphogenetic process starting from a single cell to the development of a multicellular, spherical structure composed of polarized epithelial cells surrounding a hollow lumen. We report that in addition to actively maintaining apicobasal polarity, the structures underwent rotational motions at rates of 15-20 μm/h and the structures rotated 360° every 4 h during the early phase of morphogenesis. Rotational motion was independent of the cell cycle, but was blocked by loss of the epithelial polarity proteins Scribble or Pard3, or by inhibition of dynein-based microtubule motors. Interestingly, none of the structures derived from human cancer underwent rotational motion. We found a direct relationship between rotational motion and assembly of endogenous basement membrane matrix around the 3D structures, and that structures that failed to rotate were defective in weaving exogenous laminin matrix. Dissolution of basement membrane around mature, nonrotating acini restored rotational movement and the ability to assemble exogenous laminin. Thus, coordinated rotational movement is a unique mechanophysical process observed during normal 3D morphogenesis that regulates laminin matrix assembly and is lost in cancer-derived epithelial cells.

Nezha/CAMSAP3 and CAMSAP2 cooperate in epithelial-specific organization of noncentrosomal microtubules

Major microtubules in epithelial cells are not anchored to the centrosome, in contrast to the centrosomal radiation of microtubules in other cell types. It remains to be discovered how these epithelial microtubules are generated and stabilized at noncentrosomal sites. Here, we found that Nezha [also known as calmodulin-regulated spectrin-associated protein 3 (CAMSAP3)] and its related protein, CAMSAP2, cooperate in organization of noncentrosomal microtubules. These two CAMSAP molecules coclustered at the minus ends of noncentrosomal microtubules and thereby stabilized them. Depletion of CAMSAPs caused a marked reduction of microtubules with polymerizing plus ends, concomitantly inducing the growth of microtubules from the centrosome. In CAMSAP-depleted cells, early endosomes and the Golgi apparatus exhibited irregular distributions. These effects of CAMSAP depletion were maximized when both CAMSAPs were removed. These findings suggest that CAMSAP2 and -3 work together to maintain noncentrosomal microtubules, suppressing the microtubule-organizing ability of the centrosome, and that the network of CAMSAP-anchored microtubules is important for proper organelle assembly.

Cracking the elusive alignment hypothesis: the microtubule-cellulose synthase nexus unraveled

Directed plant cell growth is governed by deposition and alterations of cell wall components under turgor pressure. A key regulatory element of anisotropic growth, and hence cell shape, is the directional deposition of cellulose microfibrils. The microfibrils are synthesized by plasma membrane-located cellulose synthase complexes that co-align with and move along cortical microtubules. That the parallel relation between cortical microtubules and extracellular microfibrils is causal has been named the alignment hypothesis. Three recent studies revealed that the previously identified pom2 mutant codes for a large cellulose synthases interacting (CSI1) protein which also binds cortical microtubules. This review summarizes these findings, provides structure-function models and discusses the inferred mechanisms in the context of plant growth.

Modulation of Golgi-associated microtubule nucleation throughout the cell cycle

A microtubule (MT) subpopulation that emanates from Golgi membrane has been recently shown to comprise a significant part of MT network in interphase cells. In this study, we address whether Golgi membrane, which is being extensively remodeled throughout the cell cycle, retains its ability to nucleate MTs at diverse cell cycle stages. Live cell imaging and immunofluorescence microscopy reveals that Golgi-derived MTs form at multiple stages of the cell cycle, including G(1), G(2), and distinct phases of mitosis. However, the capacity of Golgi to nucleate MTs in mitosis is strongly down-regulated as compared with interphase, indicating that this property is cell cycle regulated. We demonstrate that Golgi-derived MTs are indispensable for efficient Golgi assembly in telophase, and speculate that these noncentrosomal MTs may hold specific functions at other cell cycle stages.

Rear-side localization of the centrosome in migrating neuroblastoma Neuro-2a cells and its roles in process elongation

Axon elongation is usually performed by the migration of growth cones that leave axons. Axon microtubules are generated by enhanced polymerization of tubulin in the growth cones. Some kinds of neurons like cerebellar granule cells, however, generate axons as a result of migration of the cell body leaving axons at the rear. The mechanism to generate microtubules during such growth cone-independent elongation of axons is not well understood. To establish an experimental model to study this mechanism, we cultured neuroblastoma (Neuro-2a) cells on substrates that facilitate cell migration. When cultured on laminin-treated substrate, cells migrated actively and left processes at the rear. We investigated the role of the centrosome in this process formation. The centrosomes were always located at the base of the processes, i.e., at the rear side of the migrating cell body. Close observation of cytoskeletons revealed microtubules limited around the centrosomes, but concentrated at the periphery of the cells or within the processes. Microtubule regrowth experiments showed the ability of the centrosomes to nucleate microtubules. We thus examined the role of microtubule release from the centrosomes, by knocking down the expression of spastin, a microtubule-severing enzyme. Introducing siRNA for spastin into Neuro-2a cells reduced both the migration speed and the length of the processes. Taken together, Neuro-2a cells on laminin proved useful as a model to study the alternative type of axon elongation in which cell migration leaves axons at the rear. This model provided evidence for the involvement of microtubule release from centrosomes in the mechanisms for this type of process elongation.

Cep70 promotes microtubule assembly in vitro by increasing microtubule elongation

Microtubules are dynamic cytoskeletal polymers present in all eukaryotic cells. In animal cells, they are organized by the centrosome, the major microtubule-organizing center. Many centrosomal proteins act coordinately to modulate microtubule assembly and organization. Our previous work has shown that Cep70, a novel centrosomal protein regulates microtubule assembly and organization in mammalian cells. However, the molecular details remain to be investigated. In this study, we investigated the molecular mechanism of how Cep70 regulates microtubule assembly using purified proteins. Our data showed that Cep70 increased the microtubule length without affecting the microtubule number in the purified system. These results demonstrate that Cep70 could directly regulate microtubule assembly by promoting microtubule elongation instead of microtubule nucleation.

A role for the centrosome and PAR-3 in the hand-off of MTOC function during epithelial polarization

BACKGROUND

The centrosome is the major microtubule organizing center (MTOC) in dividing cells and in many postmitotic, differentiated cells. In other cell types, however, MTOC function is reassigned from the centrosome to noncentrosomal sites. Here, we analyze how MTOC function is reassigned to the apical membrane of C. elegans intestinal cells.

RESULTS

After the terminal intestinal cell division, the centrosomes and nuclei move near the future apical membranes, and the postmitotic centrosomes lose all, or nearly all, of their associated microtubules. We show that microtubule-nucleating proteins such as γ-tubulin and CeGrip-1 that are centrosome components in dividing cells become localized to the apical membrane, which becomes highly enriched in microtubules. Our results suggest that centrosomes are critical to specify the apical membrane as the new MTOC. First, γ-tubulin appears to redistribute directly from the migrating centrosome onto the lateral then apical membrane. Second, γ-tubulin fails to accumulate apically in wild-type cells following laser ablation of the centrosome. We show that centrosomes localize apically by first moving toward lateral foci of the conserved polarity proteins PAR-3 and PAR-6 and then move together with these foci toward the future apical surface. Embryos lacking PAR-3 fail to localize their centrosomes apically and have aberrant localization of γ-tubulin and CeGrip-1.

CONCLUSIONS

These data suggest that PAR proteins contribute to apical polarity in part by determining centrosome position and that the reassignment of MTOC function from centrosomes to the apical membrane is associated with a physical hand-off of nucleators of microtubule assembly.

Computer simulation and mathematical models of the noncentrosomal plant cortical microtubule cytoskeleton

There is rising interest in modeling the noncentrosomal cortical microtubule cytoskeleton of plant cells, particularly its organization into ordered arrays and the mechanisms that facilitate this organization. In this review, we discuss quantitative models of this highly complex and dynamic structure both at a cellular and molecular level. We report differences in methodologies and assumptions of different models as well as their controversial results. Our review provides insights for future studies to resolve these controversies, in addition to underlining the common results between various models. We also highlight the need to compare the results from simulation and mathematical models with quantitative data from biological experiments in order to test the validity of the models and to further improve them. It is our hope that this review will serve to provide guidelines for how to combine quantitative and experimental techniques to develop higher-level models of the plant cytoskeleton in the future.

The where, when and how of microtubule nucleation – one ring to rule them all

The function of microtubules depends on their arrangement into highly ordered arrays. Spatio-temporal control over the formation of new microtubules and regulation of their properties are central to the organization of these arrays. The nucleation of new microtubules requires γ-tubulin, an essential protein that assembles into multi-subunit complexes and is found in all eukaryotic organisms. However, the way in which γ-tubulin complexes are regulated and how this affects nucleation and, potentially, microtubule behavior, is poorly understood. γ-tubulin has been found in complexes of various sizes but several lines of evidence suggest that only large, ring-shaped complexes function as efficient microtubule nucleators. Human γ-tubulin ring complexes (γTuRCs) are composed of γ-tubulin and the γ-tubulin complex components (GCPs) 2, 3, 4, 5 and 6, which are members of a conserved protein family. Recent work has identified additional unrelated γTuRC subunits, as well as a large number of more transient γTuRC interactors. In this Commentary, we discuss the regulation of γTuRC-dependent microtubule nucleation as a key mechanism of microtubule organization. Specifically, we focus on the regulatory roles of the γTuRC subunits and interactors and present an overview of other mechanisms that regulate γTuRC-dependent microtubule nucleation and organization.

Microtubules are organized independently of the centrosome in Drosophila neurons

Background

The best-studied arrangement of microtubules is that organized by the centrosome, a cloud of microtubule nucleating and anchoring proteins is clustered around centrioles. However, noncentrosomal microtubule arrays are common in many differentiated cells, including neurons. Although microtubules are not anchored at neuronal centrosomes, it remains unclear whether the centrosome plays a role in organizing neuronal microtubules. We use Drosophila as a model system to determine whether centrosomal microtubule nucleation is important in mature neurons.

Results

In developing and mature neurons, centrioles were not surrounded by the core nucleation protein γ-tubulin. This suggests that the centrioles do not organize functional centrosomes in Drosophila neurons in vivo. Consistent with this idea, centriole position was not correlated with a specific region of the cell body in neurons, and growing microtubules did not cluster around the centriole, even after axon severing when the number of growing plus ends is dramatically increased. To determine whether the centrosome was required for microtubule organization in mature neurons, we used two approaches. First, we used DSas-4 centriole duplication mutants. In these mutants, centrioles were present in many larval sensory neurons, but they were not fully functional. Despite reduced centriole function, microtubule orientation was normal in axons and dendrites. Second, we used laser ablation to eliminate the centriole, and again found that microtubule polarity in axons and dendrites was normal, even 3 days after treatment.

Conclusion

We conclude that the centrosome is not a major site of microtubule nucleation in Drosophila neurons, and is not required for maintenance of neuronal microtubule organization in these cells.

Interplay between microtubule dynamics and intracellular organization

Microtubules are hollow tubes essential for many cellular functions such as cell polarization and migration, intracellular trafficking and cell division. They are polarized polymers composed of α and β tubulin that are, in most cells, nucleated at the centrosome at the center of the cell. Microtubule plus-ends are oriented towards the periphery of the cell and explore the cytoplasm in a very dynamic manner. Microtubule alternate between phases of growth and shrinkage in a manner described as dynamic instability. Their dynamics is highly regulated by multiple factors: tubulin post-translational modifications such as detyrosination or acetylation, and microtubule-associated proteins, among them the plus-tip tracking proteins. This regulation is necessary for microtubule functions in the cell. In this review, we will focus on the role of microtubules in intracellular organization. After an overview of the mechanisms responsible for the regulation of microtubule dynamics, the major roles of microtubules dynamics in organelle positioning and organization in interphase cells will be discussed. Conversely, the role of certain organelles, like the nucleus and the Golgi apparatus as microtubule organizing centers will be reviewed. We will then consider the role of microtubules in the establishment and maintenance of cell polarity using few examples of cell polarization: epithelial cells, neurons and migrating cells. In these cells, the microtubule network is reorganized and undergoes specific and local regulation events; microtubules also participate in the intracellular reorganization of different organelles to ensure proper cell differentiation.

Shaping microtubules into diverse patterns: molecular connections for setting up both ends

Microtubules serve as rails for intracellular trafficking and their appropriate organization is critical for the generation of cell polarity, which is a foundation of cell differentiation, tissue morphogenesis, ontogenesis and the maintenance of homeostasis. The microtubule array is not just a static railway network; it undergoes repeated collapse and reassembly in diverse patterns during cell morphogenesis. In the last decade much progress has been made toward understanding the molecular mechanisms governing complex microtubule patterning. This review first revisits the basic principle of microtubule dynamics, and then provides an overview of how microtubules are arranged in highly shaped and functional patterns in cells changing their morphology by factors controlling the fate of microtubule ends.

Complex cellular functions of the von Hippel-Lindau tumor suppressor gene: insights from model organisms

The von Hippel-Lindau tumor suppressor gene (VHL) has attracted intensive interest not only because its mutations predispose carriers to devastating tumors, but also because it is involved in oxygen sensing under physiological conditions. VHL loss-of-function mutations result in organ-specific tumors, such as hemangioblastoma of the central nervous system and renal cell carcinoma, both untreatable with conventional chemotherapies. The VHL protein is best known as an E3 ubiquitin ligase that targets hypoxia-inducible factor-α (HIF-α), but many diverse, non-canonical cellular functions have also been assigned to VHL, mainly based on studies in cell culture systems. As such, although the HIF-dependent role of VHL is critical, the full spectrum of pathophysiological functions of VHL is still unresolved. Such understanding requires careful cross-referencing with physiologically relevant experimental models. Studies in model systems, such as Caenorhabditis elegans, Drosophila, zebrafish and mouse have provided critical in vivo confirmation of the VHL-HIF pathway, and verification of potentially important cellular functions including microtubule stabilization and epithelial morphogenesis. More recently, animal models have also suggested systemic roles of VHL in hematopoiesis, metabolic homeostasis and inflammation. In this review, the studies performed in model organisms will be summarized and placed in context with existing clinical and in vitro data.

Orientation and function of the nuclear-centrosomal axis during cell migration

A hallmark of polarity in most migrating cells is the orientation of the nuclear centrosomal (NC) axis relative to the front-back cellular axis. Here, we review 'effector functions' associated with the NC axis during cell migration. We highlight recent research that has demonstrated that the orientation of the NC axis depends upon the coordinated, but separate positioning of the nucleus and the centrosome. We stress the importance of environmental factors such as cell-cell contacts and substrate topology for NC axis orientation. Finally, we summarize tests of the significance of this axis for cell migration and disease.

Lis1 is essential for cortical microtubule organization and desmosome stability in the epidermis

Desmoplakin recruits the centrosomal protein Lis1 to the epidermal cell cortex, where it regulates cortical microtubule organization and desmosome stability.

Desmosomes are cell–cell adhesion structures that integrate cytoskeletal networks. In addition to binding intermediate filaments, the desmosomal protein desmoplakin (DP) regulates microtubule reorganization in the epidermis. In this paper, we identify a specific subset of centrosomal proteins that are recruited to the cell cortex by DP upon epidermal differentiation. These include Lis1 and Ndel1, which are centrosomal proteins that regulate microtubule organization and anchoring in other cell types. This recruitment was mediated by a region of DP specific to a single isoform, DPI. Furthermore, we demonstrate that the epidermal-specific loss of Lis1 results in dramatic defects in microtubule reorganization. Lis1 ablation also causes desmosomal defects, characterized by decreased levels of desmosomal components, decreased attachment of keratin filaments, and increased turnover of desmosomal proteins at the cell cortex. This contributes to loss of epidermal barrier activity, resulting in completely penetrant perinatal lethality. This work reveals essential desmosome-associated components that control cortical microtubule organization and unexpected roles for centrosomal proteins in epidermal function.

Alphaherpesviruses and the cytoskeleton in neuronal infections

Following infection of exposed peripheral tissues, neurotropic alphaherpesviruses invade nerve endings and deposit their DNA genomes into the nuclei of neurons resident in ganglia of the peripheral nervous system. The end result of these events is the establishment of a life-long latent infection. Neuroinvasion typically requires efficient viral transmission through a polarized epithelium followed by long-distance transport through the viscous axoplasm. These events are mediated by the recruitment of the cellular microtubule motor proteins to the intracellular viral particle and by alterations to the cytoskeletal architecture. The focus of this review is the interplay between neurotropic herpesviruses and the cytoskeleton.

Differential distribution of posttranslationally modified microtubules in osteoclasts

The differential distribution of microtubules in osteoclasts in culture was examined by using antibodies against acetylated, tyrosinated, or detyrosinated tubulins. Tyrosinated tubulin was found throughout the cytoplasmic microtubules in all cells examined. An expanding protrusion that contained tyrosinated tubulin but none of the detyrosinated or acetylated form was seen in the immature osteoclasts. Detyrosinated or acetylated tubulin was detectable in the peripheral cytoplasm of the mature osteoclasts displaying the loss of the expanding protrusion. Although most of the microtubules were derived from the centrosome, noncentrosomal microtubules were distributed in the expanding protrusion, which was predominantly positive for tyrosinated tubulin. By tracing single microtubules, the authors found that their growing ends were always rich in tyrosinated tubulin subunits. End binding protein 1 bound preferentially to the microtubule ends. Both acetylated and tyrosinated microtubules were shown to be closely associated with podosomes. Microtubules appeared to grow over or into the podosomes; in addition, the growing ends of single microtubules could be observed to target the podosomes. Moreover, a microtubule-associated histone deacetylase 6 was localized in the podosomes of the osteoclast. On the basis of these results, the authors conclude that posttranslational modifications of microtubules may correlate with characteristic changes in podosome dynamics in osteoclasts.

The posttranslational modification of tubulin undergoes a switch from detyrosination to acetylation as epithelial cells become polarized

Polarity leads to a shift in tubulin modification and microtubule organization. In unpolarized epithelial cells, detyrosinated microtubules point to the spreading edge, but in polarized cells, acetylated microtubules point to the apical domain. In both cases the modified microtubules are oriented to support cargo transport to areas of high need.

Tubulin posttranslational modifications (PTMs) have been suggested to provide navigational cues for molecular motors to deliver cargo to spatially segregated subcellular domains, but the molecular details of this process remain unclear. Here we show that in Madin-Darby Canine Kidney (MDCK) epithelial cells, microtubules express several tubulin PTMs. These modifications, however, are not coordinated, and cells have multiple subpopulations of microtubules that are marked by different combinations of PTMs. Furthermore these subpopulations show differential sensitivity to both drug- and cold-induced depolymerization, suggesting that they are functionally different as well. The composition and distribution of modified microtubules change as cells undergo the morphogenesis associated with polarization. Two-dimensionally polarized spreading cells have more detyrosinated microtubules that are oriented toward the leading edge, but three-dimensionally polarized cells have more acetylated microtubules that are oriented toward the apical domain. These data suggest that the transition from 2D polarity to 3D polarity involves both a reorganization of the microtubule cytoskeleton and a change in tubulin PTMs. However, in both 2D polarized and 3D polarized cells, the modified microtubules are oriented to support vectorial cargo transport to areas of high need.

Genetic interaction of centrosomin and bazooka in apical domain regulation in Drosophila photoreceptor

Background

Cell polarity genes including Crumbs (Crb) and Par complexes are essential for controlling photoreceptor morphogenesis. Among the Crb and Par complexes, Bazooka (Baz, Par-3 homolog) acts as a nodal component for other cell polarity proteins. Therefore, finding other genes interacting with Baz will help us to understand the cell polarity genes' role in photoreceptor morphogenesis.

Methodology/Principal Findings

Here, we have found a genetic interaction between baz and centrosomin (cnn). Cnn is a core protein for centrosome which is a major microtubule-organizing center. We analyzed the effect of the cnn mutation on developing eyes to determine its role in photoreceptor morphogenesis. We found that Cnn is dispensable for retinal differentiation in eye imaginal discs during the larval stage. However, photoreceptors deficient in Cnn display dramatic morphogenesis defects including the mislocalization of Crumbs (Crb) and Bazooka (Baz) during mid-stage pupal eye development, suggesting that Cnn is specifically required for photoreceptor morphogenesis during pupal eye development. This role of Cnn in apical domain modulation was further supported by Cnn's gain-of-function phenotype. Cnn overexpression in photoreceptors caused the expansion of the apical Crb membrane domain, Baz and adherens junctions (AJs).

Conclusions/Significance

These results strongly suggest that the interaction of Baz and Cnn is essential for apical domain and AJ modulation during photoreceptor morphogenesis, but not for the initial photoreceptor differentiation in the Drosophila photoreceptor.

Ankyrin-B interactions with spectrin and dynactin-4 are required for dystrophin-based protection of skeletal muscle from exercise injury

Costameres are cellular sites of mechanotransduction in heart and skeletal muscle where dystrophin and its membrane-spanning partner dystroglycan distribute intracellular contractile forces into the surrounding extracellular matrix. Resolution of a functional costamere interactome is still limited but likely to be critical for understanding forms of muscular dystrophy and cardiomyopathy. Dystrophin binds a set of membrane-associated proteins (the dystrophin-glycoprotein complex) as well as γ-actin and microtubules and also is required to align sarcolemmal microtubules with costameres. Ankyrin-B binds to dystrophin, dynactin-4, and microtubules and is required for sarcolemmal association of these proteins as well as dystroglycan. We report here that ankyrin-B interactions with β2 spectrin and dynactin-4 are required for localization of dystrophin, dystroglycan, and microtubules at costameres as well as protection of muscle from exercise-induced injury. Knockdown of dynactin-4 in adult mouse skeletal muscle phenocopied depletion of ankyrin-B and resulted in loss of sarcolemmal dystrophin, dystroglycan, and microtubules. Moreover, mutations of ankyrin-B and of dynactin-4 that selectively impaired binary interactions between these proteins resulted in loss of their costamere-localizing activity and increased muscle fiber fragility as a result of loss of costamere-associated dystrophin and dystroglycan. In addition, costamere-association of dynactin-4 did not require dystrophin but did depend on β2 spectrin and ankyrin-B, whereas costamere association of ankyrin-B required β2 spectrin. Together, these results are consistent with a functional hierarchy beginning with β2 spectrin recruitment of ankyrin-B to costameres. Ankyrin-B then interacts with dynactin-4 and dystrophin, whereas dynactin-4 collaborates with dystrophin in coordinating costamere-aligned microtubules.

Directed microtubule growth, +TIPs, and kinesin-2 are required for uniform microtubule polarity in dendrites

BACKGROUND

in many differentiated cells, microtubules are organized into polarized noncentrosomal arrays, yet few mechanisms that control these arrays have been identified. For example, mechanisms that maintain microtubule polarity in the face of constant remodeling by dynamic instability are not known. Drosophila neurons contain uniform-polarity minus-end-out microtubules in dendrites, which are often highly branched. Because undirected microtubule growth through dendrite branch points jeopardizes uniform microtubule polarity, we have used this system to understand how cells can maintain dynamic arrays of polarized microtubules.

RESULTS

we find that growing microtubules navigate dendrite branch points by turning the same way, toward the cell body, 98% of the time and that growing microtubules track along stable microtubules toward their plus ends. Using RNAi and genetic approaches, we show that kinesin-2, and the +TIPS EB1 and APC, are required for uniform dendrite microtubule polarity. Moreover, the protein-protein interactions and localization of Apc2-GFP and Apc-RFP to branch points suggests that these proteins work together at dendrite branches. The functional importance of this polarity mechanism is demonstrated by the failure of neurons with reduced kinesin-2 to regenerate an axon from a dendrite.

CONCLUSIONS

we conclude that microtubule growth is directed at dendrite branch points and that kinesin-2, APC, and EB1 are likely to play a role in this process. We propose that kinesin-2 is recruited to growing microtubules by +TIPS and that the motor protein steers growing microtubules at branch points. This represents a newly discovered mechanism for maintaining polarized arrays of microtubules.

Cytoplasmic gamma-tubulin complex from brain contains nonerythroid spectrin

The newer member of the tubulin superfamily, gamma-tubulin, is known to mediate microtubule nucleation from the centrosome of eukaryotic cells with the aid of some other proteins. The major amount of gamma-tubulin is believed to be located in the centrosome before the onset of mitotic division. However, a considerable amount has been found in the cytoplasm in the form of a complex whose function is not well known. Microtubules are most abundant in brain tissues and brain microtubules have been extensively used in many in vitro studies. Thus, it is relevant to use brain tissue to characterize cytoplasmic gamma-tubulin complex. Here we show that cytoplasmic gamma-tubulin in brain tissues exists as a ring complex as in other tissues. Interestingly, along with the common members of the gamma-TuRC reported from several tissues and species, the purified brain cytoplasmic complex contains some high molecular weight proteins including alpha and beta nonerythroid spectrin which are not found in other tissues. Immunohistochemical studies of brain tissue sections also show the co-localization of gamma-tubulin and spectrin. The possible implications have been discussed.