Regional regulation of microtubule dynamics in polarized, motile cells

Rasal1 regulates calcium dependent neuronal maturation by modifying microtubule dynamics

BACKGROUND

Rasal1 is a Ras GTPase-activating protein which contains C2 domains necessary for dynamic membrane association following intracellular calcium elevation. Membrane-bound Rasal1 inactivates Ras signaling through its RasGAP activity, and through such mechanisms has been implicated in regulating various cellular functions in the context of tumors. Although highly expressed in the brain, the contribution of Rasal1 to neuronal development and function has yet to be explored.

RESULTS

We examined the contributions of Rasal1 to neuronal development in primary culture of hippocampal neurons through modulation of Rasal1 expression using molecular tools. Fixed and live cell imaging demonstrate diffuse expression of Rasal1 throughout the cell soma, dendrites and axon which localizes to the neuronal plasma membrane in response to intracellular calcium fluctuation. Pull-down and co-immunoprecipitation demonstrate direct interaction of Rasal1 with PKC, tubulin, and CaMKII. Consequently, Rasal1 is found to stabilize microtubules, through post-translational modification of tubulin, and accordingly inhibit dendritic outgrowth and branching. Through imaging, molecular, and electrophysiological techniques Rasal1 is shown to promote NMDA-mediated synaptic activity and CaMKII phosphorylation.

CONCLUSIONS

Rasal1 functions in two separate roles in neuronal development; calcium regulated neurite outgrowth and the promotion of NMDA receptor-mediated postsynaptic events which may be mediated both by interaction with direct binding partners or calcium-dependent regulation of down-stream pathways. Importantly, the outlined molecular mechanisms of Rasal1 may contribute notably to normal neuronal development and synapse formation.

Microtubule detyrosination drives symmetry breaking to polarize cells for directed cell migration

To initiate directed movement, cells must become polarized, establishing a protrusive leading edge and a contractile trailing edge. This symmetry-breaking process involves reorganization of cytoskeleton and asymmetric distribution of regulatory molecules. However, what triggers and maintains this asymmetry during cell migration remains largely elusive. Here, we established a micropatterning-based 1D motility assay to investigate the molecular basis of symmetry breaking required for directed cell migration. We show that microtubule (MT) detyrosination drives cell polarization by directing kinesin-1-based transport of the adenomatous polyposis coli (APC) protein to cortical sites. This is essential for the formation of cell's leading edge during 1D and 3D cell migration. These data, combined with biophysical modeling, unveil a key role for MT detyrosination in the generation of a positive feedback loop linking MT dynamics and kinesin-1-based transport. Thus, symmetry breaking during cell polarization relies on a feedback loop driven by MT detyrosination that supports directed cell migration.

Factor quinolinone inhibitors alter cell morphology and motility by destabilizing interphase microtubules

Factor quinolinone inhibitors are promising anti-cancer compounds, initially characterized as specific inhibitors of the oncogenic transcription factor LSF (TFCP2). These compounds exert anti-proliferative activity at least in part by disrupting mitotic spindles. Herein, we report additional interphase consequences of the initial lead compound, FQI1, in two telomerase immortalized cell lines. Within minutes of FQI1 addition, the microtubule network is disrupted, resulting in a substantial, although not complete, depletion of microtubules as evidenced both by microtubule sedimentation assays and microscopy. Surprisingly, this microtubule breakdown is quickly followed by an increase in tubulin acetylation in the remaining microtubules. The sudden breakdown and partial depolymerization of the microtubule network precedes FQI1-induced morphological changes. These involve rapid reduction of cell spreading of interphase fetal hepatocytes and increase in circularity of retinal pigment epithelial cells. Microtubule depolymerization gives rise to FH-B cell compaction, as pretreatment with taxol prevents this morphological change. Finally, FQI1 decreases the rate and range of locomotion of interphase cells, supporting an impact of FQI1-induced microtubule breakdown on cell motility. Taken together, our results show that FQI1 interferes with microtubule-associated functions in interphase, specifically cell morphology and motility.

Spatial regulation of MCAK promotes cell polarization and focal adhesion turnover to drive robust cell migration

The asymmetric distribution of microtubule (MT) dynamics in migrating cells is important for cell polarization, yet the underlying regulatory mechanisms remain underexplored. Here, we addressed this question by studying the role of the MT depolymerase, MCAK (mitotic centromere-associated kinesin), in the highly persistent migration of RPE-1 cells. MCAK knockdown leads to slowed migration and poor directional movement. Fixed and live cell imaging revealed that MCAK knockdown results in excessive membrane ruffling as well as defects in cell polarization and the maintenance of a major protrusive front. Additionally, loss of MCAK increases the lifetime of focal adhesions by decreasing their disassembly rate. These functions correlate with a spatial distribution of MCAK activity, wherein activity is higher in the trailing edge of cells compared with the leading edge. Overexpression of Rac1 has a dominant effect over MCAK activity, placing it downstream of or in a parallel pathway to MCAK function in migration. Together, our data support a model in which the polarized distribution of MCAK activity and subsequent differential regulation of MT dynamics contribute to cell polarity, centrosome positioning, and focal adhesion dynamics, which all help facilitate robust directional migration.

Persistent growth of microtubules at low density

Microtubules (MTs) often form a polarized array with minus ends anchored at the centrosome and plus ends extended toward the cell margins. Plus ends display behavior known as dynamic instability—transitions between rapid shortening and slow growth. It is known that dynamic instability is regulated locally to ensure entry of MTs into nascent areas of the cytoplasm, but details of this regulation remain largely unknown. Here, we test an alternative hypothesis for the local regulation of MT behavior. We used microsurgery to isolate a portion of peripheral cytoplasm from MTs growing from the centrosome, creating cytoplasmic areas locally depleted of MTs. We found that in sparsely populated areas MT plus ends persistently grew or paused but never shortened. In contrast, plus ends that entered regions of cytoplasm densely populated with MTs frequently transitioned to shortening. Persistent growth of MTs in sparsely populated areas could not be explained by a local increase in concentration of free tubulin subunits or elevation of Rac1 activity proposed to enhance MT growth at the cell leading edge during locomotion. These observations suggest the existence of a MT density–dependent mechanism regulating MT dynamics that determines dynamic instability of MTs in densely populated areas of the cytoplasm and persistent growth in sparsely populated areas.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

Analysis of Microtubule Dynamics Heterogeneity in Cell Culture

Microtubules (MTs) are dynamic components of the cytoskeleton playing an important role in a large number of cell functions. Individual MTs in living cells undergo stochastic switching between alternate states of growth, shortening and attenuated phase, a phenomenon known as tempered dynamic instability. Dynamic instability of MTs is usually analyzed by labeling MTs with +TIPs, namely, EB proteins. Tracking of +TIP trajectories allows analyzing MT growth in cells with a different density of MTs. Numerous labs now use +TIP to track growing MTs in a variety of cell cultures. However, heterogeneity of MT dynamics is usually underestimated, and rather small sampling for the description of dynamic instability parameters is often used. The strategy described in this chapter is the method for repetitive quantitative analysis of MT growth rate within the same cell that allows minimization of the variation in MT dynamics measurement. We show that variability in MT dynamics within a cell when using repeated measurements is significantly less than between different cells in the same chamber. This approach allows better estimation of the heterogeneity of cells' responses to different treatments. To compare the effects of different MT inhibitors, the protocol using normalized values for MT dynamics and repetitive measurements for each cell is employed. This chapter provides detailed methods for analysis of MT dynamics in tissue cultures. We describe protocols for imaging MT dynamics by fluorescent microscopy, contrast enhancement technique, and MT dynamics analysis using triple color-coded display based on sequential subtraction analysis.

CLASP1 regulates endothelial cell branching morphology and directed migration

Endothelial cell (EC) branching is critically dependent upon the dynamic nature of the microtubule (MT) cytoskeleton. Extracellular matrix (ECM) mechanosensing is a prominent mechanism by which cytoskeletal reorganization is achieved; yet how ECM-induced signaling is able to target cytoskeletal reorganization intracellularly to facilitate productive EC branching morphogenesis is not known. Here, we tested the hypothesis that the composition and density of the ECM drive the regulation of MT growth dynamics in ECs by targeting the MT stabilizing protein, cytoplasmic linker associated protein 1 (CLASP1). High-resolution fluorescent microscopy coupled with computational image analysis reveal that CLASP1 promotes slow MT growth on glass ECMs and promotes short-lived MT growth on high-density collagen-I and fibronectin ECMs. Within EC branches, engagement of either high-density collagen-I or high-density fibronectin ECMs results in reduced MT growth speeds, while CLASP1-dependent effects on MT dynamics promotes elevated numbers of short, branched protrusions that guide persistent and directed EC migration.

Summary: CLASP1 modulates microtubule dynamics with sub-cellular specificity in response to extracellular matrix density and composition. CLASP1 effects on microtubules promote short, branched protrusions that guide persistent and directional EC migration. This article has an associated First Person interview with the first author of the paper as part of the supplementary information.

Self-repair promotes microtubule rescue

The dynamic instability of microtubules is characterised by slow growth phases stochastically interrupted by rapid depolymerisations called catastrophes. Rescue events can arrest the depolymerisation and restore microtubule elongation. However the origin of these rescue events remain unexplained. Here we show that microtubule lattice self-repair, in structurally damaged sites, is responsible for the rescue of microtubule growth. Tubulin photo-conversion in cells revealed that free tubulin dimers can incorporate along the shafts of microtubules, especially in regions where microtubules cross each other, form bundles or become bent due to mechanical constraints. These incorporation sites appeared to act as effective rescue sites ensuring microtubule rejuvenation. By securing damaged microtubule growth, the self-repair process supports a mechanosensitive growth by specifically promoting microtubule assembly in regions where they are subjected to physical constraints.

Identification of microtubule growth deceleration and its regulation by conserved and novel proteins

Live imaging of microtubule dynamics in Caenorhabditis elegans muscle cells reveals a novel microtubule behavior characterized by an abrupt change in growth rate, named “microtubule growth deceleration.” The conserved protein ZYG-9^TOGp and two novel ORFs, cylc-1 and cylc-2, are involved in the regulation of this novel microtubule behavior.

Microtubules (MTs) are cytoskeletal polymers that participate in diverse cellular functions, including cell division, intracellular trafficking, and templating of cilia and flagella. MTs undergo dynamic instability, alternating between growth and shortening via catastrophe and rescue events. The rates and frequencies of MT dynamic parameters appear to be characteristic for a given cell type. We recently reported that all MT dynamic parameters vary throughout differentiation of a smooth muscle cell type in intact Caenorhabditis elegans. Here we describe local differences in MT dynamics and a novel MT behavior: an abrupt change in growth rate (deceleration) of single MTs occurring in the cell periphery of these cells. MT deceleration occurs where there is a decrease in local soluble tubulin concentration at the cell periphery. This local regulation of tubulin concentration and MT deceleration are dependent on two novel homologues of human cylicin. These novel ORFs, which we name cylc-1 and -2, share sequence homology with stathmins and encode small, very basic proteins containing several KKD/E repeats. The TOG domain–containing protein ZYG-9^TOGp is responsible for the faster polymerization rate within the cell body. Thus we have defined two contributors to the molecular regulation for this novel MT behavior.

Centrosome nucleates numerous ephemeral microtubules and only few of them participate in the radial array

It is generally accepted that long microtubules (MTs) grow from the centrosome with their minus ends anchored there and plus ends directed towards cell membrane. However, recent findings show this scheme to be an oversimplification. To further analyze the relationship between the centrosome and the MT array we undertook a detailed study on the MTs growing from the centrosome after microinjection of Cy3 labeled tubulin and transfection of cells with EB1-GFP. To evaluate MTs around the centrosome two approaches were used: path photobleaching across the centrosome area (Komarova et al., ) and sequential image subtraction analysis (Vorobjev et al., ). We show that about 50% of MTs had been nucleated at the centrosome are short-living: their mean length was 1.8 ± 0.8 μm and their life span - 7 ± 2 s. MTs initiated from the centrosome also rarely reach cell margin, since their elongation was limited and growth after shortening (rescue) was rare. After initial growth all MTs associated with the centrosome converted to pause or shortening. After pause MTs associated with the centrosome mainly depolymerized via the plus end shortening. Stability of the minus ends of cytoplasmic MTs was the same as for centrosomal ones. We conclude that in fibroblasts (1) the default behavior of free MTs in the cell interior is biased dynamic instability (i.e., random walk of the plus ends with significant positive drift); (2) MTs born at the centrosome show "dynamic instability" type behavior with no boundary; and (3) that the extended radial array is formed predominantly by MTs not associated with the centrosome.

Robust intestinal homeostasis relies on cellular plasticity in enteroblasts mediated by miR-8-Escargot switch

The intestinal epithelium is remarkably robust despite perturbations and demand uncertainty. Here, we investigate the basis of such robustness using novel tracing methods that allow simultaneously capturing the dynamics of stem and committed progenitor cells (called enteroblasts) and intestinal cell turnover with spatiotemporal resolution. We found that intestinal stem cells (ISCs) divide "ahead" of demand during Drosophila midgut homeostasis. Their newborn enteroblasts, on the other hand, take on a highly polarized shape, acquire invasive properties and motility. They extend long membrane protrusions that make cell-cell contact with mature cells, while exercising a capacity to delay their final differentiation until a local demand materializes. This cellular plasticity is mechanistically linked to the epithelial-mesenchymal transition (EMT) programme mediated by escargot, a snail family gene. Activation of the conserved microRNA miR-8/miR-200 in "pausing" enteroblasts in response to a local cell loss promotes timely terminal differentiation via a reverse MET by antagonizing escargot. Our findings unveil that robust intestinal renewal relies on hitherto unrecognized plasticity in enteroblasts and reveal their active role in sensing and/or responding to local demand.

LINKIN, a new transmembrane protein necessary for cell adhesion

In epithelial collective migration, leader and follower cells migrate while maintaining cell–cell adhesion and tissue polarity. We have identified a conserved protein and interactors required for maintaining cell adhesion during a simple collective migration in the developing C. elegans male gonad. LINKIN is a previously uncharacterized, transmembrane protein conserved throughout Metazoa. We identified seven atypical FG–GAP domains in the extracellular domain, which potentially folds into a β-propeller structure resembling the α-integrin ligand-binding domain. C. elegans LNKN-1 localizes to the plasma membrane of all gonadal cells, with apical and lateral bias. We identified the LINKIN interactors RUVBL1, RUVBL2, and α-tubulin by using SILAC mass spectrometry on human HEK 293T cells and testing candidates for lnkn-1-like function in C. elegans male gonad. We propose that LINKIN promotes adhesion between neighboring cells through its extracellular domain and regulates microtubule dynamics through RUVBL proteins at its intracellular domain.

DOI:http://dx.doi.org/10.7554/eLife.04449.001

In animals, cells can move from one place to another to shape tissues, heal wounds, or defend against invading microbes. A cell may move alone or it may be attached to others and move as part of a group. One member of the group leads this ‘collective migration’, but it is not known how the cells are able to stick to each other and move together.

Collective migration takes place in the male gonad—the organ that makes sperm cells—in larvae of the nematode worm C. elegans. As the gonad matures, a group of cells form a simple chain that can move together. Kato et al. found that a protein called LINKIN must be present for this to happen.

LINKIN is found in the membrane that surrounds animal cells. One section of the protein—called the β-propeller—sits on the outside surface of the membrane. The structure of the β-propeller is similar to a section of another protein—called α-integrin—that also allows cells to attach, suggesting LINKIN may work in a similar way.

LINKIN is found in many animals, so Kato et al. searched for proteins that can interact with it in human cells. This search revealed three proteins that can interact with LINKIN and are required for the cells to move together. Two of the proteins control elements of the internal scaffolding of the cell: this scaffolding, which is known as the cytoskeleton, is involved in moving the cells.

The experiments suggest that LINKIN coordinates the process of binding together with the changes in the cytoskeleton that are needed to allow the cells to move as one. The next challenge is to understand how LINKIN changes the internal program of the cells to achieve this.

DOI:http://dx.doi.org/10.7554/eLife.04449.002

Microfabricated Systems and Assays for Studying the Cytoskeletal Organization, Micromechanics, and Motility Patterns of Cancerous Cells

Cell motions are driven by coordinated actions of the intracellular cytoskeleton - actin, microtubules (MTs) and substrate/focal adhesions (FAs). This coordination is altered in metastatic cancer cells resulting in deregulated and increased cellular motility. Microfabrication tools, including photolithography, micromolding, microcontact printing, wet stamping and microfluidic devices have emerged as a powerful set of experimental tools with which to probe and define the differences in cytoskeleton organization/dynamics and cell motility patterns in non-metastatic and metastatic cancer cells. In this review, we discuss four categories of microfabricated systems: (i) micropatterned substrates for studying of cell motility sub-processes (for example, MT targeting of FAs or cell polarization); (ii) systems for studying cell mechanical properties, (iii) systems for probing overall cell motility patterns within challenging geometric confines relevant to metastasis (for example, linear and ratchet geometries), and (iv) microfluidic devices that incorporate co-cultures of multiple cells types and chemical gradients to mimic in vivo intravasation/extravasation steps of metastasis. Together, these systems allow for creating controlled microenvironments that not only mimic complex soft tissues, but are also compatible with live cell high-resolution imaging and quantitative analysis of single cell behavior.

Rac1 and Aurora A regulate MCAK to polarize microtubule growth in migrating endothelial cells

A Rac1–Aurora A–MCAK signaling pathway mediates endothelial cell polarization and directional migration by promoting regional differences in microtubule dynamics in the leading and trailing cell edges.

Endothelial cells (ECs) migrate directionally during angiogenesis and wound healing by polarizing to extracellular cues to guide directional movement. EC polarization is controlled by microtubule (MT) growth dynamics, which are regulated by MT-associated proteins (MAPs) that alter MT stability. Mitotic centromere-associated kinesin (MCAK) is a MAP that promotes MT disassembly within the mitotic spindle, yet its function in regulating MT dynamics to promote EC polarity and migration has not been investigated. We used high-resolution fluorescence microscopy coupled with computational image analysis to elucidate the role of MCAK in regulating MT growth dynamics, morphology, and directional migration of ECs. Our results show that MCAK-mediated depolymerization of MTs is specifically targeted to the trailing edge of polarized wound-edge ECs. Regulation of MCAK function is dependent on Aurora A kinase, which is regionally enhanced by signaling from the small guanosine triphosphatase, Rac1. Thus, a Rac1–Aurora A–MCAK signaling pathway mediates EC polarization and directional migration by promoting regional differences in MT dynamics in the leading and trailing cell edges.

Engineered kinase activation reveals unique morphodynamic phenotypes and associated trafficking for Src family isoforms

The Src kinase family comprises nine homologous members whose distinct expression patterns and cellular distributions indicate that they have unique roles. These roles have not been determined because genetic manipulation has not produced clearly distinct phenotypes, and the kinases' homology complicates generation of specific inhibitors. Through insertion of a modified FK506 binding protein (insertable FKBP12, iFKBP) into the protein kinase isoforms Fyn, Src, Lyn, and Yes, we engineered kinase analogs that can be activated within minutes in living cells (RapR analogs). Combining our RapR analogs with computational tools for quantifying and characterizing cellular dynamics, we demonstrate that Src family isoforms produce very different phenotypes, encompassing cell spreading, polarized motility, and production of long, thin cell extensions. Activation of Src and Fyn led to patterns of kinase translocation that correlated with morphological changes in temporally distinct stages. Phenotypes were dependent on N-terminal acylation, not on Src homology 3 (SH3) and Src homology 2 (SH2) domains, and correlated with movement between a perinuclear compartment, adhesions, and the plasma membrane.

The interplay between G protein-coupled receptor kinase 2 (GRK2) and histone deacetylase 6 (HDAC6) at the crossroads of epithelial cell motility

G protein-coupled receptor kinase 2 (GRK2) is emerging as a key integrative node in cell migration control. In addition to its canonical role in the desensitization of G protein-coupled receptors involved in chemotaxis, novel recently identified GRK2 substrates and interacting partners appear to mediate the GRK2-dependent modulation of diverse molecular processes involved in motility, such as gradient sensing, cell polarity or cytoskeletal reorganization. We have recently identified an interaction between GRK2 and histone deacetylase 6 (HDAC6), a major cytoplasmic α-tubulin deacetylase involved in cell motility and adhesion. GRK2 dynamically associates with and phosphorylates HDAC6 to stimulate its α-tubulin deacetylase activity at specific cellular localizations such as the leading edge of migrating cells, thus promoting local tubulin deacetylation and enhanced motility. This GRK2-HDAC6 functional interaction may have important implications in pathological contexts related to aberrant epithelial cell migration.

Automated screening of microtubule growth dynamics identifies MARK2 as a regulator of leading edge microtubules downstream of Rac1 in migrating cells

Polarized microtubule (MT) growth in the leading edge is critical to directed cell migration, and is mediated by Rac1 GTPase. To find downstream targets of Rac1 that affect MT assembly dynamics, we performed an RNAi screen of 23 MT binding and regulatory factors and identified RNAi treatments that suppressed changes in MT dynamics induced by constitutively activated Rac1. By analyzing fluorescent EB3 dynamics with automated tracking, we found that RNAi treatments targeting p150^glued, APC2, spastin, EB1, Op18, or MARK2 blocked Rac1-mediated MT growth in lamellipodia. MARK2 was the only protein whose RNAi targeting additionally suppressed Rac1 effects on MT orientation in lamellipodia, and thus became the focus of further study. We show that GFP-MARK2 rescued effects of MARK2 depletion on MT growth lifetime and orientation, and GFP-MARK2 localized in lamellipodia in a Rac1-activity-dependent manner. In a wound-edge motility assay, MARK2-depleted cells failed to polarize their centrosomes or exhibit oriented MT growth in the leading edge, and displayed defects in directional cell migration. Thus, automated image analysis of MT assembly dynamics identified MARK2 as a target regulated downstream of Rac1 that promotes oriented MT growth in the leading edge to mediate directed cell migration.

Kinetic stabilization of microtubule dynamics by indanocine perturbs EB1 localization, induces defects in cell polarity and inhibits migration of MDA-MB-231 cells

Cell motility is an essential aspect of metastatic spread of cancer. Microtubule-targeted agents exhibit anti-metastatic properties, the underlying mechanism of which remains understudied. In this study, we have investigated the role of microtubule dynamics in migration of cancer cells using indanocine, a synthetic small molecule inhibitor of tubulin. We found that indanocine, at concentrations that did not visibly affect microtubule organization, suppressed dynamic instability of microtubules and reduced the rate of migration of highly metastatic MDA-MB-231 cells. Indanocine-treated cells were defective in lamellipodium formation and could not develop polarized morphology. The kinetic stabilization of microtubules was associated with a marked increase in their acetylation level and a perturbation in the localization of EB1, a microtubule plus end binding protein. Using standard scratch wound healing assay and immunofluorescence analysis; we found that microtubule acetylation occurred in the direction of migration in vehicle-treated cells, whereas indanocine treatment led to a global acetylation of microtubules. The results together suggested that selective stabilization of microtubules was perturbed in the presence of indanocine that possibly resulted in lack of cell polarization and a concurrent reduction in migration of cells. Moreover, microtubule stabilization by indanocine affected adhesion turnover and impaired the polarized pattern of adhesion sites in cells. Together the results indicated that the regulation of microtubule dynamics is required to coordinate cell polarization as well as adhesion asymmetry and support the hypothesis that the perturbation of microtubule dynamics by tubulin-targeted agents can be exploited to restrict the migration of tumor cells.

A role for microtubules in endothelial cell protrusion in three-dimensional matrices

BACKGROUND INFORMATION

Most cells reside in vivo in a three-dimensional (3D) environment surrounded by extracellular matrix and other neighbouring cells, conditions that are different from those found by cells cultured in vitro on two-dimensional (2D) substrata. Cell morphology and behaviour are very different under these two different conditions, but the structural basis for these differences is still not understood, especially the role of microtubules (MTs). To address this issue, we studied the early spreading behaviour of bovine aortic endothelial cells (BAECs) cultured in 3D collagen matrices and on 2D substrata, in the presence of MT-disrupting drugs.

RESULTS

We found that depolymerisation of MTs greatly reduces the ability of BAECs to form large and stable protrusions inside 3D collagen matrices, an effect that is less pronounced when the cells are cultured on 2D substrata. Colcemid-treated BAECs inside 3D matrices begin assembling protrusions and pull on the matrix, but they fail to extend those protrusions deep into the matrix. It has been previously reported that MT disruption affects Rho signalling which may result in increased cell rigidity and adhesiveness to 2D matrices. Accordingly, we demonstrate that colcemid treatment indeed leads to activation of Rho-kinase (ROCK) targets, which in turn results in activation of regulatory myosin light chains, and that blocking of ROCK mitigates some of the effects of MT disruption in cell spreading in 3D.

CONCLUSIONS

Our results show that MT depolymerisation is particularly disruptive when cells interact with pliable 3D matrices, suggesting a role for MTs and the Rho pathway in the fine-tuning of contractile and adhesive forces necessary to sustain cell motility in vivo.

plusTipTracker: Quantitative image analysis software for the measurement of microtubule dynamics

Here we introduce plusTipTracker, a Matlab-based open source software package that combines automated tracking, data analysis, and visualization tools for movies of fluorescently-labeled microtubule (MT) plus end binding proteins (+TIPs). Although +TIPs mark only phases of MT growth, the plusTipTracker software allows inference of additional MT dynamics, including phases of pause and shrinkage, by linking collinear, sequential growth tracks. The algorithm underlying the reconstruction of full MT trajectories relies on the spatially and temporally global tracking framework described in Jaqaman et al. (2008). Post-processing of track populations yields a wealth of quantitative phenotypic information about MT network architecture that can be explored using several visualization modalities and bioinformatics tools included in plusTipTracker. Graphical user interfaces enable novice Matlab users to track thousands of MTs in minutes. In this paper, we describe the algorithms used by plusTipTracker and show how the package can be used to study regional differences in the relative proportion of MT subpopulations within a single cell. The strategy of grouping +TIP growth tracks for the analysis of MT dynamics has been introduced before (Matov et al., 2010). The numerical methods and analytical functionality incorporated in plusTipTracker substantially advance this previous work in terms of flexibility and robustness. To illustrate the enhanced performance of the new software we thus compare computer-assembled +TIP-marked trajectories to manually-traced MT trajectories from the same movie used in Matov et al. (2010).

Quantification of protein group coherence and pathway assignment using functional association

Background

Genomics and proteomics experiments produce a large amount of data that are awaiting functional elucidation. An important step in analyzing such data is to identify functional units, which consist of proteins that play coherent roles to carry out the function. Importantly, functional coherence is not identical with functional similarity. For example, proteins in the same pathway may not share the same Gene Ontology (GO) terms, but they work in a coordinated fashion so that the aimed function can be performed. Thus, simply applying existing functional similarity measures might not be the best solution to identify functional units in omics data.

Results

We have designed two scores for quantifying the functional coherence by considering association of GO terms observed in two biological contexts, co-occurrences in protein annotations and co-mentions in literature in the PubMed database. The counted co-occurrences of GO terms were normalized in a similar fashion as the statistical amino acid contact potential is computed in the protein structure prediction field. We demonstrate that the developed scores can identify functionally coherent protein sets, i.e. proteins in the same pathways, co-localized proteins, and protein complexes, with statistically significant score values showing a better accuracy than existing functional similarity scores. The scores are also capable of detecting protein pairs that interact with each other. It is further shown that the functional coherence scores can accurately assign proteins to their respective pathways.

Conclusion

We have developed two scores which quantify the functional coherence of sets of proteins. The scores reflect the actual associations of GO terms observed either in protein annotations or in literature. It has been shown that they have the ability to accurately distinguish biologically relevant groups of proteins from random ones as well as a good discriminative power for detecting interacting pairs of proteins. The scores were further successfully applied for assigning proteins to pathways.

Determination of microtubule dynamic instability in living cells

The precise regulation of microtubules and their dynamics is critical for cell cycle progression, cell signaling, intracellular transport, cell polarization, and organismal development. For example, mitosis, cell migration, and axonal outgrowth all involve rapid and dramatic changes in microtubule organization and dynamics. Microtubule-associated proteins (MAPs) such as MAP2 and tau (Bunker et al., 2004; Dhamodharan and Wadsworth, 1995) and microtubule-interacting proteins such as stathmin, the kinesin MCAK, and EB1 (Cassimeris, 1999; Moore and Wordeman, 2004; Ringhoff and Cassimeris, 2009; Rusan et al., 2001) as well as numerous clinically approved or experimental anti-mitotic drugs including the taxanes, vinca alkaloids, and colchicine-like compounds modulate microtubule dynamic in cells (Jordan, 2002; Jordan and Kamath, 2007). In this chapter, we describe methods to analyze the dynamic instability of microtubules in living cells by microscopy of microinjected or expressed fluorescent tubulin, time-lapse microscopy, and analysis of time-dependent microtubule length changes.

The use of fluorescence redistribution after photobleaching for analysis of cellular microtubule dynamics

Microtubules (MTs) are highly dynamic polymers that serve as tracks for vesicular movement during interphase and as structural components of the mitotic spindle, which is used to segregate the genetic material. MT dynamics are highly regulated wherein MTs turnover differentially between interphase and mitosis. Within the mitotic spindle, there are distinct classes of MTs with different dynamic properties. To understand how cellular proteins regulate the dynamics of MTs, it is necessary to have methods to assess their turnover properties. In this chapter we present approaches to assess MT dynamics in cultured mammalian cells using fluorescence redistribution after photobleaching. We include a discussion of cell culture and imaging conditions that maintain cell viability. We also provide an extensive discussion of both data collection and analysis that are utilized to estimate the turnover dynamics of MTs.

Significance of microtubule catastrophes at focal adhesion sites

Directional cell migration requires cell polarization and asymmetric distribution of cell signaling. Focal adhesions and microtubules are two systems which are essential for these. It was shown that these two systems closely interact with each other. It is known that microtubule targeting stimulates focal adhesion dissociation. Our recent study shows that focal adhesions, in turn, specifically induce microtubule catastrophe via a biochemical mechanism. We were able to track down one of the focal adhesion proteins paxillin which is involved in this process. Paxillin phosphorylation was previously shown to be the key component in the regulation of focal adhesion assembly or disassembly. Since microtubule catastrophe dynamic differs at the leading edge and cell rear, similar to paxillin phosphorylation levels, we suggest a model connecting asymmetric distribution of focal adhesions and asymmetric distribution of microtubule catastrophes at adhesion sites as a feedback loop.

Microtubule network asymmetry in motile cells: role of Golgi-derived array

Cell migration requires polarization of the cell into the leading edge and the trailing edge. Microtubules (MTs) are indispensable for polarized cell migration in the majority of cell types. To support cell polarity, MT network has to be functionally and structurally asymmetric. How is this asymmetry achieved? In interphase cells, MTs form a dynamic system radiating from a centrosome-based MT-organizing center (MTOC) to the cell edges. Symmetry of this radial array can be broken according to four general principles. Asymmetry occurs due to differential modulation of MT dynamics, relocation of existing MTs within a cell, adding an asymmetric nucleation site, and/or repositioning of a symmetric nucleation site to one side of a cell. Combinations of these asymmetry regulation principles result in a variety of asymmetric MT networks typical for diverse motile cell types. Importantly, an asymmetric MT array is formed at a non-conventional MT nucleation site, the Golgi. Here, we emphasize the contribution of this array to the asymmetry of MT network.

Par3 and dynein associate to regulate local microtubule dynamics and centrosome orientation during migration

BACKGROUND

Centrosome orientation toward the leading edge of migrating cells depends on dynein and microtubules (MTs), as well as a number of signaling factors at the leading edge. However, centrosomes are maintained at the cell center during orientation in fibroblasts, suggesting that factors working at sites other than the leading edge may also be involved.

RESULTS

In a search for factors that function with dynein in centrosome orientation, we found that the polarity protein Par3 associated with dynein and that knockdown of Par3 inhibited centrosome orientation by disrupting the position of the centrosome at the cell center; this disrupted centrosome positioning is the same phenotype as that observed with dynein inhibition. Par3 associated with dynein through its N-terminal dimerization and PDZ1 domains and interacted specifically with dynein light intermediate chain 2 (LIC2). siRNA knockdown of LIC2, but not LIC1, or overexpression of LIC2 or the N-terminal domain of Par3, also inhibited centrosome orientation by disrupting centrosome position. In wound-edge fibroblasts, Par3 specifically localized to cell-cell contacts where it overlapped with MT ends and dynein puncta in a LIC2-dependent fashion. Live imaging showed that MTs exhibited increased pausing at cell-cell contacts compared to the leading edge and that this elevated pausing was dependent on Par3 and LIC2.

CONCLUSIONS

Par3 associates with dynein and contributes to the local regulation of MT dynamics at cell-cell contacts and proper positioning of the centrosome at the cell center. We propose that Par3 acts as a cortical factor that tethers MTs through its association with LIC2 dynein.

ROCK-regulated cytoskeletal dynamics participate in the inhibitory effect of melatonin on cancer cell migration

Cell movement is generated by a driving force provided by dynamic cytoskeletal organization. Two main cytoskeletal-dependent features, essential for migration, are the highly cell polarized structure and focal adhesion complexes. Cell migration and substrate anchorage are finely regulated by external signaling exerted by growth factors and hormones. In particular, the serine threonine kinase activated by the small GTPase Rho, the Rho-associated protein kinase (ROCK), participate in both processes through regulation of actin rearrangements in lamellipodia, filopodia, ruffles, and stress fibers. Melatonin, the main product secreted by the pineal gland has oncostatic properties. In MCF-7 cells, 1 nm melatonin reduces migration and invasiveness through increased expression of two cell surface adhesion proteins, E-cadherin and beta(1)-integrin. In this work, we studied the microfilament and microtubule rearrangements elicited by melatonin in migrating leader MCF-7 cells by a wound-healing assay. Additionally, cell anchorage was estimated by quantification of focal adhesions in MCF-7 cells cultured with melatonin. ROCK participation in the indole effects on anchorage and migration was explored by inhibition of the kinase activity with the specific inhibitor of ROCK, the Y-27632 compound. The results indicate that ROCK participates in the melatonin inhibitory effects on cell migration by changing cytoskeletal organization of leader MCF-7 cells. Also, they indicated that indole increased the number of focal contacts through ROCK. These results support the notion that melatonin inhibits cancer cell invasion and metastasis formation via ROCK-regulated microfilament and microtubule organization that converge in a migration/anchorage switch.

The Leishmania infantum cytosolic SIR2-related protein 1 (LiSIR2RP1) is an NAD+-dependent deacetylase and ADP-ribosyltransferase

Proteins of the SIR2 (Silent Information Regulator 2) family are characterized by a conserved catalytic domain that exerts unique NAD+-dependent deacetylase activity on histones and various other cellular substrates. Previous reports from us have identified a Leishmania infantum gene encoding a cytosolic protein termed LiSIR2RP1 (Leishmania infantum SIR2-related protein 1) that belongs to the SIR2 family. Targeted disruption of one LiSIR2RP1 gene allele led to decreased amastigote virulence, in vitro as well as in vivo. In the present study, attempts were made for the first time to explore and characterize the enzymatic functions of LiSIR2RP1. The LiSIR2RP1 exhibited robust NAD+-dependent deacetylase and ADP-ribosyltransferase activities. Moreover, LiSIR2RP1 is capable of deacetylating tubulin, either in dimers or, when present, in taxol-stabilized microtubules or in promastigote and amastigote extracts. Furthermore, the immunostaining of parasites revealed a partial co-localization of α-tubulin and LiSIR2RP1 with punctate labelling, seen on the periphery of both promastigote and amastigote stages. Isolated parasite cytoskeleton reacted with antibodies showed that part of LiSIR2RP1 is associated to the cytoskeleton network of both promastigote and amastigote forms. Moreover, the Western blot analysis of the soluble and insoluble fractions of the detergent of promastigote and amastigote forms revealed the presence of α-tubulin in the insoluble fraction, and the LiSIR2RP1 distributed in both soluble and insoluble fractions of promastigotes as well as amastigotes. Collectively, the results of the present study demonstrate that LiSIR2RP1 is an NAD+-dependent deacetylase that also exerts an ADP-ribosyltransferase activity. The fact that tubulin could be among the targets of LiSIR2RP1 may have significant implications during the remodelling of the morphology of the parasite and its interaction with the host cell.

Antiangiogenic vinflunine affects EB1 localization and microtubule targeting to adhesion sites

The motile behavior of endothelial cells is a crucial event for neoangiogenesis. We previously showed that noncytotoxic concentrations of vinflunine inhibit capillary-like tube formation on Matrigel and endothelial cell migration with a concomitant increase in interphase microtubule dynamic instability. In this article, we further investigated the effects of vinflunine on migration and cytoskeleton interaction dynamics in HMEC-1 endothelial cells. We confirmed that vinflunine, at low and noncytotoxic concentrations (0.01-1 nmol/L), inhibited endothelial cell random motility by 54%. This effect was associated with a decrease in the percentage of stable microtubules and in the mean duration of pauses for dynamic ones. Moreover, we found that vinflunine altered adhesion site targeting by microtubules and suppressed the microtubule (+) end pause that occurs at adhesion sites during cell migration (from 151 +/- 20 seconds in control cells to 38 +/- 7 seconds in vinflunine-treated cells, P < 0.001). This effect was associated with the inhibition of adhesion site dynamics and the formation of long-lived stress fibers. Importantly, we found that vinflunine altered EB1 localization at microtubule (+) ends. These results highlight a new mechanism of action of vinflunine, which act by disrupting the mutual control between microtubule and adhesion site dynamics and strengthen the role of +TIPs proteins such as EB1 as key regulators of endothelial cell motility.

Paxillin-dependent stimulation of microtubule catastrophes at focal adhesion sites

An organized microtubule array is essential for the polarized motility of fibroblasts. Dynamic microtubules closely interact with focal adhesion sites in migrating cells. Here, we examined the effect of focal adhesions on microtubule dynamics. We observed that the probability of microtubule catastrophes (transitions from growth to shrinkage) was seven times higher at focal adhesions than elsewhere. Analysis of the dependence between the microtubule growth rate and catastrophe probability throughout the cytoplasm revealed that a nonspecific (mechanical or spatial) factor provided a minor contribution to the catastrophe induction by decreasing microtubule growth rate at adhesions. Strikingly, at the same growth rate, the probability of catastrophes was significantly higher at adhesions than elsewhere, indicative of a site-specific biochemical trigger. The observed catastrophe induction occurred at adhesion domains containing the scaffolding protein paxillin that has been shown previously to interact with tubulin. Furthermore, replacement of full-length paxillin at adhesion sites by microinjected paxillin LIM2-LIM3 domains suppressed microtubule catastrophes exclusively at adhesions. We suggest that paxillin influences microtubule dynamics at focal adhesions by serving as a scaffold for a putative catastrophe factor and/or regulating its exposure to microtubules.

Asymmetric focal adhesion disassembly in motile cells

Cell migration requires the integration and coordination of specific focal adhesion dynamics at the cell front, center and rear. In this review, we will present our understanding of the regulation of adhesion turnover and disassembly in various regions of the cell. Adhesion turnover involves a number of tyrosine kinases and phosphatases, most of which are engaged in FAK signaling pathways. Additionally, adhesions are regulated by tensile forces that depend on dynamic coupling with the actin cytoskeleton. The distribution of adhesion disassembly throughout a motile cell is likely coordinated by the asymmetry of the microtubule network. We present a model that suggests two stages of microtubule-driven adhesion disassembly: destabilization and detachment.

Cytoskeleton as a potential target in the neuropathology of maple syrup urine disease: insight from animal studies

In this short review we provide evidence that the branched-chain keto acids accumulating in the neurometabolic disorder maple syrup urine disease disturb rat cerebral cytoskeleton in a developmentally regulated manner. Alterations of protein phosphorylation leading to brain cytoskeletal misregulation and neural cell death caused by these metabolites are associated with energy deprivation, oxidative stress and excitotoxicity that may ultimately disrupt normal cell function and viability.

Laser‐Based Measurements in Cell Biology

In this chapter, we review the imaging techniques and methods of molecular interrogation made possible by integrating laser light sources with microscopy. We discuss the advantages of exciting fluorescence by laser illumination and review commonly used laser-based imaging techniques such as confocal, multiphoton, and total internal reflection microcopy. We also discuss emerging imaging modalities based on intrinsic properties of biological macromolecules such as second harmonic generation imaging and coherent anti-Raman resonance spectroscopy. Super resolution techniques are presented that exceed the theoretical diffraction-limited resolution of a microscope objective. This chapter also focuses on laser-based techniques that can report biophysical parameters of fluorescently labeled molecules within living cells. Photobleaching techniques, fluorescence lifetime imaging, and fluorescence correlation methods can measure kinetic rates, molecular diffusion, protein-protein interactions, and concentration of a fluorophore-bound molecule. This chapter provides an introduction to the field of laser-based microscopy enabling readers to determine how best to match their research questions to the current suite of techniques.

Plus end-specific depolymerase activity of Kip3, a kinesin-8 protein, explains its role in positioning the yeast mitotic spindle

The budding yeast protein Kip3p is a member of the conserved kinesin-8 family of microtubule motors, which are required for microtubule-cortical interactions, normal spindle assembly and kinetochore dynamics. Here, we demonstrate that Kip3p is both a plus end-directed motor and a plus end-specific depolymerase--a unique combination of activities not found in other kinesins. The ATPase activity of Kip3p was activated by both microtubules and unpolymerized tubulin. Furthermore, Kip3p in the ATP-bound state formed a complex with unpolymerized tubulin. Thus, motile kinesin-8s may depolymerize microtubules by a mechanism that is similar to that used by non-motile kinesin-13 proteins. Fluorescent speckle analysis established that, in vivo, Kip3p moved toward and accumulated on the plus ends of growing microtubules, suggesting that motor activity brings Kip3p to its site of action. Globally, and more dramatically on cortical contact, Kip3p promoted catastrophes and pausing, and inhibited microtubule growth. These findings explain the role of Kip3p in positioning the mitotic spindle in budding yeast and potentially other processes controlled by kinesin-8 family members.

Cell distribution of stress fibres in response to the geometry of the adhesive environment

Cells display a large variety of shapes when plated in classical culture conditions despite their belonging to a common cell type. These shapes are transitory, since cells permanently disassemble and reassemble their cytoskeleton while moving. Adhesive micropatterns are commonly used to confine cell shape within a given geometry. In addition the micropattern can be designed so as to impose cells to spread upon adhesive and nonadhesive areas. Modulation of the pattern geometry allows the analysis of the mechanisms governing the determination of cell shape in response to external adhesive conditions. In this study, we show that the acquisition of cell shape follows two stages where initially the cell forms contact with the micropattern. Here, the most distal contacts made by the cell with the micropattern define the apices of the cell shape. Then secondly, the cell borders that link two apices move so as to minimise the distance between the two apices. In these cell borders, the absence of an underlying adhesive substrate is overcome by stress fibres forming between the apices, which in turn are marked by an accumulation of focal adhesions. By inhibiting myosin function, cell borders on nonadhesive zones become more concave, suggesting that the stress fibres work against the membrane tension in the cell border. Moreover, this suggested that traction forces are unevenly distributed in stationary, nonmigrating, cells. By comparing the stress fibres in cells with one, two, or three nonadherent cell borders it was reasoned that stress fibre strength is inversely proportional to number. We conclude that cells of a given area can generate the same total sum of tractional forces but that these tractional forces are differently spaced depending on the spatial distribution of its adherence contacts.

Adenomatous polyposis coli on microtubule plus ends in cell extensions can promote microtubule net growth with or without EB1

In interphase cells, the adenomatous polyposis coli (APC) protein accumulates on a small subset of microtubules (MTs) in cell protrusions, suggesting that APC may regulate the dynamics of these MTs. We comicroinjected a nonperturbing fluorescently labeled monoclonal antibody and labeled tubulin to simultaneously visualize dynamics of endogenous APC and MTs in living cells. MTs decorated with APC spent more time growing and had a decreased catastrophe frequency compared with non-APC-decorated MTs. Endogenous APC associated briefly with shortening MTs. To determine the relationship between APC and its binding partner EB1, we monitored EB1-green fluorescent protein and endogenous APC concomitantly in living cells. Only a small fraction of EB1 colocalized with APC at any one time. APC-deficient cells and EB1 small interfering RNA showed that EB1 and APC localized at MT ends independently. Depletion of EB1 did not change the growth-stabilizing effects of APC on MT plus ends. In addition, APC remained bound to MTs stabilized with low nocodazole, whereas EB1 did not. Thus, we demonstrate that the association of endogenous APC with MT ends correlates directly with their increased growth stability, that this can occur independently of its association with EB1, and that APC and EB1 can associate with MT plus ends by distinct mechanisms.

Histone deacetylase 6 regulates human immunodeficiency virus type 1 infection

Efficient human immunodeficiency virus (HIV)-1 infection depends on multiple interactions between the viral gp41/gp120 envelope (Env) proteins and cell surface receptors. However, cytoskeleton-associated proteins that modify membrane dynamics may also regulate the formation of the HIV-mediated fusion pore and hence viral infection. Because the effects of HDAC6-tubulin deacetylase on cortical alpha-tubulin regulate cell migration and immune synapse organization, we explored the possible role of HDAC6 in HIV-1-envelope-mediated cell fusion and infection. The binding of the gp120 protein to CD4+-permissive cells increased the level of acetylated alpha-tubulin in a CD4-dependent manner. Furthermore, overexpression of active HDAC6 inhibited the acetylation of alpha-tubulin, and remarkably, prevented HIV-1 envelope-dependent cell fusion and infection without affecting the expression and codistribution of HIV-1 receptors. In contrast, knockdown of HDAC6 expression or inhibition of its tubulin deacetylase activity strongly enhanced HIV-1 infection and syncytia formation. These results demonstrate that HDAC6 plays a significant role in regulating HIV-1 infection and Env-mediated syncytia formation.

Quantification of microtubule nucleation, growth and dynamics in wound-edge cells

Mammalian cells develop a polarized morphology and migrate directionally into a wound in a monolayer culture. To understand how microtubules contribute to these processes, we used GFP-tubulin to measure dynamic instability and GFP-EB1, a protein that marks microtubule plus-ends, to measure microtubule growth events at the centrosome and cell periphery. Growth events at the centrosome, or nucleation, do not show directional bias, but are equivalent toward and away from the wound. Cells with two centrosomes nucleated approximately twice as many microtubules/minute as cells with one centrosome. The average number of growing microtubules per microm2 at the cell periphery is similar for leading and trailing edges and for cells containing one or two centrosomes. In contrast to microtubule growth, measurement of the parameters of microtubule dynamic instability demonstrate that microtubules in the trailing edge are more dynamic than those in the leading edge. Inhibition of Rho with C3 transferase had no detectable effect on microtubule dynamics in the leading edge, but stimulated microtubule turnover in the trailing edge. Our data demonstrate that in wound-edge cells, microtubule nucleation is non-polarized, in contrast to microtubule dynamic instability, which is highly polarized, and that factors in addition to Rho contribute to microtubule stabilization.

G protein control of microtubule assembly

Microtubules are dynamic polymers required for many aspects of eukaryotic cell function. The interphase microtubule network is essential for intracellular transport, organization, and cell polarization, whereas the mitotic spindle is required for chromosome segregation and cell division. Studies in different areas such as cell migration, mitosis, and asymmetric cell division have shown that Ran, Rho, and heterotrimeric G proteins regulate many aspects of microtubule functions. This review surveys how G protein-signaling coordinates microtubule polymerization and organization with specific cellular activities.

p27(Kip1)-stathmin interaction influences sarcoma cell migration and invasion

Emerging evidences suggest that cyclin-dependent kinase inhibitors (CKIs) can regulate cellular functions other than cell cycle progression, such as differentiation and migration. Here, we report that cytoplasmic expression of p27(kip1) affects microtubule (MT) stability following cell adhesion on extracellular matrix (ECM) constituents. This p27(kip1) activity is due to its ability to bind and impair the function of the MT-destabilizing protein stathmin. Accordingly, upregulation of p27(kip1) or downregulation of stathmin expression results in the inhibition of mesenchymal cell motility. Moreover, high stathmin and low cytoplasmic p27(kip1) expression correlate with the metastatic phenotype of human sarcomas in vivo. This study provides a functional link between proliferation and invasion of tumor cells based on diverse activities of p27(kip1) in different subcellular compartments.

Confocal and video imaging of cytoskeleton dynamics in the leech zygote

Ooplasmic segregation in the late interphase zygote of the leech Theromyzon trizonare is accomplished by reorganization of an ectoplasmic cytoskeleton formed by polar rings and meridional bands. The dynamic properties of this cytoskeleton were explored by time-lapse confocal and video microscopy. Cytoskeleton assembly was investigated in zygotes pulse-labeled with microinjected fluorophore-tagged or biotin-tagged dimeric tubulin and G-actin. Cytoskeleton disassembly was studied by comparing the linear dimensions of the cytoskeleton at different time points during late interphase. The relative distributions of F- and-G-actin were determined after microinjection of rhodamine-labeled actin and fluorescein-labeled DNase I. Results showed that labeled precursors were readily incorporated into a network of microtubules or actin filaments. Bipolar translocation of the rings and meridional bands was accompanied by the rapid assembly and disassembly of microtubules and actin filaments. Because labeled microtubules and microfilaments gradually decreased, the rate of cytoskeleton disassembly was greater than the rate of cytoskeleton assembly. Hence, ooplasmic segregation was accompanied by the rapid turnover of cytoskeletal components. Co-distribution of F- and-G-actin during mid and late interphase may favor polymer-monomer interchange. We conclude that cytoskeleton reorganization during foundation of cytoplasmic domains can be conveniently studied in the live leech zygote after microinjection of labeled precursors.

RASSF1A Interacts with Microtubule-Associated Proteins and Modulates Microtubule Dynamics

The candidate tumor suppressor gene RASSF1A is inactivated in many types of adult and childhood cancers. However, the mechanisms by which RASSF1A exerts its tumor suppressive functions have yet to be elucidated. To this end, we performed a yeast two-hybrid screen to identify novel RASSF1A-interacting proteins in a human brain cDNA library. Seventy percent of interacting clones had homology to microtubule-associated proteins, including MAP1B and VCY2IP1/C19ORF5. RASSF1A association with MAP1B and VCY2IP1/C19ORF5 was subsequently confirmed in mammalian cell lines. This suggested that RASSF1A may exert its tumor-suppressive functions through interaction with the microtubules. We demonstrate that RASSF1A associates with the microtubules, causing them to exist as hyperstabilized circular bundles. We found that two naturally occurring tumor-associated missense substitutions in the RASSF1A coding region, C65R and R257Q, perturb the association of RASSF1A with the microtubules. The C65R and R257Q in addition to VCY2IP1/C19ORF5 showed reduced ability to induce microtubule acetylation and were unable to protect the microtubules against the depolymerizing action of nocodazole. In addition, wild-type RASSF1A but not the C65R or the R257Q is able to block DNA synthesis. Our data identify a role for RASSF1A in the regulation of microtubules and cell cycle dynamics that could be part of the mechanism(s) by which RASSF1A exerts its growth inhibition on cancer cells.

EB1-microtubule interactions in Xenopus egg extracts: role of EB1 in microtubule stabilization and mechanisms of targeting to microtubules

EB1 targets to polymerizing microtubule ends, where it is favorably positioned to regulate microtubule polymerization and confer molecular recognition of the microtubule end. In this study, we focus on two aspects of the EB1-microtubule interaction: regulation of microtubule dynamics by EB1 and the mechanism of EB1 association with microtubules. Immunodepletion of EB1 from cytostatic factor-arrested M-phase Xenopus egg extracts dramatically reduced microtubule length; this was complemented by readdition of EB1. By time-lapse microscopy, EB1 increased the frequency of microtubule rescues and decreased catastrophes, resulting in increased polymerization and decreased depolymerization and pausing. Imaging of EB1 fluorescence revealed a novel structure: filamentous extensions on microtubule plus ends that appeared during microtubule pauses; loss of these extensions correlated with the abrupt onset of polymerization. Fluorescent EB1 localized to comets at the polymerizing plus ends of microtubules in cytostatic factor extracts and uniformly along the lengths of microtubules in interphase extracts. The temporal decay of EB1 fluorescence from polymerizing microtubule plus ends predicted a dissociation half-life of seconds. Fluorescence recovery after photobleaching also revealed dissociation and rebinding of EB1 to the microtubule wall with a similar half-life. EB1 targeting to microtubules is thus described by a combination of higher affinity binding to polymerizing ends and lower affinity binding along the wall, with continuous dissociation. The latter is likely to be attenuated in interphase. The highly conserved effect of EB1 on microtubule dynamics suggests it belongs to a core set of regulatory factors conserved in higher organisms, and the complex pattern of EB1 targeting to microtubules could be exploited by the cell for coordinating microtubule behaviors.

Conserved microtubule-actin interactions in cell movement and morphogenesis

Interactions between microtubules and actin are a basic phenomenon that underlies many fundamental processes in which dynamic cellular asymmetries need to be established and maintained. These are processes as diverse as cell motility, neuronal pathfinding, cellular wound healing, cell division and cortical flow. Microtubules and actin exhibit two mechanistic classes of interactions--regulatory and structural. These interactions comprise at least three conserved 'mechanochemical activity modules' that perform similar roles in these diverse cell functions.

Regulation of leading edge microtubule and actin dynamics downstream of Rac1

Actin in migrating cells is regulated by Rho GTPases. However, Rho proteins might also affect microtubules (MTs). Here, we used time-lapse microscopy of PtK1 cells to examine MT regulation downstream of Rac1. In these cells, "pioneer" MTs growing into leading-edge protrusions exhibited a decreased catastrophe frequency and an increased time in growth as compared with MTs further from the leading edge. Constitutively active Rac1(Q61L) promoted pioneer behavior in most MTs, whereas dominant-negative Rac1(T17N) eliminated pioneer MTs, indicating that Rac1 is a regulator of MT dynamics in vivo. Rac1(Q61L) also enhanced MT turnover through stimulation of MT retrograde flow and breakage. Inhibition of p21-activated kinases (Paks), downstream effectors of Rac1, inhibited Rac1(Q61L)-induced MT growth and retrograde flow. In addition, Rac1(Q61L) promoted lamellipodial actin polymerization and Pak-dependent retrograde flow. Together, these results indicate coordinated regulation of the two cytoskeletal systems in the leading edge of migrating cells.

Organization and dynamics of growing microtubule plus ends during early mitosis

A stable cell line expressing EB1-green fluorescent protein was used to image growing microtubule plus ends at the G(2)/M transition. By late prophase growing ends no longer extend to the cell periphery and were not uniformly distributed around each centrosome. Growing ends were much more abundant in the area surrounding the nuclear envelope, and microtubules growing around the nucleus were 1.5 fold longer than those growing in the opposite direction. The growth of longer ends toward the nucleus did not result from a localized faster growth rate, because this rate was approximately 11 microm/min in all directions from the centrosome. Rather, microtubule ends growing toward the nucleus seemed stabilized by dynein/dynactin associated with the nuclear envelope. Injection of p50 into late prophase cells removed dynein from the nuclear envelope, reduced the density of growing ends near the nuclear envelope and resulted in a uniform distribution of growing ends from each centrosome. We suggest that the cell cycle-dependent binding of dynein/dynactin to the nuclear envelope locally stabilizes growing microtubules. Both dynein and microtubules would then be in a position to participate in nuclear envelope breakdown, as described in recent studies.