Modulation of substrate adhesion dynamics via microtubule targeting requires kinesin-1

ZF21 protein regulates cell adhesion and motility

Cell migration on an extracellular matrix (ECM) requires continuous formation and turnover of focal adhesions (FAs) along the direction of cell movement. However, our knowledge of the components of FAs and the mechanism of their regulation remains limited. Here, we identify ZF21, a member of a protein family characterized by the presence of a phosphatidylinositol 3-phosphate-binding FYVE domain, to be a new regulator of FAs and cell movement. Knockdown of ZF21 expression in cells increased the number of FAs and suppressed cell migration. Knockdown of ZF21 expression also led to a significant delay in FA disassembly following induction of synchronous disassembly of FAs by nocodazole treatment. ZF21 bound to focal adhesion kinase, localized to FAs, and was necessary for dephosphorylation of FAK at Tyr(397), which is important for disassembly of FAs. Thus, ZF21 represents a new component of FAs, mediates disassembly of FAs, and thereby regulates cell motility.

The heel and toe of the cell's foot: a multifaceted approach for understanding the structure and dynamics of focal adhesions

Focal adhesions (FAs) are large clusters of transmembrane receptors of the integrin family and a multitude of associated cytoplasmic "plaque" proteins, which connect the extracellular matrix-bound receptors with the actin cytoskeleton. The formation of nearly stationary FAs defines a boundary between the dense and highly dynamic actin network in lamellipodium and the sparser and more diverse cytoskeletal organization in the lamella proper, creating a template for the organization of the entire actin network. The major "mechanical" and "sensory" functions of FAs; namely, the nucleation and regulation of the contractile, myosin-II-containing stress fibers and the mechanosensing of external surfaces depend, to a major extent, on the dynamics of molecular components within FAs. A central element in FA regulation concerns the positive feedback loop, based on the most intriguing feature of FAs; that is, their dependence on mechanical tension developing by the growing stress fibers. FAs grow in response to such tension, and rapidly disassemble upon its relaxation. In this article, we address the mechanistic relationships between the process of FA development, maturation and dissociation and the dynamic molecular events, which take place in different regions of the FA, primarily in the distal end of this structure (the "toe") and the proximal "heel," and discuss the central role of local mechanical forces in orchestrating the complex interplay between FAs and the actin system.

How and why does the endoplasmic reticulum move?

The ER (endoplasmic reticulum) is a fascinating organelle that is highly dynamic, undergoing constant movement and reorganization. It has many key roles, including protein synthesis, folding and trafficking, calcium homoeostasis and lipid synthesis. It can expand in size when needed, and the balance between tubular and lamellar regions can be altered. The distribution and organization of the ER depends on both motile and static interactions with microtubules and the actin cytoskeleton. In the present paper, we review how the ER moves, and consider why this movement may be important for ER and cellular function.

Kinesin-1 regulates microtubule dynamics via a c-Jun N-terminal kinase-dependent mechanism

In the kinesin family, all the molecular motors that have been implicated in the regulation of microtubule dynamics have been shown to stimulate microtubule depolymerization. Here, we report that kinesin-1 (also known as conventional kinesin or KIF5B) stimulates microtubule elongation and rescues. We show that microtubule-associated kinesin-1 carries the c-Jun N-terminal kinase (JNK) to allow its activation and that microtubule elongation requires JNK activity throughout the microtubule life cycle. We also show that kinesin-1 and JNK promoted microtubule rescues to similar extents. Stimulation of microtubule rescues by the kinesin-1/JNK pathway could not be accounted for by the rescue factor CLIP-170. Indeed only a dual inhibition of kinesin-1/JNK and CLIP-170 completely blocked rescues and led to extensive microtubule loss. We propose that the kinesin-1/JNK signaling pathway is a major regulator of microtubule dynamics in living cells and that it is required with the rescue factor CLIP-170 to allow cells to build their interphase microtubule network.

Touch, grasp, deliver and control: functional cross-talk between microtubules and cell adhesions

Cross-talk between microtubule networks and sites of cell-matrix and cell-cell adhesion has profound impact on these structures and is essential for proper cell organization, polarization and motility. Components of adhesion sites can interact directly with microtubules or with proteins that specifically associate with microtubule plus ends and minus ends and in this way capture, stabilize or destabilize microtubules. In their turn, microtubules can serve as routes for delivery of structural and regulatory factors that control adhesion site turnover. In addition, the microtubule lattice or growing microtubule plus ends can serve as diffusional sinks that accumulate and scaffold regulatory molecules, thereby affecting their activity in the vicinity of adhesions. Combination of these mechanisms underlies the functional co-operation between microtubules and adhesion sites and defines their dynamic behavior.

ACF7 regulates cytoskeletal-focal adhesion dynamics and migration and has ATPase activity

Coordinated interactions between microtubule (MT) and actin cytoskeletons are involved in many polarized cellular processes. Spectraplakins are enormous (>500 kDa) proteins able to bind both MTs and actin filaments (F-actin) directly. To elucidate the physiological significance and functions of mammalian spectraplakin ACF7, we've conditionally targeted it in skin epidermis. Intriguingly, ACF7 deficiency compromises the targeting of microtubules along F-actin to focal adhesions (FAs), stabilizes FA-actin networks, and impairs epidermal migration. Exploring underlying mechanisms, we show that ACF7's binding domains for F-actin, MTs, and MT plus-end proteins are not sufficient to rescue the defects in FA-cytoskeletal dynamics and migration functions of ACF7 null keratinocytes. We've uncovered an intrinsic actin-regulated ATPase domain in ACF7 and demonstrate that it is both functional and essential for these roles. Our findings provide insight into the functions of this important cytoskeletal crosslinking protein in regulating dynamic interactions between MTs and F-actin to sustain directional cell movement.

Mal3, the Schizosaccharomyces pombe homolog of EB1, changes the microtubule lattice

In vitro studies of pure tubulin have suggested that tubulin heterodimers in cells assemble into B-lattice microtubules, where the 8-nm dimers in adjacent protofilaments are staggered by 0.9 nm. This arrangement requires the tube to close by forming a seam with an A-lattice, in which the protofilaments are staggered by 4.9 nm. Here we show that Mal3, an EB1 family tip-tracking protein, drives tubulin to assemble in vitro into exclusively 13-protofilament microtubules with a high proportion of A-lattice protofilament contacts. We present a three-dimensional cryo-EM reconstruction of a purely A-lattice microtubule decorated with Mal3, in which Mal3 occupies the groove between protofilaments and associates closely with one tubulin monomer. We propose that Mal3 promotes assembly by binding to freshly formed tubulin polymer and particularly favors any with A-lattice arrangement. These results reopen the question of microtubule structure in cells.

Intracellular Transport and Kinesin Superfamily Proteins, KIFs: Structure, Function, and Dynamics

Various molecular cell biology and molecular genetic approaches have indicated significant roles for kinesin superfamily proteins (KIFs) in intracellular transport and have shown that they are critical for cellular morphogenesis, functioning, and survival. KIFs not only transport various membrane organelles, protein complexes, and mRNAs for the maintenance of basic cellular activity, but also play significant roles for various mechanisms fundamental for life, such as brain wiring, higher brain functions such as memory and learning and activity-dependent neuronal survival during brain development, and for the determination of important developmental processes such as left-right asymmetry formation and suppression of tumorigenesis. Accumulating data have revealed a molecular mechanism of cargo recognition involving scaffolding or adaptor protein complexes. Intramolecular folding and phosphorylation also regulate the binding activity of motor proteins. New techniques using molecular biophysics, cryoelectron microscopy, and X-ray crystallography have detected structural changes in motor proteins, synchronized with ATP hydrolysis cycles, leading to the development of independent models of monomer and dimer motors for processive movement along microtubules.

Differential trafficking of Kif5c on tyrosinated and detyrosinated microtubules in live cells

Kinesin-1 is a molecular transporter that trafficks along microtubules. There is some evidence that kinesin-1 targets specific cellular sites, but it is unclear how this spatial regulation is achieved. To investigate this process, we used a combination of in vivo imaging of kinesin heavy-chain Kif5c (an isoform of kinesin-1) fused to GFP, in vitro analyses and mathematical modelling. GFP-Kif5c fluorescent puncta localised to a subset of microtubules in live cells. These puncta moved at speeds of up to 1 microm second(-1) and exchanged into cortically labelled clusters at microtubule ends. This behaviour depended on the presence of a functional motor domain, because a rigor-mutant GFP-Kif5c bound to microtubules but did not move along them. Further analysis indicated that the microtubule subset decorated by GFP-Kif5c was highly stable and primarily composed of detyrosinated tubulin. In vitro motility assays showed that the motor domain of Kif5c moved detyrosinated microtubules at significantly lower velocities than tyrosinated (unmodified) microtubules. Mathematical modelling predicted that a small increase in detyrosination would bias kinesin-1 occupancy towards detyrosinated microtubules. These data suggest that kinesin-1 preferentially binds to and trafficks on detyrosinated microtubules in vivo, providing a potential basis for the spatial targeting of kinesin-1-based cargo transport.

Scaffold mediated regulation of MAPK signaling and cytoskeletal dynamics: a perspective

Cell migration is critical for many physiological processes and is often misregulated in developmental disorders and pathological conditions including cancer and neurodegeneration. MAPK signaling and the Rho family of proteins are known regulators of cell migration that exert their influence on cellular cytoskeleton during cell adhesion and migration. Here we review data supporting the view that localized ERK signaling mediated through recently identified scaffold proteins may regulate cell migration.

Microtubules Tethered at Epithelial Cell Junctions by Dynein Facilitate Efficient Junction Assembly

Efficient remodeling of cell-cell adhesions is critical during development and morphogenesis. Junctional components must be specifically and rapidly transported to sites of junction assembly. In this study, we show a mechanism by which this targeted trafficking may occur. Microtubules target epithelial adherens junctions, and the number of microtubules both projecting to and tethered at sites of contact is increased during junction assembly, consistent with an increased need for new material at the nascent junction. Cytoplasmic dynein is localized to sites of cell-cell contact, and microtubules project to dynein patches where they become tethered. Microinjection of anti-dynein antibodies disrupts the tethering of microtubules, showing that the motor anchors them. Furthermore, disruption of dynein inhibits junction formation. Immunocytochemistry with antibodies to p120 catenin support the hypothesis that tethered microtubules serve as tracks for delivery of new components to forming junctions, suggesting a model in which material is targeted for delivery to sites of need through microtubules tethered by dynein.

HDAC6 deacetylation of tubulin modulates dynamics of cellular adhesions

Genetic or pharmacological alteration of the activity of the histone deacetylase 6 (HDAC6) induces a parallel alteration in cell migration. Using tubacin to block deacetylation of alpha-tubulin, and not other HDAC6 substrates, yielded a motility reduction equivalent to agents that block all NAD-independent HDACs. Accordingly, we investigated how the failure to deacetylate tubulin contributes to decreased motility in HDAC6-inhibited cells. Testing the hypothesis that motility is reduced because cellular adhesion is altered, we found that inhibiting HDAC6 activity towards tubulin rapidly increased total adhesion area. Next, we investigated the mechanism of the adhesion area increase. Formation of adhesions proceeded normally and cell spreading was more rapid in the absence of active HDAC6; however, photobleaching assays and adhesion breakdown showed that adhesion turnover was slower. To test the role of hyperacetylated tubulin in altering adhesion turnover, we measured microtubule dynamics in HDAC6-inhibited cells because dynamic microtubules are required to target adhesions for turnover. HDAC6 inhibition yielded a decrease in microtubule dynamics that was sufficient to decrease focal adhesion turnover. Thus, our results suggest a scenario in which the decreased dynamics of hyperacetylated microtubules in HDAC6-inhibited cells compromises their capacity to mediate the focal adhesion dynamics required for rapid cell migration.

Microtubule-targeting-dependent reorganization of filopodia

Interaction between the microtubule system and actin cytoskeleton has emerged as a fundamental process required for spatial regulation of cell protrusion and retraction activities. In our current studies, analysis of digital fluorescence images revealed targeting of microtubules to filopodia in B16F1 melanoma cells and fibroblasts. We investigated the functional consequence of targeting on filopodia reorganization and examined mechanisms by which microtubules may be guided to, or interact with, filopodia. Live cell imaging studies show that targeting events in lamellipodia wings temporally correlated with filopodia turning toward the lamellipodium midline and with filopodia merging. Rapid uncoupling of targeting with nocodazole decreased filopodia merging events and increased filopodia density. Total internal reflection fluorescence microscopy identified microtubules near the ventral surface and upward movement of targeted filopodia. The role of adhesion sites and microtubule plus-end proteins in targeting was investigated. Correlation of adhesion sites with microtubule targeting to filopodia was not observed and depletion of microtubule plus-end proteins did not significantly alter targeting frequency. We propose that microtubules target filopodia, independent of focal adhesions and plus-end proteins, causing filopodia movement and microtubules regulate filopodia density in lamellipodia wings through filopodia merging events.

A microtubule-based, dynein-dependent force induces local cell protrusions: Implications for neurite initiation

A key event in neurite initiation is the accumulation of microtubule bundles at the neuron periphery. We hypothesized that such bundled microtubules may generate a force at the plasma membrane that facilitates neurite initiation. To test this idea we observed the behavior of microtubule bundles that were induced by the microtubule-associated protein MAP2c. Endogenous MAP2c contributes to neurite initiation in primary neurons, and exogeneous MAP2c is sufficient to induce neurites in Neuro-2a cells. We performed nocodazol washout experiments in primary neurons, Neuro-2a cells and COS-7 cells to investigate the underlying mechanism. During nocodazol washout, small microtubule bundles formed rapidly in the cytoplasm and immediately began to move toward the cell periphery in a unidirectional manner. In neurons and Neuro-2a cells, neurite-like processes extended within minutes and concurrently accumulated bundles of repolymerized microtubules. Speckle microscopy in COS-7 cells indicated that bundle movement was due to transport, not treadmilling. At the periphery bundles remained under a unidirectional force and induced local cell protrusions that were further enhanced by suppression of Rho kinase activity. Surprisingly, this bundle motility was independent of classical actin- or microtubule-based tracks. It was, however, reversed by function-blocking antibodies against dynein. Suppression of dynein expression in primary neurons by RNA interference severely inhibited the generation of new neurites, but not the elongation of existing neurites formed prior to dynein knockdown. Together, these cell biological data suggest that neuronal microtubule-associated proteins induce microtubule bundles that are pushed outward by dynein and locally override inward contraction to initiate neurite-like cell protrusions. A similar force-generating mechanism might participate in spontaneous initiation of neurites in developing neurons.

Myosin IIA regulates cell motility and actomyosin–microtubule crosstalk

Non-muscle myosin II has diverse functions in cell contractility, cytokinesis and locomotion, but the specific contributions of its different isoforms have yet to be clarified. Here, we report that ablation of the myosin IIA isoform results in pronounced defects in cellular contractility, focal adhesions, actin stress fibre organization and tail retraction. Nevertheless, myosin IIA-deficient cells display substantially increased cell migration and exaggerated membrane ruffling, which was dependent on the small G-protein Rac1, its activator Tiam1 and the microtubule moter kinesin Eg5. Myosin IIA deficiency stabilized microtubules, shifting the balance between actomyosin and microtubules with increased microtubules in active membrane ruffles. When microtubule polymerization was suppressed, myosin IIB could partially compensate for the absence of the IIA isoform in cellular contractility, but not in cell migration. We conclude that myosin IIA negatively regulates cell migration and suggest that it maintains a balance between the actomyosin and microtubule systems by regulating microtubule dynamics.

Integrin signalling in directed cell migration

Migrating cells tend to continue moving in the same direction, a property called persistence. During migration, cells, by definition, form new adhesions at their front and break old adhesions at the rear. We hypothesize that the distinction between new adhesions at the front and older adhesions at the rear plays a major role in directional persistence. We propose specific mechanisms of persistence on the basis of known properties of integrin signals, in hope of stimulating investigation of these ideas.

Calpain 2 and Src dependence distinguishes mesenchymal and amoeboid modes of tumour cell invasion: a link to integrin function

Cancer cells can invade three-dimensional matrices by distinct mechanisms, recently defined by their dependence on extracellular proteases, including matrix metalloproteinases. Upon treatment with protease inhibitors, some tumour cells undergo a 'mesenchymal to amoeboid' transition that allows invasion in the absence of pericellular proteolysis and matrix degradation. We show here that in HT1080 cells, this transition is associated with weakened integrin-dependent adhesion, consistently reduced cell surface expression of the alpha2beta1 integrin collagen receptor and impaired signalling downstream, as judged by reduced autophosphorylation of focal adhesion kinase (FAK). On examining cancer cells that use defined invasion strategies, we show that distinct from mesenchymal invasion, amoeboid invasion is independent of intracellular calpain 2 proteolytic activity that is usually needed for turnover of integrin-linked adhesions during two-dimensional planar migration. Moreover, an inhibitor of Rho/ROCK signalling, which specifically impairs amoeboid-like invasion, restores cell surface expression of alpha2beta1 integrin, downstream FAK autophosphorylation and calpain 2 sensitivity--features of mesenchymal invasion. These findings link weakened integrin function to a lack of requirement for calpain 2-mediated integrin adhesion turnover during amoeboid invasion. In keeping with the need for integrin adhesion turnover, mesenchymal invasion is uniquely sensitive to Src inhibitors. Thus, the need for a major pathway that controls integrin adhesion turnover defines and distinguishes cancer cell invasion strategies.

The kinesin KIF1C and microtubule plus ends regulate podosome dynamics in macrophages

Microtubules are important for the turnover of podosomes, dynamic, actin-rich adhesions implicated in migration and invasion of monocytic cells. The molecular basis for this functional dependency, however, remained unclear. Here, we show that contact by microtubule plus ends critically influences the cellular fate of podosomes in primary human macrophages. In particular, we identify the kinesin KIF1C, a member of the Kinesin-3 family, as a plus-end-enriched motor that targets regions of podosome turnover. Expression of mutation constructs or small interfering RNA-/short hairpin RNA-based depletion of KIF1C resulted in decreased podosome dynamics and ultimately in podosome deficiency. Importantly, protein interaction studies showed that KIF1C binds to nonmuscle myosin IIA via its PTPD-binding domain, thus providing an interface between the actin and tubulin cytoskeletons, which may facilitate the subcellular targeting of podosomes by microtubules. This is the first report to implicate a kinesin in podosome regulation and also the first to describe a function for KIF1C in human cells.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Assembly and mechanosensory function of focal adhesions: experiments and models

Initial integrin-mediated cell-matrix adhesions (focal complexes) appear underneath the lamellipodia, in the regions of the "fast" centripetal flow driven by actin polymerization. Once formed, these adhesions convert the flow behind them into a "slow", myosin II-driven mode. Some focal complexes then turn into elongated focal adhesions (FAs) associated with contractile actomyosin bundles (stress fibers). Myosin II inhibition does not suppress formation of focal complexes but blocks their conversion into mature FAs and further FA growth. Application of external pulling force promotes FA growth even under conditions when myosin II activity is blocked. Thus, individual FAs behave as mechanosensors responding to the application of force by directional assembly. We proposed a thermodynamic model for the mechanosensitivity of FAs, taking into account that an elastic molecular aggregate subject to pulling forces tends to grow in the direction of force application by incorporating additional subunits. This simple model can explain a variety of processes typical of FA behavior. Assembly of FAs is triggered by the small G-protein Rho via activation of two major targets, Rho-associated kinase (ROCK) and the formin homology protein, Dia1. ROCK controls creation of myosin II-driven forces, while Dia1 is involved in the response of FAs to these forces. Expression of the active form of Dia1, allows the external force-induced assembly of mature FAs, even in conditions when Rho is inhibited. Conversely, downregulation of Dia1 by siRNA prevents FA maturation even if Rho is activated. Dia1 and other formins cap barbed (fast growing) ends of actin filaments, allowing insertion of the new actin monomers. We suggested a novel mechanism of such "leaky" capping based on an assumption of elasticity of the formin/barbed end complex. Our model predicts that formin-mediated actin polymerization should be greatly enhanced by application of external pulling force. Thus, the formin-actin complex might represent an elementary mechanosensing device responding to force by enhancement of actin assembly. In addition to its role in actin polymerization, Dia1 seems to be involved in formation of links between actin filaments and microtubules affecting microtubule dynamics. Alpha-tubulin deacetylase HDAC6 cooperates with Dia1 in formation of such links. Since microtubules are known to promote FA disassembly, the Dia1-mediated effect on microtubule dynamics may possibly play a role in the negative feedback loop controlling size and turnover of FAs.

Microtubule-induced focal adhesion disassembly is mediated by dynamin and focal adhesion kinase

Imaging studies implicate microtubule targeting of focal adhesions in focal adhesion disassembly, although the molecular mechanism is unknown. Here, we develop a model system of focal adhesion disassembly based on the finding that microtubule regrowth after nocodazole washout induces disassembly of focal adhesions, and that this disassembly occurs independently of Rho and Rac, but depends on focal adhesion kinase (FAK) and dynamin. During disassembly, dynamin interacts with FAK and colocalizes with focal adhesions. Inhibition of dynamin prevents migration of cells with a focal adhesion phenotype. Our results show that focal adhesion disassembly involves microtubules, dynamin and FAK, and is not simply the reversal of focal adhesion formation.

Kinesin is involved in protecting nascent microtubules from disassembly after recovery from nocodazole treatment

Upon recovery from nocodazole treatment, microtubules from cultured epithelial cells exhibit unusual properties: they re-grow as fast as any highly dynamic microtubule, but they are also protected against disassembly when challenged with nocodazole like the stable microtubules of steady-state cells. Exploring the mechanism that underlies this protection, we found that it was sensitive to ATP treatment and that it involved conventional kinesin. Kinesin localized at the growing end or along nascent microtubules. Its inhibition using a dominant-negative construct for cargo binding, or by micro-injecting an anti-kinesin heavy chain antibody that impairs motor activity, resulted in the partial or total loss of microtubule protection. Finally, in an ex vivo elongation assay, we found that kinesin also participates in the control of microtubule re-growth. Altogether, our findings suggest that kinesin is involved in an early microtubule protection process that is linked to the control of their dynamics during their early growth phase.

Mechanisms of polarized growth and organelle segregation in yeast

Cell polarity, as reflected by polarized growth and organelle segregation during cell division in yeast, appears to follow a simple hierarchy. On the basis of physical cues from previous cell cycles or stochastic processes, yeast cells select a site for bud emergence that also defines the axis of cell division. Once polarity is established, rho protein-based signal pathways set up a polarized cytoskeleton by activating localized formins to nucleate and assemble polarized actin cables. These serve as tracks for the transport of secretory vesicles, the segregation of the trans Golgi network, the vacuole, peroxisomes, endoplasmic reticulum, mRNAs for cell fate determination, and microtubules that orient the nucleus in preparation for mitosis, all by myosin-Vs encoded by the MYO2 and MYO4 genes. Most of the proteins participating in these processes in yeast are conserved throughout the kingdoms of life, so the emerging models are likely to be generally applicable. Indeed, several parallels to cellular organization in animals are evident.

N-terminal kinesins: many and various

Molecular motors are a fascinating group of proteins that have vital roles in a huge variety of cellular processes. They all share the ability to produce force through the hydrolysis of adenosine triphosphate, and fall into classes groups: the kinesins, myosins and the dyneins. The kinesin superfamily itself can be split into three major groups depending on the position of the motor domain, which is localized N-terminally, C-terminally, or internally. This review focuses on the N-terminal kinesins, providing a brief overview of their roles within the cell, and illustrating recent key developments in our understanding of how these proteins function.

Cytoplasmic dynein regulates the subcellular distribution of mitochondria by controlling the recruitment of the fission factor dynamin-related protein-1

While the subcellular organisation of mitochondria is likely to influence many aspects of cell physiology, its molecular control is poorly understood. Here, we have investigated the role of the retrograde motor protein complex, dynein-dynactin, in mitochondrial localisation and morphology. Disruption of dynein function, achieved in HeLa cells either by over-expressing the dynactin subunit, dynamitin (p50), or by microinjection of an anti-dynein intermediate chain antibody, resulted in (a) the redistribution of mitochondria to the nuclear periphery, and (b) the formation of long and highly branched mitochondrial structures. Suggesting that an alteration in the balance between mitochondrial fission and fusion may be involved in both of these changes, overexpression of p50 induced the translocation of the fission factor dynamin-related protein (Drp1) from mitochondrial membranes to the cytosol and microsomes. Moreover, a dominant-negative-acting form of Drp1 mimicked the effects of p50 on mitochondrial morphology, while wild-type Drp1 almost completely restored normal mitochondrial distribution in p50 over-expressing cells. Thus, the dynein/dynactin complex plays an unexpected role in the regulation of mitochondrial morphology in living cells, by controlling the recruitment of Drp1 to these organelles.

Overexpression of autocrine motility factor in metastatic tumor cells: possible association with augmented expression of KIF3A and GDI-beta

Autocrine motility factor (AMF), which is identical to phosphohexose isomerase (PHI)/glucose-6-phosphate isomerase (GPI), a ubiquitous enzyme essential for glycolysis, neuroleukin (NLK), a neurotrophic growth factor, and maturation factor (MF) mediating the differentiation of human myeloid cells, enhances the motility and metastatic ability of tumor cells. AMF/PHI activity is elevated in the serum or urine in patients with malignant tumors. Here, we constructed an amf/phi/nlk/mf gene using adenovirus vector and transfected into two tumor cell lines. Overexpression of AMF/PHI/NLK/MF enhanced AMF secretion into the culture media in both tumor cell lines. However, upregulation of motility and metastatic ability was found only in metastatic fibrosarcoma cells expressing an AMF receptor, gp78, and was not found in gp78-undetectable osteosarcoma cells. Thus, not only serum AMF activity but also gp78-expression in tumor cells may be required for metastasis-related motility induction. With the use of microarray analyses, we detected two augmented genes, rho GDP dissociation inhibitor beta and kinesin motor 3A, as well as AMF itself. The RNA message and protein expression of these two molecules was confirmed to be upregulated, suggesting a possible association with AMF-induced signaling for cell motility and metastasis.

Signaling in leukocyte transendothelial migration

Under a variety of (patho) physiological conditions, leukocytes will leave the bloodstream by crossing the endothelial monolayer that lines the vessels and migrate into the underlying tissues. It is now clear that the process of extravasation involves a range of adhesion molecules on both leukocytes and endothelial cells, as well as extensive intracellular signaling that drives adhesion and chemotaxis on the one hand and controls a transient modulation of endothelial integrity on the other. We review here the current knowledge of the intracellular signaling pathways that are activated in the context of transendothelial migration in leukocytes and in endothelial cells. In leukocytes, polarization of receptors and of the signaling machinery is of key importance to drive adhesion and directional migration. Subsequent adhesion-induced signaling in endothelial cells, mediated by Rho-like GTPases and reactive oxygen species, induces a transient and focal loss of endothelial cell-cell adhesion to allow transmigration of the leukocyte. This review underscores the notion that we have likely just scratched the surface in revealing the complexity of the signaling that controls leukocyte transendothelial migration.

A role for cytoplasmic dynein and LIS1 in directed cell movement

Cytoplasmic dynein has been implicated in numerous aspects of intracellular movement. We recently found dynein inhibitors to interfere with the reorientation of the microtubule cytoskeleton during healing of wounded NIH3T3 cell monolayers. We now find that dynein and its regulators dynactin and LIS1 localize to the leading cell cortex during this process. In the presence of serum, bright diffuse staining was observed in regions of active ruffling. This pattern was abolished by cytochalasin D, and was not observed in cells treated with lysophosphatidic acid, conditions which allow microtubule reorientation but not forward cell movement. Under the same conditions, using total internal reflection fluorescence microscopy, clear punctate dynein/dynactin containing structures were observed along the sides and at the tips of microtubules at the leading edge. Overexpression of dominant negative dynactin and LIS1 cDNAs or injection of antidynein antibody interfered with the rate of cell migration. Together, these results implicate a leading edge cortical pool of dynein in both early and persistent steps in directed cell movement.

A Novel Actin-bundling Kinesin-related Protein from Dictyostelium discoideum

Actin filaments and microtubules are two major cytoskeletal systems involved in wide cellular processes, and the organizations of their filamentous networks are regulated by a large number of associated proteins. Recently, evidence has accumulated for the functional cooperation between the two filament systems via associated proteins. However, little is known about the interactions of the kinesin superfamily proteins, a class of microtubule-based motor proteins, with actin filaments. Here, we describe the identification and characterization of a novel kinesin-related protein named DdKin5 from Dictyostelium. DdKin5 consists of an N-terminal conserved motor domain, a central stalk region, and a C-terminal tail domain. The motor domain showed binding to microtubules in an ATP-dependent manner that is characteristic of kinesin-related proteins. We found that the C-terminal tail domain directly interacts with actin filaments and bundles them in vitro. Immunofluorescence studies showed that DdKin5 is specifically enriched at the actin-rich surface protrusions in cells. Overexpression of the DdKin5 protein affected the organization of actin filaments in cells. We propose that a kinesin-related protein, DdKin5, is a novel actin-bundling protein and a potential cross-linker of actin filaments and microtubules associated with specific actin-based structures in Dictyostelium.

Bicaudal D induces selective dynein-mediated microtubule minus end-directed transport

Bicaudal D is an evolutionarily conserved protein, which is involved in dynein-mediated motility both in Drosophila and in mammals. Here we report that the N-terminal portion of human Bicaudal D2 (BICD2) is capable of inducing microtubule minus end-directed movement independently of the molecular context. This characteristic offers a new tool to exploit the relocalization of different cellular components by using appropriate targeting motifs. Here, we use the BICD2 N-terminal domain as a chimera with mitochondria and peroxisome-anchoring sequences to demonstrate the rapid dynein-mediated transport of selected organelles. Surprisingly, unlike other cytoplasmic dynein-mediated processes, this transport shows very low sensitivity to overexpression of the dynactin subunit dynamitin. The dynein-recruiting activity of the BICD2 N-terminal domain is reduced within the full-length molecule, indicating that the C-terminal part of the protein might regulate the interaction between BICD2 and the motor complex. Our findings provide a novel model system for dissection of the molecular mechanism of dynein motility.

What kinesin does at roadblocks: the coordination mechanism for molecular walking

Competing models for the coordination of processive stepping in kinesin can be tested by introducing a roadblock to prevent lead head attachment. We used T93N, an irreversibly binding mutant monomer, as a roadblock, and measured the rates of nucleotide-induced detachment of kinesin monomers or dimers with and without the T93N roadblock using microflash photolysis combined with stopped flow. Control nucleotide-induced monomer (rK340) unbinding was 73.6 s(-1) for ATP and 40.5 s(-1) for ADP. Control ADP-induced dimer (rK430) unbinding was 18.6 s(-1). Added 20 mM Pi slowed both monomer and dimer unbinding. With the roadblock in place, lead head attachment of dimers is prevented and ATP-induced trail head unbinding was then 42 s(-1). This is less than two-fold slower than the stepping rate of unimpeded rK430 dimers (50-70 s(-1)), indicating that during walking, lead head attachment induces at most only a slight (less than two-fold) acceleration of trail head detachment. As we discuss, this implies a coordination model having very fast (>2000 s(-1)) ATP-induced attachment of the lead head, followed by slower, strain-sensitive ADP release from the lead head.

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.

Microtubules meet substrate adhesions to arrange cell polarity

Cell movement is driven by the regulated and polarised turnover of the actin cytoskeleton and of the adhesion complexes that link it to the extracellular matrix. For most cells, polarisation requires the engagement of microtubules, which exert their effect by mediating changes in the activity of the Rho GTPases. Evidence suggests that these changes are effected in a very localised fashion at sites of substrate adhesion, via specific microtubule-targeting interactions. Targeting serves to bring molecular complexes bound at the tips and along microtubules in close proximity with adhesion complexes, to promote adhesion disassembly and remodelling of the actin cytoskeleton.

Interaction of the mitotic inhibitor monastrol with human kinesin Eg5

The microtubule-dependent kinesin-like protein Eg5 from Homo sapiens is involved in the assembly of the mitotic spindle. It shows a three-domain structure with an N-terminal motor domain, a central coiled coil, and a C-terminal tail domain. In vivo HsEg5 is reversibly inhibited by monastrol, a small cell-permeable molecule that causes cells to be arrested in mitosis. Both monomeric and dimeric Eg5 constructs have been examined in order to define the minimal monastrol binding domain on HsEg5. NMR relaxation experiments show that monastrol interacts with all of the Eg5 constructs used in this study. Enzymatic techniques indicate that monastrol partially inhibits Eg5 ATPase activity by binding directly to the motor domain. The binding is noncompetitive with respect to microtubules, indicating that monastrol does not interfere with the formation of the motor-MT complex. The binding is not competitive with respect to ATP. Both enzymology and in vivo assays show that the S enantiomer of monastrol is more active than the R enantiomer and racemic monastrol. Stopped-flow fluorometry indicates that monastrol inhibits ADP release by forming an Eg5-ADP-monastrol ternary complex. Monastrol reversibly inhibits the motility of human Eg5. Monastrol has no inhibitory effect on the following members of the kinesin superfamily: MC5 (Drosophila melanogaster Ncd), HK379 (H. sapiens conventional kinesin), DKH392 (D. melanogaster conventional kinesin), BimC1-428 (Aspergillus nidulans BimC), Klp15 (Caenorhabditis elegans C-terminal motor), or Nkin460GST (Neurospora crassa conventional kinesin).

Adhesion-dependent cell mechanosensitivity

The conversion of physical signals, such as contractile forces or external mechanical perturbations, into chemical signaling events is a fundamental cellular process that occurs at cell-extracellular matrix contacts, known as focal adhesions. At these sites, transmembrane integrin receptors are associated via their cytoplasmic domains with the actin cytoskeleton. This interaction with actin is mediated by a submembrane plaque, consisting of numerous cytoskeletal and signaling molecules. Application of intrinsic or external forces to these structures dramatically affects their assembly and triggers adhesion-mediated signaling. In this review, we discuss the structure-function relationships of focal adhesions and the possible mode of action of the putative mechanosensor associated with them. We also discuss the general phenomenon of mechanosensitivity, and the approaches used to measure local forces at adhesion sites, the cytoskeleton-mediated regulation of local contractility, and the nature of the signaling networks that both affect contractility and are affected by it.

How do microtubules guide migrating cells?

Microtubules have long been implicated in the polarization of migrating cells, but how they carry out this role is unclear. Here, we propose that microtubules determine cell polarity by modulating the pattern of adhesions that a cell develops with the underlying matrix, through focal inhibitions of contractility.

Involvement of conventional kinesin in glucose-stimulated secretory granule movements and exocytosis in clonal pancreatic beta-cells

Recruitment of secretory vesicles to the cell surface is essential for the sustained secretion of insulin in response to glucose. At present, the molecular motors involved in this movement, and the mechanisms whereby they may be regulated, are undefined. To investigate the role of kinesin family members, we labelled densecore vesicles in clonal beta-cells using an adenovirally expressed, vesicle-targeted green fluorescent protein (phogrin.EGFP), and employed immunoadsorption to obtain highly purified insulin-containing vesicles. Whereas several kinesin family members were expressed in this cell type, only conventional kinesin heavy chain (KHC) was detected in vesicle preparations. Expression of a dominant-negative KHC motor domain (KHC(mut)) blocked all vesicular movements with velocity >0.4 micro m second(-1), which demonstrates that kinesin activity was essential for vesicle motility in live beta-cells. Moreover, expression of KHC(mut) strongly inhibited the sustained, but not acute, stimulation of secretion by glucose. Finally, vesicle movement was stimulated by ATP dose-dependently in permeabilized cells, which suggests that glucose-induced increases in cytosolic [ATP] mediate the effects of the sugar in vivo, by enhancing kinesin activity. These data therefore provide evidence for a novel mechanism whereby glucose may enhance insulin release.

Association of the Ste20-like kinase (SLK) with the microtubule. Role in Rac1-mediated regulation of actin dynamics during cell adhesion and spreading

Cytoskeletal remodeling events are tightly regulated by signal transduction systems that impinge on adhesion components and modulators of cellular architecture. We have previously shown that the Ste20-like kinase (SLK) can induce apoptosis through the induction of actin disassembly and cellular retraction (Sabourin, L. A., Tamai, K., Seale, P., Wagner, J., and Rudnicki, M. A. (2000) Mol. Cell. Biol. 20, 684-696). Using immunofluorescence studies, we report that SLK is redistributed with adhesion components at large podosome-like adhesion sites in fibronectin-stimulated fibroblasts. However, in vitro kinase assays demonstrate that its activity is not modulated following fibronectin stimulation. Double immunofluorescence studies in exponentially growing or spreading fibroblasts show that SLK is associated with the microtubule network and can be coprecipitated with alpha-tubulin. Furthermore, the stimulation of adhesion site formation by microtubule-disrupting agents induces the relocalization of SLK with unpolymerized alpha-tubulin to large vinculin-containing adhesion complexes. Using microinjection studies, we show that ectopic expression of activated SLK induces the disassembly of actin stress fibers, a process that can be inhibited by dominant negative Rac1. Significantly, endogenous SLK can be colocalized with Rac1 in spreading cells on FN. Finally, the overexpression of SLK by adenoviral infection inhibits cell spreading on fibronectin. These results suggest that SLK is part of a microtubule-associated complex that is targeted to adhesion sites and implicated in the regulation of cytoskeletal dynamics.

Shmoos, rafts, and uropods- the many facets of cell polarity

The recent Juan March Foundation meeting on "Regulation and functional insights in cellular polarity" focused on cellular polarity in yeasts, Dictyostelium, epithelial cells, fibroblasts, and immune cells. The molecular systems covered included membrane rafts, actin and tubulin cytoskeleton, polarized transcription, signaling, and cell-cell adhesion. Across these diverse biological and molecular systems, important general concepts emerged, including new ideas for establishing and maintaining polarity that are likely to be applicable across models and experimental systems.

Microtubule-actin cross-talk at focal adhesions

Focal adhesions are dynamic structures in which traction forces are exerted against the substratum during cell migration and are sites for the organization of signaling complexes. Palazzo and Gundersen discuss how focal adhesions may also be the site of cross-talk between the actin-based and microtubule-based cytoskeletons. Microtubules appear to deliver factors that can regulate the formation and dissolution of focal adhesions, whereas focal adhesions contribute to microtubule localization and stability.

Tensile stress stimulates microtubule outgrowth in living cells

Cell motility is driven by the sum of asymmetric traction forces exerted on the substrate through adhesion foci that interface with the actin cytoskeleton. Establishment of this asymmetry involves microtubules, which exert a destabilising effect on adhesion foci via targeting events. Here, we demonstrate the existence of a mechano-sensing mechanism that signals microtubule polymerisation and guidance of the microtubules towards adhesion sites under increased stress. Stress was applied either by manipulating the body of cells moving on glass with a microneedle or by stretching a flexible substrate that cells were migrating on. We propose a model for this mechano-sensing phenomenon whereby microtubule polymerisation is stimulated and guided through the interaction of a microtubule tip complex with actin filaments under tension.