Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcement

Cytoskeletal microtubules have been proposed to influence cell shape and mechanics based on their ability to resist large-scale compressive forces exerted by the surrounding contractile cytoskeleton. Consistent with this, cytoplasmic microtubules are often highly curved and appear buckled because of compressive loads. However, the results of in vitro studies suggest that microtubules should buckle at much larger length scales, withstanding only exceedingly small compressive forces. This discrepancy calls into question the structural role of microtubules, and highlights our lack of quantitative knowledge of the magnitude of the forces they experience and can withstand in living cells. We show that intracellular microtubules do bear large-scale compressive loads from a variety of physiological forces, but their buckling wavelength is reduced significantly because of mechanical coupling to the surrounding elastic cytoskeleton. We quantitatively explain this behavior, and show that this coupling dramatically increases the compressive forces that microtubules can sustain, suggesting they can make a more significant structural contribution to the mechanical behavior of the cell than previously thought possible.     

Directional loading of the kinesin motor molecule as it buckles a microtubule.

Single kinesin motor molecules were observed to buckle the microtubules along which they moved in a modified in vitro gliding assay. In this assay a central portion of the microtubule was clamped to the glass substrate via biotin-streptavidin bonds, while the plus end of the microtubule was free to interact with motors adsorbed at low density to the substrate. A statistical analysis of the length of microtubules buckled by single motors showed a decreasing probability of buckling for loads greater than 4-6 pN parallel to the filament. This is consistent with kinesin stalling forces found in other experiments. A detailed analysis of some buckling events allowed us to estimate both the magnitude and direction of the loading force as it developed a perpendicular component tending to pull the motor away from the microtubule. We also estimated the motor speed as a function of this changing vector force. The kinesin motors consistently reached unexpectedly high speeds as the force became nonparallel to the direction of motor movement. Our results suggest that a perpendicular component of load does not hinder the kinesin motor, but on the contrary causes the motor to move faster against a given parallel load. Because the perpendicular force component speeds up the motor but does no net work, perpendicular force acts as a mechanical catalyst for the reaction. A simple explanation is that there is a spatial motion of the kinesin molecule during its cycle that is rate-limiting under load; mechanical catalysis results if this motion is oriented away from the surface of the microtubule.     

Coupled oscillations of a protein microtubule immersed in cytoplasm: an orthotropic elastic shell modeling

Revealing vibration characteristics of sub-cellular structural components such as membranes and microtubules has a principal role in obtaining a deeper understanding of their biological functions. Nevertheless, limitations and challenges in biological experiments at this scale necessitates the use of mathematical and computational models as an alternative solution. As one of the three major cytoskeletal filaments, microtubules are highly anisotropic structures built from tubulin heterodimers. They are hollow cylindrical shells with a ∼ 25 nm outer diameter and are tens of microns long. In this study, a mechanical model including the effects of the viscous cytosol and surrounding filaments is developed for predicting the coupled oscillations of a single microtubule immersed in cytoplasm. The first-order shear deformation shell theory for orthotropic materials is used to model the microtubule, whereas the motion of the cytosol is analyzed by considering the Stokes flow. The viscous cytosol and the microtubule are coupled through the continuity condition across the microtubule–cytosol interface. The stress and velocity fields in the cytosol induced by vibrating microtubule are analytically determined. Finally, the influences of the dynamic viscosity of the cytosol, filament network elasticity, microtubule shear modulus, and circumferential wave-number on longitudinal, radial, and torsional modes of microtubule vibration are elucidated.     

Effects of dynein on microtubule mechanics and centrosome positioning

To determine forces on intracellular microtubules, we measured shape changes of individual microtubules following laser severing in bovine capillary endothelial cells. Surprisingly, regions near newly created minus ends increased in curvature following severing, whereas regions near new microtubule plus ends depolymerized without any observable change in shape. With dynein inhibited, regions near severed minus ends straightened rapidly following severing. These observations suggest that dynein exerts a pulling force on the microtubule that buckles the newly created minus end. Moreover, the lack of any observable straightening suggests that dynein prevents lateral motion of microtubules. To explain these results, we developed a model for intracellular microtubule mechanics that predicts the enhanced buckling at the minus end of a severed microtubule. Our results show that microtubule shapes reflect a dynamic force balance in which dynein motor and friction forces dominate elastic forces arising from bending moments. A centrosomal array of microtubules subjected to dynein pulling forces and resisted by dynein friction is predicted to center on the experimentally observed time scale, with or without the pushing forces derived from microtubule buckling at the cell periphery.     

Mechanics of Individual Keratin Bundles in Living Cells

Along with microtubules and microfilaments, intermediate filaments are a major component of the eukaryotic cytoskeleton and play a key role in cell mechanics. In cells, keratin intermediate filaments form networks of bundles that are sparser in structure and have lower connectivity than, for example, actin networks. Because of this, bending and buckling play an important role in these networks. Buckling events, which occur due to compressive intracellular forces and cross-talk between the keratin network and other cytoskeletal components, are measured here in situ. By applying a mechanical model for the bundled filaments, we can access the mechanical properties of both the keratin bundles themselves and the surrounding cytosol. Bundling is characterized by a coupling parameter that describes the strength of the linkage between the individual filaments within a bundle. Our findings suggest that coupling between the filaments is mostly complete, although it becomes weaker for thicker bundles, with some relative movement allowed.     

Decoration of microtubules by fluorescently labeled microtubule-associated protein 2 (MAP2) does not interfere with their spatial organization and progress through mitosis in living fibroblasts

Microtubule-associated protein 2 (MAP2) derivatized with iodoacetamidotetramethylrhodamine or with iodoacetamidofluorescein binds to microtubules after injection into living interphase cells [Scherson et al, 1984]. The binding of derivatized MAP2 stabilized microtubules in vitro; it was therefore important to check if the binding of MAP2 in vivo perturbed the dynamics and organization of the microtubule network. We have addressed these questions by studying the effect of the injection of derivatized MAP2 on mitosis in PtK 1 cells and on the recovery of the microtubule network from low temperature incubation in interphase cells. We found that the presence of derivatized MAP2 did not change the duration of any mitotic stage and that the injected cell normally completed mitosis. We subsequently showed that the injected MAP2 bound to the microtubules within 5 minutes after injection and remained bound throughout the course of mitosis. The reorganization of the microtubule network upon cooling and rewarming was studied in the cytoplasm of human foreskin fibroblasts (356 cells). During the recovery, the distribution of the fluorescent MAP2 in living cells was identical with the microtubule pattern visualized by immunofluorescence in lysed and fixed cells. In these experiments, the fluorescent MAP2 bound to microtubules can be considered as a nonperturbing reporter of the microtubule network. This result is discussed in terms of the role of MAPs in the dynamics and organization of microtubules in living cells.     

Transient Pinning and Pulling: A Mechanism for Bending Microtubules

Microtubules have a persistence length of the order of millimeters in vitro, but inside cells they bend over length scales of microns. It has been proposed that polymerization forces bend microtubules in the vicinity of the cell boundary or other obstacles, yet bends develop even when microtubules are polymerizing freely, unaffected by obstacles and cell boundaries. How these bends are formed remains unclear. By tracking the motions of microtubules marked by photobleaching, we found that in LLC-PK1 epithelial cells local bends develop primarily by plus-end directed transport of portions of the microtubule contour towards stationary locations (termed pinning points) along the length of the microtubule. The pinning points were transient in nature, and their eventual release allowed the bends to relax. The directionality of the transport as well as the overall incidence of local bends decreased when dynein was inhibited, while myosin inhibition had no observable effect. This suggests that dynein generates a tangential force that bends microtubules against stationary pinning points. Simulations of microtubule motion and polymerization accounting for filament mechanics and dynein forces predict the development of bends of size and shape similar to those observed in cells. Furthermore, simulations show that dynein-generated bends at a pinning point near the plus end can cause a persistent rotation of the tip consistent with the observation that bend formation near the tip can change the direction of microtubule growth. Collectively, these results suggest a simple physical mechanism for the bending of growing microtubules by dynein forces accumulating at pinning points.     

The control of microtubule stability in vitro and in transfected cells by MAP1B and SCG10

In neurons, the regulation of microtubules plays an important role for neurite outgrowth, axonal elongation, and growth cone steering. SCG10 family proteins are the only known neuronal proteins that have a strong destabilizing effect, are highly enriched in growth cones and are thought to play an important role during axonal elongation. MAP1B, a microtubule-stabilizing protein, is found in growth cones as well, therefore it was important to test their effect on microtubules in the presence of both proteins. We used recombinant proteins in microtubule assembly assays and in transfected COS-7 cells to analyze their combined effects in vitro and in living cells, respectively. Individually, both proteins showed their expected activities in microtubule stabilization and destruction respectively. In MAP1B/SCG10 double-transfected cells, MAP1B could not protect microtubules from SCG10-induced disassembly in most cells, in particular not in cells that contained high levels of SCG10. This suggests that SCG10 is more potent to destabilize microtubules than MAP1B to rescue them. In microtubule assembly assays, MAP1B promoted microtubule formation at a ratio of 1 MAP1B per 70 tubulin dimers while a ratio of 1 SCG10 per two tubulin dimers was needed to destroy microtubules. In addition to its known binding to tubulin dimers, SCG10 binds also to purified microtubules in growth cones of dorsal root ganglion neurons in culture. In conclusion, neuronal microtubules are regulated by antagonistic effects of MAP1B and SCG10 and a fine tuning of the balance of these proteins may be critical for the regulation of microtubule dynamics in growth cones.     

S. pombe kinesins-8 promote both nucleation and catastrophe of microtubules

The kinesins-8 were originally thought to be microtubule depolymerases, but are now emerging as more versatile catalysts of microtubule dynamics. We show here that S. pombe Klp5-436 and Klp6-440 are non-processive plus-end-directed motors whose in vitro velocities on S. pombe microtubules at 7 and 23 nm s(-1) are too slow to keep pace with the growing tips of dynamic interphase microtubules in living S. pombe. In vitro, Klp5 and 6 dimers exhibit a hitherto-undescribed combination of strong enhancement of microtubule nucleation with no effect on growth rate or catastrophe frequency. By contrast in vivo, both Klp5 and Klp6 promote microtubule catastrophe at cell ends whilst Klp6 also increases the number of interphase microtubule arrays (IMAs). Our data support a model in which Klp5/6 bind tightly to free tubulin heterodimers, strongly promoting the nucleation of new microtubules, and then continue to land as a tubulin-motor complex on the tips of growing microtubules, with the motors then dissociating after a few seconds residence on the lattice. In vivo, we predict that only at cell ends, when growing microtubule tips become lodged and their growth slows down, will Klp5/6 motor activity succeed in tracking growing microtubule tips. This mechanism would allow Klp5/6 to detect the arrival of microtubule tips at cells ends and to amplify the intrinsic tendency for microtubules to catastrophise in compression at cell ends. Our evidence identifies Klp5 and 6 as spatial regulators of microtubule dynamics that enhance both microtubule nucleation at the cell centre and microtubule catastrophe at the cell ends.     

Vinblastine suppresses dynamics of individual microtubules in living interphase cells.

We have characterized the effects of vinblastine on the dynamic instability behavior of individual microtubules in living BS-C-1 cells microinjected with rhodamine-labeled tubulin and have found that at low concentrations (3-64 nM), vinblastine potently suppresses dynamic instability without causing net microtubule depolymerization. Vinblastine suppressed the rates of microtubule growth and shortening, and decreased the frequency of transitions from growth or pause to shortening, also called catastrophe. In vinblastine-treated cells, both the average duration of a pause (a state of attenuated dynamics where neither growth nor shortening could be detected) and the percentage of total time spent in pause were significantly increased. Vinblastine potently decreased dynamicity, a measure of the overall dynamic activity of microtubules, reducing this parameter by 75% at 32 nM. The present work, consistent with earlier in vitro studies, demonstrates that vinblastine kinetically caps the ends of microtubules in living cells and supports the hypothesis that the potent chemotherapeutic action of vinblastine as an antitumor drug is suppression of mitotic spindle microtubule dynamics. Further, the results indicate that molecules that bind to microtubule ends can regulate microtubule dynamic behavior in living cells and suggest that endogenous regulators of microtubule dynamics that work by similar mechanisms may exist in living cells.     

Lateral Motion and Bending of Microtubules Studied with a New Single-Filament Tracking Routine in Living Cells

The cytoskeleton is involved in numerous cellular processes such as migration, division, and contraction and provides the tracks for transport driven by molecular motors. Therefore, it is very important to quantify the mechanical behavior of the cytoskeletal filaments to get a better insight into cell mechanics and organization. It has been demonstrated that relevant mechanical properties of microtubules can be extracted from the analysis of their motion and shape fluctuations. However, tracking individual filaments in living cells is extremely complex due, for example, to the high and heterogeneous background. We introduce a believed new tracking algorithm that allows recovering the coordinates of fluorescent microtubules with ∼9 nm precision in in vitro conditions. To illustrate potential applications of this algorithm, we studied the curvature distributions of fluorescent microtubules in living cells. By performing a Fourier analysis of the microtubule shapes, we found that the curvatures followed a thermal-like distribution as previously reported with an effective persistence length of ∼20 μm, a value significantly smaller than that measured in vitro. We also verified that the microtubule-associated protein XTP or the depolymerization of the actin network do not affect this value; however, the disruption of intermediate filaments decreased the persistence length. Also, we recovered trajectories of microtubule segments in actin or intermediate filament-depleted cells, and observed a significant increase of their motion with respect to untreated cells showing that these filaments contribute to the overall organization of the microtubule network. Moreover, the analysis of trajectories of microtubule segments in untreated cells showed that these filaments presented a slower but more directional motion in the cortex with respect to the perinuclear region, and suggests that the tracking routine would allow mapping the microtubule dynamical organization in cells.     

Cortical microtubule labeling: Insight of AFH14 in non-dividing cells

We recently reported that AFH14 participated in microtubule and actin filament interaction in cell division, and the AFH14 (FH1FH2) was important to the directly binding activity of microtubules and microfilaments. To preliminarily understand the function and localization of AFH14 in non-dividing cells, we overexpressed FH1FH2-RFP in onion epidermal cells, and found a fluorescence labeled filamentous network. The result of double labeling with different cytoskeleton reporter proteins indicated that FH1FH2-RFP co-localized with cortical microtubules. Treatment of cells expressing FH1FH2-RFP with cytoskeleton disrupting drugs confirms that FH1FH2-RFP binds to microtubules. Moreover, the binding of FH1FH2-RFP to microtubules were revealed to be dynamic by fluorescence recovery after photobleaching (FRAP) experiment. Time-lapse confocal microscopy showed that FH1FH2-RFP could display a dynamics similar to the microtubule dynamic instability. These data suggest that FH1FH2 domain may lead AFH14 function on cortical microtubules in non-dividing cells, and FH1FH2-RFP may be utilized as a microtubule reporter protein in living onion epidermal cells.Key words: cortical microtubule, AFH14, non-dividing cell, microtubule dynamics, FRAP     

Apparent stiffness of vimentin intermediate filaments in living cells and its relation with other cytoskeletal polymers

The cytoskeleton is a complex network of interconnected biopolymers intimately involved in the generation and transmission of forces. Several mechanical properties of microtubules and actin filaments have been extensively explored in cells. In contrast, intermediate filaments (IFs) received comparatively less attention despite their central role in defining cell shape, motility and adhesion during physiological processes as well as in tumor progression. Here, we explored relevant biophysical properties of vimentin IFs in living cells combining confocal microscopy and a filament tracking routine that allows localizing filaments with ~20 nm precision. A Fourier-based analysis showed that IFs curvatures followed a thermal-like behavior characterized by an apparent persistence length (lp*) similar to that measured in aqueous solution. Additionally, we determined that certain perturbations of the cytoskeleton affect lp* and the lateral mobility of IFs as assessed in cells in which either the microtubule dynamic instability was reduced or actin filaments were partially depolymerized. Our results provide relevant clues on how vimentin IFs mechanically couple with microtubules and actin filaments in cells and support a role of this network in the response to mechanical stress.     

Vaults bind directly to microtubules via their caps and not their barrels

Vaults are large (13 Mda) ribonucleoprotein particles that are especially abundant in multidrug resistant cancer cells and have been implicated in nucleocytoplasmic drug transport. To understand how these large barrel-shaped complexes are transported through the cytosol, we examined the association of vaults with microtubules both in vitro and in vivo. Within cells, a subpopulation of vaults clearly associates with microtubules, and these vaults remain associated with tubulin dimers/oligomers when microtubules are disassembled by nocodazole treatment. In vitro, a microtubule-pull down assay using highly purified rat vaults and reassembled microtubules reveals that vaults exhibit concentration-dependent binding to microtubules that does not require the carboxyl terminal end of tubulin. Remarkably, negative staining for electron microscopy reveals that vault binding to microtubules is mediated by the vault caps; more than 82% of bound vaults attach to the microtubule lattice with their long axes perpendicular to the long axis of the microtubule. Five to six vault particles were bound per micron of microtubule, with no crosslinking of microtubules observed, suggesting that only one end of the vault can bind microtubules. Taken together, the data support the model of vaults as barrel-shaped containers that transiently interact with microtubules.     

Arabidopsis AUGMIN Subunit8 Is a Microtubule Plus-End Binding Protein That Promotes Microtubule Reorientation in Hypocotyls

In plant cells, cortical microtubules provide tracks for cellulose-synthesizing enzymes and regulate cell division, growth, and morphogenesis. The role of microtubules in these essential cellular processes depends on the spatial arrangement of the microtubules. Cortical microtubules are reoriented in response to changes in cell growth status and cell shape. Therefore, an understanding of the mechanism that underlies the change in microtubule orientation will provide insight into plant cell growth and morphogenesis. This study demonstrated that AUGMIN subunit8 (AUG8) in Arabidopsis thaliana is a novel microtubule plus-end binding protein that participates in the reorientation of microtubules in hypocotyls when cell elongation slows down. AUG8 bound to the plus ends of microtubules and promoted tubulin polymerization in vitro. In vivo, AUG8 was recruited to the microtubule branch site immediately before nascent microtubules branched out. It specifically associated with the plus ends of growing cortical microtubules and regulated microtubule dynamics, which facilitated microtubule reorientation when microtubules changed their growth trajectory or encountered obstacle microtubules during microtubule reorientation. This study thus reveals a novel mechanism underlying microtubule reorientation that is critical for modulating cell elongation in Arabidopsis.     

The Golgi Complex Is a Microtubule-organizing Organelle

We show that the Golgi complex can directly stimulate microtubule nucleation in vivo and in vitro and thus behaves as a potent microtubule-organizing organelle in interphase cells. With the use of nocodazole wash-out experiments in hepatic cells, we found that the occurrence of noncentrosomal, early stabilized microtubules is highly correlated with the subcellular localization of Golgi membranes. With the use of in vitro reconstituted microtubule assembly systems with or without cytosol, we also found that, in contrast to centrosomally attached microtubules, the distal ends of Golgi-attached microtubules are remotely stabilized in a way that requires additional cytosolic component(s). Finally, we demonstrate that Golgi-based microtubule nucleation is direct and involves a subset of gamma-tubulin bound to the cytoplasmic face of the organelle.     

Lateral Motion and Bending of Microtubules Studied with a New Single-Filament Tracking Routine in Living Cells

The cytoskeleton is involved in numerous cellular processes such as migration, division, and contraction and provides the tracks for transport driven by molecular motors. Therefore, it is very important to quantify the mechanical behavior of the cytoskeletal filaments to get a better insight into cell mechanics and organization. It has been demonstrated that relevant mechanical properties of microtubules can be extracted from the analysis of their motion and shape fluctuations. However, tracking individual filaments in living cells is extremely complex due, for example, to the high and heterogeneous background. We introduce a believed new tracking algorithm that allows recovering the coordinates of fluorescent microtubules with ∼9 nm precision in in vitro conditions. To illustrate potential applications of this algorithm, we studied the curvature distributions of fluorescent microtubules in living cells. By performing a Fourier analysis of the microtubule shapes, we found that the curvatures followed a thermal-like distribution as previously reported with an effective persistence length of ∼20 μm, a value significantly smaller than that measured in vitro. We also verified that the microtubule-associated protein XTP or the depolymerization of the actin network do not affect this value; however, the disruption of intermediate filaments decreased the persistence length. Also, we recovered trajectories of microtubule segments in actin or intermediate filament-depleted cells, and observed a significant increase of their motion with respect to untreated cells showing that these filaments contribute to the overall organization of the microtubule network. Moreover, the analysis of trajectories of microtubule segments in untreated cells showed that these filaments presented a slower but more directional motion in the cortex with respect to the perinuclear region, and suggests that the tracking routine would allow mapping the microtubule dynamical organization in cells.     

Insulin-mediated GLUT4 translocation is dependent on the microtubule network

The GLUT4 facilitative glucose transporter is recruited to the plasma membrane by insulin. This process depends primarily on the exocytosis of a specialized pool of vesicles containing GLUT4 in their membranes. The mechanism of GLUT4 vesicle exocytosis in response to insulin is not understood. To determine whether GLUT4 exocytosis is dependent on intact microtubule network, we measured insulin-mediated GLUT4 exocytosis in 3T3-L1 adipocytes in which the microtubule network was depolymerized by pretreatment with nocodazole. Insulin-mediated GLUT4 translocation was inhibited by more than 80% in nocodazole-treated cells. Phosphorylation of insulin receptor substrate 1 (IRS-1), activation of IRS-1 associated phosphatidylinositide 3-kinase, and phosphorylation of protein kinase B/Akt-1 were not inhibited by nocodazole treatment indicating that the microtubule network was not required for proximal insulin signaling. An intact microtubule network is specifically required for insulin-mediated GLUT4 translocation since nocodazole treatment did not affect insulin-mediated GLUT1 translocation or adipsin secretion. By using in vitro microtubule binding, we demonstrated that both GLUT4 vesicles and IRS-1 bind specifically to microtubules, implicating microtubules in both insulin signaling and GLUT4 translocation. Vesicle binding to microtubules was not mediated through direct binding of GLUT4 or insulin-responsive aminopeptidase to microtubules. A model microtubule-dependent translocation of GLUT4 is proposed.