Microtubule dynamics in serum-starved and serum-stimulated Swiss 3T3 mouse fibroblasts: implications for the relationship between serum-induced contractility and microtubules

Cellular quiescence in budding yeast

Cellular quiescence, the temporary and reversible exit from proliferative growth, is the predominant state of all cells. However, our understanding of the biological processes and molecular mechanisms that underlie cell quiescence remains incomplete. As with the mitotic cell cycle, budding and fission yeast are preeminent model systems for studying cellular quiescence owing to their rich experimental toolboxes and the evolutionary conservation across eukaryotes of pathways and processes that control quiescence. Here, we review current knowledge of cell quiescence in budding yeast and how it pertains to cellular quiescence in other organisms, including multicellular animals. Quiescence entails large-scale remodeling of virtually every cellular process, organelle, gene expression, and metabolic state that is executed dynamically as cells undergo the initiation, maintenance, and exit from quiescence. We review these major transitions, our current understanding of their molecular bases, and highlight unresolved questions. We summarize the primary methods employed for quiescence studies in yeast and discuss their relative merits. Understanding cell quiescence has important consequences for human disease as quiescent single-celled microbes are notoriously difficult to kill and quiescent human cells play important roles in diseases such as cancer. We argue that research on cellular quiescence will be accelerated through the adoption of common criteria, and methods, for defining cell quiescence. An integrated approach to studying cell quiescence, and a focus on the behavior of individual cells, will yield new insights into the pathways and processes that underlie cell quiescence leading to a more complete understanding of the life cycle of cells. TAKE AWAY: Quiescent cells are viable cells that have reversibly exited the cell cycle Quiescence is induced in response to a variety of nutrient starvation signals Quiescence is executed dynamically through three phases: initiation, maintenance, and exit Quiescence entails large-scale remodeling of gene expression, organelles, and metabolism Single-cell approaches are required to address heterogeneity among quiescent cells.

Responses of rainbow trout intestinal epithelial cells to different kinds of nutritional deprivation

In order to develop an in vitro system to study the cell biology of starvation in the fish intestine, rainbow trout intestinal epithelial cells were subjected to three kinds of nutrient deprivation and evaluated for 7 days. The RTgutGC cell line was grown into monolayers in Leibovitz's basal medium supplemented with fetal bovine serum (L15/FBS) and then subjected to deprivation of serum (L15); of serum, amino acids, and vitamin (L15/ex); and of all nutrients (L15/salts). After 7 days of nutrient deprivation, the cells remained attached to the plastic surface as monolayers but changes were seen in shape, with the cells becoming more polygonal, actin and α-tubulin cytoskeleton organization, and in tight junction protein-1 (ZO-1) localization. Two barrier functions, transepithelial electrical resistance (TEER) and Lucifer Yellow (LY) retention, were impaired by nutrient deprivation. In L15/FBS, cells rapidly healed a gap or wound in the monolayer. In L15 and L15/ex, some cells moved into the gap, but after 7 days, the wound remained unhealed, whereas in L15/salts, cells did not even migrate into the gap. Upon nutrient replenishment (L15/FBS) after 7 days in L15, L15/ex, or L15/salts, cells proliferated again and healed a wound. After 7 days of nutrient deprivation, monolayers were successfully passaged with trypsin and cells in L15/FBS grew to again form monolayers. Therefore, rainbow trout intestinal epithelial cells survived starvation, but barrier and wound healing functions were impaired.

Microtubule stabilization drives 3D centrosome migration to initiate primary ciliogenesis

The classical view of centrosome decentering and migration to the cell periphery during ciliogenesis is that it is pulled toward its final destination. Here, Pitaval et al. argue that microtubule stabilization in the early stages of ciliogenesis generates pushing forces that propel the centrosome toward the apical pole.

Primary cilia are sensory organelles located at the cell surface. Their assembly is primed by centrosome migration to the apical surface, yet surprisingly little is known about this initiating step. To gain insight into the mechanisms driving centrosome migration, we exploited the reproducibility of cell architecture on adhesive micropatterns to investigate the cytoskeletal remodeling supporting it. Microtubule network densification and bundling, with the transient formation of an array of cold-stable microtubules, and actin cytoskeleton asymmetrical contraction participate in concert to drive apical centrosome migration. The distal appendage protein Cep164 appears to be a key actor involved in the cytoskeleton remodeling and centrosome migration, whereas intraflagellar transport 88’s role seems to be restricted to axoneme elongation. Together, our data elucidate the hitherto unexplored mechanism of centrosome migration and show that it is driven by the increase and clustering of mechanical forces to push the centrosome toward the cell apical pole.

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.

Ballistic intracellular nanorheology reveals ROCK-hard cytoplasmic stiffening response to fluid flow

Cells in vivo are constantly subjected to mechanical shear stresses that play important regulatory roles in various physiological and pathological processes. Cytoskeletal reorganizations that occur in response to shear flow have been studied extensively, but whether the cytoplasm of an adherent cell adapts its mechanical properties to respond to shear is largely unknown. Here we develop a new method where fluorescent nanoparticles are ballistically injected into the cells to probe, with high resolution, possible local viscoelastic changes in the cytoplasm of individual cells subjected to fluid flow. This new assay, ballistic intracellular nanorheology (BIN), reveals that shear flow induces a dramatic sustained 25-fold increase in cytoplasmic viscosity in serum-starved Swiss 3T3 fibroblasts. By contrast, cells stimulated with the actin contractile agonist LPA show highly transient stiffening of much lower amplitude, despite the formation of similar cytoskeletal structures. Shear-induced cytoplasmic stiffening is attenuated by inhibiting actomyosin interactions and is entirely eliminated by specific Rho-kinase (ROCK) inhibition. Together, these results show that biochemical and biophysical stimuli may elicit the formation of qualitatively similar cytoskeleton structures (i.e. stress fibers and focal adhesions), but induces quantitatively different micromechanical responses. Our results suggest that when an adherent cell is subjected to shear stresses, its first order of action is to prevent detachment from its substratum by greatly stiffening its cytoplasm through enhanced actin assembly and Rho-kinase mediated contractility.

Regulation of microtubule dynamics in 3T3 fibroblasts by Rho family GTPases

To get insight into the action of Rho GTPases on the microtubule system we investigated the effects of Cdc42, Rac1, and RhoA on the dynamics of microtubules in Swiss 3T3 fibroblasts. In control cells microtubule ends were dynamic: plus ends frequently switched between growth, shortening and pauses; the growth phase predominated over shortening. Free minus ends of microtubules depolymerized rapidly and never grew. Free microtubules were short-lived, and the microtubule network was organized into a radial array. In serum-starved cells microtubule ends became more stable: although plus ends still transited between growth and shortening, polymerization and depolymerization excursions became shorter and balanced each other. Microtubule minus ends were also stabilized. Consequently lifespan of free microtubules increased and microtubule array changed its radial pattern into a random one. Activation of Cdc42 and Rac1 in serum-starved cells promoted dynamic behavior of microtubule plus and minus ends, while inhibition of these GTPases in serum-grown cells suppressed microtubule dynamics and mimicked all effects of serum starvation. Activation of RhoA in serum-grown cells had effects similar to Cdc42 /Rac1 inactivation: it suppressed the dynamics of plus and minus ends, reduced the length of growth and shrinking episodes, and disrupted the radial organization of microtubules. However, in contrast to Cdc42 and Rac1 inactivation, active RhoA had no effect on the balance between microtubule growth and shortening. We conclude that Cdc42 and Rac1 have similar stimulating effects on microtubule dynamics while RhoA acts in an opposite way.

Dispersal of epithelial cancer cell colonies by lysophosphatidic acid (LPA)

We describe a model system in which cancer cell colonies disperse into single, highly migratory cells in response to lysophosphatidic acid (LPA). Though LPA is known to stimulate chemotaxis and chemokinesis, a colony dispersal effect has not been reported, to our knowledge. Cancer colony dispersal by LPA is comprised of an ordered sequence of events: (1) stimulation of membrane ruffling and formation of lamellipodia, (2) dissolution of adherens junctions, (3) single cell migration in a mesenchymal-like morphology we term "ginkgo-leaf." The net result is dispersal of carcinoma cells from a compact colony. We analyzed these three steps using live-cell imaging and computer-assisted quantification and measured the following parameters: onset of lamellipodia formation, lamellipodia velocity, colony dispersal, trans-epithelial resistance, migrating cell number and speed. Because hepatocyte growth factor (HGF) was described as an epithelial scatter factor, we compared it to LPA in our system and found that HGF has no epithelial colony dispersal properties and that this effect is strictly related to LPA. Given its striking similarity to tumor cell budding observed in patients, we propose that LPA-colony dispersal may provide a cellular mechanism underlying cancer invasion and as such deserves further studies.

Comparative vibrational spectroscopy of intracellular tau and extracellular collagen I reveals parallels of gelation and fibrillar structure

The N-terminal tau 2-19 peptide undergoes gelation, syneresis, and aggregation over a period of years. These changes may be approximated on a shorter time scale by agitation and partial dehydration. The anomalously enhanced (229 nm) ultraviolet resonance Raman (UVRR) imide II band reveals a common structural feature for gels of nondehydrated tau 2-19 and collagen I and insoluble paired helical filaments (PHFs) and collagen I of weak hydrogen bonding at proline carbonyls. Anomalous UVRR enhancement of amide bands at 229 nm results from gel structure, as demonstrated by increased amide absorption at the red edge for tau 2-19 gel and implies the involvement of water in gel structure. In aged, dehydrated tau 2-19 gel, proline carbonyls lose their bonds to water and tyrosine becomes deprotonated, as demonstrated by UVRR spectroscopy. The Fourier transform infrared (FTIR) amide I band shows that antiparallel beta-sheet structure increases with syneresis in the tau 2-19 hydrogel. The comparison of FTIR results for PHFs with collagen I gel and polyproline demonstrates that the secondary structure of PHFs is polyproline II. One implication of this assignment is that the fibrillation of hydrophilic tau is thermodynamically driven by the entropy gained as hydrogen-bonded water is freed, as for collagen I. The FTIR results also show that peptide domains culled from a longer protein do not necessarily fold into identical secondary structures. A pathological, sequential mechanism of gelation, syneresis, and fibrillation for tau in AD is suggested and is supported by the observation of amorphous neurofibrillary tangle development and fibrillation in vivo.

The prostate-derived sterile 20-like kinase (PSK) regulates microtubule organization and stability

Sterile 20 (STE20) protein kinases, which include germinal center kinases and p21-activated protein kinases, are known to activate mitogen-activated protein kinase pathways (c-Jun NH(2)-terminal kinase, p38, or extracellular signal-regulated kinase), leading to changes in gene transcription. Some STE20s can also regulate the cytoskeleton, and we have shown that the germinal center kinase-like kinase prostate-derived STE20-like kinase (PSK) affects actin cytoskeletal organization. Here, we demonstrate that PSK colocalizes with microtubules; and that this localization is disrupted by the microtubule depolymerizing agent nocodazole. The association of PSK with microtubules results in the production of stabilized perinuclear microtubule cables that are nocodazole-resistant and contain increased levels of acetylated alpha-tubulin. Kinase-defective PSK (K57A) or the C terminus of PSK (amino acids 745-1235) lacking the kinase domain are sufficient for microtubule binding and stabilization, demonstrating that the catalytic activity of the protein is not required. The localization of PSK to microtubules occurs via its C terminus, and PSK binds and phosphorylates alpha- and beta-tubulin in vitro. The N terminus of PSK (1-940) is unable to bind or stabilize microtubules, demonstrating that PSK must associate with microtubules for their reorganization to occur. These results demonstrate that PSK interacts with microtubules and affects their organization and stability independently of PSK kinase activity.

Dynamic regulation of microtubule coils in ADP-induced platelet shape change by p160ROCK (Rho-kinase)

Platelet shape change is an extremely rapid process mediated by both the calcium-sensitive and p160ROCK pathways. The present study examines how different features of shape change studied by scanning electron microscopy clearly correlate to changes in the pattern of light absorbance measured in an aggregometer. Platelets change shape from the initial 'disc' form by producing: membrane 'blebs', sphere formation (cell-rounding), filopodia extension, and surface membrane folding. The presentation of these features was dramatically slower in the absence of intracellular calcium mobilization. In the presence of the p160ROCK-inhibitor, Y-27632, shape change was initially normal but platelets rapidly transformed back to smooth discs with extended filopodia. The reappearance of the disc shape is reflected by an increase in the amplitude of oscillations in the aggregometer shape change tracing. The kinetics of actin/cytoskeleton association correlated with filopodia formation but not with disc to sphere transformation. Changes in the level of tubulin polymerization correlated with changes from disc to sphere morphology. These experiments are consistent with a role for a RhoA/Rho kinase-regulated pathway in the maintenance of a spherical platelet shape after agonist-dependent activation. Continued disruption of the cytoskeletal microtubule ring, appears to be a Rhokinase-dependent event involved in the transformation of discoid platelets into spheres.

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

Recent studies have shown that the targeting of substrate adhesions by microtubules promotes adhesion site disassembly (Kaverina, I., O. Krylyshkina, and J.V. Small. 1999. J. Cell Biol. 146:1033-1043). It was accordingly suggested that microtubules serve to convey a signal to adhesion sites to modulate their turnover. Because microtubule motors would be the most likely candidates for effecting signal transmission, we have investigated the consequence of blocking microtubule motor activity on adhesion site dynamics. Using a function-blocking antibody as well as dynamitin overexpression, we found that a block in dynein-cargo interaction induced no change in adhesion site dynamics in Xenopus fibroblasts. In comparison, a block of kinesin-1 activity, either via microinjection of the SUK-4 antibody or of a kinesin-1 heavy chain construct mutated in the motor domain, induced a dramatic increase in the size and reduction in number of substrate adhesions, mimicking the effect observed after microtubule disruption by nocodazole. Blockage of kinesin activity had no influence on either the ability of microtubules to target substrate adhesions or on microtubule polymerisation dynamics. We conclude that conventional kinesin is not required for the guidance of microtubules into substrate adhesions, but is required for the focal delivery of a component(s) that retards their growth or promotes their disassembly.

Determination of the net exchange rate of tubulin dimer in steady-state microtubules by fluorescence correlation spectroscopy

The microtubule cytoskeleton plays an important role in eukaryotic cells, e. g., in cell movement or morphogenesis. Microtubules, formed by assembly of tubulin dimers, are dynamic polymers changing randomly between periods of growing and shortening, a property known as dynamic instability. Another process characterizing the dynamic behaviour is the so-called treadmilling due to different binding constants of tubulin at both microtubule ends. In this study, we used tetramethylrhodamine (TMR)-labeled tubulin added to microtubule suspensions to determine the net exchange rate (NER) of tubulin dimers by fluorescence correlation spectroscopy (FCS) as a measure for microtubule dynamics. This approach, which seems to be suitable as a screening system to detect compounds influencing the NER of tubulin dimers into microtubules at steady-state, showed that taxol, nocodazole, colchicine, and vinblastine affect microtubule dynamics at concentrations as low as 10(-9)-10(-10) M.

Microtubule disassembly increases endothelial cell barrier dysfunction: role of MLC phosphorylation

Endothelial cell (EC) barrier regulation is critically dependent on cytoskeletal components (microfilaments and microtubules). Because several edemagenic agents induce actomyosin-driven EC contraction tightly linked to myosin light chain (MLC) phosphorylation and microfilament reorganization, we examined the role of microtubule components in bovine EC barrier regulation. Nocodazole or vinblastine, inhibitors of microtubule polymerization, significantly decreased transendothelial electrical resistance in a dose-dependent manner, whereas pretreatment with the microtubule stabilizer paclitaxel significantly attenuated this effect. Decreases in transendothelial electrical resistance induced by microtubule disruption correlated with increases in lung permeability in isolated ferret lung preparations as well as with increases in EC stress fiber content and MLC phosphorylation. The increases in MLC phosphorylation were attributed to decreases in myosin-specific phosphatase activity without significant increases in MLC kinase activity and were attenuated by paclitaxel or by several strategies (C3 exotoxin, toxin B, Rho kinase inhibition) to inhibit Rho GTPase. Together, these results suggest that microtubule disruption initiates specific signaling pathways that cross talk with microfilament networks, resulting in Rho-mediated EC contractility and barrier dysfunction.

Yeast Bim1p promotes the G1-specific dynamics of microtubules

Microtubule dynamics vary during the cell cycle, and microtubules appear to be more dynamic in vivo than in vitro. Proteins that promote dynamic instability are therefore central to microtubule behavior in living cells. Here, we report that a yeast protein of the highly conserved EB1 family, Bim1p, promotes cytoplasmic microtubule dynamics specifically during G1. During G1, microtubules in cells lacking BIM1 showed reduced dynamicity due to a slower shrinkage rate, fewer rescues and catastrophes, and more time spent in an attenuated/paused state. Human EB1 was identified as an interacting partner for the adenomatous polyposis coli (APC) tumor suppressor protein. Like human EB1, Bim1p localizes to dots at the distal ends of cytoplasmic microtubules. This localization, together with data from electron microscopy and a synthetic interaction with the gene encoding the kinesin Kar3p, suggests that Bim1p acts at the microtubule plus end. Our in vivo data provide evidence of a cell cycle-specific microtubule-binding protein that promotes microtubule dynamicity.

Microtubules and signal transduction

Although molecular components of signal transduction pathways are rapidly being identified, how elements of these pathways are positioned spatially and how signals traverse the intracellular environment from the cell surface to the nucleus or to other cytoplasmic targets are not well understood. The discovery of signaling molecules that interact with microtubules (MTs), as well as the multiple effects on signaling pathways of drugs that destabilize or hyperstabilize MTs, indicate that MTs are likely to be critical to the spatial organization of signal transduction. MTs themselves are also affected by signaling pathways and this may contribute to the transmission of signals to downstream targets.