Microtubule disruption induces the formation of actin stress fibers and focal adhesions in cultured cells: possible involvement of the rho signal cascade

Coordination of microtubules and the actin cytoskeleton by the Rho effector mDia1

Coordination of microtubules and the actin cytoskeleton is important in several types of cell movement. mDia1 is a member of the formin-homology family of proteins and an effector of the small GTPase Rho. It contains the Rho-binding domain in its amino terminus and two distinct regions of formin homology, FH1 in the middle and FH2 in the carboxy terminus. Here we show that expression of mDia1(DeltaN3), an active mDia1 mutant containing the FH1 and FH2 regions without the Rho-binding domain, induces bipolar elongation of HeLa cells and aligns microtubules in parallel to F-actin bundles along the long axis of the cell. The cell elongation and microtubule alignment caused by this mutant is abolished by co-expression of an FH2-region fragment, and expression of mDia1(DeltaN3) containing point mutations in the FH2 region causes an increase in the amount of disorganized F-actin without cell elongation and microtubule alignment. These results indicate that mDia1 may coordinate microtubules and F-actin through its FH2 and FH1 regions, respectively.

Microtubule-dependent formation of podosomal adhesion structures in primary human macrophages

Podosomes are unique actin-rich adhesion structures of monocyte-derived cells such as macrophages and osteoclasts. They clearly differ from other substratum-contacting organelles like focal adhesions in morphological and functional regards. Formation of podosomes has been shown to be dependent on the small GTPase CDC42Hs and its effector Wiskott-Aldrich syndrome protein (WASp). In this study, we investigated the functional relation between podosomes and the microtubule system in primary human macrophages. We demonstrate that, in contrast to focal adhesions, assembly of podosomes in macrophages and their monocytic precursors is dependent on an intact microtubule system. In contrast, experiments using Wiskott-Aldrich syndrome (WAS) macrophages indicate that the microtubule system is not reciprocally dependent on podosomes. A potential linker between podosomes and microtubules may be WASp itself, considering that microinjection of the WASp polyproline domain prevents podosome reassembly. This polyproline domain is thought to link WASp to microtubules via CDC42 interacting protein 4 (CIP4). Consistently, macrophages microinjected with CIP4 constructs deficient in either the microtubule- or the WASp-binding domain also fail to reassemble podosomes. In sum, our findings show that microtubules are essential for podosome formation in primary human macrophages and that WASp and CIP4 may be involved in this phenomenon.

Overexpression of wild-type RhoA produces growth arrest by disrupting actin cytoskeleton and microtubules

We have investigated the role of Rho GTPase in cell growth by generating stable cells that express the wild-type RhoA (RhoA(wt)) under the control of an inducible promoter. Induction of RhoA(wt) had a biphasic effect on the actin cytoskeleton. At low levels of expression, RhoA(wt) stimulated the assembly of actin stress fibers without affecting cell growth. At high levels, there was a paradoxical disruption of the actin cytoskeleton accompanied by a growth arrest. Cell cycle analysis revealed a dual block at the G(1)/S and G(2)/M checkpoints. The G(1)/S arrest correlated with the accumulation of p21(Cip1), resulting in the inhibition of cdk2 activity, whereas the G(2)/M block correlated with the loss of microtubules. The cyclin B level and the cdc2 kinase activity, however, were increased, suggesting that the progression through mitosis rather than entry into the G(2)/M is defective when RhoA(wt) is overexpressed. Similar cell cycle defects and the loss of microtubules were observed after a cytochalasin D treatment, indicating that the ability of RhoA to regulate the integrity of actin cytoskeleton may be critical for the cell cycle transition through both the G(1)/S and M phase checkpoints.

Adhesion complexes implicated in intestinal epithelial cell-matrix interactions

This article review summarizes data on cell-substratum adhesion complexes involved in the regulation of cellular functions in the intestine. We first focus on the molecular composition of the two main adhesion structures-the beta1 integrin-adhesion complex and the hemidesmosome-found in vivo and in two human intestinal cell lines. We also report the key findings on the cellular behavior and response to the extracellular matrix that involve integrins, the main transmembrane anchors of these complexes. How the dynamics of cell/extracellular matrix interactions contribute to cell migration, proliferation, differentiation, and tumorigenicity is discussed in the light of the data provided by the human intestinal cells.

Regulation of the formation of tumor cell pseudopodia by the Na(+)/H(+) exchanger NHE1

The Na(+)/H(+) exchanger NHE1 is involved in intracellular pH homeostasis and cell volume regulation and accumulates with actin in the lamellipodia of fibroblasts. In order to determine the role of NHE1 following epithelial transformation and the acquisition of motile and invasive properties, we studied NHE1 expression in polarized MDCK cells, Moloney Sarcoma virus (MSV) transformed MDCK cells and an invasive MSV-MDCK cell variant (MSV-MDCK-INV). Expression of NHE1 was significantly increased in MSV-MDCK-INV cells relative to MSV-MDCK and MDCK cells. NHE1 was localized with b-actin to the tips of MSV-MDCK-INV cell pseudopodia by immunofluorescence. Sensitivity of NHE1-mediated (22)Na uptake to ethylisopropylamiloride, a specific inhibitor of NHE1, was increased in MSV-MDCK cells relative to MDCK cells. Changes in intracellular pH induced upon EIPA treatment were also of higher magnitude in MSV-MDCK and MSV-MDCK-INV cells compared to wild-type MDCK cells, especially in Hepes-buffered DMEM medium. Inhibition of NHE1 by 50 microM ethylisopropylamiloride induced the disassembly of actin stress fibers and redistribution of the actin cytoskeleton in all cell types. However, in MSV-MDCK-INV cells, the effect of ethylisopropylamiloride treatment was more pronounced and associated with the increased reversible detachment of the cells from the substrate. Videomicroscopy of MSV-MDCK-INV cells revealed that within 20 minutes of addition, ethylisopropylamiloride induced pseudopodial retraction and inhibited cell motility. The ability of ethylisopropylamiloride to prevent nocodazole-induced formation of actin stress fibers in MSV-MDCK cells was more pronounced in Hepes medium relative to NaHCO(3) medium, showing that NHE1 can regulate actin stress fiber assembly in transformed MSV-MDCK cells via its intracellular pH regulatory effect. These results implicate NHE1 in the regulation of the actin cytoskeleton dynamics necessary for the adhesion and pseudopodial protrusion of motile, invasive tumor cells.

Actin-dependent lamellipodia formation and microtubule-dependent tail retraction control-directed cell migration

Migrating cells are polarized with a protrusive lamella at the cell front followed by the main cell body and a retractable tail at the rear of the cell. The lamella terminates in ruffling lamellipodia that face the direction of migration. Although the role of actin in the formation of lamellipodia is well established, it remains unclear to what degree microtubules contribute to this process. Herein, we have studied the contribution of microtubules to cell motility by time-lapse video microscopy on green flourescence protein-actin- and tubulin-green fluorescence protein-transfected melanoma cells. Treatment of cells with either the microtubule-disrupting agent nocodazole or with the stabilizing agent taxol showed decreased ruffling and lamellipodium formation. However, this was not due to an intrinsic inability to form ruffles and lamellipodia because both were restored by stimulation of cells with phorbol 12-myristate 13-acetate in a Rac-dependent manner, and by stem cell factor in melanoblasts expressing the receptor tyrosine kinase c-kit. Although ruffling and lamellipodia were formed without microtubules, the microtubular network was needed for advancement of the cell body and the subsequent retraction of the tail. In conclusion, we demonstrate that the formation of lamellipodia can occur via actin polymerization independently of microtubules, but that microtubules are required for cell migration, tail retraction, and modulation of cell adhesion.

Microtubules remodel actomyosin networks in Xenopus egg extracts via two mechanisms of F-actin transport

Interactions between microtubules and filamentous actin (F-actin) are crucial for many cellular processes, including cell locomotion and cytokinesis, but are poorly understood. To define the basic principles governing microtubule/F-actin interactions, we used dual-wavelength digital fluorescence and fluorescent speckle microscopy to analyze microtubules and F-actin labeled with spectrally distinct fluorophores in interphase Xenopus egg extracts. In the absence of microtubules, networks of F-actin bundles zippered together or exhibited serpentine gliding along the coverslip. When microtubules were nucleated from Xenopus sperm centrosomes, they were released and translocated away from the aster center. In the presence of microtubules, F-actin exhibited two distinct, microtubule-dependent motilities: rapid ( approximately 250-300 nm/s) jerking and slow ( approximately 50 nm/s), straight gliding. Microtubules remodeled the F-actin network, as F-actin jerking caused centrifugal clearing of F-actin from around aster centers. F-actin jerking occurred when F-actin bound to motile microtubules powered by cytoplasmic dynein. F-actin straight gliding occurred when F-actin bundles translocated along the microtubule lattice. These interactions required Xenopus cytosolic factors. Localization of myosin-II to F-actin suggested it may power F-actin zippering, while localization of myosin-V on microtubules suggested it could mediate interactions between microtubules and F-actin. We examine current models for cytokinesis and cell motility in light of these findings.

Neuroprotection mediated by changes in the endothelial actin cytoskeleton

Cerebral blood flow is regulated by endothelium-derived nitric oxide (NO), and endothelial NO synthase-deficient (eNOS-deficient; eNOS(-/-)) mice develop larger cerebral infarctions following middle cerebral artery (MCA) occlusion. We report that disruption of Rho-mediated endothelial actin cytoskeleton leads to the upregulation of eNOS expression and reduces the severity of cerebral ischemia following MCA occlusion. Mice treated with the Rho inhibitor Clostridium botulinum C3 transferase (10 microgram/d) or the actin cytoskeleton disrupter cytochalasin D (1 mg/kg) showed a two- to fourfold increase in vascular eNOS expression and activity. This increase in eNOS expression was not due to increases in eNOS gene transcription, but to prolongation of eNOS mRNA half-life from 10 +/- 3 hours to 24 +/- 4 hours. Indeed, endothelial cells overexpressing a dominant-negative Rho mutant (N19RhoA) exhibited decreased actin stress fiber formation and increased eNOS expression. Inhibition of vascular Rho guanosine-5'-triphosphate binding activity by the 3-hydroxy-3-methylglutaryl-coenzyme A reductase inhibitor simvastatin increased cerebral blood flow to ischemic regions of the brain, and mice treated with simvastatin, C3 transferase, or cytochalasin D showed smaller cerebral infarctions following MCA occlusion. No neuroprotection was observed with these agents in eNOS(-/-) mice. These findings suggest that therapies which target the endothelial actin cytoskeleton may have beneficial effects in ischemic stroke.

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

It has been established that cell contractility can be stimulated with low or depolymerizing doses of microtubule (MT) poisons. In addition, low doses of nocodazole and vinblastine have recently been shown to decrease MT dynamics in vivo. In this study, investigated whether there is a direct, or reciprocal feedback-type relationship between contractility and microtubule dynamics, by examining MT dynamic behavior in live cells under conditions where contractility is known to be altered. Quiescent, serum-starved Swiss 3T3 mouse fibroblasts have been shown to be weakened in their contractility; serum stimulation increases cell contractility and causes the formation of stress fibers and adhesion plaques. Growing (control), quiescent (Go), and serum-stimulated cells were injected with rhodamine-tubulin, and MT dynamics were determined by analysis of MT length changes obtained from digitized images of the extreme periphery of the cells, where the MT ends were readily apparent. The MTs in quiescent cells were less dynamic than those in control cells: the growth and shortening rates were reduced by 30% and 45%, respectively. Dynamicity decreased by 47%, and the MTs spent more time in pause. After serum stimulation, MT growth rate, dynamicity, and time spent in pause returned to control cell levels. Although the shortening rate increased by 28%, it remained significantly lower than in control cells. In this system, the serum-induced increase in contractility was accompanied by an increase in MT dynamics. However, increased contractility stimulated with low doses of MT poisons is known to be accompanied by a decrease in MT dynamics. These results suggest that the relationship between MT dynamics and contractility is an indirect one.

Relationship between microtubule dynamics and lamellipodium formation revealed by direct imaging of microtubules in cells treated with nocodazole or taxol

Microtubules (MTs) contribute to the directional locomotion of many cell types through an unknown mechanism. Previously, we showed that low concentrations (<200 nM) of nocodazole or taxol reduced the rate of locomotion of NRK fibroblasts over 60% without altering MT polymer level [Liao et al., 1995: J. Cell Sci. 108:3473-3483]. In this paper, we directly measured the dynamics of MTs in migrating NRK cells injected with rhodamine tubulin and treated with low concentrations of nocodazole or taxol. Both drug treatments caused statistically significant reductions (approx. twofold) in growth and shortening rates and less dramatic effects on rescue and catastrophe transition frequencies. The percent time MTs were inactive (i.e., paused) increased greater than twofold in nocodazole- and taxol-treated cells, while the percent time growing was substantially reduced. Three parameters of MT dynamics were linearly related to the rates of locomotion determined previously: rate of shortening, percent time pausing and percent time growing. The number of MTs that came within 1 microm of the leading edge was reduced in drug-treated cells, suggesting that reduced MT dynamics may affect actin arrays necessary for cell locomotion. We examined two such structures, lamellipodium and adhesion plaques, and found that lamellipodia area was coordinately reduced with MT dynamics. No effect was detected on adhesion plaque density or distribution. In time-lapse recordings, MTs did not penetrate into the lamellipodium of untreated cells, suggesting that MTs affect lamellipodia either through their interaction with factors at the base of the lamellipodium or by releasing factors that diffuse into the lamellipodia. In support of the latter hypothesis, when all MTs were rapidly depolymerized by 20 microM nocodazole, we detected the rapid formation of exaggerated protrusions from the leading edge of the cell. Our results show for the first time a linear relationship between MT dynamics and the formation of the lamellipodium and support the idea that MT dynamics may contribute to cell locomotion by regulating the size of the lamellipodium, perhaps through diffusable factors.

Bni1p regulates microtubule-dependent nuclear migration through the actin cytoskeleton in Saccharomyces cerevisiae

The RHO1 gene encodes a yeast homolog of the mammalian RhoA protein. Rho1p is localized to the growth sites and is required for bud formation. We have recently shown that Bni1p is one of the potential downstream target molecules of Rho1p. The BNI1 gene is implicated in cytokinesis and the establishment of cell polarity in Saccharomyces cerevisiae but is not essential for cell viability. In this study, we screened for mutations that were synthetically lethal in combination with a bni1 mutation and isolated two genes. They were the previously identified PAC1 and NIP100 genes, both of which are implicated in nuclear migration in S. cerevisiae. Pac1p is a homolog of human LIS1, which is required for brain development, whereas Nip100p is a homolog of rat p150(Glued), a component of the dynein-activated dynactin complex. Disruption of BNI1 in either the pac1 or nip100 mutant resulted in an enhanced defect in nuclear migration, leading to the formation of binucleate mother cells. The arp1 bni1 mutant showed a synthetic lethal phenotype while the cin8 bni1 mutant did not, suggesting that Bni1p functions in a kinesin pathway but not in the dynein pathway. Cells of the pac1 bni1 and nip100 bni1 mutants exhibited a random distribution of cortical actin patches. Cells of the pac1 act1-4 mutant showed temperature-sensitive growth and a nuclear migration defect. These results indicate that Bni1p regulates microtubule-dependent nuclear migration through the actin cytoskeleton. Bni1p lacking the Rho-binding region did not suppress the pac1 bni1 growth defect, suggesting a requirement for the Rho1p-Bni1p interaction in microtubule function.

Caldesmon inhibits nonmuscle cell contractility and interferes with the formation of focal adhesions

Caldesmon is known to inhibit the ATPase activity of actomyosin in a Ca(2+)-calmodulin-regulated manner. Although a nonmuscle isoform of caldesmon is widely expressed, its functional role has not yet been elucidated. We studied the effects of nonmuscle caldesmon on cellular contractility, actin cytoskeletal organization, and the formation of focal adhesions in fibroblasts. Transient transfection of nonmuscle caldesmon prevents myosin II-dependent cell contractility and induces a decrease in the number and size of tyrosine-phosphorylated focal adhesions. Expression of caldesmon interferes with Rho A-V14-mediated formation of focal adhesions and stress fibers as well as with formation of focal adhesions induced by microtubule disruption. This inhibitory effect depends on the actin- and myosin-binding regions of caldesmon, because a truncated variant lacking both of these regions is inactive. The effects of caldesmon are blocked by the ionophore A23187, thapsigargin, and membrane depolarization, presumably because of the ability of Ca(2+)-calmodulin or Ca(2+)-S100 proteins to antagonize the inhibitory function of caldesmon on actomyosin contraction. These results indicate a role for nonmuscle caldesmon in the physiological regulation of actomyosin contractility and adhesion-dependent signaling and further demonstrate the involvement of contractility in focal adhesion formation.

Microtubule targeting of substrate contacts promotes their relaxation and dissociation

We recently showed that substrate contact sites in living fibroblasts are specifically targeted by microtubules (Kaverina, I., K. Rottner, and J.V. Small. 1998. J. Cell Biol. 142:181-190). Evidence is now provided that microtubule contact targeting plays a role in the modulation of substrate contact dynamics. The results are derived from spreading and polarized goldfish fibroblasts in which microtubules and contact sites were simultaneously visualized using proteins conjugated with Cy-3, rhodamine, or green fluorescent protein. For cells allowed to spread in the presence of nocodazole the turnover of contacts was retarded, as compared with controls and adhesions that were retained under the cell body were dissociated after microtubule reassembly. In polarized cells, small focal complexes were found at the protruding cell front and larger adhesions, corresponding to focal adhesions, at the retracting flanks and rear. At retracting edges, multiple microtubule contact targeting preceded contact release and cell edge retraction. The same effect could be observed in spread cells, in which microtubules were allowed to reassemble after local disassembly by the application of nocodazole to one cell edge. At the protruding front of polarized cells, focal complexes were also targeted and as a result remained either unchanged in size or, more rarely, were disassembled. Conversely, when contact targeting at the cell front was prevented by freezing microtubule growth with 20 nM taxol and protrusion stimulated by the injection of constitutively active Rac, peripheral focal complexes became abnormally enlarged. We further found that the local application of inhibitors of myosin contractility to cell edges bearing focal adhesions induced the same contact dissociation and edge retraction as observed after microtubule targeting. Our data are consistent with a mechanism whereby microtubules deliver localized doses of relaxing signals to contact sites to retard or reverse their development. We propose that it is via this route that microtubules exert their well-established control on cell polarity.

Regulation of the type II hemidesmosomal plaque assembly in intestinal epithelial cells

Hemidesmosomes (HDs) are cellular junctions that anchor epithelial cells to the extracellular matrix (ECM) and are associated morphologically with the cytoskeleton. Hemidesmosomal molecular components include two proteins involved in linking intermediate filaments, HD1/plectin and BP230, and two transmembrane proteins, BP180 and the alpha6beta4 integrin, a laminin receptor. In cells lacking BP230 and BP180, HD1/plectin still associates with alpha6beta4 integrin, forming HD-like structures, called type II HDs. In the present study, we used an intestinal epithelial cell line that expresses HD1/plectin and the alpha6beta4 integrin to investigate the regulation of assembly of these proteins in type II HDs. These compounds were found to be clustered at sites of cell-ECM contact and their polarized localization was influenced by either cell confluency or extracellular matrix deposition. Conventional and immunoelectron microscopy showed that HD1/plectin and the beta4 integrin subunit are colocalized in an adhesion structure. Using cytoskeleton-disrupting drugs and confocal microscopy, we demonstrated that type II HDs are made up of numerous individual plaques whose assembly into a cluster requires actin filaments, but not microtubules.

The polarization of the motile cell

Polarization of the motile cell is associated with the formation of a distinct plasma membrane domain, the pseudopod, whose stabilization determines the directionality of cell movement. The rapid movement of cells over a substrate requires that an essential aspect of cell motility must be the supply of the necessary molecular machinery to the site of pseudopodial extension. Renewal of this pseudopodial domain requires the directed delivery to the site of pseudopodial protrusion of proteins which regulate actin cytoskeleton dynamics, cell-substrate adhesion, and localized degradation of the extracellular matrix. Polarized targeting mechanisms include the targeted delivery of beta-actin mRNA to the leading edge and microtubule-based vesicular traffic. The latter may include Golgi-derived vesicles of the biosynthetic pathway as well as clathrin-dependent and clathrin-independent endocytosis and recycling. Coordination of protrusive activities and supply mechanisms is critical for efficient cellular displacement and may implicate small GTPases of the Rho family. While the specific molecular mechanisms underlying pseudopodial protrusion of the motile cell are well-characterized, discussion of these diverse mechanisms in the context of cellular polarization has been limited.

Cytoskeleton cross-talk during cell motility

Cell crawling entails the co-ordinated creation and turnover of substrate contact sites that interface with the actin cytoskeleton. The initiation and maturation of contact sites involves signalling via the Rho family of small G proteins, whereas their turnover is under the additional influence of the microtubule cytoskeleton. By exerting relaxing effects on substrate contact assemblies in a site- and dose-specific manner, microtubules can promote both protrusion at the front and retraction at the rear, and thereby control cell polarity.

Calreticulin affects focal contact-dependent but not close contact-dependent cell-substratum adhesion

We used two cell lines expressing fast (RPEfast) and slow (RPEslow) attachment kinetics to investigate mechanisms of cell-substratum adhesion. We show that the abundance of a cytoskeletal protein, vinculin, is dramatically decreased in RPEfast cells. This coincides with the diminished expression level of an endoplasmic reticulum chaperone, calreticulin. Both protein and mRNA levels for calreticulin and vinculin were decreased in RPEfast cells. After RPEfast cells were transfected with cDNA encoding calreticulin, both the expression of endoplasmic reticulum-resident calreticulin and cytoplasmic vinculin increased. The abundance of other adhesion-related proteins was not affected. RPEfast cells underexpressing calreticulin displayed a dramatic increase in the abundance of total cellular phosphotyrosine suggesting that the effects of calreticulin on cell adhesiveness may involve modulation of the activities of protein tyrosine kinases or phosphatases which may affect the stability of focal contacts. The calreticulin and vinculin underexpressing RPEfast cells lacked extensive focal contacts and adhered weakly but attached fast to the substratum. In contrast, the RPEslow cells that expressed calreticulin and vinculin abundantly developed numerous and prominent focal contacts slowly, but adhered strongly. Thus, while the calreticulin overexpressing RPEslow cells "grip" the substratum with focal contacts, calreticulin underexpressing RPEfast cells use close contacts to "stick" to it.

Microtubule growth activates Rac1 to promote lamellipodial protrusion in fibroblasts

Microtubules are involved in actin-based protrusion at the leading-edge lamellipodia of migrating fibroblasts. Here we show that the growth of microtubules induced in fibroblasts by removal of the microtubule destabilizer nocodazole activates Rac1 GTPase, leading to the polymerization of actin in lamellipodial protrusions. Lamellipodial protrusions are also activated by the rapid growth of a disorganized array of very short microtubules induced by the microtubule-stabilizing drug taxol. Thus, neither microtubule shortening nor long-range microtubule-based intracellular transport is required for activating protrusion. We suggest that the growth phase of microtubule dynamic instability at leading-edge lamellipodia locally activates Rac1 to drive actin polymerization and lamellipodial protrusion required for cell migration.

Expression of an isoform of the novel signal transduction protein ST5 is linked to cell morphology

The human ST5 gene is expressed as 4.6, 3.1 and 2.8 kb transcripts encoding putative 126, 82 and 70 kDa proteins that function in the MAP kinase signaling pathway in transient expression assays. Expression of the 2.8 kb transcript correlates with reduced tumorigenicity in HeLa-fibroblast hybrids, suggesting a role in tumor suppression. We now report the detection of ST5 proteins in cellular extracts, demonstrate specific expression of p70 in non-tumorigenic HeLa-fibroblast hybrids, extend the correlation between p70 expression and cellular morphology to a wide variety of cell lines, and provide direct evidence that p70 can effect changes in cell growth and morphology. ST5 proteins were identified in extracts of human, mouse and simian epithelial cells and fibroblasts, but were absent from lymphoid cells. Transfection of the 2.8 kb cDNA into a p70-negative mouse fibroblast line yielded stable transfectants with a flattened, less refractile morphology relative to controls. The p70 expressing clones had initial growth rates similar to those of control cells but their saturation density was reduced threefold, suggesting a restoration of contact-regulated growth. In conjunction with previous findings, these results suggest that ST5 proteins participate directly in events affecting cytoskeletal organization and tumorigenicity.

Bidirectional signaling between the cytoskeleton and integrins

Clustering of integrins into focal adhesions and focal complexes is regulated by the actin cytoskeleton. In turn, actin dynamics are governed by Rho family GTPases. Integrin-mediated adhesion activates these GTPases, triggering assembly of filopodia, lamellipodia and stress fibers. In the past few years, signaling pathways have begun to be identified that promote focal adhesion disassembly and integrin dispersal. Many of these pathways result in decreased myosin-mediated cell contractility.

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.

Functional design in the actin cytoskeleton

Changes in cell shape, anchorage and motility are all associated with the dynamic reorganisation of the architectural arrays of actin filaments that make up the actin cytoskeleton. The relative expression of these functionally different actin filament arrays is intimately linked to the pattern of contacts that a cell develops with its extracellular substrate. Cell polarity is acquired by the development of an asymmetric pattern of substrate contacts, effected in a specific, site-directed manner by the delivery of adhesion-site modulators along microtubules.

Regulation of the small GTP-binding protein Rho by cell adhesion and the cytoskeleton

Soluble factors from serum such as lysophosphatidic acid (LPA) are thought to activate the small GTP-binding protein Rho based on their ability to induce actin stress fibers and focal adhesions in a Rho-dependent manner. Cell adhesion to extracellular matrices (ECM) has also been proposed to activate Rho, but this point has been controversial due to the difficulty of distinguishing changes in Rho activity from the structural contributions of ECM to the formation of focal adhesions. To address these questions, we established an assay for GTP-bound cellular Rho. Plating Swiss 3T3 cells on fibronectin-coated dishes elicited a transient inhibition of Rho, followed by a phase of Rho activation. The activation phase was greatly enhanced by serum. In serum-starved adherent cells, LPA induced transient Rho activation, whereas in suspended cells Rho activation was sustained. Furthermore, suspended cells showed higher Rho activity than adherent cells in the presence of serum. These data indicate the existence of an adhesion-dependent negative-feedback loop. We also observed that both cytochalasin D and colchicine trigger Rho activation despite their opposite effects on stress fibers and focal adhesions. Our results show that ECM, cytoskeletal structures and soluble factors all contribute to regulation of Rho activity.

Positive feedback interactions between microtubule and actin dynamics during cell motility

The migration of tissue cells requires interplay between the microtubule and actin cytoskeletal systems. Recent reports suggest that interactions of microtubules with actin dynamics creates a polarization of microtubule assembly behavior in cells, such that microtubule growth occurs at the leading edge and microtubule shortening occurs at the cell body and rear. Microtubule growth and shortening may activate Rac1 and RhoA signaling, respectively, to control actin dynamics. Thus, an actin-dependent gradient in microtubule dynamic-instability parameters in cells may feed back through the activation of specific signalling pathways to perpetuate the polarized actin-assembly dynamics required for cell motility.

The M-cadherin catenin complex interacts with microtubules in skeletal muscle cells: implications for the fusion of myoblasts

M-cadherin, a calcium-dependent intercellular adhesion molecule, is expressed in skeletal muscle cells. Its pattern of expression, both in vivo and in cell culture as well as functional studies, have implied that M-cadherin is important for skeletal muscle development, in particular the fusion of myoblasts into myotubes. M-cadherin formed complexes with the catenins in skeletal muscle cells similar to E-cadherin in epithelial cells. This suggested that the muscle-specific function of the M-cadherin catenin complex might be mediated by additional interactions with yet unidentified cellular components, especially cytoskeletal elements. These include the microtubules which also have been implicated in the fusion process of myoblasts. Here we present evidence that the M-cadherin catenin complex interacts with microtubules in myogenic cells by using three independent experimental approaches. (1) Analysis by laser scan microscopy revealed that the destruction of microtubules by nocodazole leads to an altered cell surface distribution of M-cadherin in differentiating myogenic cells. In contrast, disruption of actin filaments had little effect on the surface distribution of M-cadherin. (2) M-cadherin antibodies coimmunoprecipitated tubulin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated epithelial cells ectopically expressing M-cadherin. Vice versa, tubulin antibodies coimmunoprecipitated M-cadherin from extracts of nocodazole-treated myogenic cells but not of nocodazole-treated M-cadherin-expressing epithelial cells. (3) M-cadherin and the catenins, but not a panel of control proteins, were copolymerized with tubulin from myogenic cell extracts even after repeated cycles of assembly and disassemly of tubulin. Moreover, neither M-cadherin nor E-cadherin could be found in a complex with microtubules in epithelial cells ectopically expressing M-cadherin. Our data are consistent with the idea that the interaction of M-cadherin with microtubules might be essential to keep the myoblasts aligned during fusion, a process in which both M-cadherin and microtubules have been implicated.

RhoA-binding kinase alpha translocation is facilitated by the collapse of the vimentin intermediate filament network

The regulation of morphological changes in eukaryotic cells is a complex process involving major components of the cytoskeleton including actin microfilaments, microtubules, and intermediate filaments (IFs). The putative effector of RhoA, RhoA-binding kinase alpha (ROKalpha), is a serine/threonine kinase that has been implicated in the reorganization of actin filaments and in myosin contractility. Here, we show that ROKalpha also directly affects the structural integrity of IFs. Overexpression of active ROKalpha, like that of RhoA, caused the collapse of filamentous vimentin, a type III IF. A RhoA-binding-deficient, kinase-inactive ROKalpha inhibited the collapse of vimentin IFs induced by RhoA in HeLa cells. In vitro, ROKalpha bound and phosphorylated vimentin at its head-rod domain, thereby inhibiting the assembly of vimentin. ROKalpha colocalized predominantly with the filamentous vimentin network, which remained intact in serum-starved cells. Treatment of cells with vinblastine, a microtubule-disrupting agent, also resulted in filamentous vimentin collapse and concomitant ROKalpha translocation to the cell periphery. ROKalpha translocation did not occur when the vimentin network remained intact in vinblastine-treated cells at 4 degreesC or in the presence of the dominant-negative RhoAN19 mutant. Transient translocation of ROKalpha was also observed in cells subjected to heat shock, which caused the disassembly of the vimentin network. Thus, the translocation of ROKalpha to the cell periphery upon overexpression of RhoAV14 or growth factor treatment is associated with disassembly of vimentin IFs. These results indicate that Rho effectors known to act on microfilaments may be involved in regulating the assembly of IFs. Vimentin when phosphorylated also exhibits reduced affinity for the inactive ROKalpha. The translocation of ROKalpha from IFs to the cell periphery upon action by activated RhoA and ROKalpha suggests that ROKalpha may initiate its own cascade of activation.

Targeting, capture, and stabilization of microtubules at early focal adhesions

By co-injecting fluorescent tubulin and vinculin into fish fibroblasts we have revealed a "cross talk" between microtubules and early sites of substrate contact. This mutuality was first indicated by the targeting of vinculin-rich foci by microtubules during their growth towards the cell periphery. In addition to passing directly over contact sites, the ends of single microtubules could be observed to target several contacts in succession or the same contact repetitively, with intermittent withdrawals. Targeting sometimes involved side-stepping, or the major re-routing of a microtubule, indicative of a guided, rather than a random process. The paths that microtubules followed into contacts were unrelated to the orientation of stress fiber assemblies and targeting occurred also in mouse fibroblasts that lacked a system of intermediate filaments. Further experiments with microtubule inhibitors showed that adhesion foci can: (a) capture microtubules and stabilize them against disassembly by nocodazole; and (b), act as preferred sites of microtubule polymerization, during either early recovery from nocodazole, or brief treatment with taxol. From these and other findings we speculate that microtubules are guided into substrate contact sites and through the motor-dependent delivery of signaling molecules serve to modulate their development. It is further proposed this modulation provides the route whereby microtubules exert their influence on cell shape and polarity.

Molecular dissection of the Rho-associated protein kinase (p160ROCK)-regulated neurite remodeling in neuroblastoma N1E-115 cells

A critical role for the small GTPase Rho and one of its targets, p160ROCK (a Rho-associated coiled coil-forming protein kinase), in neurite remodeling was examined in neuroblastoma N1E-115 cells. Using wild-type and a dominant-negative form of p160ROCK and a p160ROCK-specific inhibitor, Y-27632, we show here that p160ROCK activation is necessary and sufficient for the agonist-induced neurite retraction and cell rounding. The neurite retraction was accompanied by elevated phosphorylation of myosin light chain and the disassembly of the intermediate filaments and microtubules. Y-27632 blocked both neurite retraction and the elevation of myosin light chain phosphorylation in a similar concentration-dependent manner. On the other hand, suppression of p160ROCK activity by expression of a dominant-negative form of p160ROCK induced neurites in the presence of serum by inducing the reassembly of the intermediate filaments and microtubules. The neurite outgrowth by the p160ROCK inhibition was blocked by coexpression of dominant-negative forms of Cdc42 and Rac, indicating that p160ROCK constitutively and negatively regulates neurite formation at least in part by inhibiting activation of Cdc42 and Rac. The assembly of microtubules and intermediate filaments to form extended processes by inhibitors of the Rho-ROCK pathway was also observed in Swiss 3T3 cells. These results indicate that Rho/ROCK-dependent tonic inhibition of cell process extension is exerted via activation of the actomysin-based contractility, in conjunction with a suppression of assembly of intermediate filaments and microtubules in many cell types including, but not exclusive to, neuronal cells.

Microtubule depolymerization induces stress fibers, focal adhesions, and DNA synthesis via the GTP-binding protein Rho

Microtubule depolymerization has multiple consequences that include actin stress fiber and focal adhesion assembly, increased tyrosine phosphorylation and DNA synthesis. Similar effects induced by serum, or agents such as lysophosphatidic acid, have previously been shown to be mediated by the GTP-binding protein Rho. We have investigated whether the effects of microtubule depolymerization are similarly mediated by Rho and show that they are blocked by the specific Rho inhibitor, C3 transferase. Because microtubule depolymerization induces these effects in quiescent cells, in which Rho is largely inactive, we conclude that microtubule depolymerization leads to activation of Rho. The activation of Rho in response to microtubule depolymerization and the consequent stimulation of contractility suggest a mechanism by which microtubules may regulate microfilament function in various motile phenomena. These range from growth cone extension to the development of the contractile ring during cytokinesis, in which there are interactions between the microtubule and microfilament systems.

Effect of microtubule disruption on cell adhesion and spreading

Microtubules have been involved in a variety of cellular processes. In this study, we examined the role of the microtubular system in the adhesion and spreading of the adenocarcinoma cell line HT29-D4. Disruption of microtubules by nocodazole or navelbine resulted in an increase in cell adhesion to purified ECM proteins. This enhanced cell adhesion is mediated by integrins, but is not attributable to quantitative changes in the number of integrin receptors at the cell surface, as determined by flow cytometric analysis. In contrast to attachment, spreading of HT29-D4 cells was reduced by nocodazole treatment in a dose-dependent manner. Thus, microtubule depolymerization appears to increase initial attachment of cells to extracellular matrix, while impeding subsequent cell spreading.

Rho guanosine triphosphatase mediates the selective stabilization of microtubules induced by lysophosphatidic acid

The asymmetric distribution of stable, posttranslationally modified microtubules (MTs) contributes to the polarization of many cell types, yet the factors controlling the formation of these MTs are not known. We have found that lysophosphatidic acid (LPA) is a major serum factor responsible for rapidly generating stable, detyrosinated (Glu) MTs in serum-starved 3T3 cells. Using C3 toxin and val14 rho we showed that rho was both necessary and sufficient for the induction of Glu MTs by LPA and serum. Unlike previously described factors that induce MT stability, rho induced the stabilization of only a subset of the MTs and, in wound-edge cells, these stable MTs were appropriately oriented toward the leading edge of the cell. LPA had little effect on individual parameters of MT dynamics, but did induce long states of pause in a subset of MTs near the edge of the cell. Rho stimulation of MT stability was independent of actin stress fiber formation. These results identify rho as a novel regulator of the MT cytoskeleton that selectively stabilizes MTs during cell polarization by acting as a switch between dynamic and stable states of MTs rather than as a modulator of MT assembly and disassembly.