Lysophosphatidic acid and microtubule-destabilizing agents stimulate fibronectin matrix assembly through Rho-dependent actin stress fiber formation and cell contraction

Live-cell monitoring of tyrosine phosphorylation in focal adhesions following microtubule disruption

Tyrosine phosphorylation of focal adhesion components is involved in the regulation of focal adhesion formation and turnover, yet the underlying molecular mechanisms are still poorly defined. In the present study, we have used quantitative fluorescence microscopy to investigate the dynamic relationships between the incorporation of new components into growing focal adhesions and tyrosine phosphorylation of these sites. For this purpose, a new approach for monitoring phosphotyrosine levels in live cells was developed, based on a 'phosphotyrosine reporter' consisting of yellow fluorescent protein fused to two consecutive phosphotyrosine-binding Src-homology 2 (SH2)-domains derived from pp60(c-Src). This YFP-dSH2 localized to cell-matrix adhesions and its intensity was linearly correlated with that of an anti-phosphotyrosine antibody labeling. The differential increase in vinculin and phosphotyrosine levels was examined in live cells by two-color time-lapse movies of CFP-vinculin and YFP-dSH2. In this study, focal adhesion growth was triggered by microtubule disruption, which was previously shown to stimulate focal adhesion development by inducing cellular contraction. We show here that, 2 minutes after addition of the microtubule-disrupting drug nocodazole, the local densities of the focal adhesion-associated proteins vinculin, paxillin and focal adhesion kinase (FAK) are significantly elevated and the focal adhesion area is increased, whereas elevation in tyrosine phosphorylation inside the growing adhesions occurs only a few minutes later. Phosphotyrosine and FAK density reach their maximum levels after 10 minutes of treatment, whereas vinculin and paxillin levels as well as focal adhesion size continue to grow, reaching a plateau at about 30 minutes. Our findings suggest that protein recruitment and growth of focal adhesions are an immediate and direct result of increased contractility induced by microtubule disruption, whereas tyrosine phosphorylation is activated later.

RhoA is required for cortical retraction and rigidity during mitotic cell rounding

Mitotic cell rounding is the process of cell shape change in which a flat interphase cell becomes spherical at the onset of mitosis. Rearrangement of the actin cytoskeleton, de-adhesion, and an increase in cortical rigidity accompany mitotic cell rounding. The molecular mechanisms that contribute to this process have not been defined. We show that RhoA is required for cortical retraction but not de-adhesion during mitotic cell rounding. The mitotic increase in cortical rigidity also requires RhoA, suggesting that increases in cortical rigidity and cortical retraction are linked processes. Rho-kinase is also required for mitotic cortical retraction and rigidity, indicating that the effects of RhoA on cell rounding are mediated through this effector. Consistent with a role for RhoA during mitotic entry, RhoA activity is elevated in rounded, preanaphase mitotic cells. The activity of the RhoA inhibitor p190RhoGAP is decreased due to its serine/threonine phosphorylation at this time. Cumulatively, these results suggest that the mitotic increase in RhoA activity leads to rearrangements of the cortical actin cytoskeleton that promote cortical rigidity, resulting in mitotic cell rounding.

Microtubule-dependent regulation of Rho GTPases during internalisation of Yersinia pseudotuberculosis

Internalisation of the human pathogen Yersinia pseudotuberculosis via interaction of bacterial invasin with host beta1 integrins depends on the actin cytoskeleton and involves Src family kinases, focal adhesion kinase, p130Crk-associated substrate, proline-rich tyrosine kinase 2, Rac, Arp 2/3 complex and WASP family members. We show here that Rho GTPases are regulated by the microtubule system during bacterial uptake. Interfering with microtubule organisation using nocodazole or paclitaxel suppressed uptake by HeLa cells. The nocodazole effect on microtubule depolymerisation was partially inhibited through overexpression of Rac, Cdc42, RhoG or RhoA and completely prevented by expression of Vav2. This suggests that microtubules influence Rho GTPases during invasin-mediated phagocytosis and in the absence of functional microtubules Vav2 can mimic their effect on one, or more, of the Rho family GTPases. Lastly, overexpression of p50 dynamitin partially inhibited bacterial uptake and this effect was also blocked by co-expression of Vav2, thus further implicating this guanine nucleotide exchange factor in activating Rho GTPases for internalisation during loss of microtubule function.

The fibronectin-binding integrins alpha5beta1 and alphavbeta3 differentially modulate RhoA-GTP loading, organization of cell matrix adhesions, and fibronectin fibrillogenesis

We have studied the formation of different types of cell matrix adhesions in cells that bind to fibronectin via either alpha5beta1 or alphavbeta3. In both cases, cell adhesion to fibronectin leads to a rapid decrease in RhoA activity. However, alpha5beta1 but not alphavbeta3 supports high levels of RhoA activity at later stages of cell spreading, which are associated with a translocation of focal contacts to peripheral cell protrusions, recruitment of tensin into fibrillar adhesions, and fibronectin fibrillogenesis. Expression of an activated mutant of RhoA stimulates alphavbeta3-mediated fibrillogenesis. Despite the fact that alpha5beta1-mediated adhesion to the central cell-binding domain of fibronectin supports activation of RhoA, other regions of fibronectin are required for the development of alpha5beta1-mediated but not alphavbeta3-mediated focal contacts. Using chimeras of beta1 and beta3 subunits, we find that the extracellular domain of beta1 controls RhoA activity. By expressing both beta1 and beta3 at high levels, we show that beta1-mediated control of the levels of beta3 is important for the distribution of focal contacts. Our findings demonstrate that the pattern of fibronectin receptors expressed on a cell dictates the ability of fibronectin to stimulate RhoA-mediated organization of cell matrix adhesions.

Tenascin-C modulates matrix contraction via focal adhesion kinase- and Rho-mediated signaling pathways

A provisional matrix consisting of fibrin and fibronectin (FN) is deposited at sites of tissue damage and repair. This matrix serves as a scaffold for fibroblast migration into the wound where these cells deposit new matrix to replace lost or damaged tissue and eventually contract the matrix to bring the margins of the wound together. Tenascin-C is expressed transiently during wound repair in tissue adjacent to areas of injury and contacts the provisional matrix in vivo. Using a synthetic model of the provisional matrix, we have found that tenascin-C regulates cell responses to a fibrin-FN matrix through modulation of focal adhesion kinase (FAK) and RhoA activation. Cells on fibrin-FN+tenascin-C redistribute their actin to the cell cortex, downregulate focal adhesion formation, and do not assemble a FN matrix. Cells surrounded by a fibrin-FN+tenascin-C matrix are unable to induce matrix contraction. The inhibitory effect of tenascin-C is circumvented by downstream activation of RhoA. FAK is also required for matrix contraction and the absence of FAK cannot be overcome by activation of RhoA. These observations show dual requirements for both FAK and RhoA activities during contraction of a fibrin-FN matrix. The effects of tenascin-C combined with its location around the wound bed suggest that this protein regulates fundamental processes of tissue repair by limiting the extent of matrix deposition and contraction to fibrin-FN-rich matrix in the primary wound area.

Fibronectin: an interesting vocal fold protein

A great deal of information is accruing regarding the function of the extracellular matrix. Once thought to be simply a structural entity to surround cells, it is now known to do much more. Fibronectin in particular has received specific attention. Fibronectin is a ubiquitous glycoprotein found most abundantly in the extracellular matrix of regenerating, healing, and embryonic tissue. Vast evidence supports the fact that fibronectin participates in many diverse functions throughout the body that are relevant to vocal fold biology. This article introduces the structure of fibronectin and its isoforms and provides an introduction to some of the many functions it plays. It also reviews the evidence about fibronectin's place in vocal folds and vocal fold pathology. It discusses fibronectin's presence in vocal nodules, vocal polyps, vocal scarring, and Reinke's edema, and reviews the data on its role in mucosal wave impairment. Lastly, it discusses preliminary microarray data that show gene expression for fibronectin to be upregulated in true vocal folds when compared to false vocal folds.

Interaction of G alpha(12) with G alpha(13) and G alpha(q) signaling pathways

The G(12) subfamily of heterotrimeric G-proteins consists of two members, G(12) and G(13). Gene-targeting studies have revealed a role for G(13) in blood vessel development. Mice lacking the alpha subunit of G(13) die around embryonic day 10 as the result of an angiogenic defect. On the other hand, the physiological role of G(12) is still unclear. To address this issue, we generated G alpha(12)-deficient mice. In contrast to the G alpha(13)-deficient mice, G alpha(12)-deficient mice are viable, fertile, and do not show apparent abnormalities. However, G alpha(12) does not seem to be entirely redundant, because in the offspring generated from G alpha(12)+/- G alpha(13)+/- intercrosses, at least one intact G alpha(12) allele is required for the survival of animals with only one G alpha(13) allele. In addition, G alpha(12) and G alpha(13) showed a difference in mediating cell migratory response to lysophosphatidic acid in embryonic fibroblast cells. Furthermore, mice lacking both G alpha(12) and G alpha(q) die in utero at about embryonic day 13. These data indicate that the G alpha(12)-mediated signaling pathway functionally interacts not only with the G alpha(13)- but also with the G alpha(q/11)-mediated signaling systems.

The Rho exchange factor Net1 is regulated by nuclear sequestration

Net1 is a guanine nucleotide exchange factor specific for the small GTPase Rho. Oncogenic activation of Net1 occurs by truncation of the N-terminal part of the protein, which functions as a negative regulatory domain. Here, we have investigated the mechanism of Net1 regulation via its N terminus. We find that Net1 localizes to the nucleus, whereas oncogenic Net1 is found in the cytoplasm. Nuclear import of Net1 is mediated by two nuclear localization signals present in the N terminus of the protein, and forced cytoplasmic localization of Net1 is sufficient to activate Rho. In addition, the pleckstrin homology (PH) domain of Net1 acts as a nuclear export signal. Because an amino acid substitution in the PH domain that inhibits guanine nucleotide exchange factor activity does not inhibit nuclear export, we conclude that this PH domain has at least two functions. Together, our results suggest that Net1 can shuttle in and out of the nucleus, and that activation of Rho by Net1 is controlled by changes in its subcellular localization.

Fibronectin extension and unfolding within cell matrix fibrils controlled by cytoskeletal tension

Evidence is emerging that mechanical stretching can alter the functional states of proteins. Fibronectin (Fn) is a large, extracellular matrix protein that is assembled by cells into elastic fibrils and subjected to contractile forces. Assembly into fibrils coincides with expression of biological recognition sites that are buried in Fn's soluble state. To investigate how supramolecular assembly of Fn into fibrillar matrix enables cells to mechanically regulate its structure, we used fluorescence resonance energy transfer (FRET) as an indicator of Fn conformation in the fibrillar matrix of NIH 3T3 fibroblasts. Fn was randomly labeled on amine residues with donor fluorophores and site-specifically labeled on cysteine residues in modules FnIII(7) and FnIII(15) with acceptor fluorophores. Intramolecular FRET was correlated with known structural changes of Fn in denaturing solution, then applied in cell culture as an indicator of Fn conformation within the matrix fibrils of NIH 3T3 fibroblasts. Based on the level of FRET, Fn in many fibrils was stretched by cells so that its dimer arms were extended and at least one FnIII module unfolded. When cytoskeletal tension was disrupted using cytochalasin D, FRET increased, indicating refolding of Fn within fibrils. These results suggest that cell-generated force is required to maintain Fn in partially unfolded conformations. The results support a model of Fn fibril elasticity based on unraveling and refolding of FnIII modules. We also observed variation of FRET between and along single fibrils, indicating variation in the degree of unfolding of Fn in fibrils. Molecular mechanisms by which mechanical force can alter the structure of Fn, converting tensile forces into biochemical cues, are discussed.

The incorporation of fibrinogen into extracellular matrix is dependent on active assembly of a fibronectin matrix

Fibrinogen is a soluble protein produced by hepatocytes and secreted into plasma, where it functions in hemostasis. During inflammation, the hepatic synthesis of fibrinogen is induced 2-10 fold. Recent studies demonstrate that after an inflammatory stimulus, fibrinogen gene expression and protein production is upregulated in lung epithelial cells, where it is secreted basolaterally and consequently deposited into the extracellular matrix in fibrils that extensively colocalize with fibronectin fibrils. In this study, we show that the deposition of fibrinogen into the matrix of fibroblasts occurred rapidly and in a Rho-dependent manner in response to serum or lysophosphatidic acid; RhoA GTPase signaling is also required for fibronectin matrix assembly. Using mouse embryonic fibronectin-null cells, we show that incorporation of exogenous fibrinogen into matrix fibrils occurred only in the presence of exogenous fibronectin, which is also assembled into matrix fibrils. Furthermore, treatment of fibroblasts and fibronectin-null cells with an antibody that inhibits fibronectin matrix assembly impaired incorporation of fibrinogen into matrix fibrils. Collectively, these data suggest that incorporation of fibrinogen into the extracellular matrix requires active fibronectin polymer elongation into matrix fibrils. From these data, we hypothesize that fibrinogen deposition rapidly changes the topology of the extracellular matrix to provide a surface for cell migration and matrix remodeling during tissue repair.

Microtubule disruption modulates the Rho-kinase pathway in vascular smooth muscle

Microtubules constitute one of the main cytoskeletal components in eukaryotic cells. Recent studies have shown that microtubule disruption induced significant vasoconstriction or enhanced agonist-induced contraction in vascular smooth muscle. However, the underlying mechanisms are not clear. We hypothesize that microtubule disruption may affect contractile signaling in vascular smooth muscle and lead to the enhanced contraction. The present study demonstrates that both colchicine and nocodazole induced a small but sustained contraction (4-6% P0) in rat aortic rings. This microtubule disruption-induced contraction was abolished by co-treatment with either HA 1077 or Y-27632, both of which are relatively specific Rho-kinase inhibitors. However, co-treatment with ML-9, an inhibitor of myosin light chain kinase, (MLCK) did not have a significant effect on the colchicine-induced contraction. The enhanced KCl-induced contraction due to treatment with colchicine was also blocked by inhibition of Rho-kinase, but not by inhibition of MLCK. These results indicate that microtubule disruption modulates contractile signaling in vascular smooth muscle, mainly through the Rho-kinase pathway, but not MLCK. Interestingly, the colchicine-enhanced, phenylephrine-induced contraction was not completely blocked by inhibition of Rho-kinase suggesting that other signaling pathways might also be involved.

Chondroitin sulfate and cytoplasmic domain-dependent membrane targeting of the NG2 proteoglycan promotes retraction fiber formation and cell polarization

Targeting of the NG2 proteoglycan to cellular retraction fibers was studied by expressing mutant NG2 molecules lacking specific structural elements of the proteoglycan. Both the cytoplasmic domain and the chondroitin sulfate chain of NG2 appear to have roles in sorting NG2 to subcellular microdomains destined to become retraction fibers. Neither of these structural features alone is sufficient to allow optimal targeting of NG2 to retraction fibers, but together they promote efficient localization of the proteoglycan to these sites. This pattern of NG2 sorting seems to be necessary for optimal retraction fiber formation, as cells expressing poorly targeted NG2 mutants are noticeably deficient in their ability to extend retraction fibers. Furthermore, retraction fiber formation correlates strongly with the tendency of cells to assume a polarized morphology with NG2-positive retraction fibers at one pole of the cell and actin-rich lamellipodia at the other. This polarization can be triggered either through engagement of NG2 by the substratum or by exposure to lysophosphatidic acid, a potent activator of the rho GTPase. These results suggest a possible role for NG2 in regulating rho-dependent mechanisms in the trailing processes of motile cells.

Assembly of laminin polymers is dependent on beta1-integrins

Recent reports suggest that laminin deposition is controlled by the cell via specific receptors, one of which is dystroglycan. In this study, the involvement of beta1-integrins in this process was investigated by comparing beta1-integrin-deficient cells of different phenotypes with their normal counterparts. Normal embryonic stem (ES) cells and embryoid bodies (EBs) derived from them were found to deposit cell-associated laminin into fibrillar networks, and in the EBs a basement membrane was assembled under the primitive endoderm. beta1-deficient ES cells and their EBs formed only small amounts of dot-like laminin deposits. Skeletal myotubes formed after prolonged differentiation in EBs were found to be surrounded by laminin, nidogen, and perlecan by immunofluorescent staining irrespective of the presence of beta1-integrins on the myotubes. However, at the electron microscope level only very thin sheet-like structures were detected close to the beta1-deficient myotubes, while the wt myotubes formed thick basement membranes. An epithelial cell line, GE11, derived from the beta1-integrin-deficient ES cells was also unable to assemble laminin on the cell surface, while transfection of the cells with the integrin beta1 subunit resulted in formation of a dense laminin network. Taken together, these results suggest that dystroglycan and beta1-integrins can both contribute to the recruitment of laminin to cell surfaces and that integrins are required at a subsequent step in the formation of basement membranes.

Modulation of cell interactions with extracellular matrix by lysophosphatidic acid and sphingosine 1-phosphate

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (SPP) are lipid mediators released upon platelet activation. The concentration of LPA in serum is estimated at 1-10 microM whereas the concentration in plasma is considerably less. The SPP concentration in serum is 0.5 microM, approximately two-fold higher than the plasma concentration. The lipids are present during tissue injury and promote cellular processes involved in wound repair. LPA and SPP have multiple effects on cells, many of which are pertinent to wound healing and require that the cells interact in some fashion with components of the extracellular matrix. This review focuses on modulation of cell adhesion, cell migration, collagen gel contraction, and fibronectin matrix assembly by LPA and SPP.

Extracellular matrix fibronectin alters connexin43 expression by alveolar epithelial cells

Alveolar type II epithelial cells undergo phenotypic changes and establish gap junction intercellular communication as they reach confluence in primary culture. The pattern of gap junction protein (connexin) expression changes in parallel. Although connexin (Cx)43 mRNA and protein increase significantly by culture day 2, Cx26 and Cx32 expression decline. Along with increasing Cx43 expression, the cells assemble fibronectin derived both from serum in the culture medium and from de novo synthesis into the extracellular matrix (ECM). The present studies indicate that this ECM regulates Cx43 expression. Culture of type II cells in DMEM containing 8-10% fetal bovine serum (FBS) promotes assembly of a fibronectin-rich ECM that stimulates expression of both Cx43 mRNA and protein. Although Cx43 protein expression increased in response to FBS in a dose-dependent manner, fibronectin also elevated Cx43 protein in the absence of FBS. Anti-fibronectin antibody significantly reduced the serum-dependent increase in Cx43 expression. These results support the premise that fibronectin in the ECM contributes to the regulation of Cx43 expression by alveolar epithelial cells in primary culture.

c-Cbl facilitates fibronectin matrix production by v-Abl-transformed NIH3T3 cells via activation of small GTPases

The protooncogenic protein c-Cbl has been shown to act as a multivalent adaptor and a negative regulator of protein tyrosine kinase-mediated signaling. Recent studies have implicated it in the regulation of cell adhesion-related events. We have previously shown that c-Cbl facilitates adhesion and spreading of v-Abl-transformed fibroblasts, and that these effects are dependent on its tyrosine phosphorylation. However, the mechanisms mediating effects of c-Cbl on fibroblast adhesion remain poorly understood. In this study we demonstrate that the tyrosine phosphorylation-dependent effect of c-Cbl on adhesion of v-Abl-transformed fibroblasts is primarily mediated by an increase in fibronectin matrix deposition by these cells. This increase in fibronectin matrix deposition and, hence, in cell adhesion is dependent on cytoskeletal rearrangements induced by RhoA, Rac1 and, possibly, Rap1 activation caused by c-Cbl. The observed activation of these GTPases is mediated by the recruitment of phosphatidylinositol-3' kinase, CrkL and Vav2 to the C-terminal tyrosine residues of c-Cbl. The results of this study also demonstrate that ubiquitination is essential for the observed effects of c-Cbl on fibronectin matrix production and cell adhesion.

Cytoskeletal changes in cell transformation and tumorigenesis

Research during the past couple of years has provided important new information as to how the actin cytoskeleton contributes to growth control in both normal and transformed cells. The cytoskeleton can no longer be viewed as simply a structural framework playing a role in cell shape and motile events such as cell movement, intracellular transport, contractile-ring formation and chromosome movement. More recent experiments show that the cytoskeleton plays a critical role in the regulation of various cellular processes linked to transformation including proliferation, contact inhibition, anchorage-independent cell growth, and apoptosis.

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.

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.

Protein crosslinking in assembly and remodelling of extracellular matrices: the role of transglutaminases

Transglutaminases form a family of proteins that have evolved for specialized functions such as protein crosslinking in haemostasis, semen coagulation, or keratinocyte cornified envelope formation. In contrast to the other members of this protein family, tissue transglutaminase is a multifunctional enzyme apparently involved in very disparate biological processes. By virtue of its reciprocal Ca2+-dependent crosslinking activity or GTP-dependent signal transducing activity, tissue transglutaminase exhibits true multifunctionality at the molecular level. The crosslinking activity can subserve disparate biological phenomena depending on the location of the target proteins. Intracellular activation of tissue transglutaminase can give rise to crosslinked protein envelopes in apoptotic cells, whereas extracellular activation contributes to stabilization of the extracellular matrix and promotes cell-substrate interaction. While tissue transglutaminase synthesis and activation is normally part of a protective cellular response contributing to tissue homeostasis, the enzyme has also been implicated in a number of pathological conditions including fibrosis, atherosclerosis, neurodegenerative diseases, celiac disease, and cancer metastasis. This review discusses the role of transglutaminases in extracellular matrix crosslinking with a focus on the multifunctional enzyme tissue transglutaminase.

Regulation of fibronectin matrix assembly by activated Ras in transformed cells

Fibronectin extracellular matrix plays a critical role in the microenvironment of cells. Loss of this matrix frequently accompanies oncogenic transformation, allowing changes in cell growth, morphology, and tissue organization. The HT1080 human fibrosarcoma cell line is deficient in formation of fibronectin matrix fibrils but assembly can be induced by the glucocorticoid dexamethasone. Here we show that fibronectin assembly can also be restored by stimulation of alpha5beta1 integrin with activating antibody or with Mn2+ suggesting that integrin activity is reduced in these cells. While dexamethasone promoted actin stress fiber formation, actin filaments remained cortical following Mn2+ treatment showing that the dexamethasone effect is not due solely to cytoskeletal changes. HT1080 cells have one activated allele of N-ras and PD98059 inhibition of signaling from Ras through ERK increased fibronectin matrix accumulation. Conversely, the p38 MAP kinase inhibitor SB203580 blocked induction of matrix and increased ERK phosphorylation. Thus, two MAP kinase pathways contribute to the control of integrin-mediated fibronectin assembly. ERK activity and fibronectin assembly were linked in three different ras-transformed cell lines but not in SV40- or RSV-transformed cells indicating that oncogenic Ras uses a distinct mechanism to down-regulate cell-fibronectin interactions.

Stimulation of integrin-mediated cell contractility by fibronectin polymerization

Ligation of integrins with extracellular matrix molecules induces the clustering of actin and actin-binding proteins to focal adhesions, which serves to mechanically couple the matrix with the cytoskeleton. During wound healing and development, matrix deposition and remodeling may impart additional tensile forces that modulate integrin-mediated cell functions, including cell migration and proliferation. We have utilized the ability of cells to contract floating collagen gels to determine the effect of fibronectin polymerization on mechanical tension generation by cells. Our data indicate that fibronectin polymerization promotes cell spreading in collagen gels and stimulates cell contractility by a Rho-dependent mechanism. Fibronectin-stimulated contractility was dependent on integrin ligation; however, integrin ligation by fibronectin fragments was not sufficient to induce either tension generation or cell spreading. Furthermore, treatment of cells with polyvalent RGD peptides or pre-polymerized fibronectin did not stimulate cell contractility. Fibronectin-induced contractility was blocked by agents that inhibit fibronectin polymerization, suggesting that the process of fibronectin polymerization is critical in triggering cytoskeletal tension generation. These data indicate that Rho-mediated cell contractility is regulated by the process of fibronectin polymerization and suggest a novel mechanism by which extracellular matrix fibronectin regulates cytoskeletal organization and cell function.

Integrin dynamics and matrix assembly: tensin-dependent translocation of alpha(5)beta(1) integrins promotes early fibronectin fibrillogenesis

Fibronectin matrix assembly is a multistep, integrin-dependent process. To investigate the role of integrin dynamics in fibronectin fibrillogenesis, we developed an antibody-chasing technique for simultaneous tracking of two integrin populations by different antibodies. We established that whereas the vitronectin receptor alpha(v)beta(3) remains within focal contacts, the fibronectin receptor alpha(5)beta(1) translocates from focal contacts into and along extracellular matrix (ECM) contacts. This escalator-like translocation occurs relative to the focal contacts at 6.5 +/- 0.7 microm/h and is independent of cell migration. It is induced by ligation of alpha(5)beta(1) integrins and depends on interactions with a functional actin cytoskeleton and vitronectin receptor ligation. During cell spreading, translocation of ligand-occupied alpha(5)beta(1) integrins away from focal contacts and along bundles of actin filaments generates ECM contacts. Tensin is a primary cytoskeletal component of these ECM contacts, and a novel dominant-negative inhibitor of tensin blocked ECM contact formation, integrin translocation, and fibronectin fibrillogenesis without affecting focal contacts. We propose that translocating alpha(5)beta(1) integrins induce initial fibronectin fibrillogenesis by transmitting cytoskeleton-generated tension to extracellular fibronectin molecules. Blocking this integrin translocation by a variety of treatments prevents the formation of ECM contacts and fibronectin fibrillogenesis. These studies identify a localized, directional, integrin translocation mechanism for matrix assembly.

alpha5beta1 integrin expression and luminal edge fibronectin matrix assembly by smooth muscle cells after arterial injury

Fibronectin is secreted from the cell as a soluble protein that must then polymerize to regulate cell function. To elucidate the process of fibronectin matrix assembly in vascular disease, we immunostained sections of balloon-injured rat carotid artery for the fibronectin-binding alpha5beta1 integrin. Whereas alpha5beta1 integrin was not evident in the normal carotid artery, its expression was induced after a vascular injury. By 14 days, the alpha5beta1 integrin was localized exclusively to the less differentiated smooth muscle cells (SMCs) at the luminal surface of the neointima. Platelet-derived growth factor-BB, dominant in neointimal formation, selectively increased the expression of the alpha5beta1 integrin by human SMCs in culture. To track the assembly of fibronectin fibers, fluorescence-labeled soluble fibronectin protomers were added to cultured SMCs and to fresh segments of normal and balloon-injured rat carotid arteries. Fibronectin fiber formation in cultured SMCs could be detected within 10 minutes, and was blocked by an RGD peptide, an anti-beta1 integrin antibody, and an anti-alpha5beta1 integrin antibody, but not by an anti-beta3 integrin antibody. En face confocal microscopy of arterial segments revealed that soluble fibronectin had polymerized on the alpha5beta1 integrin-expressing SMCs of the luminal surface of the injured arterial neointima, but not on the alpha5beta1 integrin-negative neointimal SMCs below this or on the endothelial cells of uninjured arteries. Furthermore, in situ fibronectin assembly by the neointimal SMCs was inhibited by an RGD peptide and by an anti-beta1 integrin antibody. These studies indicate that a subpopulation of SMCs in the repairing artery wall orchestrates integrin-mediated fibronectin assembly.

Bioactive lysophospholipids and their G protein-coupled receptors

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are serum-borne lysophospholipids that signal through their cognate G protein-coupled receptors to evoke a great variety of responses in numerous cell types. In addition to stimulating cell proliferation and survival, LPA and S1P induce profound cytoskeletal changes through Rho-mediated signaling pathways, leading to such diverse responses as cell rounding, neurite retraction, and modulation of tumor cell invasiveness (transcellular migration). A major recent advance is the identification of a subfamily of heptahelical receptors for LPA and S1P.

Signaling to Rho GTPases

Rho GTPases regulate many important processes in all eukaryotic cells, including the organization of the actin cytoskeleton, gene transcription, cell cycle progression, and membrane trafficking. Their activity is regulated by signals originating from different classes of surface receptors including G-protein-coupled receptors, tyrosine kinase receptors, cytokine receptors, and adhesion receptors. Recent work has identified multiple mechanisms by which receptors can signal to Rho GTPases and this will be the major focus of this review. In addition, there is growing evidence for cross-talk within the Rho GTPase family as well as between the Rho and Ras GTPase families. These signaling networks are thought to provide the cooperative and coordinated interactions that are crucial for regulating complex biological processes such as cell migration.

Promotion of skeletal muscle differentiation by K252a with tyrosine phosphorylation of focal adhesion: a possible involvement of small GTPase Rho

K252a, a protein kinase inhibitor, acts as a neurotrophic factor in several neuronal cells. In this study we show that K252a enhanced the differentiation of C2C12 myoblasts as well as tyrosine phosphorylation of several focal adhesion-associated proteins including p130(Cas), focal adhesion kinase, and paxillin. The tyrosine phosphorylation of these proteins, reaching a maximum at 30 min after K252a treatment, closely correlated with the colocalization of these proteins in focal adhesion complexes and the coimmunoprecipitation of these proteins with p130(Cas). In addition, K252a stimulated longitudinal development of stress fiber-like structures and cell-matrix interaction in postmitotic myoblasts and eventually formation of well-developed myofibrils in multinucleated myotubes. Herbimycin A, a potent inhibitor of Src family kinases, and cytochalasin D, a selective disrupting-agent of actin filament, completely inhibited K252a-induced tyrosine phosphorylation as well as myoblast differentiation. Similar inhibitory effect was observed in the cells scrape loaded with a Rho inhibitor, C3 transferase, and the treatment of K252a induced a rapid translocation of Rho. These results are consistent with the model that Rho-dependent tyrosine phosphorylation of focal adhesion-associated proteins plays an important role in skeletal muscle differentiation.

Self-assembly of fibronectin into fibrillar networks underneath dipalmitoyl phosphatidylcholine monolayers: role of lipid matrix and tensile forces

The cell-mediated assembly of fibronectin (Fn) into fibrillar matrices is a complex multistep process that is incompletely understood because of the chemical complexity of the extracellular matrix and a lack of experimental control over molecular interactions and dynamic events. We have identified conditions under which Fn assembles into extended fibrillar networks after adsorption to a dipalmitoyl phosphatidylcholine (DPPC) monolayer in contact with physiological buffer. We propose a sequential model for the Fn assembly pathway, which involves the orientation of Fn underneath the lipid monolayer by insertion into the liquid expanded (LE) phase of DPPC. Attractive interactions between these surface-anchored proteins and the liquid condensed (LC) domains leads to Fn enrichment at domain edges. Spontaneous self-assembly into fibrillar networks, however, occurs only after expansion of the DPPC monolayer from the LC phase though the LC/LE phase coexistence. Upon monolayer expansion, the domain boundaries move apart while attractive interactions among Fn molecules and between Fn and domain edges produce a tensile force on the proteins that initiates fibril assembly. The resulting fibrils have been characterized in situ by using fluorescence and light-scattering microscopy. We have found striking similarities between fibrils produced under DPPC monolayers and those found on cellular surfaces, including their assembly pathways.

The amino-terminal matrix assembly domain of fibronectin stabilizes cell shape and prevents cell cycle progression

Adhesion to the extracellular matrix modulates the cellular response to growth factors and is critical for cell cycle progression. The present study was designed to address the relationship between fibronectin matrix assembly and cell shape or shape dependent cellular processes. The binding of fibronectin's amino-terminal matrix assembly domain to adherent cells represents the initial step in the assembly of exogenous fibronectin into the extracellular matrix. When added to monolayers of pulmonary artery endothelial cells, the 70 kDa fragment of fibronectin (which contains the matrix assembly domain) stabilized both the extracellular fibronectin matrix as well as the actin cytoskeleton against cytochalasin D-mediated structural reorganization. This activity appeared to require specific fibronectin sequences as fibronectin fragments containing the cell adhesion domain as well as purified vitronectin were ineffective inhibitors of cytochalasin D-induced cytoarchitectural restructuring. Such pronounced morphologic consequences associated with exposure to the 70 kDa fragment suggested that this region of the fibronectin molecule may affect specific growth traits known to be influenced by cell shape. To assess this possibility, the 70 kDa fragment was added to scrape-wounded monolayers of bovine microvessel endothelium and the effects on two shape-dependent processes (i.e. migration and proliferation) were measured as a function of time after injury and location from the wound. The addition of amino-terminal fragments of fibronectin to the monolayer significantly inhibited (by >50%) wound closure. Staining of wounded monolayers with BrdU, moreover, indicated that either the 70 kDa or 25 kDa amino-terminal fragments of fibronectin, but not the 40 kDa collagen binding fragment, also inhibited cell cycle progression. These results suggest that the binding of fibronectin's amino-terminal region to endothelial cell layers inhibits cell cycle progression by stabilizing cell shape.

Fibronectin fibrillogenesis: a paradigm for extracellular matrix assembly

Fibronectin matrix assembly is a regulated stepwise process. In the past year, analyses of fibronectin domains, integrin and cytoskeletal contributions, and fibril architecture have provided new insights into assembly mechanisms and matrix control of cell functions. Like fibronectin, laminin polymerization is cell-mediated. Thus a common pathway for extracellular matrix assembly is emerging.

Inhibition of fibronectin matrix assembly by the heparin-binding domain of vitronectin

The deposition of fibronectin into the extracellular matrix is an integrin-dependent, multistep process that is tightly regulated in order to ensure controlled matrix deposition. Reduced fibronectin deposition has been associated with altered embryonic development, tumor cell invasion, and abnormal wound repair. In one of the initial steps of fibronectin matrix assembly, the amino-terminal region of fibronectin binds to cell surface receptors, termed matrix assembly sites. The present study was undertaken to investigate the role of extracellular signals in the regulation of fibronectin deposition. Our data indicate that the interaction of cells with the extracellular glycoprotein, vitronectin, specifically inhibits matrix assembly site expression and fibronectin deposition. The region of vitronectin responsible for the inhibition of fibronectin deposition was localized to the heparin-binding domain. Vitronectin's heparin-binding domain inhibited both beta(1) and non-beta(1) integrin-dependent matrix assembly site expression and could be overcome by treatment of cells with lysophosphatidic acid, an agent that promotes actin polymerization. The interaction of cells with the heparin-binding domain of vitronectin resulted in changes in actin microfilament organization and the subcellular distribution of the actin-associated proteins alpha-actinin and talin. These data suggest a mechanism whereby the heparin-binding domain of vitronectin regulates the deposition of fibronectin into the extracellular matrix through alterations in the organization of the actin cytoskeleton.

Regulation of F-actin binding to platelet moesin in vitro by both phosphorylation of threonine 558 and polyphosphatidylinositides

Activation of human platelets with thrombin transiently increases phosphorylation at (558)threonine of moesin as determined with phosphorylation state-specific antibodies. This specific modification is completely inhibited by the kinase inhibitor staurosporine and maximally promoted by the phosphatase inhibitor calyculin A, making it possible to purify the two forms of moesin to homogeneity. Blot overlay assays with F-actin probes labeled with either [32P]ATP or 125I show that only phosphorylated moesin interacts with F-actin in total platelet lysates, in moesin antibody immunoprecipitates, and when purified. In the absence of detergents, both forms of the isolated protein are aggregated. Phosphorylated, purified moesin co-sediments with alpha- or beta/gamma-actin filaments in cationic, but not in anionic, nonionic, or amphoteric detergents. The interaction affinity is high (Kd, approximately 1.5 nM), and the maximal moesin:actin stoichiometry is 1:1. This interaction is also observed in platelets extracted with cationic but not with nonionic detergents. In 0.1% Triton X-100, F-actin interacts with phosphorylated moesin only in the presence of polyphosphatidylinositides. Thus, both polyphosphatidylinositides and phosphorylation can activate moesin's high-affinity F-actin binding site in vitro. Dual regulation by both mechanisms may be important for proper cellular control of moesin-mediated linkages between the actin cytoskeleton and the plasma membrane.

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.

Synergism among lysophosphatidic acid, beta1A integrins, and epidermal growth factor or platelet-derived growth factor in mediation of cell migration

GD25 cells lacking the beta1 integrin subunit or expressing beta1A with certain cytoplasmic mutations have poor directed cell migration to platelet-derived growth factor (PDGF) or epidermal growth factor (EGF), ligands of receptor tyrosine kinases, or to lysophosphatidic acid (LPA), a ligand of G-protein-coupled receptors (Sakai, T., Zhang, Q., Fässler, R., and Mosher, D. F. (1998) J. Cell Biol. 141, 527-538 and Sakai, T., Peyruchaud, O., Fässler, R., and Mosher, D. F. (1998) J. Biol. Chem. 273, 19378-19382). We demonstrate here that LPA synergizes with signals induced by beta1A integrins and ligated EGF or PDGF receptors to modulate migration. When LPA was mixed with EGF or PDGF, migration was greater than with EGF or PDGF alone. The enhancement was greater for beta1A-expressing cells than for beta1-null cells. Cells expressing beta1A with mutations of prolines or tyrosines in conserved cytoplasmic NPXY motifs had blunted migratory responses to mixtures of LPA and EGF or PDGF. The major effects on beta1A-expressing cells of LPA when combined with EGF or PDGF were to sensitize cells so that maximal responses were obtained with >10-fold lower concentrations of growth factor and increase the chemokinetic component of migration. Sensitization by LPA was lost when cells were preincubated with pertussis toxin or C3 exotransferase. There was no evidence for transactivation or sensitization of receptors for EGF or PDGF by LPA. EGF or PDGF and LPA caused activation of mitogen-activated protein kinase by pertussis toxin-insensitive and -sensitive pathways respectively, but activation was not additive. These findings indicate that signaling pathways initiated by the cytoplasmic domains of ligated beta1A integrins and tyrosine kinase receptors interact with signaling pathways initiated by LPA to facilitate directed cell migration.

Sphingosine 1-Phosphate Stimulates Fibronectin Matrix Assembly Through a Rho-Dependent Signal Pathway

Fibronectin matrix assembly is a cell-dependent process mediated by cell surface binding sites for the 70-kD N-terminal portion of fibronectin. We have shown that Rho-dependent cytoskeleton reorganization induced by lysophosphatidic acid (LPA) or the microtubule-disrupting agent nocodazole increases fibronectin binding (Zhang et al, Mol Biol Cell 8:1415, 1997). Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid implicated in mitogenesis and cytoskeletal remodelling. Both LPA and S1P are present in increased amounts in serum as compared with plasma as a result of platelet activation. Addition of S1P to human osteosarcoma MG63 cells or human foreskin fibroblasts increased cell-mediated binding and assembly of fibronectin. MG63 cells expressed the Edg-2 and Edg-4 G-protein–coupled receptors for bioactive lipids, whereas foreskin fibroblasts expressed Edg-2, Edg-3, and Edg-4. The stimulatory effect of S1P on the binding of fibronectin or the N-terminal 70-kD fragment of fibronectin was dynamic and due to increases in both the number and affinity of binding sites. The stimulation of 70-kD fragment binding by nanomolar S1P, like stimulation of binding by LPA or nocodazole, was blocked by inactivation of Rho with C3 exotoxin but not by pertussis toxin-mediated inactivation of Gi. These results indicate a common signal pathway leading to control of cellular fibronectin matrix assembly by bioactive lipids generated during blood coagulation.

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.

Sphingosine 1-Phosphate Stimulates Fibronectin Matrix Assembly Through a Rho-Dependent Signal Pathway

Fibronectin matrix assembly is a cell-dependent process mediated by cell surface binding sites for the 70-kD N-terminal portion of fibronectin. We have shown that Rho-dependent cytoskeleton reorganization induced by lysophosphatidic acid (LPA) or the microtubule-disrupting agent nocodazole increases fibronectin binding (Zhang et al, Mol Biol Cell 8:1415, 1997). Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid implicated in mitogenesis and cytoskeletal remodelling. Both LPA and S1P are present in increased amounts in serum as compared with plasma as a result of platelet activation. Addition of S1P to human osteosarcoma MG63 cells or human foreskin fibroblasts increased cell-mediated binding and assembly of fibronectin. MG63 cells expressed the Edg-2 and Edg-4 G-protein–coupled receptors for bioactive lipids, whereas foreskin fibroblasts expressed Edg-2, Edg-3, and Edg-4. The stimulatory effect of S1P on the binding of fibronectin or the N-terminal 70-kD fragment of fibronectin was dynamic and due to increases in both the number and affinity of binding sites. The stimulation of 70-kD fragment binding by nanomolar S1P, like stimulation of binding by LPA or nocodazole, was blocked by inactivation of Rho with C3 exotoxin but not by pertussis toxin-mediated inactivation of Gi. These results indicate a common signal pathway leading to control of cellular fibronectin matrix assembly by bioactive lipids generated during blood coagulation.

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.

RhoA activity is required for fibronectin assembly and counteracts beta1B integrin inhibitory effect in FRT epithelial cells

FRT thyroid epithelial cells synthesize fibronectin and organize a network of fibronectin fibrils at the basal surface of the cells. Fibronectin fibril formation is enhanced by the overexpression of the ubiquitous beta1A integrin and is inhibited by the expression of the dominant-negative beta1B subunit. We tested the hypotheses that RhoA activity might mediate the integrin-dependent fibronectin fibrillogenesis and might counteract beta1B integrin inhibitory effect. FRT-beta1A cells were transfected with a vector carrying a dominant negative form of RhoA (RhoAN19) or treated with the C3 transferase exoenzyme. Both treatments inhibited fibronectin assembly and caused loss of actin microfilaments and adhesion plaques. On the other hand, FRT-beta1B cells were transfected with the constitutively activated form of RhoA (RhoAV14) or treated with the E. coli cytotoxic necrotizing factor 1, which directly activates RhoA. Either treatment restored microfilament and adhesion plaque assembly and promoted fibronectin fibril organization. A great increase in fibronectin fibril assembly was also obtained by treatment of FRT-beta1B cells with TGF-beta. Our data indicate that RhoA is required to promote fibronectin matrix assembly in FRT cells and that the activation of the signal transduction pathway downstream of RhoA can overcome the inhibitory effect of beta1B integrin.

Identification of protein-disulfide isomerase activity in fibronectin

Assembly and degradation of fibronectin-containing extracellular matrices are dynamic processes that are up-regulated during wound healing, embryogenesis, and metastasis. Although several of the early steps leading to fibronectin deposition have been identified, the mechanisms leading to the accumulation of fibronectin in disulfide-stabilized multimers are largely unknown. Disulfide-stabilized fibronectin multimers are thought to arise through intra- or intermolecular disulfide exchange. Several proteins involved in disulfide exchange reactions contain the sequence Cys-X-X-Cys in their active sites, including thioredoxin and protein-disulfide isomerase. The twelfth type I module of fibronectin (I12) contains a Cys-X-X-Cys motif, suggesting that fibronectin may have the intrinsic ability to catalyze disulfide bond rearrangement. Using an established protein refolding assay, we demonstrate here that fibronectin has protein-disulfide isomerase activity and that this activity is localized to the carboxyl-terminal type I module I12. I12 was as active on an equal molar basis as intact fibronectin, indicating that most of the protein-disulfide isomerase activity of fibronectin is localized to I12. Moreover, the protein-disulfide isomerase activity of fibronectin appears to be partially cryptic since limited proteolysis of I10-I12 increased its isomerase activity and dramatically enhanced the rate of RNase refolding. This is the first demonstration that fibronectin contains protein-disulfide isomerase activity and suggests that cross-linking of fibronectin in the extracellular matrix may be catalyzed by a disulfide isomerase activity contained within the fibronectin molecule.

Transformation mediated by RhoA requires activity of ROCK kinases

BACKGROUND

The Ras-related GTPase RhoA controls signalling processes required for cytoskeletal reorganisation, transcriptional regulation, and transformation. The ability of RhoA mutants to transform cells correlates not with transcription but with their ability to bind ROCK-I, an effector kinase involved in cytoskeletal reorganisation. We used a recently developed specific ROCK inhibitor, Y-27632, and ROCK truncation mutants to investigate the role of ROCK kinases in transcriptional activation and transformation.

RESULTS

In NIH3T3 cells, Y-27632 did not prevent the activation of serum response factor, transcription of c-fos or cell cycle re-entry following serum stimulation. Repeated treatment of NIH3T3 cells with Y-27632, however, substantially disrupted their actin fibre network but did not affect their growth rate. Y-27632 blocked focus formation by RhoA and its guanine-nucleotide exchange factors Dbl and mNET1. It did not affect the growth rate of cells transformed by Dbl and mNET1, but restored normal growth control at confluence and prevented their growth in soft agar. Y-27632 also significantly inhibited focus formation by Ras, but had no effect on the establishment or maintenance of transformation by Src. Furthermore, it significantly inhibited anchorage-independent growth of two out of four colorectal tumour cell lines. Consistent with these data, a truncated ROCK derivative exhibited weak ability to cooperate with activated Raf in focus formation assays.

CONCLUSIONS

ROCK signalling is required for both the establishment and maintenance of transformation by constitutive activation of RhoA, and contributes to the Ras-transformed phenotype. These observations provide a potential explanation for the requirement for Rho in Ras-mediated transformation. Moreover, the inhibition of ROCK kinases may be of therapeutic use.

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.

Functional beta1-integrins release the suppression of fibronectin matrix assembly by vitronectin

beta1-null GD25 fibroblasts adherent to vitronectin fail to bind the N-terminal 70-kDa matrix assembly domain of fibronectin or to assemble fibronectin (Sakai, T., Zhang, Q., Fässler, R., and Mosher, D. F. (1998) J. Cell Biol. 141, 527-538). We have made four observations that extend this finding. First, the presence of vitronectin on a substrate that otherwise can support fibronectin assembly has a dominant-negative effect on assembly. Second, the dominant-negative effect is lost when active beta1A is expressed. Third, beta1A containing the extracellular D130A inactivating mutation has a dominant-negative effect on fibronectin assembly. Fourth, beta1-null cells adherent to vitronectin are flat and lack filopodia, whereas beta1-null cells adherent to fibronectin or beta1A-expressing cells adherent to either vitronectin or fibronectin are contracted and exhibit numerous filopodia. These results reveal, therefore, that GD25 cells adherent to vitronectin can only assume a shape suitable for assembly of fibronectin when there is a countervailing signal from functional beta1-integrins.

Identification of a signaling pathway activated specifically in the somatodendritic compartment by a heparan sulfate that regulates dendrite growth

In two earlier reports we demonstrated that natural heparan sulfate, but not dermatan or chondroitin sulfate glycosaminoglycans, stimulate axonal elongation and inhibit dendrite growth in vitro (Lafont et al., 1992). The latter specific effect on dendrite elongation was reproduced by chemically synthesized heparan sulfates and by SR 80037A, a purified sulfated and hexanoylated heparin fragment (Lafont et al., 1994). Adding radioactive SR 80037A to purified neurons demonstrated the existence, at the neuronal surface, of heparan sulfate-specific and saturable binding sites, suggesting that SR 80037A activates specific signal transduction pathways. In the present study, using rat or mouse neurons from the embryonic cortex, we show that SR 80037A signaling involves one or several G-coupled receptor or receptors, small GTPases rhoA and/or rhoC, and one or several PKCs. We also demonstrate that the rapid soma rounding elicited by SR 80037A does not require protein synthesis but that the long-term effect on dendrite initiation requires protein synthesis in a short period after the addition of the heparan sulfate. Finally, by preparing membranes from the somatodendritic or axonal compartments we demonstrate that the identified signaling pathway is activated by SR 80037A primarily in the somatodendritic compartment and is not sensitive to the addition of a dermatan sulfate glycosaminoglycan that does not induce the axonal phenotype by impairing dendrite initiation and elongation.

A role for dystroglycan in basement membrane assembly

Basement membranes are composed of ordered arrays of characteristic extracellular matrix proteins, but little is known about the assembly of these structures in vivo. We have investigated the function of dystroglycan, a cell-surface laminin receptor expressed by cells contacting basement membranes in developing and adult tissues. We find that dystroglycan is required for the formation of a basement membrane in embryoid bodies. Our results further indicate that dystroglycanlaminin interactions are prerequisite for the deposition of other basement membrane proteins. Dystroglycan may exert its influence on basement membrane assembly by binding soluble laminin and organizing it on the cell surface. These data establish a role for dystroglycan in the assembly of basement membranes and suggest fundamental mechanisms underlying this process.