Formation of stable microspikes containing actin and the 55 kDa actin bundling protein, fascin, is a consequence of cell adhesion to thrombospondin-1: implications for the anti-adhesive activities of thrombospondin-1

The role of fascin in the migration and invasiveness of malignant glioma cells

Malignant glioma is the most common primary brain tumor, and its ability to invade the surrounding brain parenchyma is a leading cause of tumor recurrence and treatment failure. Whereas the molecular mechanisms of glioma invasion are incompletely understood, there is growing evidence that cytoskeletal-matrix interactions contribute to this process. Fascin, an actin-bundling protein, induces parallel actin bundles in cell protrusions and increases cell motility in multiple human malignancies. The role of fascin in glioma invasion remains unclear. We demonstrate that fascin is expressed in a panel of human malignant glioma cell lines, and downregulation of fascin expression in glioma cell lines by small interfering RNA (siRNA) is associated with decreased cellular attachment to extracellular matrix (ECM) and reduced migration. Using immunofluorescence analysis, we show that fascin depletion results in a reduced number of filopodia as well as altered glioma cell shape. In vitro invasiveness of U251, U87, and SNB19 glioma cells was inhibited by fascin siRNA treatment by 52.2%, 40.3%, and 23.8% respectively. Finally, we show a decreased invasiveness of U251-GFP cells by fascin knockdown in an ex vivo rat brain slice model system. This is the first study to demonstrate a role for fascin in glioma cell morphology, motility, and invasiveness.

Actin stress fibres

Animal cell movement is effected through a combination of protrusive and contractile events. Non-muscle cells contain stress fibres - bundles of actomyosin that are the major mediators of cell contraction and that can be compared to the highly organised actomyosin arrays of muscle cells. Recent studies have defined regulatory mechanisms that control stress fibre formation, placing the ROCK protein kinase at the centre of a complex signalling network controlling actomyosin contractility and stress fibre assembly. As we uncover the details of stress fibre construction, it is becoming clear that different categories of stress fibres exist. Some of these structures are less suited for cell motility and more suited to static contraction. In keeping with this, many specialised contractile cell types use stress fibres to remodel tissues and extracellular matrix.

Syndecan-1 interaction with the LG4/5 domain in laminin-332 is essential for keratinocyte migration

Laminin 5/laminin 332 (LN332) is an adhesion substrate for epithelial cells. After secretion of LN332, a regulated cleavage occurs at the carboxy-terminus of its alpha3 subunit, which releases a tandem of two globular modules named LG4/5. We show that the presence of the LG4/5 domain in precursor LN332 decreases its integrin-mediated cell adhesion properties in comparison with mature LN332. Whereas cell adhesion to the recombinant LG4/5 fragment relies solely on the heparan sulfate proteoglycan (HSPG) receptor syndecan-1, we reveal that both syndecan-1 and the alpha3beta1 integrin bind to precursor LN332. We further demonstrate that syndecan-1 mediated cell adhesion to the LG4/5 fragment and pre-LN332 allows the formation of fascin-containing protrusions, depending on the GTPases Rac and Cdc42 activation. Reducing syndecan-1 expression in normal keratinocytes prevents cell protrusions on pre-LN332 with subsequent failure of the peripheral localization of the alpha3beta1 integrin. We finally show that cell migration on pre-LN332 requires syndecan-1. Therefore, the LG4/5 domain in precursor LN332 appears to trigger intracellular signaling events, which participate in keratinocyte motility.

Proteomic identification of nitrated brain proteins in amnestic mild cognitive impairment: a regional study

Oxidative stress is an imbalance between the level of antioxidants and oxidants in a cell. Oxidative stress has been shown in brain of subjects with mild cognitive impairment (MCI) as well Alzheimer's disease (AD). MCI is considered as a transition phase between control and AD. The focus of the current study was to identify nitrated proteins in the hippocampus and inferior parietal lobule (IPL) brain regions of subjects with amnestic MCI using proteomics. The identified nitrated proteins in MCI brain were compared to those previously reported to be nitrated and oxidatively modified in AD brain, a comparison that might provide an invaluable insight into the progression of the disease.

Roles of fascin in human carcinoma motility and signaling: prospects for a novel biomarker?

Fascin is a globular actin cross-linking protein that has a major function in forming parallel actin bundles in cell protrusions that are key specialisations of the plasma membrane for environmental guidance and cell migration. Fascin is widely expressed in mesenchymal tissues and the nervous system and is low or absent in adult epithelia. Recent data from a number of laboratories have highlighted that fascin is up-regulated in many human carcinomas and, in individual tissues, correlates with the clinical aggressiveness of tumours and poor patient survival. In cell culture, over-expression or depletion of fascin modulates cell migration and alters cytoskeletal organisation. The identification of biomarkers to provide more effective early diagnosis of potentially aggressive tumours, or identify tumours susceptible to targeted therapies, is an important goal in clinical research. Here, we discuss the evidence that fascin is upregulated in carcinomas, its contributions to carcinoma cell behaviour and its potential as a candidate novel biomarker or therapeutic target.

Fascin-1 expression in papillary and invasive urothelial carcinomas of the urinary bladder

Fascin-1 is an actin-bundling protein that plays an important role in cell motility and adhesion. The level of fascin-1 is low or undetectable in normal epithelial cells. However, overexpression is reported in transformed epithelial cells and in several common types of carcinomas [Bioessays. 2002;24:359-361]. Up-regulation of fascin-1 is associated with higher grades and with aggressive tumors with poorer prognoses. We found no report on the role or the protein expression of fascin-1 in urothelial carcinomas (UCs) of the urinary bladder. In this study, we examined by immunohistochemistry the expression of fascin-1 in the normal human transitional epithelium, benign vesical lesions, and different types of UCs. We found no detectable fascin-1 in the normal transitional epithelium. There was no increase of fascin-1 expression in cystitis cystica, cystitis glandularis, nephrogenic adenoma (n = 10), inverted papilloma (n = 5), and classic exophytic papilloma (n = 4) or in adjacent transitional epithelia associated with these conditions. Patchy or diffusely weak fascin-1 expression was observed in 42% (5/12) of superficial papillary UCs (Ta), and 95% (19/20) of invasive UCs (T2 or higher) demonstrated diffuse strong staining for fascin-1. The microinvasive foci in the lamina propria of UC (T1, n = 8) were also positive for fascin-1, although they were not as strongly stained as in the deeply invasive tumors. Interestingly, the neoplastic cells in the tips of microinvasive carcinomas were distinctly positive for fascin-1. There were significant numbers of fascin-1-positive cells (>50% of the neoplastic cells) in UCs in situ (n = 10). These findings suggest an association between increased fascin-1 expression and increased invasiveness of carcinomas in the urinary bladder.

Functional role of syndecan-1 cytoplasmic V region in lamellipodial spreading, actin bundling, and cell migration

Cell protrusions contribute to cell motility and migration by mediating the outward extension and initial adhesion of cell edges. In many cells, these extensions are supported by actin bundles assembled by the actin cross-linking protein, fascin. Multiple extracellular cues regulate fascin and here we focus on the mechanism by which the transmembrane proteoglycan, syndecan-1, specifically activates lamellipodial cell spreading and fascin-and-actin bundling when clustered either by thrombospondin-1, laminin, or antibody to the syndecan-1 extracellular domain. There is almost no knowledge of the signaling mechanisms of syndecan-1 cytoplasmic domain and we have tested the hypothesis that the unique V region of syndecan-1 cytoplasmic domain has a crucial role in these processes. By four criteria--the activities of N-cadherin/V region chimeras, syndecan-1 deletion mutants, or syndecan-1 point mutants, and specific inhibition by a membrane-permeable TAT-V peptide--we demonstrate that the V region is necessary and sufficient for these cell behaviors and map the molecular basis for its activity to multiple residues located across the V region. These activities correlate with a V-region-dependent incorporation of cell-surface syndecan-1 into a detergent-insoluble form. We also demonstrate functional roles of syndecan-1 V region in laminin-dependent C2C12 cell adhesion and three-dimensional cell migration. These data identify for the first time specific cell behaviors that depend on signaling through the V region of syndecan-1.

Expression of fascin-1, the gene encoding the actin-bundling protein fascin-1, during mouse embryogenesis

Fascin-1 is an actin-bundling protein that contributes to the architecture and function of cell protrusions and microfilaments in cell adhesion, interactions and motility. Fascin-1 has been studied in cultured cells and by biophysical methods, but little is known about its distribution and functions in vertebrate development. As a first step to understanding the role of fascin-1 in embryogenesis, we have characterised the expression pattern of fascin-1 by in situ hybridisation on whole-mount and sectioned mouse embryos from embryonic day (E)8.0-E16.5. Fascin-1 was widely expressed throughout the embryo and the developing nervous system and mesenchymal tissues represented major sites of expression. Intense signals were observed in different regions of the brain, in the spinal cord and retina, and the cranial and dorsal root ganglia (DRG) appeared strongly positive. This neural expression remained strong throughout development. Fascin-1 was also present in the developing somites. High expression was detected in branchial arches and limb bud mesenchyme. At later stages, fascin-1 was expressed in different muscles of the face, skeletal muscles of the body, and in smooth muscle layers of several organs. Limb tendons appeared strongly positive. There was weak expression in heart ventricles. These results show that fascin-1 is principally expressed in neural and mesenchymal derivatives during embryonic development.

Fascin protrusions in cell interactions

Cell protrusions are outward extensions of the plasma membrane of individual cells that function in sensing the cell environment and in making initial, dynamic adhesions to extracellular matrix and other cells. Cell protrusions can be grouped into two major categories on the basis of morphology: localized, finger-like structures of highly defined shape and various lengths; or broad, irregular extensions of the plasma membrane. A key requirement of all cell protrusions is the need for a rigid cytoskeleton to support the localized extension of the plasma membrane. This is achieved either by a core unipolar bundle of actin microfilaments in finger-like protrusions, or by a combination of radial, rib-like, actin bundles integrated with a dendritic meshwork of microfilaments in the broad, lamellipodial protrusions. From studies of multiple cell types in vertebrates and invertebrates, fascin-1 has emerged as an actin-bundling protein of general importance for a diverse set of cell protrusions with functions in cell adhesion, cell interactions, and cell migration. This review discusses current knowledge of the molecular and cellular properties and functions of fascin, the roles of fascin-based protrusions in the cardiovascular system in health and disease, and areas of future interest.

Internalization but not binding of thrombospondin-1 to low density lipoprotein receptor-related protein-1 requires heparan sulfate proteoglycans

The amino-terminal domain of the extracellular matrix (ECM) protein thrombospondin-1 (TSP-1) mediates binding to cell surface heparan sulfate proteoglycans (HSPG) as well as binding to the endocytic receptor, low density lipoprotein-related protein (LRP-1). We previously found that recombinant TSP-1 containing the amino-terminal residues 1-214, retained both of these interactions (Mikhailenko et al. [1997]: J Biol Chem 272:6784-6791). Here, we examined the activity of a recombinant protein containing amino-terminal residues 1-90 of TSP-1 and found that this domain did not retain high-affinity heparin-binding. The loss of heparin-binding correlated with decreased binding to the fibroblast cell surface. However, both ligand blotting and solid phase binding studies indicate that this truncated fragment of TSP-1 retained high-affinity binding to LRP-1. Consistent with this, it also retained the ability to block the uptake and degradation of (125)I-TSP-1. However, TSP-1(1-90) itself was poorly endocytosed and this truncated amino-terminal domain was considerably more effective than the full-length heparin-binding domain (HBD) of TSP-1 in blocking the catabolism of endogenously expressed TSP-1. These results indicate that TSP-1 binding to LRP-1 does not require prior or concomitant interaction with cell surface HSPG but suggest subsequent endocytosis requires high-affinity heparin-binding.

Lectin-induced haemocyte inactivation in insects

Most multimeric lectins are adhesion molecules, promoting attachment and spreading on surface glycodeterminants. In addition, some lectins have counter-adhesion properties, detaching already spread cells which then acquire round or spindle-formed cell shapes. Since lectin-mediated adhesion and detachment is observed in haemocyte-like Drosophila cells, which have haemomucin as the major lectin-binding glycoprotein, the two opposite cell behaviours may be the result of lectin-mediated receptor rearrangements on the cell surface. To investigate oligomeric lectins as a possible extracellular driving force affecting cell shape changes, we examined lectin-mediated reactions in lepidopteran haemocytes after cytochalasin D-treatment and observed that while cell-spreading was dependent on F-actin, lectin-uptake was less dependent on F-actin. We propose a model of cell shape changes involving a dynamic balance between adhesion and uptake reactions.

Expression of fascin, an actin-bundling protein, in astrocytomas of varying grades

Malignant astrocytomas are highly infiltrative neoplasms that invade readily into regions of normal brain. On a cellular basis, the motility and invasiveness of human cancers can be ascribed in part to complex rearrangements of the actin cytoskeleton that are governed by several actinbinding proteins. One such actin-binding protein that has been linked to the invasive behavior of carcinomas is fascin, which serves to aggregate F actin into bundles. In this study, we examined the expression of fascin in a series of human malignant astrocytomas (WHO grades I-IV). Five grade I, 5 grade II, 10 grade III, and 26 grade IV human astrocytomas were examined for fascin and glial fibrillary acidic protein (GFAP) expression by double immunofluorescence confocal microscopy. Expression of fascin and GFAP was also determined by Western blot analysis. Fascin expression increased with increasing WHO grade of astrocytoma. This is in marked contrast to GFAP expression, which decreased with increasing WHO grade. In grades I and II neoplasms, and within non-neoplastic brain, fascin and GFAP were expressed diffusely within regions examined. However, in the higher-grade astrocytomas (grades III and IV), fascin and GFAP were expressed regionally in distinctly separate tumor cell populations. This is the first study to demonstrate the expression of fascin in human astrocytic neoplasms. The role that fascin plays in contributing to the invasive phenotype of anaplastic astrocytomas awaits further study and investigation.

Cartilage oligomeric matrix protein expression in hepatocellular carcinoma and the cirrhotic liver

BACKGROUND

Cartilage oligomeric matrix protein (COMP) is the fifth member of the thrombospondin family of extracellular, calcium-binding proteins. It was initially isolated and characterized in cartilage tissue, where it is thought to contribute to the extracellular matrix composition and cell-extracellular matrix interaction. In the present study the expression of COMP was investigated in normal liver (n=19), liver cirrhosis (n=14) and hepatocellular carcinoma (HCC; n=16) tissues, both at the mRNA and protein level.

METHODS AND RESULTS

By northern blot and western blot analysis, COMP was absent or rarely expressed in the normal liver and liver cirrhosis tissues, but significantly overexpressed in HCC tissue samples. The COMP mRNA overexpression in HCC was not related to the clinical stage or tumor grade. By in situ hybridization and immunohistochemistry analysis, COMP mRNA and protein expression were localized within the cytoplasm of the tumor cells.

CONCLUSION

COMP is highly expressed within the tumor cells of HCC, suggesting that COMP might play a role in the pathophysiology of this disease.

An extracellular driving force of cell-shape changes

The cellular capacity to internalise objects, involving attachment, engulfment and uptake, exists in virtually all organisms. Many uptake reactions are associated with cell signalling. However, the mechanical forces that form endocytotic vesicles are not known. We propose a 'leverage-mediated' uptake mechanism involving lateral cross-linking processes on the cell surface that can generate the configurational energy to create an inverse curvature of the membrane.

Low dose latrunculin-A inhibits dexamethasone-induced changes in the actin cytoskeleton and alters extracellular matrix protein expression in cultured human trabecular meshwork cells

We determined the effects of a low dose of the actin-disrupting agent latrunculin (LAT)-A on dexamethasone (DEX)-induced changes in actin organization, focal adhesions, and production of extracellular matrix proteins in cultured human trabecular meshwork (HTM) cells. HTM cells were cultured to a highly confluent stage with stable endothelium-like morphology and incubated with 0.1 or 0.2 microM DEX and/or 0.1 microM LAT-A. Changes in the actin cytoskeleton and vinculin-containing focal contacts were evaluated by immunofluorescence microscopy. Expression of thrombospondin-1 (TSP1) and fibronectin (FN) in HTM cells was evaluated by Western blot analysis. The results showed that DEX induced morphological changes and actin reorganization in HTM cells. The cells partly recovered after DEX withdrawal, but the addition of low dose LAT-A hastened the recovery. In addition, DEX failed to induce changes when co-incubated with LAT-A for at least 4 weeks, and for at least 2 weeks when cells were pre-treated with LAT-A for 2 weeks. HTM cells treated with 0.1 microM LAT-A only for 5 days showed mild disorganization of the actin cytoskeleton and focal adhesions, which persisted during the 4 weeks of treatment. DEX stimulated production of FN in HTM cells independent of LAT-A treatment. LAT-A and, to a lesser extent, DEX inhibited production of TSP1 by HTM cells. Although LAT-A is not a DEX receptor antagonist, it is able to prevent the effects of DEX on the actin cytoskeleton in cultured HTM cells at a dose subthreshold for increasing outflow facility in monkeys. This suggests that LAT-A at low doses may be useful in treating steroid and other glaucomas. TSP1 may be an important target of LAT-A in HTM cells and modulation of TSP may influence the actin cytoskeleton of the trabecular meshwork (TM), and consequently, intraocular pressure.

Thrombospondin signaling through the calreticulin/LDL receptor-related protein co-complex stimulates random and directed cell migration

The matricellular extracellular matrix protein thrombospondin-1 (TSP1) stimulates focal adhesion disassembly through a sequence (known as the hep I peptide) in its heparin-binding domain. This mediates signaling through a receptor co-complex involving calreticulin and low-density lipoprotein (LDL) receptor-related protein (LRP). We postulate that this transition to an intermediate adhesive state enhances cellular responses to dynamic environmental conditions. Since cell adhesion dynamics affect cell motility, we asked whether TSP1/hep I-induced intermediate adhesion alters cell migration. Using both transwell and Dunn chamber assays, we demonstrate that TSP1 and hep I gradients stimulate endothelial cell chemotaxis. Treatment with focal adhesion-labilizing concentrations of TSP1/hep I in the absence of a gradient enhances endothelial cell random migration, or chemokinesis, associated with an increase in cells migrating, migration speed, and total cellular displacement. Calreticulin-null and LRP-null fibroblasts do not migrate in response to TSP1/hep I, nor do endothelial cells treated with the LRP inhibitor receptor-associated protein (RAP). Furthermore, TSP1/hep I-induced focal adhesion disassembly is associated with reduced chemotaxis to basic fibroblast growth factor (bFGF) but enhanced chemotaxis to acidic (a)FGF, suggesting differential modulation of growth factor-induced migration. Thus, TSP1/hep I stimulation of intermediate adhesion regulates the migratory phenotype of endothelial cells and fibroblasts, suggesting a role for TSP1 in remodeling responses.

Sustained induction of ERK, protein kinase B, and p70 S6 kinase regulates cell spreading and formation of F-actin microspikes upon ligation of integrins by galectin-8, a mammalian lectin

Galectin-8, a member of the galectin family of mammalian lectins, is a secreted protein that promotes cell adhesion and migration upon binding to a subset of integrins through sugar-protein interactions. Ligation of integrins by galectin-8 triggers a distinct pattern of cytoskeletal organization, including formation of F-actin-containing microspikes. This is associated with activation of integrin-mediated signaling cascades (ERK and phosphatidylinositol 3 kinase (PI3K)) that are much more robust and are of longer duration than those induced upon cell adhesion to fibronectin. Indeed, formation of microspikes is enhanced 40% in cells that overexpress protein kinase B, the downstream effector of PI3K. Inhibition of PI3K activity induced by wortmannin partially inhibits cell adhesion and spreading while largely inhibiting microspike formation in cells adherent to galectin-8. Furthermore, the inhibitory effects of wortmannin are markedly accentuated in cells overexpressing PKB or p70S6K (CHO(PKB) and CHO(p70S6K) cells), whose adhesion and spreading on galectin-8 (but not on fibronectin) is inhibited approximately 25-35% in the presence of wortmannin. The above results suggest that galectin-8 is an extracellular matrix protein that triggers a unique repertoire of integrin-mediated signals, which leads to a distinctive cytoskeletal organization and microspike formation. They further suggest that downstream effectors of PI3K, including PKB and p70 S6 kinase, in part mediate cell adhesion, spreading, and microspike formation induced by galectin-8.

Oxidative stress causes ERK phosphorylation and cell death in cultured retinal pigment epithelium: prevention of cell death by AG126 and 15-deoxy-delta 12, 14-PGJ2

BACKGROUND

The retina, which is exposed to both sunlight and very high levels of oxygen, is exceptionally rich in polyunsaturated fatty acids, which makes it a favorable environment for the generation of reactive oxygen species. The cytotoxic effects of hydrogen peroxide (H2O2) induced oxidative stress on retinal pigment epithelium were characterized in this study.

METHODS

The MTT cell viability assay, Texas-Red phalloidin staining, immunohistochemistry and Western blot analysis were used to assess the effects of oxidative stress on primary human retinal pigment epithelial cell cultures and the ARPE-19 cell line.

RESULTS

The treatment of retinal pigment epithelial cells with H2O2 caused a dose-dependent decrease of cellular viability, which was preceded by a significant cytoskeletal rearrangement, activation of the Extracellular signal-Regulated Kinase, lipid peroxidation and nuclear condensation. This cell death was prevented partially by the prostaglandin derivative, 15d-PGJ2 and by the protein kinase inhibitor, AG126.

CONCLUSION

15d-PGJ2 and AG126 may be useful pharmacological tools in the future capable of preventing oxidative stress induced RPE cell death in human ocular diseases.

Independent prognostic value of fascin immunoreactivity in stage I nonsmall cell lung cancer

Fascin-1, the most expressed form of fascin in vertebrate tissues, is an actin-bundling protein that induces cell membrane protrusions and increases motility of normal and transformed epithelial cells. Very few data are available on the role of this protein in nonsmall cell lung cancer (NSCLC). Two hundred and twenty patients with stage I NSCLC and long-term follow-up were evaluated immunocytochemically for fascin expression. Overall, variable fascin immunoreactivity was detected in 98% of 116 squamous cell carcinomas, in 78% of 96 adenocarcinomas, in 83% of six large cell carcinomas, and in the two adenosquamous carcinomas under study. Neoplastic emboli were commonly decorated by the antifascin antibody (P<0.001), also when the surrounding invasive carcinoma was unreactive. Fascin immunoreactivity correlated with high tumour grade (P=0.017) and, in adenocarcinomas, with high Ki-67 labelling index (P=0.021). Adenocarcinomas with a prevalent bronchiolo-alveolar in situ component were less commonly immunoreactive for fascin than invasive tumours (P=0.005). Contralateral thoracic or distant metastases were associated significantly with diffuse (>60% immunoreactive tumour cells) fascin expression in adenocarcinomas (P=0.043), and marginally with strong fascin immunostaining in squamous cell carcinomas (P=0.13). No associations were noted with any other clinicopathological variables tested. Patients with tumours showing diffuse (>60% immunoreactive neoplastic cells) and/or strong immunoreactivity for fascin had a shorter survival (P=0.006 for adenocarcinomas and P=0.026 for squamous cell carcinomas), even after multivariate analysis (P=0.014 and 0.050, respectively). The current study documents for the first time that fascin is upregulated in invasive and more aggressive NSCLC, being an independent prognostic predictor of unfavourable clinical course of the disease. Targetting the fascin pathway could be a novel therapeutic strategy of NSCLC.

Fascin, an actin-bundling protein, modulates colonic epithelial cell invasiveness and differentiation in vitro

In epithelial tissue, cell-matrix and cell-cell adhesive interactions have important roles in the normal organization and stabilization of the cell layer. The malignant conversion of epithelial cells involves alterations in the expression and function of these adhesion systems that enable a switch to a migratory phenotype in tumor invasion and metastasis. Fascin is an actin-crosslinking protein that is found in the core actin bundles of cell-surface spikes and projections that are implicated in cell motility. We demonstrate that fascin is not detectable in normal colonic epithelium, but is dramatically up-regulated in colorectal adenocarcinoma. To test the hypothesis that fascin could participate in tumor invasive behavior, we developed a cell culture model to examine the effect of fascin expression on the adhesive interactions, invasiveness, and differentiation of colonic epithelial cells. We report marked effects on the organization of cell-surface protrusions, actin cytoskeleton, and focal adhesions in the absence of alterations in the protein levels of the major components of these structures. These effects correlate with alterations in cell movements on two-dimensional matrix, and increased invasiveness in three-dimensional matrix. The cells also show increased proliferation and decreased capacity for normal glandular differentiation in collagen gels. We propose that up-regulation of fascin, by promoting the formation of protrusive, actin-based, cell-motility structures, could be a significant component in the acquisition of invasive phenotype in colonic carcinoma.

Characterization of a Drosophila melanogaster orthologue of muskelin

Muskelin was identified in vertebrates as a novel, intracellular, kelch repeat protein that is needed in cell-spreading responses to the matrix adhesion molecule, thrombospondin-1. The identification and characterization of an orthologue of muskelin in Drosophila melanogaster is now reported. The Drosophila muskelin gene, located on chromosome 2R, is encoded in ten exons. Drosophila muskelin is expressed in embryos, larvae and adult flies. The protein has 45% sequence identity to vertebrate muskelins, with highest sequence identity in an amino-terminal domain and the six kelch repeats that form a beta-propeller structure. Multiple sequence alignment of human, mouse, rat and Drosophila muskelins and protein database searches revealed a novel highly conserved motif within the amino-terminal domain, lissencephaly homology motif (LisH) and C-terminal to LisH motifs in the central region of the molecule, and several conserved consensus motifs for phosphorylation by protein kinase C and casein kinase II. These findings provide new information on the modular structure of muskelin and indicate potential for conserved mechanisms of function.

Functional synergy of actin filament cross-linking proteins

The organization of filamentous actin (F-actin) in resilient networks is coordinated by various F-actin cross-linking proteins. The relative tolerance of cells to null mutations of genes that code for a single actin cross-linking protein suggests that the functions of those proteins are highly redundant. This apparent functional redundancy may, however, reflect the limited resolution of available assays in assessing the mechanical role of F-actin cross-linking/bundling proteins. Using reconstituted F-actin networks and rheological methods, we demonstrate how alpha-actinin and fascin, two F-actin cross-linking/bundling proteins that co-localize along stress fibers and in lamellipodia, could synergistically enhance the resilience of F-actin networks in vitro. These two proteins can generate microfilament arrays that "yield" at a strain amplitude that is much larger than each one of the proteins separately. F-actin/alpha-actinin/fascin networks display strain-induced hardening, whereby the network "stiffens" under shear deformations, a phenomenon that is non-existent in F-actin/fascin networks and much weaker in F-actin/alpha-actinin networks. Strain-hardening is further enhanced at high rates of deformation and high concentrations of actin cross-linking proteins. A simplified model suggests that the optimum results of the competition between the increased stiffness of bundles and their decreased density of cross-links. Our studies support a re-evaluation of the notion of functional redundancy among cytoskeletal regulatory proteins.

Induction of fascin spikes in breast cancer cells by activation of the insulin-like growth factor-I receptor

Insulin-like growth factor-I receptor (IGF-IR) signaling contributes to the formation of mammary carcinomas and has chiefly been studied with regard to the proliferative and anti-apoptotic effects of IGF-IR signaling. However, IGF-IR activation also affects the actin cytoskeleton and alterations in cell migratory behavior are of known importance for the malignant conversion and metastasis of epithelial cells. The actin-binding protein fascin is found in cell projections and spikes that are involved in the locomotion of mesenchymal cells. Fascin expression is typically low in normal epithelial cells, but is markedly upregulated in several types of carcinomas. Here, we also demonstrate increased fascin expression in breast carcinoma cell lines and adopt MCF-7 human mammary carcinoma cells that over-express wild-type or kinase-inactivated forms of the IGF-IR as a model system to test the hypothesis that IGF-IR activation induces fascin projections. We show that the time-dependent dissociation of cell colonies that occurs upon receptor activation by IGF-I involves the formation of dynamic, fascin-containing lateral cell projections that co-localize with ruffling membranes in association with protrusive activity and cell migratory phenotype. The molecular mechanism of these effects is completely dependent on IGF-IR tyrosine kinase activity and is mediated by a phosphatidylinositol (PI) 3-kinase-dependent process. In demonstrating transduction of fascin spike assembly by activation of a peptide growth factor receptor, these novel data reveal a wide role for fascin spikes in cell motility and provide new insight into the complex effects of IGF-IR signaling on actin cytoskeletal organization.

Trimeric assembly of the C-terminal region of Thrombospondin-1 or Thrombospondin-2 is necessary for cell spreading and fascin spike organisation

Thrombospondin-1 (TSP-1) and the highly related protein thrombospondin-2(TSP-2) are trimeric extracellular molecules that have complex roles in wound healing, angiogenesis and matrix organisation. At the cellular level, TSP-1 supports cell adhesion and migration by the organisation of fascin spike cytoskeletal structures. To define the molecular requirements for assembly of fascin spikes by thrombospondins, we developed a panel of recombinant protein units of TSP-1 and TSP-2; these were designed according to the domain boundaries and included matched monomeric and trimeric units. These proteins were tested for their effects on cell attachment and fascin spike organisation using C2C12 skeletal myoblasts and vascular smooth muscle cells. In monomeric units, cell attachment activity was localised to the type 1 repeats or type 3 repeats/C-terminal globule, and both regions need to be present in the same molecule for maximal activity. On a molar basis, cell-attachment activities with monomeric units were low compared with intact TSP-1, and no monomeric unit induced cell spreading. Trimeric versions of the type 1 repeats were more adhesive but did not induce cell spreading. Strikingly, trimers that contained the type 3 repeats/C-terminal globule of either TSP-1 or TSP-2 supported cell spreading and fascin spike organisation, producing a similar activity to intact TSP-1. We conclude that trimeric assembly of the highly conserved TSP C-terminal region is necessary for organisation of the fascin-based cytoskeletal structures that are needed for thrombospondin-induced cell motility.

Fascins, and their roles in cell structure and function

The fascins are a structurally unique and evolutionarily conserved group of actin cross-linking proteins. Fascins function in the organisation of two major forms of actin-based structures: dynamic, cortical cell protrusions and cytoplasmic microfilament bundles. The cortical structures, which include filopodia, spikes, lamellipodial ribs, oocyte microvilli and the dendrites of dendritic cells, have roles in cell-matrix adhesion, cell interactions and cell migration, whereas the cytoplasmic actin bundles appear to participate in cell architecture. We discuss the current understanding of the cellular mechanisms that regulate the binding of fascin to actin and how these processes contribute to the organisation or disassembly of cell protrusions. Although the in vivo roles of fascin have been studied principally in Drosophila, several human diseases are associated with inherited or acquired alterations in the expression of fascins. Strategies to modulate fascin-containing protrusions and thereby cell adhesive and migratory behaviour could have potential for therapeutic intervention in these conditions. The supplementary material referred to in this section can be found at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/2002/v24.350.html

Thrombospondins: multifunctional regulators of cell interactions

Thrombospondins are secreted, multidomain macromolecules that act as regulators of cell interactions in vertebrates. Gene knockout mice constructed for two members of this family demonstrate roles in the organization and homeostasis of multiple tissues, with particularly significant activities in the regulation of angiogenesis. This review discusses the functions of thrombospondins with regard to their cellular mechanisms of action and highlights recent advances in understanding how multifactorial molecular interactions, at the cell surface and within extracellular matrix, produce cell-type-specific effects on cell behavior and the organization of matrix and tissues.

Cell imaging and morphology: application to studies of inherited purine metabolic disorders

A number of inherited or drug-induced metabolic disorders involving dysfunctions in purines and pyrimidines are strongly associated with neurological dysfunction, e.g., Lesch Nyhan syndrome. Such disorders have been studied extensively using biochemical and molecular techniques in order to examine how such defects occur, sometimes using in vitro models based upon cultured neuroblastoma cell lines. However, these metabolic dysfunctions may manifest their effects in other ways, such as impaired synaptic transmission and gross abnormalities in neuronal growth and differentiation. This review outlines the latter novel facet of purine research. It is proposed that by employing cell imaging techniques and cultured neuroblastoma cell lines, believed to model the nervous system, significant insights into how inherited disorders of purine metabolism affect neuronal development can be obtained. This review provides an example of the application of these techniques to understand the etiology of Lesch Nyhan syndrome, and encourages further study of the role of purines and pyrimidines in the development of the nervous system.

A role for syndecan-1 in coupling fascin spike formation by thrombospondin-1

An important role of cell matrix adhesion receptors is to mediate transmembrane coupling between extracellular matrix attachment, actin reorganization, and cell spreading. Thrombospondin (TSP)-1 is a modulatory component of matrix expressed during development, immune response, or wound repair. Cell adhesion to TSP-1 involves formation of biochemically distinct matrix contacts based on stable fascin spikes. The cell surface adhesion receptors required have not been identified. We report here that antibody clustering of syndecan-1 proteoglycan specifically transduces organization of cortical actin and fascin bundles in several cell types. Transfection of COS-7 cells with syndecan-1 is sufficient to stimulate cell spreading, fascin spike assembly, and extensive protrusive lateral ruffling on TSP-1 or on syndecan-1 antibody. The underlying molecular mechanism depends on glycosaminoglycan (GAG) modification of the syndecan-1 core protein at residues S45 or S47 for cell membrane spreading and on the VC2 region of the cytoplasmic domain for spreading and fascin spike formation. Expression of the VC2 deletion mutant or GAG-negative syndecan-1 showed that syndecan-1 is necessary in spreading and fascin spike formation by C2C12 cells on TSP-1. These results establish a novel role for syndecan-1 protein in coupling a physiological matrix ligand to formation of a specific matrix contact structure.

The cell biology of thrombospondin-1

Thrombospondin-1 (TSP-1) is a matricellular protein that regulates cellular phenotype during tissue genesis and repair. It acts as a molecular facilitator by bringing together cytokines, growth factors, matrix components, membrane receptors and extracellular proteases. TSP-1 binds to a wide variety of integrin and non-integrin cell surface receptors. The binding sites for these receptors on TSP-1 are dispersed throughout the molecule, with most domains binding multiple receptors. In some cases, TSP-1 binds to multiple receptors concurrently, and recent data indicate that there is cross-talk between the receptor systems. Thus, TSP-1 may function to direct the clustering of receptors to specialized domains for adhesion and signal transduction.

Stimulation of fascin spikes by thrombospondin-1 is mediated by the GTPases Rac and Cdc42

Cell adhesion to extracellular matrix is an important physiological stimulus for organization of the actin-based cytoskeleton. Adhesion to the matrix glycoprotein thrombospondin-1 (TSP-1) triggers the sustained formation of F-actin microspikes that contain the actin-bundling protein fascin. These structures are also implicated in cell migration, which may be an important function of TSP-1 in tissue remodelling and wound repair. To further understand the function of fascin microspikes, we examined whether their assembly is regulated by Rho family GTPases. We report that expression of constitutively active mutants of Rac or Cdc42 triggered localization of fascin to lamellipodia, filopodia, and cell edges in fibroblasts or myoblasts. Biochemical assays demonstrated prolonged activation of Rac and Cdc42 in C2C12 cells adherent to TSP-1 and activation of the downstream kinase p21-activated kinase (PAK). Expression of dominant-negative Rac or Cdc42 in C2C12 myoblasts blocked spreading and formation of fascin spikes on TSP-1. Spreading and spike assembly were also blocked by pharmacological inhibition of F-actin turnover. Shear-loading of monospecific anti-fascin immunoglobulins, which block the binding of fascin to actin into cytoplasm, strongly inhibited spreading, actin cytoskeletal organization and migration on TSP-1 and also affected the motility of cells on fibronectin. We conclude that fascin is a critical component downstream of Rac and Cdc42 that is needed for actin cytoskeletal organization and cell migration responses to thrombospondin-1.

Replacing two conserved tyrosines of the EphB2 receptor with glutamic acid prevents binding of SH2 domains without abrogating kinase activity and biological responses

Eph receptor tyrosine kinases play key roles in pattern formation during embryonic development, but little is known about the mechanisms by which they elicit specific biological responses in cells. Here, we investigate the role of tyrosines 605 and 611 in the juxtamembrane region of EphB2, because they are conserved Eph receptor autophosphorylation sites and demonstrated binding sites for the SH2 domains of multiple signaling proteins. Mutation of tyrosines 605 and 611 to phenylalanine impaired EphB2 kinase activity, complicating analysis of their function as SH2 domain binding sites and their contribution to EphB2-mediated signaling. In contrast, mutation to the negatively charged glutamic acid disrupted SH2 domain binding without reducing EphB2 kinase activity. By using a panel of EphB2 mutants, we found that kinase activity is required for the changes in cell-matrix and cell - cell adhesion, cytoskeletal organization, and activation of mitogen-activated protein (MAP) kinases elicited by EphB2 in transiently transfected cells. Instead, the two juxtamembrane SH2 domain binding sites were dispensable for these effects. These results suggest that phosphorylation of tyrosines 605 and 611 is critical for EphB2-mediated cellular responses because it regulates EphB2 kinase activity.

Cell-matrix adhesions differentially regulate fascin phosphorylation

Cell adhesion to individual macromolecules of the extracellular matrix has dramatic effects on the subcellular localization of the actin-bundling protein fascin and on the ability of cells to form stable fascin microspikes. The actin-binding activity of fascin is down-regulated by phosphorylation, and we used two differentiated cell types, C2C12 skeletal myoblasts and LLC-PK1 kidney epithelial cells, to examine the hypothesis that cell adhesion to the matrix components fibronectin, laminin-1, and thrombospondin-1 differentially regulates fascin phosphorylation. In both cell types, treatment with the PKC activator 12-tetradecanoyl phorbol 13-acetate (TPA) or adhesion to fibronectin led to a diffuse distribution of fascin after 1 h. C2C12 cells contain the PKC family members alpha, gamma, and lambda, and PKCalpha localization was altered upon cell adhesion to fibronectin. Two-dimensional isoelectric focusing/SDS-polyacrylamide gels were used to determine that fascin became phosphorylated in cells adherent to fibronectin and was inhibited by the PKC inhibitors calphostin C and chelerythrine chloride. Phosphorylation of fascin was not detected in cells adherent to thrombospondin-1 or to laminin-1. LLC-PK1 cells expressing green fluorescent protein (GFP)-fascin also displayed similar regulation of fascin phosphorylation. LLC-PK1 cells expressing GFP-fascin S39A, a nonphosphorylatable mutant, did not undergo spreading and focal contact organization on fibronectin, whereas cells expressing a GFP-fascin S39D mutant with constitutive negative charge spread more extensively than wild-type cells. In contrast, C2C12 cells coexpressing S39A fascin with endogenous fascin remained competent to form microspikes on thrombospondin-1, and cells that expressed fascin S39D attached to thrombospondin-1 but did not form microspikes. Blockade of PKCalpha activity by TPA-induced down-regulation led to actin association of wild-type fascin in fibronectin-adherent C2C12 and LLC-PK1 cells but did not alter the distribution of S39A or S39D fascins. The association of fascin with actin in fibronectin-adherent cells was also evident in the presence of an inhibitory antibody to integrin alpha5 subunit. These novel results establish matrix-initiated PKC-dependent regulation of fascin phosphorylation at serine 39 as a mechanism whereby matrix adhesion is coupled to the organization of cytoskeletal structure.

Identification of an alpha(3)beta(1) integrin recognition sequence in thrombospondin-1

A synthetic peptide containing amino acid residues 190-201 of thrombospondin-1 (TSP1) promoted adhesion of MDA-MB-435 breast carcinoma cells when immobilized and inhibited adhesion of the same cells to TSP1 when added in solution. Adhesion to this peptide was enhanced by a beta(1) integrin-activating antibody, Mn(2+), and insulin-like growth factor I and was inhibited by an alpha(3)beta(1) integrin function-blocking antibody. The soluble peptide inhibited adhesion of cells to the immobilized TSP1 peptide or spreading on intact TSP1 but at the same concentrations did not inhibit attachment or spreading on type IV collagen or fibronectin. Substitution of several residues in the TSP1 peptide with Ala residues abolished or diminished the inhibitory activity of the peptide in solution, but only substitution of Arg-198 completely inactivated the adhesive activity of the immobilized peptide. The essential residues for activity of the peptide as a soluble inhibitor are Asn-196, Val-197, and Arg-198, but flanking residues enhance the inhibitory activity of this core sequence, either by altering the conformation of the active sequence or by interacting with the integrin. This functional sequence is conserved in all known mammalian TSP1 sequences and in TSP1 from Xenopus laevis. The TSP1 peptide also inhibited adhesion of MDA-MB-435 cells to the laminin-1 peptide GD6, which contains a potential integrin-recognition sequence Asn-Leu-Arg and is derived from a similar position in a pentraxin module. Adhesion studies using recombinant TSP1 fragments also localized beta1 integrin-dependent adhesion to residues 175-242 of this region, which contain the active sequence.

Adhesion-induced intracellular signalling in endothelial cells depends on the nature of the matrix

The adhesion of a human microvascular endothelial cell line to its own matrix was studied in comparison with adhesion of the same cells to fibronectin or thrombospondin-1. These endothelial cells adhered preferentially to their matrix whereas an equal cell number was attached to fibronectin or thrombospondin-1. The adhesion of cells to thrombospondin-1 was mediated by the N-terminal heparin binding domain of thrombospondin-1 as shown by the use of a recombinant fragment, N18. Cells adhering to their matrix displayed a morphology and a cytoskeleton organization very similar to that observed in vivo with an apical immunostaining for actin stress fibers and a fine basal labeling for vinculin. Cells on fibronectin were extensively spread and rapidly assembled stress fibers and focal contacts. Cells adherent to thrombospondin-1 presented large lamellae rich in actin but devoid of vinculin and only few actin fibers were observed. Depending on the substratum used, adhering endothelial cells displayed also different tyrosine phosphorylation patterns on electrophoresis. Our observations indicate that endothelial cells adhering to their matrix present an activation state intermediate between that induced by a "hyperadhesive" protein like fibronectin and that generated by a moderate, indeed anti-adhesive, protein like thrombospondin-1.

Thrombospondin-1 induces tyrosine phosphorylation of adherens junction proteins and regulates an endothelial paracellular pathway

Thrombospondin-1 (TSP) induces endothelial cell (EC) actin reorganization and focal adhesion disassembly and influences multiple EC functions. To determine whether TSP might regulate EC-EC interactions, we studied the effect of exogenous TSP on the movement of albumin across postconfluent EC monolayers. TSP increased transendothelial albumin flux in a dose-dependent manner at concentrations >/=1 microg/ml (2.2 nM). Increases in albumin flux were observed as early as 1 h after exposure to 30 microg/ml (71 nM) TSP. Inhibition of tyrosine kinases with herbimycin A or genistein protected against the TSP-induced barrier dysfunction by >80% and >50%, respectively. TSP-exposed monolayers exhibited actin reorganization and intercellular gap formation, whereas pretreatment with herbimycin A protected against this effect. Increased staining of phosphotyrosine-containing proteins was observed in plaque-like structures and at the intercellular boundaries of TSP-treated cells. In the presence of protein tyrosine phosphatase inhibition, TSP induced dose- and time-dependent increments in levels of phosphotyrosine-containing proteins; these TSP dose and time requirements were compatible with those defined for EC barrier dysfunction. Phosphoproteins that were identified include the adherens junction proteins focal adhesion kinase, paxillin, gamma-catenin, and p120(Cas). These combined data indicate that TSP can modulate endothelial barrier function, in part, through tyrosine phosphorylation of EC proteins.

Pro-adhesive and chemotactic activities of thrombospondin-1 for breast carcinoma cells are mediated by alpha3beta1 integrin and regulated by insulin-like growth factor-1 and CD98

Thrombospondin-1 (TSP1) is a matricellular protein that displays both pro- and anti-adhesive activities. Binding to sulfated glycoconjugates mediates most high affinity binding of soluble TSP1 to MDA-MB-435 cells, but attachment and spreading of these cells on immobilized TSP1 is primarily beta1 integrin-dependent. The integrin alpha3beta1 is the major mediator of breast carcinoma cell adhesion and chemotaxis to TSP1. This integrin is partially active in MDA-MB-435 cells but is mostly inactive in MDA-MB-231 and MCF-7 cells, which require beta1 integrin activation to induce spreading on TSP1. Integrin-mediated cell spreading on TSP1 is accompanied by extension of filopodia containing beta1 integrins. TSP1 binding activity of the alpha3beta1 integrin is not stimulated by CD47-binding peptides from TSP1 or by protein kinase C activation, which activate alphavbeta3 integrin function in the same cells. In MDA-MB-231 but not MDA-MB-435 cells, this integrin is activated by pertussis toxin, whereas serum, insulin, insulin-like growth factor-1, and ligation of CD98 increase activity of this integrin in both cell lines. Serum stimulation is accompanied by increased surface expression of CD98, whereas insulin-like growth factor-1 does not increase CD98 expression. Thus, the pro-adhesive activity of TSP1 for breast carcinoma cells is controlled by several signals that regulate activity of the alpha3beta1 integrin.

The distribution of the matricellular protein thrombospondin 2 in tissues of embryonic and adult mice

Mice that lack the matricellular protein thrombospondin 2 (TSP2) develop a pleiotropic phenotype characterized by morphological changes in connective tissues, an increase in vascular density, and a propensity for bleeding. Furthermore, dermal cells derived from TSP2-null mice display adhesion defects, a finding that implicates TSP2 in cell-matrix interactions. To gain a better understanding of the participation of TSP2 in the development and maturation of the mouse, we examined its distribution in embryonic and adult tissues. Special attention was paid to the presence of TSP2 in collagen fibers, because collagen fibrils in the TSP2-null mouse appear to be irregular in size and contour by electron microscopy. Immunohistochemical analysis of Day 15 and Day 18 embryos revealed TSP2 in areas of chondrogenesis, osteogenesis, and vasculogenesis, and in dermal and other connective tissue-forming cells. Distinctly different patterns of deposition of TSP2 were observed in areas of developing cartilage and bone at Days 15 and 18 of embryonic development. A survey of adult tissues revealed TSP2 in dermal fibroblasts, articular chondrocytes, Purkinje cells in the cerebellum, Leidig cells in the testis, and in the adrenal cortex. Dermal fibroblasts were also shown to synthesize TSP2 in vitro. The distribution of TSP2 during development is in keeping with its participation in the formation of a variety of connective tissues. In adult tissues, TSP2 is located in the pericellular environment, where it can potentially influence the cell-matrix interactions associated with cell movement and tissue repair.

Adhesion of mature polyploid megakaryocytes to fibronectin is mediated by beta 1 integrins and leads to cell damage

Human CD34+ bone marrow cells were committed to the megakaryocytic lineage in serum-free liquid cultures by the following cytokines: thrombopoietin, erythropoietin, and IL-6. Megakaryocyte maturation has been described as being regulated by the extracellular matrix. These cells express receptors for laminin, collagen, and vitronectin, but they selectively adhere to and spread on fibronectin, a major component of the bone marrow environment. Function-perturbing antibodies against beta 1 integrins totally abolished the adhesion of megakaryocytes on fibronectin, whereas antibodies to beta 3 did not, suggesting that beta 1 integrins were responsible for the adhesive phenotype of these polyploid cells. beta 1-positive clusters were visualized in close contact with the extremities of stress fibers at the cell surface. In the course of cell spreading, we observed morphological modifications such as the disorganization of the compact nuclei structure and the appearance of holes in the cytoplasm leading to the release of alpha IIb beta 3-positive cellular fragments. This process appeared to be a specific feature of megakaryocytes and is correlated neither to apoptosis nor to integrin signaling.

Fascin, an actin-bundling protein, induces membrane protrusions and increases cell motility of epithelial cells

Fascin is an actin-bundling protein that is found in membrane ruffles, microspikes, and stress fibers. The expression of fascin is greatly increased in many transformed cells, as well as in specialized normal cells including neuronal cells and antigen-presenting dendritic cells. A morphological characteristic common to these cells expressing high levels of fascin is the development of many membrane protrusions in which fascin is predominantly present. To examine whether fascin contributes to the alterations in microfilament organization at the cell periphery, we have expressed fascin in LLC-PK1 epithelial cells to levels as high as those found in transformed cells and in specialized normal cells. Expression of fascin results in large changes in morphology, the actin cytoskeleton, and cell motility: fascin-transfected cells form an increased number of longer and thicker microvilli on apical surfaces, extend lamellipodia-like structures at basolateral surfaces, and show disorganization of cell-cell contacts. Cell migration activity is increased by 8-17 times when assayed by modified Boyden chamber. Microinjection of a fascin protein into LLC-PK1 cells causes similar morphological alterations including the induction of lamellipodia at basolateral surfaces and formation of an increased number of microvilli on apical surfaces. Furthermore, microinjection of fascin into REF-52 cells, normal fibroblasts, induces the formation of many lamellipodia at all regions of cell periphery. These results together suggest that fascin is directly responsible for membrane protrusions through reorganization of the microfilament cytoskeleton at the cell periphery.

Mice that lack thrombospondin 2 display connective tissue abnormalities that are associated with disordered collagen fibrillogenesis, an increased vascular density, and a bleeding diathesis

Thrombospondin (TSP) 2, and its close relative TSP1, are extracellular proteins whose functions are complex, poorly understood, and controversial. In an attempt to determine the function of TSP2, we disrupted the Thbs2 gene by homologous recombination in embryonic stem cells, and generated TSP2-null mice by blastocyst injection and appropriate breeding of mutant animals. Thbs2-/- mice were produced with the expected Mendelian frequency, appeared overtly normal, and were fertile. However, on closer examination, these mice displayed a wide variety of abnormalities. Collagen fiber patterns in skin were disordered, and abnormally large fibrils with irregular contours were observed by electron microscopy in both skin and tendon. As a functional correlate of these findings, the skin was fragile and had reduced tensile strength, and the tail was unusually flexible. Mutant skin fibroblasts were defective in attachment to a substratum. An increase in total density and in cortical thickness of long bones was documented by histology and quantitative computer tomography. Mutant mice also manifested an abnormal bleeding time, and histologic surveys of mouse tissues, stained with an antibody to von Willebrand factor, showed a significant increase in blood vessels. The basis for the unusual phenotype of the TSP2-null mouse could derive from the structural role that TSP2 might play in collagen fibrillogenesis in skin and tendon. However, it seems likely that some of the diverse manifestations of this genetic disorder result from the ability of TSP2 to modulate the cell surface properties of mesenchymal cells, and thus, to affect cell functions such as adhesion and migration.

Characterization of cell-matrix adhesion requirements for the formation of fascin microspikes

Cell adhesion to thrombospondin-1 (TSP-1) correlates with assembly of cell-substratum contact structures that contain fascin microspikes. In this analysis, cell-matrix requirements for assembly of fascin microspikes were examined in detail. In six cell lines, cell spreading on a TSP-1 substratum correlated with expression of fascin protein and formation of fascin microspikes. Microspikes were not formed by H9c2 cells adherent on fibronectin, vitronectin, collagen IV, or platelet factor 4. However, both fascin microspikes and focal contacts were assembled by cells adherent on laminin-1. Using mixed substrata containing different proportions of TSP-1, and fibronectin, fascin microspike formation by H9c2 and C2C12 cells was found to be reduced on substrata containing 25% fibronectin and abolished on substrata containing 75% fibronectin. Adhesion to intermediate mixtures of TSP-1 and fibronectin resulted in coassembly of fascin microspikes and focal contacts, colocalization of fascin with actin stress fiber bundles and altered distributions of beta 1 integrins, cortical alpha-actinin, and tropomyosin. In cells adherent on 50% TSP-1:50% fibronectin, GRGDSP peptide treatment decreased focal contact assembly and altered cytoskeletal organization but did not inhibit microspike assembly. Treatment with chondroitin sulfate A or p-nitrophenol beta-D-xylopyranoside decreased microspike formation and modified cytoskeletal organization but did not inhibit focal contact formation. In polarized migratory and postmitotic C2C12 cells, fascin microspikes and ruffles were localized at leading edges and TSP matrix deposition was also concentrated in this region. Depletion of matrix TSP by heparin treatment correlated with decreased microspike formation and cell motility. Thus, the balance of adhesive receptors ligated at the cell surface during initial cell-matrix attachment serves to regulate the type of substratum adhesion contact assembled and subsequent cytoskeletal organization. A role for fascin microspikes in cell motile behavior is indicated.

Cell-adhesive responses to tenascin-C splice variants involve formation of fascin microspikes

Tenascin-C is an adhesion-modulating matrix glycoprotein that has multiple effects on cell behavior. Tenascin-C transcripts are expressed in motile cells and at sites of tissue modeling during development, and alternative splicing generates variants that encode different numbers of fibronectin type III repeats. We have examined the in vivo expression and cell adhesive properties of two full-length recombinant tenascin-C proteins: TN-190, which contains the eight constant fibronectin type III repeats, and TN-ADC, which contains the additional AD2, AD1, and C repeats. In situ hybridization with probes specific for the AD2, AD1, and C repeats shows that these splice variants are expressed at sites of active tissue modeling and fibronectin expression in the developing avian feather bud and sternum. Transcripts incorporating the AD2, AD1, and C repeats are present in embryonic day 10 wing bud but not in embryonic day 10 lung. By using a panel of nine cell lines in attachment assays, we have found that C2C12, G8, and S27 myoblastic cells undergo concentration-dependent adhesion to both variants, organize actin microspikes that contain the actin-bundling protein fascin, and do not assemble focal contacts. On a molar basis, TN-ADC is more active than TN-190 in promoting cell attachment and irregular cell spreading. The addition of either TN-190 or TN-ADC in solution to C2C12, COS-7, or MG-63 cells adherent on fibronectin decreases cell attachment and results in decreased organization of actin microfilament bundles, with formation of cortical membrane ruffles and retention of residual points of substratum contact that contain filamentous actin and fascin. These data establish a biochemical similarity in the processes of cell adhesion to tenascin-C and thrombospondin-1, also an "antiadhesive" matrix component, and also demonstrate that both the adhesive and adhesion-modulating properties of tenascin-C involve similar biochemical events in the cortical cytoskeleton. In addition to these generic properties, TN-ADC is less active in adhesion modulation than TN-190. The coordinated expression of different tenascin-C transcripts during development may, therefore, provide appropriate microenvironments for regulated changes in cell shape, adhesion, and movement.

Thrombospondin-1

Thrombospondin-1 is a glycoprotein that is released from platelet alpha-granules in response to thrombin stimulation and that is also a transient component of extracellular matrix in developing and repairing tissues. It is a 420 kDa homotrimer, each subunit of which consists of multiple structural domains. A variety of factors regulate thrombospondin-1 expression and the protein is degraded by both extracellular and intracellular routes. Thrombospondin-1 functions as a cell adhesion molecule and also modulates cell movement, cell proliferation, neurite outgrowth and angiogenesis. The molecular mechanisms underlying these activities are beginning to be examined. Medical interest in thrombospondin-1 centres on its roles in haemostasis and its effects on angiogenesis.

Identification of an actin binding region and a protein kinase C phosphorylation site on human fascin

Fascin is a 55-58-kDa actin-bundling protein, the actin binding of which is regulated by phosphorylation (Yamakita, Y., Ono, S., Matsumura, F., and Yamashiro, S. (1996) J. Biol. Chem. 271, 12632-12638). To understand the mechanism of fascin-actin interactions, we dissected the actin binding region and its regulatory site by phosphorylation of human fascin. First, we found that the C-terminal half constitutes an actin binding domain. Partial digestion of human recombinant fascin with trypsin yielded the C-terminal fragment with molecular masses of 32, 30, and 27 kDa. The 32- and 27-kDa fragments purified as a mixture formed a dimer and bound to F-actin at a saturation ratio of 1 dimer:11 actin molecules with an affinity of 1.4 x 10(6) M-1. Second, we identified the phosphorylation site of fascin as Ser-39 by sequencing a tryptic phosphopeptide purified by chelating column chromatography followed by C-18 reverse phase high performance liquid chromatography. Peptide map analyses revealed that the purified peptide represented the major phosphorylation site of in vivo as well as in vitro phosphorylated fascin. The mutation replacing Ser-39 with Ala eliminated the phosphorylation-dependent regulation of actin binding of fascin, indicating that phosphorylation at this site regulates the actin binding ability of fascin.

Cyclic GMP-dependent protein kinase is required for thrombospondin and tenascin mediated focal adhesion disassembly

Focal adhesions are specialized regions of cell membranes that are foci for the transmission of signals between the outside and the inside of the cell. Intracellular signaling events are important in the organization and stability of these structures. In previous work, we showed that the counter-adhesive extracellular matrix proteins, thrombospondin, tenascin, and SPARC, induce the disassembly of focal adhesion plaques and we identified the active regions of these proteins. In order to determine the mechanisms whereby the anti-adhesive matrix proteins modulate cytoskeletal organization and focal adhesion integrity, we examined the role of protein kinases in mediating the loss of focal adhesions by these proteins. Data from these studies show that cGMP-dependent protein kinase is necessary to mediate focal adhesion disassembly triggered by either thrombospondin or tenascin, but not by SPARC. In experiments using various protein kinase inhibitors, we observed that selective inhibitors of cyclic GMP-dependent protein kinase, KT5823 and Rp-8-Br-cGMPS, blocked the effects of both the active sequence of thrombospondin 1 (hep I) and the alternatively-spliced segment (TNfnA-D) of tenascin-C on focal adhesion disassembly. Moreover, early passage rat aortic smooth muscle cells which have high levels of cGMP-dependent protein kinase were sensitive to hep I treatment, in contrast to passaged cGMP-dependent protein kinase deficient cells which were refractory to hep I or TNfnA-D treatment, but were sensitive to SPARC. Transfection of passaged smooth muscle cells with the catalytic domain of PKG I alpha restored responsiveness to hep I and TNfnA-D. While these studies show that cGMP-dependent protein kinase activity is necessary for thrombospondin and tenascin-mediated focal adhesion disassembly, kinase activity alone is not sufficient to induce disassembly as transfection of the catalytic domain of the kinase in the absence of additional stimuli does not result in loss of focal adhesions.