Cell surface distribution of fibronectin and vitronectin receptors depends on substrate composition and extracellular matrix accumulation

Osteoblastic Cell Behavior and Gene Expression Related to Bone Metabolism on Different Titanium Surfaces

The surface topography of titanium dental implants has a great influence on osseointegration. In this work, we try to determine the osteoblastic behavior and gene expression of cells with different titanium surfaces and relate them to the physicochemical properties of the surface. For this purpose, we have used commercial titanium discs of grade 3: as-received corresponds to machined titanium without any surface treatment (MA), chemically acid etched (AE), treated via sand blasting with Al₂O₃ particles (SB) and a sand-blasting treatment with acid etching (SB+AE). The surfaces have been observed using scanning electron microscopy (SEM) and the roughness, wettability and surface energy with dispersive and polar components have been characterized. Osteoblastic cultures were performed with SaOS-2 osteoblastic cells determining cell viability as well as alkaline phosphatase levels for 3 and 21 days, and osteoblastic gene expression was determined. The roughness values of the MA discs was 0.02 μm, which increases to 0.3 μm with acid attack and becomes the maximum for the sand-blasted samples, reaching values of 1.2 μm for SB and SB+AE. The hydrophilic behavior of the MA and AE samples with contact angles of 63° and 65° is superior to that of the rougher samples, being 75° for SB and 82° for SB+AE. In all cases, they show good hydrophilicity. GB and GB+AE surfaces present a higher polar component in the surface energy values, 11.96 and 13.18 mJ/m², respectively, than AE and MA, 6.64 and 9.79 mJ/m², respectively. The osteoblastic cell viability values at three days do not show statistically significant differences between the four surfaces. However, the viability of the SB and SB+AE surfaces at 21 days is much higher than that of the AE and MA samples. From the alkaline phosphatase studies, higher values were observed for those treated with sand blasting with and without acid etching compared to the other two surfaces, indicating a greater activity in osteoblastic differentiation. In all cases except in the Osterix (Ostx) -osteoblast-specific transcription factor-a decrease in gene expression is observed in relation to the MA samples (control). The most important increase was observed for the SB+AE condition. A decrease in the gene expression of Osteoprotegerine (OPG), Runt-related transcription factor 2 (Runx2), Receptor Activator of NF-κB Ligand (RANKL) and Alkaline Phosphatase (Alp) genes was observed in the AE surface.

Extracellular vesicle integrins act as a nexus for platelet adhesion in cerebral microvessels

Circulating extracellular vesicles (EVs) regulate signaling pathways via receptor-ligand interactions and content delivery, after attachment or internalization by endothelial cells. However, they originate from diverse cell populations and are heterogeneous in composition. To determine the effects of specific surface molecules, the use of synthetic EV mimetics permits the study of specific EV receptor-ligand interactions. Here, we used endogenous EVs derived from the circulation of rats, as well as ligand-decorated synthetic microparticles (MPs) to examine the role of integrin αvβ3 in platelet adhesion under flow in structurally intact cerebral arteries. At an intraluminal pressure of 50 mmHg and flow rate of 10 µl/min, platelets were delivered to the artery lumen and imaged with whole-field fluorescent microscopy. Under basal conditions very few platelets bound to the endothelium. However, adhesion events were markedly increased following the introduction of arginine-glycine-aspartate (RGD)-labelled synthetic MPs or endogenously-derived EVs from experimental stroke animals carrying excess RGD proteins, including vitronectin, CD40-ligand and thrombospondin-1. These data, which were generated in a dynamic and physiologically relevant system, demonstrate the importance of vesicle-carried RGD ligands in platelet adherence to the cerebrovascular endothelium and highlight the ability of synthetic EVs to isolate and identify key components of the molecular handshake between EVs and their targets.

Effects of Methacrylate-Based Thermoresponsive Polymer Brush Composition on Fibroblast Adhesion and Morphology

Thermoresponsive polymers are being used increasingly in cell culture applications due to their temperature dependent surface properties. Poly(MEO2MA-co-OEGMA) (PMO) brushes offer tunable physical properties via variation in the copolymer ratio, but the effects of composition on cell-substrate interactions is unclear. To this end, a series of PMO brushes (0-8% OEGMA) was fabricated and L-929 fibroblast adhesion and morphology was quantified in the presence of serum (FBS) or after functionalization via the adsorption of fibronectin (FN) and vitronectin (VN). Quantification of the adsorption of model proteins, bovine serum albumin and FN, revealed that the extent of adsorption was correlated to the amount MEO2MA content, which represents the more hydrophobic component in PMO brushes. Cells exhibited delayed attachment and spreading on all PMO substrates in the presence of FBS. After 24 h, cell attachment was comparable; however, increased spreading was correlated with increased MEO2MA content. Adsorption of FN significantly increased initial cell attachment to all PMO surfaces after 2 h. This was not observed with VN; however, both FN and VN increased cell spreading/decreased cell circularity for all PMO substrates relative to FBS. Pure MEO2MA brushes with FN exhibited increased cell spreading/decreased cell circularity relative to other PMO substrates after 2 h, and elicited the highest cell density after 24 h. These results demonstrate that increased MEO2MA content in PMO substrates facilitates cell attachment and spreading, which can be further enhanced by adsorbing FN in the absence of other proteins.

Opposing effects of collagen I and vitronectin on fibronectin fibril structure and function

Extracellular matrix fibronectin fibrils serve as passive structural supports for the organization of cells into tissues, yet can also actively stimulate a variety of cell and tissue functions, including cell proliferation. Factors that control and coordinate the functional activities of fibronectin fibrils are not known. Here, we compared effects of cell adhesion to vitronectin versus type I collagen on the assembly of and response to, extracellular matrix fibronectin fibrils. The amount of insoluble fibronectin matrix fibrils assembled by fibronectin-null mouse embryonic fibroblasts adherent to collagen- or vitronectin-coated substrates was not significantly different 20 h after fibronectin addition. However, the fibronectin matrix produced by vitronectin-adherent cells was ~10-fold less effective at enhancing cell proliferation than that of collagen-adherent cells. Increasing insoluble fibronectin levels with the fibronectin fragment, anastellin did not increase cell proliferation. Rather, native fibronectin fibrils polymerized by collagen- and vitronectin-adherent cells exhibited conformational differences in the growth-promoting, III-1 region of fibronectin, with collagen-adherent cells producing fibronectin fibrils in a more extended conformation. Fibronectin matrix assembly on either substrate was mediated by α5β1 integrins. However, on vitronectin-adherent cells, α5β1 integrins functioned in a lower activation state, characterized by reduced 9EG7 binding and decreased talin association. The inhibitory effect of vitronectin on fibronectin-mediated cell proliferation was localized to the cell-binding domain, but was not a general property of αvβ3 integrin-binding substrates. These data suggest that adhesion to vitronectin allows for the uncoupling of fibronectin fibril formation from downstream signaling events by reducing α5β1 integrin activation and fibronectin fibril extension.

Combined effects of PEG hydrogel elasticity and cell-adhesive coating on fibroblast adhesion and persistent migration

The development and use of synthetic, cross-linked, macromolecular substrates with tunable elasticity has been instrumental in revealing the mechanisms by which cells sense and respond to their mechanical microenvironment. We here describe a hydrogel based on radical-free, cross-linked poly(ethylene glycol) to study the effects of both substrate elasticity and type of adhesive coating on fibroblast adhesion and migration. Hydrogel elasticity was controlled through the structure and concentration of branched precursors, which efficiently react via Michael-type addition to produce the polymer network. We found that cell spreading and focal adhesion characteristics are dependent on elasticity for all types of coatings (RGD peptide, fibronectin, vitronectin), albeit with significant differences in magnitude. Importantly, fibroblasts migrated slower but more persistently on stiffer hydrogels, with the effects being more pronounced on fibronectin-coated substrates. Therefore, our results validate the hydrogels presented in this study as suitable for future mechanosensing studies and indicate that cell adhesion, polarity, and associated migration persistence are tuned by substrate elasticity and biochemical properties.

Trop-2 promotes prostate cancer metastasis by modulating β(1) integrin functions

The molecular mechanisms underlying metastatic dissemination are still not completely understood. We have recently shown that β(1) integrin-dependent cell adhesion to fibronectin and signaling is affected by a transmembrane molecule, Trop-2, which is frequently upregulated in human carcinomas. Here, we report that Trop-2 promotes metastatic dissemination of prostate cancer cells in vivo and is abundantly expressed in metastasis from human prostate cancer. We also show here that Trop-2 promotes prostate cancer cell migration on fibronectin, a phenomenon dependent on β(1) integrins. Mechanistically, we demonstrate that Trop-2 and the α(5)β(1) integrin associate through their extracellular domains, causing relocalization of α(5)β(1) and the β(1)-associated molecule talin from focal adhesions to the leading edges. Trop-2 effect is specific as this molecule does not modulate migration on vitronectin, does not associate with the major vitronectin receptor, α(v)β(3) integrin, and does not affect localization of α(v)β(3) integrin as well as vinculin in focal adhesions. We show that Trop-2 enhances directional prostate cancer cell migration, through modulation of Rac1 GTPase activity. Finally, we show that Trop-2 induces activation of PAK4, a kinase that has been reported to mediate cancer cell migration. In conclusion, we provide the first evidence that β(1) integrin-dependent migratory and metastatic competence of prostate cancer cells is enhanced by Trop-2.

Dissecting focal adhesions in cells differentially expressing calreticulin: a microscopy study

BACKGROUND INFORMATION

Our previous studies have shown that calreticulin, a Ca2+-binding chaperone located in the endoplasmic reticulum, affects cell-substratum adhesions via the induction of vinculin and N-cadherin. Cells overexpressing calreticulin contain more vinculin than low expressers and make abundant contacts with the substratum. However, cells that express low levels of calreticulin exhibit a weak adhesive phenotype and make few, if any, focal adhesions. To date, the identity of the types of focal adhesions made by calreticulin overexpressing and low expressing cells has not been dissected.

RESULTS

The results of the present study show that calreticulin affects fibronectin matrix assembly in L fibroblast cell lines that differentially express the protein, and that these cells also differ profoundly in focal adhesion formation. Although the calreticulin overexpressing cells generate numerous interference-reflection-microscopy-dark, vinculin- and paxillin-containing classical focal contacts, as well as some fibrillar adhesions, the cells expressing low levels of calreticulin generate only a few weak focal adhesions. The fibronectin receptor was found to be clustered in calreticulin overexpressing cells, but diffusely distributed over the cell surface in low expressing cells. Plating L fibroblasts on fibronectin-coated substrata induced extensive spreading in all cell lines tested. However, although calreticulin overexpressing cells were induced to form classical vinculin-rich focal contacts, the low calreticulin expressing cells overcame their weak adhesive phenotype by induction of many tensin-rich fibrillar adhesions, thus compensating for the low level of vinculin in these cells.

CONCLUSIONS

We propose that calreticulin affects fibronectin production and, thereby, assembly, and it indirectly influences the formation and/or stability of focal contacts and fibrillar adhesions, both of which are instrumental in matrix assembly and remodelling.

Reflectivity and topography of cells grown on glass-coverslips measured with phase-shifted laser feedback interference microscopy

In spite of the advantages associated with the molecular specificity of fluorescence imaging, there is still a significant need to augment these approaches with label-free imaging. Therefore, we have implemented a form of interference microscopy based upon phase-shifted, laser-feedback interferometry and developed an algorithm that can be used to separate the contribution of the elastically scattered light by sub-cellular structures from the reflection at the coverslip-buffer interface. The method offers an opportunity to probe protein aggregation, index of refraction variations and structure. We measure the topography and reflection from calibration spheres and from stress fibers and adhesions in both fixed and motile cells. Unlike the data acquired with reflection interference contrast microscopy, where the reflection from adhesions can appear dark, our approach demonstrates that these regions have high reflectivity. The data acquired from fixed and live cells show the presence of a dense actin layer located ≈ 100 nm above the coverslip interface. Finally, the measured dynamics of filopodia and the lamella in a live cell supports retrograde flow as the dominate mechanism responsible for filopodia retraction.

Modulation of embryonic mesenchymal progenitor cell differentiation via control over pure mechanical modulus in electrospun nanofibers

As the potential range of stem cell applications in tissue engineering continues to grow, the appropriate scaffolding choice is necessary to create tightly defined artificial microenvironments for each target organ. These microenvironments determine stem cell fate via control over differentiation. In this study we examined the specific effects of scaffold stiffness on embryonic mesenchymal progenitor cell behavior. Mechanically distinct scaffolds having identical microstructures and surface chemistries were produced utilizing core-shell electrospinning. The modulus of core-shell poly(ether sulfone)-poly(ε-caprolactone) (PES-PCL) fibers (30.6 MPa) was more than four times that of pure PCL (7.1 MPa). The results for chondrogenic and osteogenic differentiation of progenitor cells on each scaffold indicate that the lower modulus PCL fibers provided more appropriate microenvironments for chondrogenesis, evident by a marked up-regulation of chondrocytic Sox9, collagen type 2, and aggrecan gene expression and chondrocyte-specific extracellular matrix glycosaminoglycan production. In contrast, the stiffer core-shell PES-PCL fibers supported enhanced osteogenesis by promoting osteogenic Runx2, alkaline phosphatase, and osteocalcin gene expression, as well as alkaline phosphatase activity. The findings demonstrate that the microstructural stiffness/modules of a scaffold and the pliability of individual fibers may play a critical role in controlling stem cell differentiation. Regulation of cytoskeletal organization may occur via a "dynamic scaffold" leading to the subsequent intracellular signaling events that control differentiation-specific gene expression.

Mechanical forces facilitate actin polymerization at focal adhesions in a zyxin-dependent manner

We examined the effects of mechanical forces on actin polymerization at focal adhesions (FAs). Actin polymerization at FAs was assessed by introducing fluorescence-labeled actin molecules into permeabilized fibroblasts cultured on fibronectin. When cell contractility was inhibited by the myosin-II inhibitor blebbistatin, actin polymerization at FAs was diminished, whereas α5β1 integrin remained accumulated at FAs. This suggests that actin polymerization at FAs depends on mechanical forces. To examine the action of mechanical forces more directly, the blebbistatin-treated cells were subjected to a sustained uniaxial stretch, which induced actin polymerization at FAs. These results demonstrate the novel role of mechanical forces in inducing actin polymerization at FAs. To reveal the molecular mechanism underlying the force-induced actin polymerization at FAs, we examined the distribution of zyxin, a postulated actin-regulatory protein. Actin-polymerizing activity was strong at zyxin-rich FAs. Accumulation of zyxin at FAs was diminished by blebbistatin, whereas uniaxial stretching of the cells induced zyxin accumulation. Displacing endogenous zyxin from FAs by expressing the FA-targeting region of zyxin decreased the force-induced actin polymerization at FAs. These results suggest that zyxin is involved in mechanical-force-dependent facilitation of actin polymerization at FAs.

Serumfreie Kultivierung von Osteoprogenitorzellen und Osteoblasten zur Testung von Biomaterialien

The aim of the study is to describe a model for testing biocompatibility of implant materials. Usually cells do not bind the biomaterial surface itself via integrins but adsorbed proteins of blood or interstitial fluids. To eliminate the influence of serum proteins on cell adhesion to the test materials we cultivated osteoprogenitor cells and osteoblasts with a serum replacement or with fetal calf serum, but seeded them likewise without serum or serum replacement on cell culture polystyrene, sandblasted titanium and titanium coated with the peptide c(RGDfK) or hydroxyapatite (Bonemaster) and determined cell adhesion. In addition, the surfaces were preincubated with the serum proteins albumin, fetuin, fibronectin and vitronectin to examine specifically their influence on cell adhesion. Clearly cell adhesion depended on cell culture conditions and state of differentiation, especially with prominent differences in adhesion to c(RGDfK). Precoating with serum proteins demonstrated that besides fibronectin and vitronectin fetuin can function as an adhesion protein, whereas albumin demonstrated an antiadhesive effect. Depending on the material they affected cell adhesion differently. Although osteoprogenitor cells and osteoblasts could bind to tissue culture polystyrene, titanium and especially hydroxyapatite without mediation of proteins, it has to be taken into consideration that cell spreading and proliferation of cells on a scaffold are more important than adhesion alone and may not be ensured in the absence of adhesion proteins.

Multivalent Effects of RGD Peptides Obtained by Nanoparticle Display

The binding of RGD peptides to integrins offers an excellent system to study the multivalent mediated changes in affinity that arise when peptides, displayed on the surface of a nanoparticle carrier, bind to integrins displayed on the cell membrane. The IC50 of an RGD nanoparticle for endothelial adhesion was 1.0 nM nanoparticle or 20 nM peptide (20 peptide/nanoparticle) and was associated with strong multivalent effects, defined as a multivalent enhancement factor (MVE) of 38 (MVE=IC50 (peptide)/IC50 (peptide when displayed by nanoparticle)). The attachment of RGD peptides to nanoparticles resulted in an extension of the peptide blood half-life from 13 to 180 min. Based on the multivalent enhancement of affinity and extension of blood half-life, multivalent RGD nanoparticle-sized materials should be potent inhibitors of the alpha(V)beta(3) function on endothelial cells in vivo.

Lateral spacing of integrin ligands influences cell spreading and focal adhesion assembly

Cell-extracellular matrix (cell-ECM) interactions mediated by integrin receptors are essential for providing positional and environmental information necessary for many cell functions, such as proliferation, differentiation and survival. In vitro studies on cell adhesion to randomly adsorbed molecules on substrates have been limited to sub-micrometer patches, thus preventing the detailed study of structural arrangement of integrins and their ligands. In this article, we illustrate the role of the distance between integrin ligands, namely the RGD (arginine-glycine-aspartate) sequence present in ECM proteins, in the control of cell adhesion. By using substrates, which carry cyclic RGD peptides arranged in highly defined nanopatterns, we investigated the dynamics of cell spreading and the molecular composition of adhesion sites in relation to a fixed spacing between the peptides on the surface. Our novel approach for in vitro studies on cell adhesion indicates that not only the composition, but also the spatial organization of the extracellular environment is important in regulating cell-ECM interactions.

Gravity spun polycaprolactone fibres for soft tissue engineering: Interaction with fibroblasts and myoblasts in cell culture

Poly(epsilon-caprolactone) (PCL) fibres were produced by wet spinning from solutions in acetone under low shear (gravity flow) conditions. As-spun PCL fibres exhibited a mean strength and stiffness of 7.9 MPa and 0.1 GPa, respectively and a rough, porous surface morphology. Cold drawing to an extension of 500% resulted in increases in fibre strength (43 MPa) and stiffness (0.3 GPa) and development of an oriented, fibrillar surface texture. The proliferation rate of Swiss 3T3 mouse fibroblasts and C2C12 mouse myoblasts on as-spun, 500% cold-drawn and gelatin-modified PCL fibres was determined in cell culture to provide a basic measure of the biocompatibility of the fibres. Proliferation of both cell types was consistently higher on gelatin-coated fibres relative to as-spun fibres at time points below 7 days. Fibroblast growth rates on cold-drawn PCL fibres exceeded those on as-spun fibres but myoblast proliferation was similar on both substrates. After 1 day in culture, both cell types had spread and coalesced on the fibres to form a cell layer, which conformed closely to the underlying topography. The high fibre compliance combined with a potential for modifying the fibre surface chemistry with cell adhesion molecules and the surface architecture by cold drawing to enhance proliferation of fibroblasts and myoblasts, recommends further investigation of gravity-spun PCL fibres for 3-D scaffold production in soft tissue engineering.

Normal cutaneous wound healing: clinical correlation with cellular and molecular events

BACKGROUND

Cutaneous wound healing is a normal physiologic function, observed and described for centuries by those afflicted with wounds and by those caring for them. Recently, tremendous progress has been made in discovering the cellular and molecular mechanisms responsible for wound healing. Counseling patients appropriately and planning future therapeutic interventions in delayed or abnormal wound healing may be improved by a thorough understanding of the relationship between clinical, cellular, and subcellular events occurring during the normal healing process.

MATERIALS AND METHODS

A review of the wound healing literature from the past several decades, with a focus on the past 5 to 10 years in particular, along with illustrative case examples from our clinical practice over the past decade.

RESULTS

Traditional clinical stages of wounding healing are still relevant, but more overlap between stages is likely a more accurate depiction of events. The role of cells such as platelets, macrophages, leukocytes, fibroblasts, endothelial cells, and keratinocytes is much better known, particularly during the inflammatory and proliferation stages of healing. Molecules such as interferon, integrins, proteoglycans and glycosaminoglycans, matrix metalloproteinases, and other regulatory cytokines play a critical role in the regulation of healing mechanisms.

CONCLUSION

Cutaneous wound healing in normal hosts follows an orderly clinical process. The scientific underpinnings for healing are better understood than ever, although much remains to be discovered. Eventually, such improved understanding of cellular and subcellular physiology may lead to new or better forms of therapy for patients with acute, chronic, and surgical skin wounds.

Normal cutaneous wound healing: clinical correlation with cellular and molecular events

BACKGROUND

Cutaneous wound healing is a normal physiologic function, observed and described for centuries by those afflicted with wounds and by those caring for them. Recently, tremendous progress has been made in discovering the cellular and molecular mechanisms responsible for wound healing. Counseling patients appropriately and planning future therapeutic interventions in delayed or abnormal wound healing may be improved by a thorough understanding of the relationship between clinical, cellular, and subcellular events occurring during the normal healing process.

MATERIALS AND METHODS

A review of the wound healing literature from the past several decades, with a focus on the past 5 to 10 years in particular, along with illustrative case examples from our clinical practice over the past decade.

RESULTS

Traditional clinical stages of wounding healing are still relevant, but more overlap between stages is likely a more accurate depiction of events. The role of cells such as platelets, macrophages, leukocytes, fibroblasts, endothelial cells, and keratinocytes is much better known, particularly during the inflammatory and proliferation stages of healing. Molecules such as interferon, integrins, proteoglycans and glycosaminoglycans, matrix metalloproteinases, and other regulatory cytokines play a critical role in the regulation of healing mechanisms.

CONCLUSION

Cutaneous wound healing in normal hosts follows an orderly clinical process. The scientific underpinnings for healing are better understood than ever, although much remains to be discovered. Eventually, such improved understanding of cellular and subcellular physiology may lead to new or better forms of therapy for patients with acute, chronic, and surgical skin wounds.

Mediation of biomaterial-cell interactions by adsorbed proteins: a review

An appropriate cellular response to implanted surfaces is essential for tissue regeneration and integration. It is well described that implanted materials are immediately coated with proteins from blood and interstitial fluids, and it is through this adsorbed layer that cells sense foreign surfaces. Hence, it is the adsorbed proteins, rather than the surface itself, to which cells initially respond. Diverse studies using a range of materials have demonstrated the pivotal role of extracellular adhesion proteins--fibronectin and vitronectin in particular--in cell adhesion, morphology, and migration. These events underlie the subsequent responses required for tissue repair, with the nature of cell surface interactions contributing to survival, growth, and differentiation. The pattern in which adhesion proteins and other bioactive molecules adsorb thus elicits cellular reactions specific to the underlying physicochemical properties of the material. Accordingly, in vitro studies generally demonstrate favorable cell responses to charged, hydrophilic surfaces, corresponding to superior adsorption and bioactivity of adhesion proteins. This review illustrates the mediation of cell responses to biomaterials by adsorbed proteins, in the context of osteoblasts and selected materials used in orthopedic implants and bone tissue engineering. It is recognized, however, that the periimplant environment in vivo will differ substantially from the cell-biomaterial interface in vitro. Hence, one of the key issues yet to be resolved is that of the interface composition actually encountered by osteoblasts within the sequence of inflammation and bone regeneration.

Integrin Expression and Osteopontin Regulation in Human Fetal Osteoblastic Cells Mediated by Substratum Surface Characteristics

Integrin-mediated adhesion of anchorage-dependent cells to scaffolds is a critical component of tissue engineering. We investigated integrin expression by the human fetal osteoblastic cell line, hFOB 1.19 (hFOB), as a function of substratum surface wettability. The influence of surface wettability on bone cell phenotype was also examined. Plasma-treated quartz (PTQ) and glass (PTG) (hydrophilic, contact angles of 0 degrees), octadecyltrichlorosilane-treated quartz (STQ) and glass (STG) (hydrophobic, contact angles above about 100 degrees), and tissue culture polystyrene were used for cell culture. hFOB cells cultured on hydrophilic substrata displayed well-developed actin stress fibers relative to cells on hydrophobic substrata. Western blot analysis revealed that hFOB cells cultured on hydrophobic substrata (STQ or STG) express lower levels of alphav and beta3 integrin subunits than do cells on hydrophilic substrata (PTQ or PTG). This effect was more pronounced in cells on STQ than on STG. These variations in integrin expression were lessened by extended culture time. Double- labeled integrin/actin immunofluorescence confirmed Western blot results, that is, cells cultured on PTQ displayed distinct, large plaques of alphav and beta3 subunits and integrin alphavbeta3, as well as their colocalization with actin stress fiber ends, whereas cells on STQ did not display integrin plaques after 24 h and displayed only minimal plaque formation after 3 days. Vinculin, a focal adhesion protein that mediates binding between the integrin and actin cytoskeleton, appeared in Western blots to mimic the variations of alphav and beta3 expression with respect to surface wettability. Interestingly, real-time RT-PCR analysis showed that hFOB cultured on hydrophobic substrata, which have downregulated alphav and beta3 integrin subunits, displayed greater steady state mRNA levels of osteopontin, an extracellular matrix (ECM) protein containing the Arg-Gly-Asp (RGD) integrin recognition sequence, than did cells cultured on hydrophilic substrata. Our results imply that substratum surface wettability regulates integrin-mediated bone cell adhesion and further influences the expression of bone cell-ECM complexes.

Growth behavior, matrix production, and gene expression of human osteoblasts in defined cylindrical titanium channels

The purpose of the current study was to investigate the effect of different diameters of cylindrical titanium channels on human osteoblasts. Titanium samples having continuous drill channels with diameters of 300, 400, 500, 600, and 1000 microm were put into osteoblast cell cultures that were isolated from 12 adult human trauma patients. Cell migration into the drill channels was investigated by transmitted-light microscopy. The DNA content in the drill channels was measured photometrically, collagen type I production was analyzed by enzyme-linked immunosorbent assay (ELISA) and osteocalcin gene expression by reverse transcriptase-polymerase chain reaction (RT-PCR). Formation of mineralized tissue was assessed by microradiographs of histological sections. Within 20 days, cells grew an average of 838 microm (+/-128 microm) into the drill channels with a diameter of 600 microm and were significantly faster (p < 0.05) than in all other channels. Cells produced significantly more osteocalcin messenger RNA (mRNA) in 600-microm channels (p < 0.05) than they did in 1000-microm channels and demonstrated the highest osteogenic differentiation. The channel diameter did not influence collagen type I production. The highest cell density was found in 300-microm channels (p < 0.05). The DNA content of the channels linearly decreased with increasing channel diameters. After 40 days of culture, the proportion of mineralized tissue at the mouth section amounted to 6% in 300-microm channels and to 9-11% in 400-600-microm channels. In 1000-microm channels, only traces of mineralization were detected. Our data suggest that the diameter of cylindrical titanium channels has a significant effect on migration, gene expression, and mineralization of human osteoblasts.

RGD modified polymers: biomaterials for stimulated cell adhesion and beyond

Since RGD peptides (R: arginine; G: glycine; D: aspartic acid) have been found to promote cell adhesion in 1984 (Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule, Nature 309 (1984) 30), numerous materials have been RGD functionalized for academic studies or medical applications. This review gives an overview of RGD modified polymers, that have been used for cell adhesion, and provides information about technical aspects of RGD immobilization on polymers. The impacts of RGD peptide surface density, spatial arrangement as well as integrin affinity and selectivity on cell responses like adhesion and migration are discussed.

A novel mode for integrin-mediated signaling: tethering is required for phosphorylation of FAK Y397

The common model for integrin mediated signaling is based on integrin clustering and the potential for that clustering to recruit signaling molecules including FAK and src. The clustering model for transmembrane signaling originated with the analysis of the EGF receptor signaling and remains the predominant model. The roles for substrate-bound ligand and ligand occupancy in integrin-mediated signaling are less clear. A kinetic model was established using HT1080 cells in which there was a linear relationship between the strength of adhesion, the proportion of alpha5beta1 integrin that could be chemically cross-linked, and the number of receptor-ligand bonds. This graded signal produced a similarly graded response measured by the level of specific phosphorylation of FAK Y397. FAK Y397 phosphorylation could also be induced by antibody bound to the substrate. In contrast, clustering of alpha5beta1 on suspended cells with either antibody to beta1 or by clustering of soluble ligand bound to alpha5beta1 induced the phosphorylation of FAK Y861 but not Y397. There were no differences in signaling when activating antibodies were compared with blocking antibodies, presence or absence of ligand. Only tethering of alpha5beta1 to the substrate was required for induction of FAK Y397 phosphorylation.

β1integrins are distributed in adhesion structures with fibronectin and caveolin and in coated pits

Integrins are found in adhesion structures, which link the extracelullar matrix to cytoskeletal proteins. Here, we attempt to further define the distribution of β1integrins in the context of their association with matrix proteins and other cell surface molecules relevant to the endocytic process. We find that β1integrins colocalize with fibronectin in fibrillar adhesion structures. A fraction of caveolin is also organized along these adhesion structures. The extracellular matrix protein laminin is not concentrated in these structures. The α4β1integrin exhibits a distinct distribution from other β1integrins after cells have adhered for 1 h to extracellular matrix proteins but is localized in adhesion structures after 24 h of adhesion. There are differences between the fibronectin receptors: α5β1integrins colocalize with adaptor protein-2 in coated pits, while α4β1integrins do not. This parallels our earlier observation that of the two laminin receptors, α1β1and α6β1, only αaβ1integrins colocalize with adaptor protein-2 in coated pits. Calcium chelation or inhibition of mitogen-activated protein kinase kinase, protein kinase C, or src did not affect localization of α1β1and α5β1integrins in coated pits. Likewise, the integrity of coated-pit structures or adhesion structures is not required for integrin and adaptor protein-2 colocalization. This suggests a robust and possibly constitutive interaction between these integrins and coated pits.Key words: adhesion, endocytosis, extracellular matrix, microscopy, confocal, signalling.

Acute wound healing an overview

The ability to heal an injury is a biologic necessity for all organisms, with mammals lagging in proficiency when compared with lower life forms that have the ability to regenerate differentiated structures. Technology and increased scientific knowledge have established a coordinated interplay that has improved the ability to manage wounds in a logical manner, and, on occasion, to accelerate the healing process. Insight into the complex chain of events leading to the formation of scar is a necessity for every individual who attempts wound management.

Fibronectin polymerization regulates the composition and stability of extracellular matrix fibrils and cell-matrix adhesions

Remodeling of extracellular matrices occurs during development, wound healing, and in a variety of pathological processes including atherosclerosis, ischemic injury, and angiogenesis. Thus, identifying factors that control the balance between matrix deposition and degradation during tissue remodeling is essential for understanding mechanisms that regulate a variety of normal and pathological processes. Using fibronectin-null cells, we found that fibronectin polymerization into the extracellular matrix is required for the deposition of collagen-I and thrombospondin-1 and that the maintenance of extracellular matrix fibronectin fibrils requires the continual polymerization of a fibronectin matrix. Further, integrin ligation alone is not sufficient to maintain extracellular matrix fibronectin in the absence of fibronectin deposition. Our data also demonstrate that the retention of thrombospondin-1 and collagen I into fibrillar structures within the extracellular matrix depends on an intact fibronectin matrix. An intact fibronectin matrix is also critical for maintaining the composition of cell-matrix adhesion sites; in the absence of fibronectin and fibronectin polymerization, neither alpha5beta1 integrin nor tensin localize to fibrillar cell-matrix adhesion sites. These data indicate that fibronectin polymerization is a critical regulator of extracellular matrix organization and stability. The ability of fibronectin polymerization to act as a switch that controls the organization and composition of the extracellular matrix and cell-matrix adhesion sites provides cells with a means of precisely controlling cell-extracellular matrix signaling events that regulate many aspects of cell behavior including cell proliferation, migration, and differentiation.

Integrin-dependent interaction of human vascular endothelial cells on biomimetic peptide surfactant polymers

Biomimetic surfactant polymers designed by molecular grafting of pendant RGD peptides (Pep) and dextran oligosaccharides (Dex) in different ratios onto the backbone of poly(vinyl amine) (PVAm) were examined for their ability to promote endothelial cell (EC) growth. Adhesion, formation of focal contacts, and expression of integrin receptors were examined in EC seeded onto a series of novel surfactants containing 100% dextran (PVAm[Pep (0%)]) to 100% peptide (PVAm[Pep (100%)]) compared to fibronectin control. Interaction of EC on polymer was specific, as soluble GRGDSP, but not GRGESP, was able to inhibit both adhesion and spreading of EC. At three hours, EC attachment and spreading were rapid and comparable on fibronectin and PVAm[Pep (100%)], rounded on PVAm[Pep (0%)], and intermediate on PVAm[Pep (25%)], (PVAm[Pep (50%)], and PVAm[Pep (75%)], with increasing peptide ratio favoring more spreading, although all the substrates had similar hydrophilicity. Cells that spread well on fibronectin and PVAm[Pep (100%)] had sharp spikes of vinculin localized at the termination point of actin stress fibers. Formation of stress fibers and focal adhesions on other substrates were correlated with spreading pattern of EC and the peptide content. EC seeded on fibronectin expressed alpha5beta1 integrins all along the stress fibers and throughout the entire cytoskeleton, but this distribution pattern was less prominent on PVAm[Pep (100%)]. However, expression and distribution of vitronectin receptors (alpha(v)beta3) were similar on both fibronectin and PVAm[Pep (100%)], suggesting a strong cell adhesion on PVAm[Pep (100%)]. Viability of EC was also comparable on both fibronectin and PVAm[Pep (100%)] at 24 h. Substrates with high proportion of dextran limited cell adhesion, probably by decreasing protein adsorption. These results suggest that it may be possible to engineer substrates that promote cell adhesion in a receptor-dependent manner while blocking nonspecific protein adsorption, which may have potential as interface materials for prostheses used in cardiovascular system.

The distribution and regulation of integrin-linked kinase in normal and diabetic kidneys

Alteration in cell adhesion and extracellular matrix deposition is a hallmark of diabetic glomerulosclerosis. Integrin-linked kinase (ILK) is a recently identified integrin cytoplasmic-binding protein that has been implicated in the regulation of cell adhesion and extracellular matrix deposition. To begin to investigate whether ILK is involved in the pathogenesis of diabetic glomerulosclerosis, we have analyzed the distribution and regulation of ILK in normal and diabetic kidneys as well as in isolated mesangial cells. We have found that ILK is normally expressed at high concentration in visceral epithelial cells. In diabetic glomeruli, ILK expression in the mesangium is dramatically increased. The increase in ILK level is associated with diffuse mesangial expansion. In glomeruli where advanced nodular sclerosis and global sclerosis were dominant, ILK level was reduced, suggesting that the increase in ILK expression likely associates with relatively early glomerulosclerosis. Additionally, we have found that exposure of mesangial cells to high concentrations of glucose significantly increased the ILK level. Finally, we show that ILK localizes to regions of cell membranes that are in close contact with mesangial fibronectin matrix. These results suggest that ILK is likely involved in mesangial matrix expansion in response to hyperglycemia in the pathogenesis of diabetic glomerulosclerosis.

Analysis of integrin expression in U2OS cells cultured on various calcium phosphate ceramic substrates

Earlier we observed that calcium phosphate (Ca-P)-coated implant substrates stimulated the differentiation of osteoblast-like cells compared to uncoated substrates. This suggests that this difference in osteogenic induction is due to the chemical composition of the substratum. We hypothesized that Ca-P coatings modulate integrin expression patterns, because those receptors are the sensors of the cell. Therefore, in the present study we quantitatively analyzed integrin expression of osteosarcoma cells and their proliferation behavior on various well-defined Ca-P substrates. For this study we used the osteosarcoma cell line U2OS. Five groups of substrates were used: thermanox (Th), uncoated titanium (Ti), dense sintered hydroxyapatite (HA), and two Ca-P-coated titanium discs (TiHA-O% and TiHA-5%). At day 5, cell numbers were significantly lower (p < 0.05) for both types of Ca-P-coated titanium substrates compared to the other substrates. There were no significant differences between HA and uncoated titanium. From day 5 to 8, accumulated cell number was ranking highest to lowest HA > Th = Ti > TiHA-0% > TiHA-5%. Integrin expression at day 5 and day 8 of incubation was analyzed by flow cytometry for integrin subunits beta 1, alpha 3, alpha 4, alpha 5, alpha 6, and alpha v. Fluorescence-activated cell sorting (FACS) analysis showed that the cells express high levels of beta 1, low levels of alpha 4, alpha 5, and alpha 6, and moderate levels of alpha 3 and alpha v integrin subunits on the various biomaterial substrates. Minor differences in integrin expression between the various substrates were seen. Therefore, the observed differences in proliferation between the coatings may reside in modulating the functional properties of integrins.

Disruption of transforming growth factor beta-regulated laminin receptor function by expression of antisense laminin, a chain RNA in human colon cancer cells

Transforming growth factor beta1 (TGFbeta) simultaneously induces the expression of fibronectin, fibronectin receptor, laminin, and laminin receptor (alpha6beta1 integrin) in the human colon cancer cell line Moser (Int J Cancer, 57:742, 1994). Induction of fibronectin and induction of fibronectin receptor by TGFB are tightly coupled, and disrupting fibronectin induction disrupts the induction of fibronectin receptor and cellular adhesion to fibronectin (J Cellular Physiol, 170:138, 1997). We recently demonstrated the efficacy of using antisense chain-specific laminin RNA expression vectors to disrupt the induction by TGFP of the multichain laminin molecule (J Cellular Physiol, 178:296, 1999). We now show in this report that Moser cells used alpha6 and beta1 integrins to adhere to laminin, and, as is the fibronectin and fibronectin receptor system, disrupting the induction by TGFbeta of the ligand laminin by the expression of antisense laminin A chain RNA disrupted the induction of 125I-laminin binding and cellular adhesion to laminin. Disrupting laminin induction also blocked the induction of alpha6 and beta1 integrin laminin receptor by TGFbeta. We conclude that disrupting the induction of the ligand laminin by TGFbeta disrupts TGFbeta-regulated laminin receptor function by suppressing the induction of alpha6 and beta1 integrins. Therefore, targeted disruption of the ligand laminin may be an effective means in disrupting the function of both the ligand and its receptor in cells that utilize the laminin and laminin receptor system in malignant cell behavior.

Clathrin-mediated endocytosis and recycling of autocrine motility factor receptor to fibronectin fibrils is a limiting factor for NIH-3T3 cell motility

Autocrine motility factor receptor (AMF-R) is internalized via a clathrin-independent pathway to smooth endoplasmic reticulum tubules. This endocytic pathway is shown here to be inhibited by methyl-(beta)-cyclodextrin (m(beta)CD) implicating caveolae or caveolae-like structures in AMF internalization to smooth ER. AMF-R is also internalized via a clathrin-dependent pathway to a transferrin receptor-negative, LAMP-1/lgpA-negative endocytic compartment identified by electron microscopy as a multivesicular body (MVB). Endocytosed AMF recycles to cell surface fibrillar structures which colocalize with fibronectin; AMF-R recycling is inhibited at 20 degrees C, which blocks endocytosis past the early endosome, but not by m(beta)CD demonstrating that AMF-R recycling to fibronectin fibrils is mediated by clathrin-dependent endocytosis to MVBs. Microtubule disruption with nocodazole did not affect delivery of bAMF to cell surface fibrils indicating that recycling bAMF traverses the MVB but not a later endocytic compartment. Plating NIH-3T3 cells on an AMF coated substrate did not specifically affect cell adhesion but prevented bAMF delivery to cell surface fibronectin fibrils and reduced cell motility. AMF-R internalization and recycling via the clathrin-mediated pathway are therefore rate-limiting for cell motility. This recycling pathway to the site of deposition of fibronectin may be implicated in the de novo formation of cellular attachments or the remodeling of the extracellular matrix during cell movement.

Dynamics and segregation of cell-matrix adhesions in cultured fibroblasts

Here we use time-lapse microscopy to analyse cell-matrix adhesions in cells expressing one of two different cytoskeletal proteins, paxillin or tensin, tagged with green fluorescent protein (GFP). Use of GFP-paxillin to analyse focal contacts and GFP-tensin to study fibrillar adhesions reveals that both types of major adhesion are highly dynamic. Small focal contacts often translocate, by extending centripetally and contracting peripherally, at a mean rate of 19 micrometers per hour. Fibrillar adhesions arise from the medial ends of stationary focal contacts, contain alpha5beta1 integrin and tensin but not other focal-contact components, and associate with fibronectin fibrils. Fibrillar adhesions translocate centripetally at a mean rate of 18 micrometers per hour in an actomyosin-dependent manner. We propose a dynamic model for the regulation of cell-matrix adhesions and for transitions between focal contacts and fibrillar adhesions, with the ability of the matrix to deform functioning as a mechanical switch.

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.

Physical state of the extracellular matrix regulates the structure and molecular composition of cell-matrix adhesions

This study establishes that the physical state of the extracellular matrix can regulate integrin-mediated cytoskeletal assembly and tyrosine phosphorylation to generate two distinct types of cell-matrix adhesions. In primary fibroblasts, alpha(5)beta(1) integrin associates mainly with fibronectin fibrils and forms adhesions structurally distinct from focal contacts, independent of actomyosin-mediated cell contractility. These "fibrillar adhesions" are enriched in tensin, but contain low levels of the typical focal contact components paxillin, vinculin, and tyrosine-phosphorylated proteins. However, when the fibronectin is covalently linked to the substrate, alpha(5)beta(1) integrin forms highly tyrosine-phosphorylated, "classical" focal contacts containing high levels of paxillin and vinculin. These experiments indicate that the physical state of the matrix, not just its molecular composition, is a critical factor in defining cytoskeletal organization and phosphorylation at adhesion sites. We propose that molecular organization of adhesion sites is controlled by at least two mechanisms: 1) specific integrins associate with their ligands in transmembrane complexes with appropriate cytoplasmic anchor proteins (e.g., fibronectin-alpha(5)beta(1) integrin-tensin complexes), and 2) physical properties (e.g., rigidity) of the extracellular matrix regulate local tension at adhesion sites and activate local tyrosine phosphorylation, recruiting a variety of plaque molecules to these sites. These mechanisms generate structurally and functionally distinct types of matrix adhesions in fibroblasts.

Soft tissue and epithelial models

The applicability of a biomaterial for the manufacturing of oral implants is determined by its physicochemical and geometric surface properties. Research, therefore, is concerned with the cellular reactions that occur when an implant material comes into contact with body tissues. For permucosal oral implants, this involves both the reaction of bone and gingival cells. In vitro cell culturing--including the use of various analytical techniques like light microscopy, scanning and transmission electron microscopy, confocal laser scanning microscopy, and digital image analysis--is a good tool whereby investigators can obtain more insight into the relevant components of implant-tissue adhesion. In the current overview, the role of cell models in oral implant research is discussed, specifically with reference to responses of epithelial cells and fibroblasts.

Cell adhesion to textured silicone surfaces: the influence of time of adhesion and texture on focal contact and fibronectin fibril formation

Cell adhesion and spreading on biomaterials is a key issue in the study of cell-biomaterial interactions. With the development of new disciplines within biomaterials research such as tissue engineering and cellular therapy, information at molecular and structural levels is needed in order to conceive and design biomaterials that elicit specific, functional cell responses. In this study we determined the formation of focal adhesions and fibronectin fibrillar structures by human fibroblasts and human umbilical vein endothelial cells adhered to fibronectin-precoated, smooth, and textured silicones as a function of time. Textures consisted of parallel ridges and 0.5 mm deep grooves with a width of 2, 5, and 10 mm. In addition, pillar and well constructs were used. Cells assembled focal adhesions within the first 24 h of adhesion. Fibronectin production and assembly resulted in a dense fibrillar network at day 6. Initial focal adhesion density and size were dictated by the presence of the texture. Topography also influenced initial fibronectin deposition, although the differences did not result in apparent differences in fibronectin networks after 6 days of incubation. Without fibronectin preadsorption, cells did not proliferate on the silicone surfaces. Cells adhered to glass removed all the preabsorbed fibronectin, whereas on silicone they did not. The present study shows that different textures initially give rise to differences in focal contact and fibronectin fibril assembly. The effects of the small, initial in vitro differences on in vivo tissue biocompatibility remains to be studied.

Altered vitronectin receptor (alphav integrin) function in fibroblasts adhering on hydrophobic glass

Function of integrins is crucial for adhesion, movement, proliferation, and survival of cells. In a recent study we found impaired fibronectin receptor function on hydrophobic substrata (G. Altankov et al. J Biomater Sci Polym Edn 1997;8:712-740). Here, we have studied the distribution and function of the vitronectin receptor (alphav integrin) in fibroblasts adhering on hydrophilic glass and hydrophobic octadecyl glass (ODS). The morphology of fibroblasts and the organization of actin cytoskeleton were studied and found to be altered on ODS, where the cells did not spread and possessed condensed actin. Pretreatment of the surfaces with serum or pure vitronectin improved cell morphology on both substrata, resulting in the development of longitudinal actin stress fibers. It was found with biotinylated vitronectin that comparable quantities of vitronectin were adsorbed from single vitronectin solutions or serum on glass and on hydrophobic ODS. The organization of the vitronectin receptors on the ventral cell surface was investigated in permeabilized cells showing normal focal adhesions in fibroblasts plated on glass but none of these structures on ODS. The distribution of alphav integrin on the dorsal cell surface was studied on nonpermeabilized living cells after antibody tagging. While fibroblasts adhering on plain or serum-treated glass developed a linear organization of alphav integrin, cells on plain and serum-treated ODS were not able to reorganize the vitronectin receptor. Studies on signal transduction with antiphosphotyrosine antibodies revealed co-localization of alphav integrin and phosphotyrosine in focal adhesions on glass and serum-treated glass. However, signaling was almost absent on plain ODS and weak on serum-treated ODS. It was concluded that alterations in vitronectin receptor function on the ventral cell surface caused by the hydrophobic material surface inhibit signal transfer and subsequent intracellular events that are important for the organization and function of integrins.

Role of fibronectin-stimulated tumor cell migration in glioma invasion in vivo: clinical significance of fibronectin and fibronectin receptor expressed in human glioma tissues

In order to clarify the role of fibronectin in glioma invasion in vivo, we analyzed the relationship between fibronectin-stimulated cell migration and adhesion in 14 primary glioma cells and the expression of fibronectin and the fibronectin receptor in the corresponding tumor tissues. The tumors comprised nine glioblastomas (GB) and five anaplastic gliomas (AG) consisting of two astrocytomas, two oligoastrocytomas and one ependymoma. All glioma cells tested in the primary cell culture were found to migrate to fibronectin in a dose-dependent manner. The extent of cell migration to fibronectin was not significantly different for the GB and AG groups. On the other hand, cell adhesion to fibronectin in the AG was much stronger than that in the GB group. Immunohistochemistry demonstrated that fibronectin positively stained in the extracellular matrix (ECM) in eight cases and that the fibronectin receptor was positive in tumor cell membranes in 10 cases. In addition, cellular fibronectin isoforms containing ED-A and ED-B sequences were found to be immunolocalized in the tumor cells and the ECM of GB. These isoforms were also specifically expressed in tumor vessels within tumor tissues, but not in those within normal brain tissues. Cell migration tended to be expressed more strongly by glioma cells derived from tumor tissues in which fibronectin was positively immunolocalized in the ECM than from tissues with negative fibronectin in the ECM. Four glioma cells derived from GB whose tumor cells did not positively stain for fibronectin receptors migrated much less extensively to fibronectin than other glioma cells whose tissues showed positive staining for the fibronectin receptor. Of these four GB, two had loss of heterozygosity in the locus of fibronectin receptor beta1 gene. These results suggest that fibronectin deposited in the extracellular matrix of tumors, which can be derived from both plasma and the tumor cell itself, strongly promotes the migration of glioma cells, and that expression of the fibronectin receptor may play a critical role in the biological behavior of the tumor cells, particularly in fibronectin-stimulated cell migration in vivo.

The role of alpha2 beta1 and alpha3 beta1 integrin receptors in the initial anchoring of MDA-MB-231 human breast cancer cells to cortical bone matrix

Molecular mechanisms involved in the rapid attachment of the human breast cancer cell line MDA-MD-231 to cortical bone matrix were studied. The attachment of MDA-MD-231 cells to cortical bone disks could be blocked by 75% when cells were pretreated with a monoclonal antibody to the beta1-subunit of the integrin family. Monoclonal antibodies against the alpha2, alpha3, and alpha5 integrin subunits inhibited the attachment by 76, 26, and 8 % respectively. Collagenase type I and collagen type I antibody blocked the cell attachment by 45 and 50 % whereas pretreatment of the cells with soluble collagen type I blocked the attachment by 85 %. Our study with a panel of cancer cell lines further showed a close correlation between alpha2 beta1 and alpha3 beta1 integrin receptor expression and the capability to attach to cortical bone. These receptors appear to be the key receptors utilized by cancer cells for the initial attachment to cortical bone, and this could facilitate the localization of alpha2 beta1 and alpha3 beta1 expressing cancer cells to the skeleton.

The vitronectin receptor associates with clathrin-coated membrane domains via the cytoplasmic domain of its beta5 subunit

Rat myotubes cultured in fetal calf serum adhere to vitronectin-coated substrates through two distinct structures, focal contacts and clathrin-coated membrane domains. We studied the integrins in myotubes to learn how they associate with these two domains. Double label immunofluorescence studies with antibodies specific for clathrin, vinculin and several forms of integrin showed that focal contacts and clathrin-coated membrane domains contain both vitronectin receptors (VnR, containing beta-3 and beta-5integrins) and fibronectin receptors (FnR, containing beta1-integrin). VnR but not FnR associates tightly with the substrate in both domains, as the VnR alone remains attached to the coverslip when the lipid bilayer and other membrane proteins are removed by detergent. Ultrastructural studies confirmed the localization of the beta5 subunit of the VnR at both domains. We used intracellular injection and affinity chromatography to test the possibility that clathrin at coated membrane domains associates with the cytoplasmic sequence of the beta5 subunit of the VnR. Injection of a synthetic peptide containing the NPXY motif from the cytoplasmic domain of the human beta5 subunit, SRARYEMASNPLYRKPIST, depleted clathrin from coated membrane domains without affecting clathrin in perinuclear structures or vinculin at focal contacts. Injection of the homologous beta1 peptide, MNAKWDTGENPIYKSAVITT, also containing an NPXY motif, had no significant effect on any of these structures. Affinity matrices containing the beta5 but not the beta1 peptide selectively retained clathrin from myotube extract, and bound clathrin could be selectively eluted by soluble forms of the beta5 but not the beta1 peptide. Thus, a sequence including the NPXY motif in the integrin beta5 subunit is involved in the specific anchoring of the VnR, but not the FnR, to clathrin-coated membrane.

Orientation of ECM protein deposition, fibroblast cytoskeleton, and attachment complex components on silicone microgrooved surfaces

The microfilaments and vinculin-containing attachment complexes of rat dermal fibroblasts (RDF) incubated on microtextured surfaces were investigated with confocal laser scanning microscopy (CLSM) and digital image analysis (DIA). In addition, depositions of bovine and endogenous fibronectin and vitronectin were studied. Smooth and microtextured silicone substrata were produced that possessed parallel surface grooves with a groove and ridge width of 2.0, 5.0, and 10.0 microns. The groove depth was approximately 0.5 micron. CLSM and DIA make it possible to visualize and analyze intracellular and extracellular proteins and the underlying surface simultaneously. It was observed that the microfilaments and vinculin aggregates of the RDFs on the 2.0 microns grooved substrata were oriented along the surface grooves after 1, 3, 5, and 7 days of incubation while these proteins were significantly less oriented on the 5.0 and 10.0 microns grooved surfaces. Vinculin was located mainly on the surface ridges on all textured surfaces. In contrast, bovine and endogenous fibronectin and vitronectin were oriented along the surface grooves on all textured surfaces. These proteins did not seem to be hindered by the surface grooves since many groove-spanning filaments were found on all the microgrooved surfaces. In conclusion, it can be said that microtextured surfaces influence the orientation of intracellular and extracellular proteins. Although results corroborate three earlier published hypotheses, they do not justify a specific choice of any one of these hypotheses.

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.

Integrin receptor signaling: the propagation of an alpha-helix model

Upon ligand binding to integrin receptors, a transmembrane conformation change occurs, which is required for the engagement of the actin cytoskeleton. Integrin receptor latency clearly involves the proximal portions of the alpha and beta cytoplasmic domains. Several experiments suggest that these two regions, which are highly conserved among integrins, may be associated, and this association is the structural basis for latency. We propose that ligand binding leads to a disruption of this association, which allows for the folding of the proximal beta cytoplasmic domain. Thus, in this model, the alpha chain association keeps the beta unfolded, and ligand binding leads to the propagation of an alpha helix from the transmembrane domain through the proximal beta cytoplasmic domain, leading to signal transduction.

Human fibroblasts bind directly to fibrinogen at RGD sites through integrin alpha(v)beta3

Fibroblast migration into the blood clot initially filling a wound requires close interaction between fibroblasts and the matrix of the fibrin clot. However, very little is known about the specific receptor-ligand interactions that mediate fibroblast attachment to fibrin. Using an attachment assay developed to measure even relatively weak interactions, we demonstrate here that normal human dermal fibroblasts can attach to substrates coated with fibrinogen, fibrin, or the fibrinogen breakdown product I-9D. Fibroblast attachment to these ligands did not require the presence of fibronectin on the cell surface or as a component of the substrate. Cells treated with cycloheximide and monensin, to limit the synthesis and secretion of endogenous fibronectin, attached as well as untreated cells. The synthetic peptide GRGDS inhibited adhesion to fibrinogen, fibrin, and fibrinogen I-9D by about 60%, while the control peptide GRGES had no substantial effect. We conclude that attachment to these ligands is mediated at least partially by direct interactions between the substrates and one specific receptor, the integrin alpha(v)beta3. Affinity chromatography demonstrated that alpha(v)beta3 from detergent lysates of fibroblasts bound to a fibrinogen matrix and was eluted with EDTA. Furthermore, antibodies against the alpha(v)beta3 complex or against the alpha(v) subunit inhibited fibroblast attachment to fibrinogen and fibrin by 50-70%. An inhibitory antibody against the integrin beta1 subunit had no effect. The observation that integrin antagonists could not produce complete inhibition suggests that there may be other fibroblast cell surface proteins that can bind directly to fibrinogen.