Matrix valency regulates integrin-mediated lymphoid adhesion via Syk kinase

Integrin-Targeting Strategies for Adenovirus Gene Therapy

Numerous human adenovirus (AdV) types are endowed with arginine-glycine-aspartic acid (RGD) sequences that enable them to recognize vitronectin-binding (αv) integrins. These RGD-binding cell receptors mediate AdV entry into host cells, a crucial early step in virus infection. Integrin interactions with adenoviruses not only initiate receptor-mediated endocytosis but also facilitate AdV capsid disassembly, a prerequisite for membrane penetration by AdV protein VI. This review discusses fundamental aspects of AdV-host interactions mediated by integrins. Recent efforts to re-engineer AdV vectors and non-viral nanoparticles to target αv integrins for bioimaging and the eradication of cancer cells will also be discussed.

Lessons learned from adenovirus (1970-2019)

Animal viruses are well recognized for their ability to uncover fundamental cell and molecular processes, and adenovirus certainly provides a prime example. This review illustrates the lessons learned from studying adenovirus over the past five decades. We take a look back at the key studies of adenovirus structure and biophysical properties, which revealed the mechanisms of adenovirus association with antibody, cell receptor, and immune molecules that regulate infection. In addition, we discuss the critical contribution of studies of adenovirus gene expression to elucidation of fundamental reactions in pre-mRNA processing and its regulation. Other pioneering studies furnished the first examples of protein-primed initiation of DNA synthesis and viral small RNAs. As a nonenveloped virus, adenoviruses have furnished insights into the modes of virus attachment, entry, and penetration of host cells, and we discuss the diversity of cell receptors that support these processes, as well as membrane penetration. As a result of these extensive studies, adenovirus vectors were among the first to be developed for therapeutic applications. We highlight some of the early (unsuccessful) trials and the lessons learned from them.

Insights into the allosteric regulation of Syk association with receptor ITAM, a multi-state equilibrium

The phosphorylation of interdomain A (IA), a linker region between tandem SH2 domains of Syk tyrosine kinase, regulates the binding affinity for association of Syk with doubly-phosphorylated ITAM regions of the B cell receptor. The mechanism of this allosteric regulation has been suggested to be a switch from the high-affinity bifunctional binding, mediated through both SH2 domains binding two phosphotyrosine residues of ITAM, to a substantially lower-affinity binding of only one SH2 domain. IA phosphorylation triggers the switch by inducing disorder in IA and weakening the SH2-SH2 interaction. The postulated switch to a single-SH2-domain binding mode is examined using NMR to monitor site-specific binding to each SH2 domain of Syk variants engineered to have IA regions that differ in conformational flexibility. The combined analysis of titration curves and NMR line-shapes provides sufficient information to determine the energetics of inter-molecular binding at each SH2 site along with an intra-molecular binding or isomerization step. A less favorable isomerization equilibrium associated with the changes in the SH2-SH2 conformational ensemble and IA flexibility accounts for the inhibition of Syk association with membrane ITAM regions when IA is phosphorylated, and refutes the proposed switch to single-SH2-domain binding. Syk localizes in the cell through its SH2 interactions, and this basis for allosteric regulation of ITAM association proposes for the first time a phosphorylation-dependent model to regulate Syk binding to alternate receptors and other signaling proteins that differ either in the number of residues separating ITAM phosphotyrosines or by having only one phosphotyrosine, a half ITAM.

CCN4 induces vascular cell adhesion molecule-1 expression in human synovial fibroblasts and promotes monocyte adhesion

CCN4 is a cysteine-rich protein that belongs to the Cyr61, CTGF, Nov family of matricellular proteins. Here, we investigated the intracellular signaling pathways involved in CCN4-induced vascular cell adhesion molecule-1 expression in human osteoarthritis synovial fibroblasts. Stimulation of OASFs with CCN4 induced VCAM-1 expression. CCN4-induced VCAM-1 expression was attenuated by αvβ5 or α6β1 integrin antibody, Syk inhibitor, PKCδ inhibitor (rottlerin), JNK inhibitor (SP600125), and AP-1 inhibitors (curcumin and tanshinone). Stimulation of cells with CCN4 increased Syk, PKCδ, and JNK activation. Treatment of OASFs with CCN4 also increased c-Jun phosphorylation, AP-1-luciferase activity, and c-Jun binding to the AP-1 element in the VCAM-1 promoter. Moreover, up-regulation of VCAM-1 increased the adhesion of monocytes to OASF monolayers, and this adhesion was attenuated by transfection with a VCAM-1 siRNA. Our results suggest that CCN4 increases VCAM-1 expression in human OASFs via the Syk, PKCδ, JNK, c-Jun, and AP-1 signaling pathways. The CCN4-induced VCAM-1 expression promoted monocyte adhesion to human OASFs.

Autoinhibition and adapter function of Syk

Development, survival, and activation of B lymphocytes are controlled by signals emanating from the B-cell antigen receptor (BCR). The BCR has an autonomous signaling function also known as tonic signaling that allows for long-term survival of B cells in the immune system. Upon binding of antigen to the BCR, the tonic signal is amplified and diversified, leading to alteration in gene expression and B-cell activation. The spleen tyrosine kinase (Syk) intimately cooperates with the signaling subunits of the BCR and plays a central role in the amplification and diversification of BCR signals. In this review, we discuss the molecular mechanisms by which Syk activity is inhibited and activated at the BCR. Importantly, Syk acts not only as a kinase that phosphorylates downstream substrates but also as an adapter that can bind to a diverse set of signaling proteins. Depending on its interactions and localization, Syk can signal opposing cell fate decisions such as proliferation or differentiation of B cells.

Role of the protein tyrosine kinase Syk in regulating cell-cell adhesion and motility in breast cancer cells

The expression of the Syk protein tyrosine kinase in breast cancer cells is inversely correlated with invasive growth and metastasis. The expression of Syk inhibits cell motility while supporting the formation of cell clusters by enhancing cell-cell contacts and promoting the redistribution of the adhesion proteins cortactin and vinculin to these contacts. Syk associates physically with cortactin and catalyzes its phosphorylation on tyrosine. The clustering of integrins leads to the phosphorylation of Syk and of numerous cellular proteins in a manner dependent on the activity of the kinase and on the presence of tyrosine 342 located in the linker B region. The ability of Syk to participate in integrin-mediated protein tyrosine phosphorylation correlates well with its ability to inhibit cell motility.

Syk and pTyr'd: Signaling through the B cell antigen receptor

The B cell receptor (BCR) transduces antigen binding into alterations in the activity of intracellular signaling pathways through its ability to recruit and activate the cytoplasmic protein-tyrosine kinase Syk. The recruitment of Syk to the receptor, its activation and its subsequent interactions with downstream effectors are all regulated by its phosphorylation on tyrosine. This review discusses our current understanding of how this phosphorylation regulates the activity of Syk and its participation in signaling through the BCR.

Tyr130 phosphorylation triggers Syk release from antigen receptor by long-distance conformational uncoupling

The Syk protein-tyrosine kinase plays a major role in signaling through the B cell receptor for antigen (BCR). Syk binds the receptor via its tandem pair of SH2 domains interacting with a doubly phosphorylated immunoreceptor tyrosine-based activation motif (dp-ITAM) of the BCR complex. Upon phosphorylation of Tyr-130, which lies between the two SH2 domains distant to the phosphotyrosine binding sites, Syk dissociates from the receptor. To understand the structural basis for this dissociation, we investigated the structural and dynamic characteristics of the wild type tandem SH2 region (tSH2) and a variant tandem SH2 region (tSH2(pm)) with Tyr-130 substituted by Glu to permanently introduce a negative charge at this position. NMR heteronuclear relaxation experiments, residual dipolar coupling measurements and analytical ultracentrifugation revealed substantial differences in the hydrodynamic behavior of tSH2 and tSH2(pm). Although the two SH2 domains in tSH2 are tightly associated, the two domains in tSH2(pm) are partly uncoupled and tumble in solution with a faster correlation time. In addition, the equilibrium dissociation constant for the binding of tSH2(pm) to dp-ITAM (1.8 microM) is significantly higher than that for the interaction between dp-ITAM and tSH2 but is close to that for a singly tyrosine-phosphorylated peptide binding to a single SH2 domain. Experimental data and hydrodynamic calculations both suggest a loss of domain-domain contacts and change in relative orientation upon the introduction of a negative charge on residue 130. A long-distance structural mechanism by which the phosphorylation of Y130 negatively regulates the interaction of Syk with immune receptors is proposed.

Current advances and future challenges in Adenoviral vector biology and targeting

Gene delivery vectors based on Adenoviral (Ad) vectors have enormous potential for the treatment of both hereditary and acquired disease. Detailed structural analysis of the Ad virion, combined with functional studies has broadened our knowledge of the structure/function relationships between Ad vectors and host cells/tissues and substantial achievement has been made towards a thorough understanding of the biology of Ad vectors. The widespread use of Ad vectors for clinical gene therapy is compromised by their inherent immunogenicity. The generation of safer and more effective Ad vectors, targeted to the site of disease, has therefore become a great ambition in the field of Ad vector development. This review provides a synopsis of the structure/function relationships between Ad vectors and host systems and summarizes the many innovative approaches towards achieving Ad vector targeting.

Model of integrin-mediated cell adhesion strengthening

Cell adhesion to extracellular matrix components involves integrin binding, receptor clustering, and recruitment of cytoskeletal elements, leading to the formation of discrete adhesive structures (focal adhesions). A force balance, macroscopic-to-microscopic model of these adhesive events is presented in the context of experimentally measured parameters. Integrin bond force, bond numbers, and distribution along the contact area strongly modulated the resulting adhesive force. Furthermore, focal adhesion assembly enhanced adhesion strength by 30% over integrin clustering alone. Predicted values are in excellent agreement with experimental results. This model provides a simple framework to systematically analyze the contributions of different adhesive parameters to overall adhesion strength.

Cutting edge: constitutive B cell receptor signaling is critical for basal growth of B lymphoma

B lymphomas account for the majority of the lymphoma cases. BCR expression appears to be important for B lymphoma because most oncogenes are translocated to nonrearranged Ig loci and because all of the variants that arise in anti-idiotypic Ab-treated lymphoma patients remain BCR positive. Based on this and the fact that BCR is required for mature B cell survival, we tested the requirement for continued expression of BCR for the growth and survival of B lymphoma cells. Using Igalpha or Igbeta-specific small interfering RNA (siRNA) to inhibit BCR expression, we demonstrate for the first time that constitutive signaling by BCR is critical for survival and proliferation of both murine and human B lymphoma cells. The BCR signals in lymphoma appear to be mediated by Syk, as it is constitutively active in a variety of B lymphoma cells. Blocking Syk activity by selective inhibitors suppresses growth of several murine and human B lymphomas.

Phosphorylation of focal adhesion kinase (FAK) on Ser732 is induced by rho-dependent kinase and is essential for proline-rich tyrosine kinase-2-mediated phosphorylation of FAK on Tyr407 in response to vascular endothelial growth factor

Focal adhesion kinase (FAK) is phosphorylated on tyrosine and serine residues after cell activation. In the present work, we investigated the relationship between tyrosine and serine phosphorylation of FAK in promoting endothelial cell migration in response to vascular endothelial growth factor (VEGF). We found that VEGF induces the activation of the Rho-dependent kinase (ROCK) downstream from vascular endothelial growth factor receptor (VEGFR) 2. In turn, activated ROCK directly phosphorylates FAK on Ser732. Proline-rich tyrosine kinase-2 (Pyk2) is also activated in response to VEGF. Its activation requires the clustering of integrin alphavbeta3 and triggers directly the phosphorylation of Tyr407 within FAK, an event necessary for cell migration. Interestingly, ROCK-mediated phosphorylation of Ser732 is essential for Pyk2-dependent phosphorylation of Tyr407, because the latter is abrogated in cells expressing a FAK mutant that is nonphosphorylatable on Ser732. We suggest that VEGF elicits the activation of the VEGFR2-ROCK pathway, leading to phosphorylation of Ser732 within FAK. In turn, phosphorylation of Ser732 would change the conformation of FAK, making it accessible to Pyk2 activated in response to its association with integrin beta3. Then, activated Pyk2 triggers the phosphorylation of FAK on Tyr407, promoting cell migration.

Synthetic multivalent ligands as probes of signal transduction

Cell-surface receptors acquire information from the extracellular environment and coordinate intracellular responses. Many receptors do not operate as individual entities, but rather as part of dimeric or oligomeric complexes. Coupling the functions of multiple receptors may endow signaling pathways with the sensitivity and malleability required to govern cellular responses. Moreover, multireceptor signaling complexes may provide a means of spatially segregating otherwise degenerate signaling cascades. Understanding the mechanisms, extent, and consequences of receptor co-localization and interreceptor communication is critical; chemical synthesis can provide compounds to address the role of receptor assembly in signal transduction. Multivalent ligands can be generated that possess a variety of sizes, shapes, valencies, orientations, and densities of binding elements. This Review focuses on the use of synthetic multivalent ligands to characterize receptor function.

The Syk tyrosine kinase: a new negative regulator in tumor growth and progression

The spleen tyrosine kinase Syk was long thought to be a hematopoietic cell-specific signaling molecule. Recent evidence demonstrated that it is also expressed by many non-hematopoietic cell types and that it plays a negative role in cancer. A significant drop in its expression was first observed during breast cancer progression, but an anomalous Syk expression has now also been evidenced in many other tumor types. Mechanistic studies using Syk re-expression demonstrated its suppressive function in tumorigenesis and metastasis formation, which is surprising for a tyrosine kinase. Loss of Syk expression is regulated, albeit not exclusively, by its promoter hypermethylation. The molecular mechanism of its tumor-suppressive function remains largely unknown; the identification of its activators and effectors in non-hematopoietic cells will be a challenge for the years to come. An increasing number of clinical studies reveal a correlation between reduced Syk expression and an increased risk for metastasis formation, and assign Syk as a potential new prognostic marker in different tumor types.

The Syk tyrosine kinase localizes to the centrosomes and negatively affects mitotic progression

We showed previously that the spleen tyrosine kinase Syk is expressed by mammary epithelial cells and that it suppresses malignant growth of breast cancer cells. The exact molecular mechanism of its tumor-suppressive activity remains, however, to be identified. Here, we show that Syk colocalizes and copurifies with the centrosomal component gamma-tubulin and exhibits a catalytic activity within the centrosomes. Moreover, its centrosomal localization depends on its intact kinase activity. Centrosomal Syk expression is persistent in interphase but promptly drops during mitosis, obviously resulting from its ubiquitinylation and proteasomal degradation. Conversely, unrestrained exogenous expression of a fluorescently tagged Discosoma sp. red fluorescent protein (DsRed)-Syk chimera engenders abnormal cell division and cell death. Transient DsRed-Syk overexpression triggers an abrupt cell death lacking hallmarks of classic apoptosis but reminiscent of mitotic catastrophe. Surviving stable DsRed-Syk-transfected cells exhibit multipolar mitotic spindles and contain multiple abnormally sized nuclei and supernumerary centrosomes, revealing anomalous cell division. Taken together, these results show that Syk is a novel centrosomal kinase that negatively affects cell division. Its expression is strictly controlled in a spatiotemporal manner, and centrosomal Syk levels need to decline to allow customary progression of mitosis.

Cell adhesion strengthening: contributions of adhesive area, integrin binding, and focal adhesion assembly

Mechanical interactions between a cell and its environment regulate migration, contractility, gene expression, and cell fate. We integrated micropatterned substrates to engineer adhesive area and a hydrodynamic assay to analyze fibroblast adhesion strengthening on fibronectin. Independently of cell spreading, integrin binding and focal adhesion assembly resulted in rapid sevenfold increases in adhesion strength to steady-state levels. Adhesive area strongly modulated adhesion strength, integrin binding, and vinculin and talin recruitment, exhibiting linear increases for small areas. However, above a threshold area, adhesion strength and focal adhesion assembly reached a saturation limit, whereas integrin binding transitioned from a uniform distribution to discrete complexes. Adhesion strength exhibited exponential increases with bound integrin numbers as well as vinculin and talin recruitment, and the relationship between adhesion strength and these biochemical events was accurately described by a simple mechanical model. Furthermore, adhesion strength was regulated by the position of an adhesive patch, comprised of bound integrins and cytoskeletal elements, which generated a constant 200-nN adhesive force. Unexpectedly, focal adhesion assembly, in particular vinculin recruitment, contributed only 30% of the adhesion strength. This work elucidates the roles of adhesive complex size and position in the generation of cell-extracellular matrix forces.

Integrins and angiogenesis

The growth of new blood vessels is a dynamic yet highly regulated process that depends on coordinated signaling by growth factor and cell adhesion receptors. As part of the molecular program regulating angiogenesis, endothelial cells acquire a proliferative and invasive phenotype but also show increased susceptibility to apoptotic stimuli. Integrins are the principle adhesion receptors used by endothelial cells to interact with their extracellular microenvironment, and integrin-mediated interactions play a critical role in regulating cell proliferation, migration, and survival. Alterations in the repertoire and?or activity of integrins, as well as the availability and structural property of their ligands, regulate the vascular cell during the growth or repair of blood vessels.

Beta-1 integrin-mediated adhesion may be initiated by multiple incomplete bonds, thus accounting for the functional importance of receptor clustering

The regulation of cell integrin receptors involves modulation of membrane expression, shift between different affinity states, and topographical redistribution on the cell membrane. Here we attempted to assess quantitatively the functional importance of receptor clustering. We studied beta-1 integrin-mediated attachment of THP-1 cells to fibronectin-coated surfaces under low shear flow. Cells displayed multiple binding events with a half-life of the order of 1 s. The duration of binding events after the first second after arrest was quantitatively accounted for by a model assuming the existence of a short-time intermediate binding state with 3.6 s(-1) dissociation rate and 1.3 s(-1) transition frequency toward a more stable state. Cell binding to surfaces coated with lower fibronectin densities was concluded to be mediated by single molecular interactions, whereas multiple bonds were formed <1 s after contact with higher fibronectin surface densities. Cell treatment with microfilament inhibitors or a neutral antiintegrin antibody decreased bond number without changing aforementioned kinetic parameters whereas a function enhancing antibody increased the rate of bond formation and/or the lifetime of intermediate state. Receptor aggregation was induced by treating cells with neutral antiintegrin antibody and antiimmunoglobulin antibodies. A semiquantitative confocal microscopy study suggested that this treatment increased between 40% and 100% the average number of integrin receptors located in a volume of approximately 0.045 microm(3) surrounding each integrin. This aggregation induced up to 2.7-fold increase of the average number of bonds. Flow cytometric analysis of fluorescent ligand binding showed that THP-1 cells displayed low-affinity beta-1 integrins with a dissociation constant in the micromolar range. It is concluded that the initial step of cell adhesion was mediated by multiple incomplete bonds rather than a single equilibrium-state ligand receptor association. This interpretation accounts for the functional importance of integrin clustering.

Hypermethylation of Syk gene in promoter region associated with oncogenesis and metastasis of gastric carcinoma

AIM: To investigate the rrelationship between methylation of Syk (spleen tyrosine kinase) gene in promoter region and oncogenesis, metastasis of gastric carcinoma. The relation between silencing of the Syk gene and methylation of Syk promoter region was also studied.

METHODS: By using methylation-specific PCR (MSP) technique, the methylation of Syk promoter region in specimens from 61 gastric cancer patients (tumor tissues and adjacent normal tissues) was detected. Meanwhile, RT-PCR was used to analyse syk expression exclusively.

RESULTS: The expression of the Syk gene was detected in all normal gastric tissues. Syk expression in gastric carcinoma was lower in 14 out of 61 gastric cancer samples than in adjacent normal tissues (χ² = 72.3, P < 0.05). No methylation of Syk promoter was found in adjacent normal tissues. hypermethylation of Syk gene in promoter was detected 21 cases in 61 gastric carcinoma patients. The rate of methylation of Syk promoter in gastric carcinoma was higher than that in adjacent normal tissues (χ² = 25.1, P < 0.05). In 31 patients with lymph node metastasis, 17 were found with Syk promoter methylation. A significant difference was noted between two groups (χ² = 11.4,P < 0.05).

CONCLUSION: Hypermethylation leads to silencing of the Syk gene in human gastric carcinoma. Methylation of Syk promoter is correlated to oncogenesis and metastasis of gastric carcinoma. Syk is considered to be a potential tumor suppressor and anti-metastasis gene in human gastric cancer.

Endocytosis of adenovirus and adenovirus capsid proteins

Key proteins of the icosahedral-shaped adenovirus (Ad) capsid mediate infection, and interact with cellular proteins to coordinate stepwise events of cell entry that produce successful gene transfer. Infection is mediated predominantly by the penton and fiber capsid proteins. The fiber initiates cell binding while the penton binds integrin coreceptors, triggering integrin-mediated endocytosis. Penton integrin signaling precedes viral escape from the endosomal vesicle. After cell binding, the virus undergoes stepwise disassembly of the capsid, shedding proteins during cell entry. Intracellular trafficking of the remaining capsid shell is mediated by the interaction of naked particles with the cytoskeleton. The capsid translocates toward the nucleus, with the majority of capsid proteins accumulating at the nuclear periphery, while viral DNA and associated protein VII are extruded through the nuclear pore. This discussion will encompass the current knowledge on Ad cell entry and trafficking, with an emphasis on the contribution of Ad capsid proteins to these processes. A greater understanding of the highly effective Ad cell entry pathway may lend itself to the development of safer drug and gene delivery alternatives utilizing similar pathways.

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.

Regulation of activin A expression in mast cells and asthma: its effect on the proliferation of human airway smooth muscle cells

Activin A, a homodimeric protein (betaAbetaA) and a member of the TGF-beta superfamily, is involved in the inflammatory repair process. Using cDNA microarray analysis, we discovered strong induction of the activin betaA gene in human mast cells (MC) on stimulation with PMA and calcium ionophore (A23187). Activin betaA mRNA was also highly induced in primary cultured murine bone marrow MC (BMMC) after stimulation by IgE receptor cross-linking. Secretion of activin A was evident in human mast cell-1 line cells 3 h after stimulation and progressively increased over time. Activin A was present in the cytoplasm of activated but not unstimulated murine bone marrow MC as demonstrated by immunofluorescence studies, suggesting that secretion of activin A by MC was due to de novo synthesis rather than secretion of preformed protein. Activin A also colocalized with human lung MC from patients with asthma by double-immunofluorescence staining. Furthermore, secretion of activin A was significantly increased in the airway of wild-type mice after OVA sensitization followed by intranasal challenge. Secretion of activin A, however, was greatly reduced in MC-deficient WBB6F(1)-W/W(v) mice as compared with wild-type mice, indicating that MC are an important contributor of activin A in the airways of a murine asthma model. Additionally, activin A promoted the proliferation of human airway smooth muscle cells. Taken together, these data suggest that MC-derived activin A may play an important role in the process of airway remodeling by promoting the proliferation of airway smooth muscle.

Visualizing lipid raft dynamics and early signaling events during antigen receptor-mediated B-lymphocyte activation

Recent biochemical evidence indicates that an early event in signal transduction by the B-cell antigen receptor (BCR) is its translocation to specialized membrane subdomains known as lipid rafts. We have taken a microscopic approach to image lipid rafts and early events associated with BCR signal transduction. Lipid rafts were visualized on primary splenic B lymphocytes from wild-type or anti-hen egg lysozyme BCR transgenic mice, and on a mature mouse B-cell line Bal 17 by using fluorescent conjugates of cholera toxin B subunit or a Lyn-based chimeric protein, which targets green fluorescent protein to the lipid raft compartment. Time-lapse imaging of B cells stimulated via the BCR with the antigen hen egg lysozyme, or surrogate for antigen anti-IgM, demonstrated that lipid rafts are highly dynamic entities, which move laterally on the surface of these cells and coalesce into large regions. These regions of aggregated lipid rafts colocalized with the BCR and tyrosine-phosphorylated proteins. Microscopic imaging of live B cells also revealed an inducible colocalization of lipid rafts with the tyrosine kinase Syk and the receptor tyrosine phosphatase CD45. These two proteins play indispensable roles in BCR-mediated signaling but are not detectable in biochemically purified lipid raft fractions. Strikingly, BCR stimulation also induced the formation of long, thread-like filopodial projections, similar to previously described structures called cytonemes. These B-cell cytonemes are rich in lipid rafts and actin filaments, suggesting that they might play a role in long-range communication and/or transportation of signaling molecules during an immune response. These results provide a window into the morphological and molecular organization of the B-cell membrane during the early phase of BCR signaling.

Integrin alphavbeta3, requirement for VEGFR2-mediated activation of SAPK2/p38 and for Hsp90-dependent phosphorylation of focal adhesion kinase in endothelial cells activated by VEGF

Endothelial cell migration, a key process in angiogenesis, requires the coordinated integration of motogenic signals elicited by the adhesion of endothelial cells to extracellular matrices and by angiogenic cytokines such as the vascular endothelial growth factor (VEGF). In this study, we found that addition of VEGF to human umbilical vein endothelial cells cultivated on vitronectin triggers a synergistic interaction between the VEGF receptor VEGFR2 and the clustered integrin receptor alphavbeta3. The interaction between VEGFR2 and alphavbeta3 is required for full phosphorylation of VEGFR2 and to drive the activation of motogenic pathways involving focal adhesion kinase (FAK) and stress-activated protein kinase-2/p38 (SAPK2/p38). The signal emanating from the VEGFR2 and alphavbeta3 interaction and leading to SAPK2/p38 activation proceeds directly from VEGFR2. The chaperone Hsp90 is found in a complex that coprecipitates with inactivated VEGFR2, and the association is increased by VEGF and decreased by geldanamycin, a specific inhibitor of Hsp90-mediated events. Geldanamycin also impairs the phosphorylation of FAK that results from the interaction between VEGFR2 and alphavbeta3, and this is accompanied by an inhibition of the recruitment of vinculin to VEGFR2. We conclude that a necessary cross talk should occur between VEGFR2 and the integrin alphavbeta3, to transduce the VEGF signals to SAPK2/p38 and FAK and that Hsp90 is instrumental in the building up of focal adhesions by allowing the phosphorylation of FAK and the recruitment of vinculin to VEGFR2.

Get a ligand, get a life: integrins, signaling and cell survival

Programmed cell death is crucial for the development and maintenance of multicellular organisms. The decision to live, or to die, depends, at the cellular level, upon the cell's interaction with extracellular cues that trigger cell signaling pathways promoting survival or death. The extracellular matrix (ECM) influences the execution of the apoptotic program through the actions of adhesion receptors. Among these, integrins initiate a variety of downstream signaling events in response to ECM ligation. Integrins directly activate survival pathways via the PI 3-kinase and MAPK pathways and act as essential cofactors for their stimulation by growth factors. Conversely, elevated integrin expression in the absence of appropriate ligands, or in the presence of natural or synthetic antagonists, can promote apoptosis under otherwise permissive growth conditions. Integrins thus act in a crucial biosensory role, coordinating survival or death responses as a function of ECM composition. This dual function provides an elegant mechanism through which tissue-remodeling events may regulate cell death or survival in a temporal, ECM-governed manner.

Trimers of the fibronectin cell adhesion domain localize to actin filament bundles and undergo rearward translocation

Previous studies have shown that small beads coated with FN7-10, a four-domain cell adhesion fragment of fibronectin, bind to cell surfaces and translocate rearward. Here we investigate whether soluble constructs containing two to five FN7-10 units might be sufficient for activity. We have produced a monomer, three forms of dimers, a trimer and a pentamer of FN7-10,on the end of spacer arms. These oligomers could bind small clusters of up to five integrins. Fluorescence microscopy showed that the trimer and pentamer bound strongly to the cell surface, and within 5 minutes were prominently localized to actin fiber bundles. Monomers and dimers showed only diffuse localization. Beads coated with a low concentration (probably one complex per bead) of trimer or pentamer showed prolonged binding and rearward translocation, presumably with the translocating actin cytskeleton. Beads containing monomer or dimer showed only brief binding and diffusive movements. We conclude that clusters of three integrin-binding ligands are necessary and sufficient for coupling to and translocating with the actin cytoskeleton.

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.

The microenvironment of immobilized Arg-Gly-Asp peptides is an important determinant of cell adhesion

This paper uses self-assembled monolayers on gold as a model system to demonstrate that the attachment and spreading of Swiss 3T3 fibroblasts depends strongly on the microenvironment of immobilized RGD peptides. This work utilized monolayers that present mixtures of Arg-Gly-Asp peptides, which are ligands for cellular integrin receptors, and oligo(ethylene glycol) groups, which resist the nonspecific adsorption of protein. The microenvironment of the peptide ligands was controlled by altering the length of the surrounding oligo(ethylene glycol) groups on the monolayer. By using thiols that present either tri-, tetra-, penta-, or hexa(ethylene glycol) units, the average distance separating the glycol groups and the peptide ligand is altered while the structure and properties of the background remain unchanged. Cell attachment to monolayers presenting a fixed density of peptide decreased as the length of the oligo(ethylene glycol) group increased. The average projected area of attached cells showed a similar trend. At lower densities of immobilized peptide, decreases in both cell attachment and projected cell area were more pronounced. Attachment and spreading did not depend on density of peptide on monolayers presenting tri(ethylene glycol) groups, but showed a high sensitivity to density of ligand on monolayers presenting longer glycol oligomers. Experiments that used a soluble peptide to inhibit the attachment of cells to monolayers demonstrated that the strength of the cell-substrate interaction decreased on monolayers presenting longer glycol groups. Together, these results suggest that the microenvironment of the peptide ligand influences the affinity of the integrin-peptide interaction and that weaker interactions display a density-dependent enhancement of binding during cell attachment and spreading. This finding is an important consideration in studies that correlate biological function with the composition of ligands on a substrate. This finding also represents an important principle for the design of biologically active materials because it illustrates the degree to which the presentation of adhesion motifs can modify the response of mammalian cells.

Subversion of integrins by enteropathogenic Yersinia

Enteropathogenic Yersinia are gram-negative bacterial species that translocate from the lumen of the intestine and are able to grow within deep tissue sites. During the earliest stages of disease, the organism is able to bind integrin receptors that are presented on the apical surface of M cells in the intestine, which allows its internalization and subsequent translocation into regional lymph nodes. The primary integrin substrate is the outer-membrane protein invasin, which binds with extraordinarily high affinity to at least five different integrins that have the (beta)(1) chain. Bacterial uptake into host cells is modulated by the affinity of receptor-substrate interaction, receptor concentration and the ability of the substrate to aggregate target receptors.

Molecular signaling mechanisms of cell migration and invasion

The ability of immune cells to migrate and invade the extracellular matrix (ECM) is a central process involved in immunologic surveillance as well as inflammation. We have shown that interaction of cells with adhesive proteins or growth factors (chemokines) present in the ECM control cell migration/invasion through activation of mitogen-activated protein kinases ERK1 and ERK2 and molecular coupling of the adapter proteins p130CAS and c-CrkII. During cell migration, ERK and CAS/Crk coupling operate as distinct signaling pathways that facilitate actin-myosin motor assembly and actin membrane ruffles, respectively. Furthermore, activation of these signaling pathways protects cells from apoptosis during invasion of the ECM, which is necessary as migratory cells colonize foreign sites in the body.

Plasmin-sensitive dibasic sequences in the third fibronectin-like domain of L1-cell adhesion molecule (CAM) facilitate homomultimerization and concomitant integrin recruitment

L1 is a multidomain transmembrane neural recognition molecule essential for neurohistogenesis. While moieties in the immunoglobulin-like domains of L1 have been implicated in both heterophilic and homophilic binding, the function of the fibronectin (FN)-like repeats remains largely unresolved. Here, we demonstrate that the third FN-like repeat of L1 (FN3) spontaneously homomultimerizes to form trimeric and higher order complexes. Remarkably, these complexes support direct RGD-independent interactions with several integrins, including alpha(v)beta(3) and alpha(5)beta(1). A pep- tide derived from the putative C-C' loop of FN3 (GSQRKHSKRHIHKDHV(852)) also forms trimeric complexes and supports alpha(v)beta(3) and alpha(5)beta(1) binding. Substitution of the dibasic RK(841) and KR(845) sequences within this peptide or the FN3 domain limited multimerization and abrogated integrin binding. Evidence is presented that the multimerization of, and integrin binding to, the FN3 domain is regulated both by conformational constraints imposed by other domains and by plasmin- mediated cleavage within the sequence RK( downward arrow)HSK( downward arrow)RH(846). The integrin alpha(9)beta(1), which also recognizes the FN3 domain, colocalizes with L1 in a manner restricted to sites of cell-cell contact. We propose that distal receptor ligation events at the cell-cell interface may induce a conformational change within the L1 ectodomain that culminates in receptor multimerization and integrin recruitment via interaction with the FN3 domain.

Regulation of tissue injury responses by the exposure of matricryptic sites within extracellular matrix molecules

Extracellular matrix (ECM) is known to provide signals controlling cell shape, migration, proliferation, differentiation, morphogenesis, and survival. Recent data shows that some of these signals are derived from biologically active cryptic sites within matrix molecules (matricryptic sites) that are revealed after structural or conformational alteration of these molecules. We propose the name, matricryptins, for enzymatic fragments of ECM containing exposed matricryptic sites. Mechanisms regulating the exposure of matricryptic sites within ECM molecules include the major mechanism of enzymatic breakdown as well as others including ECM protein multimerization, adsorption to other molecules, cell-mediated mechanical forces, and ECM denaturation. Such matrix alterations occur during or as a result of tissue injury, and thus, the appearance of matricryptic sites within an injury site may provide important new signals to regulate the repair process. Here, we review the data supporting this concept and provide insight into why the increased exposure of matricryptic sites may be an important regulatory step in tissue responses to injury.

Stimulation of integrin-mediated cell contractility by fibronectin polymerization

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

Regulation of adenovirus membrane penetration by the cytoplasmic tail of integrin beta5

Adenovirus (Ad) cell entry involves sequential interactions with host cell receptors that mediate attachment (CAR), internalization (alphavbeta3 and alphavbeta5), and penetration (alphavbeta5) of the endosomal membrane. These events allow the virus to deliver its genome to the nucleus. While integrins alphavbeta3 and alphavbeta5 both promote Ad internalization into cells, integrin alphavbeta5 selectively facilitates Ad-mediated membrane permeabilization and endosome rupture. In the experiments reported herein, we demonstrate that the intracellular domain of the integrin beta5 subunit specifically regulates Ad-mediated membrane permeabilization and gene delivery. CS-1 melanoma cells expressing a truncated integrin beta5 or a chimeric (beta5-beta3) cytoplasmic tail (CT) supported normal levels of Ad endocytosis but had reduced Ad-mediated gene delivery and membrane permeabilization relative to cells expressing a wild-type integrin beta5. Thin-section electron microscopy revealed that virion particles were capable of being endocytosed into cells expressing a truncated beta5CT, but they failed to escape cytoplasmic vesicles and translocate to the nucleus. Site-specific mutagenesis studies suggest that a C-terminal TVD motif in the beta5CT plays a major role in Ad membrane penetration.

LFA-1 to LFA-1 Signals Involve ζ-Associated Protein-70 (ZAP-70) Tyrosine Kinase: Relevance for Invasion and Migration of a T Cell Hybridoma

We previously showed that LFA-1-dependent in vitro invasion and in vivo migration of a T cell hybridoma was blocked in cells overexpressing a truncated dominant-negative ζ-associated protein (ZAP)-70. The truncated ZAP-70 also blocked LFA-1-dependent chemotaxis through ICAM-1-coated filters induced by 1 ng/ml stromal cell-derived factor-1, but not LFA-1-independent chemotaxis induced by 100 ng/ml stromal cell-derived factor-1. This suggested that LFA-1 engagement triggers a signal that amplifies a weak chemokine signal and that dominant-negative ZAP-70 blocks this LFA-1 signal. Here we show that cross-linking of part of the LFA-1 molecules with Abs causes activation of free LFA-1 molecules (not occupied by the Ab) on the same cell, which then bind to ICAM-2 on other cells. This causes cell aggregation that was also blocked by dominant-negative ZAP-70. Thus, an LFA-1 signal involving ZAP-70 activates other LFA-1 molecules, suggesting that the chemokine signal can be amplified by multiple cycles of LFA-1 activation. The chemokine and the LFA-1 signal were both blocked by a phospholipase C inhibitor and a calpain inhibitor, suggesting that one of the amplified signals is the phospholipase C-dependent activation of calpain. Finally, we show that both Src-homology 2 domains are required for inhibition of invasion, chemotaxis, and aggregation by the truncated ZAP-70, suggesting that ZAP-70 interacts with a phosphorylated immunoreceptor tyrosine-based activation motif (ITAM) sequence. Remarkably, this is not an ITAM in the TCR/CD3 complex because this is not expressed by this T cell hybridoma.

Role of alpha(v) integrins in adenovirus cell entry and gene delivery

Adenoviruses (Ad) are a significant cause of acute infections in humans; however, replication-defective forms of this virus are currently under investigation for human gene therapy. Approximately 20 to 25% of all the gene therapy trials (phases I to III) conducted over the past 10 years involve the use of Ad gene delivery for treatment inherited or acquired diseases. At present, the most promising applications involve the use of Ad vectors to irradicate certain nonmetastatic tumors and to promote angiogenesis in order to alleviate cardiovascular disease. While specific problems of using Ad vectors remain to be overcome (as is true for almost all viral and nonviral delivery methods), a distinct advantage of Ad is the extensive knowledge of its macromolecular structure, genome organization, sequence, and mode of replication. Moreover, significant information has also been acquired on the interaction of Ad particles with distinct host cell receptors, events which strongly affect virus tropism. This review provides an overview of the structure and function of Ad attachment (coxsackievirus and Ad receptor [CAR]) and internalization (alpha(v) integrins) receptors and discusses their precise role in virus infection and gene delivery. Recent structure studies of integrin-Ad complexes by cryoelectron microscopy are also highlighted. Finally, unanswered questions arising from the current state of knowledge of Ad-receptor interactions are presented in the context of improving Ad vectors for future human gene therapy applications.

Structure of adenovirus complexed with its internalization receptor, alphavbeta5 integrin

The three-dimensional structure of soluble recombinant integrin alphavbeta5 bound to human adenovirus types 2 and 12 (Ad2 and -12) has been determined at approximately 21-A resolution by cryoelectron microscopy (cryo-EM). The alphavbeta5 integrin is known to promote Ad cell entry. Cryo-EM has shown that the integrin-binding RGD (Arg-Gly-Asp) protrusion of the Ad2 penton base protein is highly mobile (P. L. Stewart, C. Y. Chiu, S. Huang, T. Muir, Y. Zhao, B. Chait, P. Mathias, and G. R. Nemerow, EMBO J. 16:1189-1198, 1997). Sequence analysis indicated that the Ad12 RGD surface loop is shorter than that of Ad2 and probably less flexible, hence more suitable for structural characterization of the Ad-integrin complex. The cryo-EM structures of the two virus-receptor complexes revealed a ring of integrin density above the penton base of each virus serotype. As expected, the integrin density in the Ad2 complex was diffuse while that in the Ad12 complex was better defined. The integrin consists of two discrete subdomains, a globular domain with an RGD-binding cleft approximately 20 A in diameter and a distal domain with extended, flexible tails. Kinetic analysis of Ad2 interactions with alphavbeta5 indicated approximately 4.2 integrin molecules bound per penton base at close to saturation. These results suggest that the precise spatial arrangement of five RGD protrusions on the penton base promotes integrin clustering and the signaling events required for virus internalization.