Isolation and characterization of Chinese hamster ovary cell variants deficient in the expression of fibronectin receptor

Exploring Integrin α5β1 as a Potential Therapeutic Target for Pulmonary Arterial Hypertension: Insights from Comprehensive Multicenter Preclinical Studies

Pulmonary arterial hypertension (PAH) is characterized by obliterative vascular remodeling of the small pulmonary arteries (PA) and progressive increase in pulmonary vascular resistance (PVR) leading to right ventricular (RV) failure. Although several drugs are approved for the treatment of PAH, mortality remains high. Accumulating evidence supports a pathological function of integrins in vessel remodeling, which are gaining renewed interest as drug targets. However, their role in PAH remains largely unexplored. We found that the arginine-glycine-aspartate (RGD)-binding integrin α5β1 is upregulated in PA endothelial cells (PAEC) and PA smooth muscle cells (PASMC) from PAH patients and remodeled PAs from animal models. Blockade of the integrin α5β1 or depletion of the α5 subunit resulted in mitotic defects and inhibition of the pro-proliferative and apoptosis-resistant phenotype of PAH cells. Using a novel small molecule integrin inhibitor and neutralizing antibodies, we demonstrated that α5β1 integrin blockade attenuates pulmonary vascular remodeling and improves hemodynamics and RV function in multiple preclinical models. Our results provide converging evidence to consider α5β1 integrin inhibition as a promising therapy for pulmonary hypertension.

One sentence summary

The α5β1 integrin plays a crucial role in pulmonary vascular remodeling.

Nano-clusters of ligand-activated integrins organize immobile, signalling active, nano-clusters of phosphorylated FAK required for mechanosignaling in focal adhesions

Transmembrane signalling receptors, such as integrins, organise as nanoclusters that are thought to provide several advantages including, increasing avidity, sensitivity (increasing the signal-to-noise ratio) and robustness (signalling above a threshold rather than activation by a single receptor) of the signal compared to signalling by single receptors. Compared to large micron-sized clusters, nanoclusters offer the advantage of rapid turnover for the disassembly of the signal. However, if nanoclusters function as signalling hubs remains poorly understood. Here, we employ fluorescence nanoscopy combined with photoactivation and photobleaching at sub-diffraction limited resolution of ~100nm length scale within a focal adhesion to examine the dynamics of diverse focal adhesion proteins. We show that (i) subregions of focal adhesions are enriched in immobile population of integrin β3 organised as nanoclusters, which (ii) in turn serve to organise nanoclusters of associated key adhesome proteins- vinculin, focal adhesion kinase (FAK) and paxillin, demonstrating that signalling proceeds by formation of nanoclusters rather than through individual proteins. (iii) Distinct focal adhesion protein nanoclusters exhibit distinct dynamics dependent on function. (iv) long-lived nanoclusters function as signalling hubs- wherein phosphorylated FAK and paxillin formed stable nanoclusters in close proximity to immobile integrin nanoclusters which are disassembled in response to inactivation signal by phosphatase PTPN12 (v) signalling takes place in response to an external signal such as force or geometric arrangement of the nanoclusters and when the signal is removed, these nanoclusters disassemble. Taken together, these results demonstrate that signalling downstream of transmembrane receptors is organised as hubs of signalling proteins (FAK, paxillin, vinculin) seeded by nanoclusters of the transmembrane receptor (integrin).

Intrinsic self-organization of integrin nanoclusters within focal adhesions is required for cellular mechanotransduction

Upon interaction with the extracellular matrix, the integrin receptors form nanoclusters as a first biochemical response to ligand binding. Here, we uncover a critical biodesign principle where these nanoclusters are spatially self-organized, facilitating effective mechanotransduction. Mouse Embryonic Fibroblasts (MEFs) with integrin β3 nanoclusters organized themselves with an intercluster distance of ∼550 nm on uniformly coated fibronectin substrates, leading to larger focal adhesions. We determined that this spatial organization was driven by cell-intrinsic factors since there was no pre-existing pattern on the substrates. Altering this spatial organization using cyclo-RGD functionalized Titanium nanodiscs (of 100 nm, corroborating to the integrin nanocluster size) spaced at intervals of 300 nm (almost half), 600 nm (normal) or 1000 nm (almost double) resulted in abrogation in mechanotransduction, indicating that a new parameter i.e., an optimal intercluster distance is necessary for downstream function. Overexpression of α-actinin, which induces a kink in the integrin tail, disrupted the establishment of the optimal intercluster distance, while simultaneous co-overexpression of talin head with α-actinin rescued it, indicating a concentration-dependent competition, and that cytoplasmic activation of integrin by talin head is required for the optimal intercluster organization. Additionally, talin head-mediated recruitment of FHOD1 that facilitates local actin polymerization at nanoclusters, and actomyosin contractility were also crucial for establishing the optimal intercluster distance and a robust mechanotransduction response. These findings demonstrate that cell-intrinsic machinery plays a vital role in organizing integrin receptor nanoclusters within focal adhesions, encoding essential information for downstream mechanotransduction signalling.

CC17 group B Streptococcus exploits integrins for neonatal meningitis development

Group B Streptococcus (GBS) is the major cause of human neonatal infections. A single clone, designated CC17-GBS, accounts for more than 80% of meningitis cases, the most severe form of the infection. However, the events allowing blood-borne GBS to penetrate the brain remain largely elusive. In this study, we identified the host transmembrane receptors α5β1 and αvβ3 integrins as the ligands of Srr2, a major CC17-GBS-specific adhesin. Two motifs located in the binding region of Srr2 were responsible for the interaction between CC17-GBS and these integrins. We demonstrated in a blood-brain-barrier cellular model that both integrins contributed to the adhesion and internalization of CC17-GBS. Strikingly, both integrins were overexpressed during the postnatal period in the brain vessels of the blood-brain barrier and blood-cerebrospinal fluid barrier and contributed to juvenile susceptibility to CC17 meningitis. Finally, blocking these integrins decreased the ability of CC17-GBS to cross into the CNS of juvenile mice in an in vivo model of meningitis. Our study demonstrated that CC17-GBS exploits integrins in order to cross the brain vessels, leading to meningitis. Importantly, it provides host molecular insights into neonate's susceptibility to CC17-GBS meningitis, thereby opening new perspectives for therapeutic and prevention strategies of GBS-elicited meningitis.

Integrin nanoclusters can bridge thin matrix fibres to form cell-matrix adhesions

Integrin-mediated cell-matrix adhesions are key to sensing the geometry and rigidity of extracellular environments and influence vital cellular processes. In vivo, the extracellular matrix is composed of fibrous arrays. To understand the fibre geometries that are required for adhesion formation, we patterned nanolines of various line widths and arrangements in single, crossing or paired arrays with the integrin-binding peptide Arg-Gly-Asp. Single thin lines (width ≤30 nm) did not support cell spreading or formation of focal adhesions, despite the presence of a high density of Arg-Gly-Asp, but wide lines (>40 nm) did. Using super-resolution microscopy, we observed stable, dense integrin clusters formed on parallel (within 110 nm) or crossing thin lines (mimicking a matrix mesh) similar to those on continuous substrates. These dense clusters bridged the line pairs by recruiting activated but unliganded integrins, as verified by integrin mutants unable to bind ligands that coclustered with ligand-bound integrins when present in an active extended conformation. Thus, in a fibrous extracellular matrix mesh, stable integrin nanoclusters bridge between thin (≤30 nm) matrix fibres and bring about downstream consequences of cell motility and growth.

RNA Aptamers Targeting Integrin α5β1 as Probes for Cyto- and Histofluorescence in Glioblastoma

Nucleic acid aptamers are often referred to as chemical antibodies. Because they possess several advantages, like their smaller size, temperature stability, ease of chemical modification, lack of immunogenicity and toxicity, and lower cost of production, aptamers are promising tools for clinical applications. Aptamers against cell surface protein biomarkers are of particular interest for cancer diagnosis and targeted therapy. In this study, we identified and characterized RNA aptamers targeting cells expressing integrin α5β1. This αβ heterodimeric cell surface receptor is implicated in tumor angiogenesis and solid tumor aggressiveness. In glioblastoma, integrin α5β1 expression is associated with an aggressive phenotype and a decrease in patient survival. We used a complex and original hybrid SELEX (selective evolution of ligands by exponential enrichment) strategy combining protein-SELEX cycles on the recombinant α5β1 protein, surrounded by cell-SELEX cycles using two different cell lines. We identified aptamer H02, able to differentiate, in cyto- and histofluorescence assays, glioblastoma cell lines, and tissues from patient-derived tumor xenografts according to their α5 expression levels. Aptamer H02 is therefore an interesting tool for glioblastoma tumor characterization.

Expression of integrins to control migration direction of electrotaxis

Proper control of cell migration is critically important in many biologic processes, such as wound healing, immune surveillance, and development. Much progress has been made in the initiation of cell migration; however, little is known about termination and sometimes directional reversal. During active cell migration, as in wound healing, development, and immune surveillance, the integrin expression profile undergoes drastic changes. Here, we uncovered the extensive regulatory and even opposing roles of integrins in directional cell migration in electric fields (EFs), a potentially important endogenous guidance mechanism. We established cell lines that stably express specific integrins and determined their responses to applied EFs with a high throughput screen. Expression of specific integrins drove cells to migrate to the cathode or to the anode or to lose migration direction. Cells expressing αMβ2, β1, α2, αIIbβ3, and α5 migrated to the cathode, whereas cells expressing β3, α6, and α9 migrated to the anode. Cells expressing α4, αV, and α6β4 lost directional electrotaxis. Manipulation of α9 molecules, one of the molecular directional switches, suggested that the intracellular domain is critical for the directional reversal. These data revealed an unreported role for integrins in controlling stop, go, and reversal activity of directional migration of mammalian cells in EFs, which might ensure that cells reach their final destination with well-controlled speed and direction.-Zhu, K., Takada, Y., Nakajima, K., Sun, Y., Jiang, J., Zhang, Y., Zeng, Q., Takada, Y., Zhao, M. Expression of integrins to control migration direction of electrotaxis.

Coupling integrin dynamics to cellular adhesion behaviors

Visualizing fluorescent proteins is essential for understanding cellular function. While advances in microscopy can now resolve individual molecules, determining whether the labeled molecules report native behaviors and how the measured behaviors can be coupled to cellular outputs remains challenging. Here, we used integrin alpha-beta heterodimers - which connect extracellular matrix (ECM) and the cytoskeleton - to quantify the mobility and conformation of labeled integrins. We found that while unlabeled and labeled integrins all localized to adhesions and support anchorage-dependent cell function, integrin mobility decreased when the beta rather than the alpha subunit was labeled. In contrast to unlabeled and alpha labeled subunits, beta labeled subunits changed cellular behavior; decreasing protrusive activity and increasing adhesion size and the extent of cell spreading. Labeling the beta subunit changed the integrin conformation, extending the molecule and exposing an epitope that is revealed by activation with Mn²⁺ treatment. Our findings indicate labeling induced changes in dynamic integrin behavior alter molecular conformation as well as cellular adhesion-dependent function to demonstrate a coupling between molecular inputs and distinct cellular outputs.This article has an associated First Person interview with the first author of the paper.

The membrane-distal regions of integrin α cytoplasmic domains contribute differently to integrin inside-out activation

Functioning as signal receivers and transmitters, the integrin α/β cytoplasmic tails (CT) are pivotal in integrin activation and signaling. 18 α integrin subunits share a conserved membrane-proximal region but have a highly diverse membrane-distal (MD) region at their CTs. Recent studies demonstrated that the presence of α CTMD region is essential for talin-induced integrin inside-out activation. However, it remains unknown whether the non-conserved α CTMD regions differently regulate the inside-out activation of integrin. Using αIIbβ3, αLβ2, and α5β1 as model integrins and by replacing their α CTMD regions with those of α subunits that pair with β3, β2, and β1 subunits, we analyzed the function of CTMD regions of 17 α subunits in talin-mediated integrin activation. We found that the α CTMD regions play two roles on integrin, which are activation-supportive and activation-regulatory. The regulatory but not the supportive function depends on the sequence identity of α CTMD region. A membrane-proximal tyrosine residue present in the CTMD regions of a subset of α integrins was identified to negatively regulate integrin inside-out activation. Our study provides a useful resource for investigating the function of α integrin CTMD regions.

Direct binding to integrins and loss of disulfide linkage in interleukin-1β (IL-1β) are involved in the agonistic action of IL-1β

There is a strong link between integrins and interleukin-1β (IL-1β), but the specifics of the role of integrins in IL-1β signaling are unclear. We describe that IL-1β specifically bound to integrins αvβ3 and α5β1. The E128K mutation in the IL1R-binding site enhanced integrin binding. We studied whether direct integrin binding is involved in IL-1β signaling. We compared sequences of IL-1β and IL-1 receptor antagonist (IL1RN), which is an IL-1β homologue but has no agonistic activity. Several surface-exposed Lys residues are present in IL-1β, but not in IL1RN. A disulfide linkage is present in IL1RN, but is not in IL-1β because of natural C117F mutation. Substitution of the Lys residues to Glu markedly reduced integrin binding of E128K IL-1β, suggesting that the Lys residues mediate integrin binding. The Lys mutations reduced, but did not completely abrogate, agonistic action of IL-1β. We studied whether the disulfide linkage plays a role in agonistic action of IL-1β. Reintroduction of the disulfide linkage by the F117C mutation did not affect agonistic activity of WT IL-1β, but effectively reduced the remaining agonistic activity of the Lys mutants. Also, deletion of the disulfide linkage in IL1RN by the C116F mutation did not make it agonistic. We propose that the direct binding to IL-1β to integrins is primarily important for agonistic IL-1β signaling, and that the disulfide linkage indirectly affects signaling by blocking conformational changes induced by weak integrin binding to the Lys mutants. The integrin-IL-1β interaction is a potential target for drug discovery.

Generation and characterization of a novel candidate gene therapy and vaccination vector based on human species D adenovirus type 56

The vectorization of rare human adenovirus (HAdV) types will widen our knowledge of this family and their interaction with cells, tissues and organs. In this study we focus on HAdV-56, a member of human Ad species D, and create ease-of-use cloning systems to generate recombinant HAdV-56 vectors carrying foreign genes. We present in vitro transduction profiles for HAdV-56 in direct comparison to the most commonly used HAdV-5-based vector. In vivo characterizations demonstrate that when it is delivered intravenously (i.v.) HAdV-56 mainly targets the spleen and, to a lesser extent, the lungs, whilst largely bypassing liver transduction in mice. HAdV-56 triggered robust inflammatory and cellular immune responses, with higher induction of IFNγ, TNFα, IL5, IL6, IP10, MCP1 and MIG1 compared to HAdV-5 following i.v. administration. We also investigated its potential as a vaccine vector candidate by performing prime immunizations in mice with HAdV-56 encoding luciferase (HAdV-56-Luc). Direct comparisons were made to HAdV-26, a highly potent human vaccine vector currently in phase II clinical trials. HAdV-56-Luc induced luciferase 'antigen'-specific IFNγ-producing cells and anti-HAdV-56 neutralizing antibodies in Balb/c mice, demonstrating a near identical profile to that of HAdV-26. Taken together, the data presented provides further insight into human Ad receptor/co-receptor usage, and the first report on HAdV-56 vectors and their potential for gene therapy and vaccine applications.

Vimentin filament controls integrin α5β1-mediated cell adhesion by binding to integrin through its Ser38 residue

Regulation of integrin affinity for its ligand is essential for cell adhesion and migration. Here, we found that direct interaction of vimentin with integrin β1 can enhance binding of integrin α5β1 to its ligand, fibronectin. Conversely, blocking the interaction reduced fibronectin binding, cell migration on a fibronectin-coated surface, and neural tube closure during Xenopus embryogenesis. We also found that withaferin A (WFA), a natural compound known to inhibit vimentin function, can suppress the vimentin-integrin interaction and abolish fibronectin binding. Finally, we identified Ser38 of vimentin as a critical residue for integrin binding. Our results suggest that phosphorylation of vimentin at Ser38 may regulate the integrin-ligand interaction, thus providing a molecular basis for antivimentin therapeutic strategies.

■ REVIEW : Gene Expression in Nerve Regeneration

Injury of peripheral nerve in mammals leads to a complex but stereotypical pattern of histological events that comprise a highly reproducible sequence of degenerative reactions (Wallerian degeneration) succeeded by regenerative responses. These reactions are based on a corresponding sequence of cellular and mo lecular interactions that, in turn, reflect the differential expression of specific genes with functions in nerve degeneration and repair. We report on more than 60 genes and their products that show a specific pattern of regulation following peripheral nerve lesion. The group of regulated genes encoding, e.g., transcription factors, growth factors and their receptors, cytokines, neuropeptides, myelin proteins and lipid carriers, and cytoskeletal proteins as well as extracellular matrix and cell adhesion molecules. We describe and compare the distinct time-courses and cellular origin of expression and further discuss established or putative mo lecular interrelationships and functions with respect to the contribution of these genes/gene products to the molecular regeneration program of the PNS. NEUROSCIENTIST 3:112-122, 1997

Confinement Sensing and Signal Optimization via Piezo1/PKA and Myosin II Pathways

Cells adopt distinct signaling pathways to optimize cell locomotion in different physical microenvironments. However, the underlying mechanism that enables cells to sense and respond to physical confinement is unknown. Using microfabricated devices and substrate-printing methods along with FRET-based biosensors, we report that, as cells transition from unconfined to confined spaces, intracellular Ca²⁺ level is increased, leading to phosphodiesterase 1 (PDE1)-dependent suppression of PKA activity. This Ca²⁺ elevation requires Piezo1, a stretch-activated cation channel. Moreover, differential regulation of PKA and cell stiffness in unconfined versus confined cells is abrogated by dual, but not individual, inhibition of Piezo1 and myosin II, indicating that these proteins can independently mediate confinement sensing. Signals activated by Piezo1 and myosin II in response to confinement both feed into a signaling circuit that optimizes cell motility. This study provides a mechanism by which confinement-induced signaling enables cells to sense and adapt to different physical microenvironments.

Integrin α5 Suppresses the Phosphorylation of Epidermal Growth Factor Receptor and Its Cellular Signaling of Cell Proliferation via N-Glycosylation

Integrin α5β1-mediated cell adhesion regulates a multitude of cellular responses, including cell proliferation, survival, and cross-talk between different cellular signaling pathways. Integrin α5β1 is known to convey permissive signals enabling anchorage-dependent receptor tyrosine kinase signaling. However, the effects of integrin α5β1 on cell proliferation are controversial, and the molecular mechanisms involved in the regulation between integrin α5β1 and receptor tyrosine kinase remain largely unclear. Here we show that integrin α5 functions as a negative regulator of epidermal growth factor receptor (EGFR) signaling through its N-glycosylation. Expression of WT integrin α5 suppresses the EGFR phosphorylation and internalization upon EGF stimulation. However, expression of the N-glycosylation mutant integrin α5, S3-5, which contains fewer N-glycans, reversed the suppression of the EGFR-mediated signaling and cell proliferation. In a mechanistic manner, WT but not S3-5 integrin α5 forms a complex with EGFR and glycolipids in the low density lipid rafts, and the complex formation is disrupted upon EGF stimulation, suggesting that the N-glycosylation of integrin α5 suppresses the EGFR activation through promotion of the integrin α5-glycolipids-EGFR complex formation. Furthermore, consistent restoration of those N-glycans on the Calf-1,2 domain of integrin α5 reinstated the inhibitory effects as well as the complex formation with EGFR. Taken together, these data are the first to demonstrate that EGFR activation can be regulated by the N-glycosylation of integrin α5, which is a novel molecular paradigm for the cross-talk between integrins and growth factor receptors.

MAP4K4 regulates integrin-FERM binding to control endothelial cell motility

Cell migration is a stepwise process that coordinates multiple molecular machineries. Using in vitro angiogenesis screens with short interfering RNA and chemical inhibitors, we define here a MAP4K4-moesin-talin-β1-integrin molecular pathway that promotes efficient plasma membrane retraction during endothelial cell migration. Loss of MAP4K4 decreased membrane dynamics, slowed endothelial cell migration, and impaired angiogenesis in vitro and in vivo. In migrating endothelial cells, MAP4K4 phosphorylates moesin in retracting membranes at sites of focal adhesion disassembly. Epistasis analyses indicated that moesin functions downstream of MAP4K4 to inactivate integrin by competing with talin for binding to β1-integrin intracellular domain. Consequently, loss of moesin (encoded by the MSN gene) or MAP4K4 reduced adhesion disassembly rate in endothelial cells. Additionally, α5β1-integrin blockade reversed the membrane retraction defects associated with loss of Map4k4 in vitro and in vivo. Our study uncovers a novel aspect of endothelial cell migration. Finally, loss of MAP4K4 function suppressed pathological angiogenesis in disease models, identifying MAP4K4 as a potential therapeutic target.

Cloning and characterization of chicken α5 integrin: endogenous and experimental expression in early chicken embryos

Key roles for fibronectin and its integrin receptors have been postulated in the multiple cell-matrix interactions essential for chick embryo morphogenesis. However, mechanistic studies of these processes have been hampered by the current absence of sequence data and chicken cDNA clones for the major fibronectin receptor subunit, integrin α5 (ITGA5). We report here the sequence, endogenous expression pattern, and transfection of full-length chicken integrin α5. During early chicken embryonic development, α5 is highly expressed in cranial neural folds and migrating neural crest cells, suggesting potential roles in neural crest formation and migration. In fact, over-expression of this integrin in early neural tube selectively induces BMP5, a growth factor recently implicated in neural crest formation. Availability of these α5 integrin tools should facilitate studies of its functions in early embryonic development.

Directional cell migration in an extracellular pH gradient: a model study with an engineered cell line and primary microvascular endothelial cells

Extracellular pH (pH(e)) gradients are characteristic of tumor and wound environments. Cell migration in these environments is critical to tumor progression and wound healing. While it has been shown previously that cell migration can be modulated in conditions of spatially invariant acidic pH(e) due to acid-induced activation of cell surface integrin receptors, the effects of pH(e) gradients on cell migration remain unknown. Here, we investigate cell migration in an extracellular pH(e) gradient, using both model α(v)β(3) CHO-B2 cells and primary microvascular endothelial cells. For both cell types, we find that the mean cell position shifts toward the acidic end of the gradient over time, and that cells preferentially polarize toward the acidic end of the gradient during migration. We further demonstrate that cell membrane protrusion stability and actin-integrin adhesion complex formation are increased in acidic pH(e), which could contribute to the preferential polarization toward acidic pH(e) that we observed for cells in pH(e) gradients. These results provide the first demonstration of preferential cell migration toward acid in a pH(e) gradient, with intriguing implications for directed cell migration in the tumor and wound healing environments.

Identification of immunoreactive regions of homology between soluble epidermal growth factor receptor and α5-integrin

Proteins encoded by the epidermal growth factor receptor (EGFR/HER1/ERBB1) gene are being studied as diagnostic, prognostic, and theragnostic biomarkers for numerous human cancers. The clinical application of these tissue/tumor biomarkers has been limited, in part, by discordant results observed for epidermal growth factor receptor (EGFR) expression using different immunological reagents. Previous studies have used EGFR-directed antibodies that cannot distinguish between full-length and soluble EGFR (sEGFR) expression. We have generated and characterized an anti-sEGFR polyclonal antiserum directed against a 31-mer peptide (residues 604-634) located within the unique 78-amino acid carboxy-terminal sequence of sEGFR. Here, we use this antibody to demonstrate that sEGFR is coexpressed with EGFR in a number of carcinoma-derived cell lines. In addition, we show that a second protein of ~140 kDa (p140) also is detected by this antibody. Rigorous biochemical characterization identifies this second protein to be α5-integrin. We show that a 26-amino acid peptide in the calf domain of α5-integrin (residues 710-735) is 35% identical in sequence with a 31-mer carboxy-terminal sEGFR peptide and exhibits an approximately 5-fold lower affinity for anti-sEGFR than the homologous 31-mer sEGFR peptide does. We conclude that the carboxy terminus of sEGFR and the calf-1 domain of α5-integrin share a region of sequence identity, which results in their mutual immunological reactivity with anti-sEGFR. We also demonstrate that anti-sEGFR promotes three-dimensional tissue cohesion and compaction in vitro, further suggesting a functional link between sEGFR and α5-integrin and a role of the calf-1 domain in cell adhesion. These results have implications for the study of both EGFR and sEGFR as cancer biomarkers and also provide new insight into the mechanisms of interaction between cell surface EGFR isoforms and integrins in complex processes such as cell adhesion and survival signaling.

Two cDNAs coding for the porcine CD51 (αv) integrin subunit: cloning, expression analysis, adhesion assays and chromosomal localization

CD51 (α(v)) is an integrin chain that associates with multiple β integrin chains to form different receptor complexes that mediate important human processes. Pigs show substantial physiological, immunological and anatomical similarities to humans, and are therefore a good model system to study immunological and pathological processes. Here we report the cloning and characterization of two cDNAs produced by alternative splicing that encode two different porcine CD51 proteins that differ in five amino acid residues. Pig CD51 cDNAs encode polypeptides of 1046 or 1041 amino acid residues, respectively, that share with other mammalian homologous proteins a high percentage amino acid identity and the functional domains. Expression analysis of CD51 was carried out at two different levels. RT-PCR analysis revealed that both CD51 transcripts were expressed ubiquitously but heterogeneously, with the exception of some platelets in which only the smallest CD51 transcript was detected. A specific monoclonal antibody against a pig CD51 recombinant protein was made and used in the immunohistochemical localization of CD51 proteins. It showed that CD51 was mainly expressed in hematopoietic cells of myeloid linage, epithelial and endothelial cells, osteoclasts, nervous fibers and smooth muscle. Adhesion assays showed that in the presence of Mn(++) pig α(v)-CHO-B2 transfected cells increased their attachment to fibronectin and vitonectin, but not to fibrinogen. Finally, we localized the CD51 gene on the porcine chromosome 15 (SSC15), q23-q26.

Tumor Cell - Substrate Stabilization Mediated by Integrins

Metastasis formation is dependent on the arrest and stabilization of adhesive interactions to prevent detachment from secondary sites. Primary receptor-ligand interactions are not sufficient to maintain prolonged adhesive contacts without secondary events that lead to stabilization. Tumor cell arrest and stabilization were studied under physiologically relevant shear conditions. We used a parallel-plate flow chamber with surfaces coated with human plasma fibronectin or vitronectin. Our previous work suggested that stabilization of cells to immobilized proteins is in part attributed to transglutaminase covalently cross-linking cytoskeletal-integrin-fibronectin multiprotein complexes via lysine-glutamine linkages. To study the role of integrins in mediating arrest and initiating stabilization we used a human melanoma line (70w) and polyclonal antibodies that inhibit the function of the fibronectin (α5β1) and vitronectin (αvβ3/β5) integrin receptors. To confirm the role of integrins in initiating stabilization we used CHO (Chinese hamster ovary) cells selected for low levels of α5β1integrin expression and integrin transfected CHO cells selected for α5βloverexpression. The level of fibronectin receptor surface expression was inversely related to the adhesion stabilization lag time. These studies confirmed that integrins are essential for mediating arrest and initiating stabilization. They also confirm that secondary events are necessary for complete stabilization to occur. Finally, it is important to note that the arrest and stabilization methods we have developed are capable of detecting biologic effects at far greater sensitivity than static adhesion assays. Some examples of pharmacologic agents or biomaterials effects that can be detected using stabilization assays include: 1) very low drug doses, 2) very low levels of peptide, carbohydrate, and antibody inhibitors, 3) slight modification of endogenous protein expression by antisense oligonucleotides or transfected genetic expression constructs.

Acidic extracellular pH promotes activation of integrin α(v)β(3)

Acidic extracellular pH is characteristic of the cell microenvironment in several important physiological and pathological contexts. Although it is well established that acidic extracellular pH can have profound effects on processes such as cell adhesion and migration, the underlying molecular mechanisms are largely unknown. Integrin receptors physically connect cells to the extracellular matrix, and are thus likely to modulate cell responses to extracellular conditions. Here, we examine the role of acidic extracellular pH in regulating activation of integrin α_vβ₃. Through computational molecular dynamics simulations, we find that acidic extracellular pH promotes opening of the α_vβ₃ headpiece, indicating that acidic pH can thereby facilitate integrin activation. This prediction is consistent with our flow cytometry and atomic force microscope-mediated force spectroscopy assays of integrin α_vβ₃ on live cells, which both demonstrate that acidic pH promotes activation at the intact cell surface. Finally, quantification of cell morphology and migration measurements shows that acidic extracellular pH affects cell behavior in a manner that is consistent with increased integrin activation. Taken together, these computational and experimental results suggest a new and complementary mechanism of integrin activation regulation, with associated implications for cell adhesion and migration in regions of altered pH that are relevant to wound healing and cancer.

Binding of extracellular maspin to beta1 integrins inhibits vascular smooth muscle cell migration

Maspin is a serpin that has multiple effects on cell behavior, including inhibition of migration. How maspin mediates these diverse effects remains unclear, as it is devoid of protease inhibitory activity. We have previously shown that maspin rapidly inhibits the migration of vascular smooth muscle cells (VSMC), suggesting the involvement of direct interactions with cell surface proteins. Here, using immunofluorescence microscopy, we demonstrate that maspin binds specifically to the surface of VSMC in the dedifferentiated, but not the differentiated, phenotype. Ligand blotting of VSMC lysates revealed the presence of several maspin-binding proteins, with a protein of 150 kDa differentially expressed between the two VSMC phenotypes. Western blotting suggested that this protein was the beta1 integrin subunit, and subsequently both alpha3beta1 and alpha5beta1, but not alphavbeta3, were shown to associate with maspin by coimmunoprecipitation. Specific binding of these integrins was also observed using maspin-affinity chromatography, using HT1080 cell lysates. Direct binding of maspin to alpha5beta1 was confirmed using a recombinant alpha5beta1-Fc fusion protein. Using conformation-dependent anti-beta1 antibodies, maspin binding to VSMC was found to lead to a decrease in the activation status of the integrin. The functional involvement of alpha5beta1 in mediating the effect of maspin was established by the inhibition of migration of CHO cells overexpressing human alpha5 integrin, but not those lacking alpha5 expression. Our observations suggest that maspin engages in specific interactions with a limited number of integrins on VSMC, leading to their inactivation, and that these interactions are responsible for the effects of maspin in the pericellular environment.

Accessibility to the Fibronectin Synergy Site in a 3D Matrix Regulates Engagement of α 5β 1 versus α vβ 3 Integrin Receptors

Cell adhesion and migration on fibronectin (FN) extracellular matrix are mediated by integrin receptors. Integrins alpha5beta1 and alphavbeta3 require the RGD cell-binding sequence in FN, but alpha5beta1 also requires the nearby synergy site for maximal binding. In this study, we investigated how differences in the numbers of RGD or synergy sites within a three-dimensional (3D) FN-rich matrix influence cell adhesion and migration. CHO cell adhesion, spreading, and migration were reduced on 3D chimeric matrix containing FN lacking RGD (FN(RGD-)). Incorporation of FN with mutation of the synergy site (FN(syn-)), however, resulted in selective usage of integrins. CHO cells expressing alpha5beta1 showed decreased interactions with FN(syn-) chimeric matrix. In contrast, the presence of FN(syn-) had no effect on CHOalphavbeta3 cell migration. Interestingly, CHOalpha5/alphavbeta3 cells expressing both integrins selectively used alpha5beta1 for migration on wild type FN matrix but preferred alphavbeta3 for migration on FN(syn-) chimeric matrix. Thus sequestration or exposure of the FN synergy site within a 3D matrix may represent a novel mechanism for regulating cell functions through differential usage of integrin receptors. [Supplementary materials are available for this article. Go to the publisher's online edition of Cell Communication and Adhesion for the following free supplemental resource: a video recording shows migration of HT1080 cells on 3D matrix. HT1080 cells were allowed to attach to the matrix in serum-free DMEM for 2 h. FBS was then added to the medium to a final concentration of 10% and video recording was started. Images were taken every 5 min for 2 h. The video plays at 6 frames/s.].

Molecular interplay between endostatin, integrins, and heparan sulfate

Endostatin is an endogenous inhibitor of angiogenesis. Although several endothelial cell surface molecules have been reported to interact with endostatin, its molecular mechanism of action is not fully elucidated. We used surface plasmon resonance assays to characterize interactions between endostatin, integrins, and heparin/heparan sulfate. alpha5beta1 and alphavbeta3 integrins form stable complexes with immobilized endostatin (KD=approximately 1.8x10(-8) M, two-state model). Two arginine residues (Arg27 and Arg139) are crucial for the binding of endostatin to integrins and to heparin/heparan sulfate, suggesting that endostatin would not bind simultaneously to integrins and to heparan sulfate. Experimental data and molecular modeling support endostatin binding to the headpiece of the alphavbeta3 integrin at the interface between the beta-propeller domain of the alphav subunit and the betaA domain of the beta3 subunit. In addition, we report that alpha5beta1 and alphavbeta3 integrins bind to heparin/heparan sulfate. The ectodomain of the alpha5beta1 integrin binds to haparin with high affinity (KD=15.5 nM). The direct binding between integrins and heparin/heparan sulfate might explain why both heparan sulfate and alpha5beta1 integrin are required for the localization of endostatin in endothelial cell lipid rafts.

Alpha5beta1-integrin controls ebolavirus entry by regulating endosomal cathepsins

Integrins are involved in the binding and internalization of both enveloped and nonenveloped viruses. By using 3 distinct cell systems-CHO cells lacking expression of alpha(5)beta(1)-integrin, HeLa cells treated with siRNA to alpha(5)-integrin, and mouse beta(1)-integrin knockout fibroblasts, we show that alpha(5)beta(1)-integrin is required for efficient infection by pseudovirions bearing the ebolavirus glycoprotein (GP). These integrins are necessary for viral entry but not for binding or internalization. Given the need for endosomal cathepsins B and L (CatB and CatL) to prime GPs for fusion, we investigated the status of CatB and CatL in integrin-positive and integrin-negative cell lines. Alpha(5)beta(1)-Integrin-deficient cells lacked the double-chain (DC) forms of CatB and CatL, and this correlated with decreased CatL activity in integrin-negative CHO cells. These data indicate that alpha(5)beta(1)-integrin-negative cells may be refractory to infection by GP pseudovirions because they lack the necessary priming machinery (the double-chain forms of CatB and CatL). In support of this model, we show that GP pseudovirions that have been preprimed in vitro to generate the 19-kDa form of GP overcome the requirement for alpha(5)beta(1)-integrin for infection. These results provide further support for the requirement for endosomal cathepsins for ebolavirus infection, identify the DC forms of these cathepsins as previously unrecognized factors that contribute to cell tropism of this virus, and reveal a previously undescribed role for integrins during viral entry as regulators of endosomal cathepsins, which are required to prime the entry proteins of ebolavirus and other pathogenic viruses.

An N-glycosylation site on the beta-propeller domain of the integrin alpha5 subunit plays key roles in both its function and site-specific modification by beta1,4-N-acetylglucosaminyltransferase III

Recently we reported that N-glycans on the beta-propeller domain of the integrin alpha5 subunit (S-3,4,5) are essential for alpha5beta1 heterodimerization, expression, and cell adhesion. Herein to further investigate which N-glycosylation site is the most important for the biological function and regulation, we characterized the S-3,4,5 mutants in detail. We found that site-4 is a key site that can be specifically modified by N-acetylglucosaminyltransferase III (GnT-III). The introduction of bisecting GlcNAc into the S-3,4,5 mutant catalyzed by GnT-III decreased cell adhesion and migration on fibronectin, whereas overexpression of N-acetylglucosaminyltransferase V (GnT-V) promoted cell migration. The phenomenon is similar to previous observations that the functions of the wild-type alpha5 subunit were positively and negatively regulated by GnT-V and GnT-III, respectively, suggesting that the alpha5 subunit could be duplicated by the S-3,4,5 mutant. Interestingly GnT-III specifically modified the S-4,5 mutant but not the S-3,5 mutant. This result was confirmed by erythroagglutinating phytohemagglutinin lectin blot analysis. The reduction in cell adhesion was consistently observed in the S-4,5 mutant but not in the S-3,5 mutant cells. Furthermore mutation of site-4 alone resulted in a substantial decrease in erythroagglutinating phytohemagglutinin lectin staining and suppression of cell spread induced by GnT-III compared with that of either the site-3 single mutant or wild-type alpha5. These results, taken together, strongly suggest that N-glycosylation of site-4 on the alpha5 subunit is the most important site for its biological functions. To our knowledge, this is the first demonstration that site-specific modification of N-glycans by a glycosyltransferase results in functional regulation.

Human decay-accelerating factor and CEACAM receptor-mediated internalization and intracellular lifestyle of Afa/Dr diffusely adhering Escherichia coli in epithelial cells

We used transfected epithelial CHO-B2 cells as a model to identify the mechanism mediating internalization of Afa/Dr diffusely adhering Escherichia coli. We provide evidence that neither the alpha5 or beta1 integrin subunits nor alpha5beta1 integrin functioned as a receptor mediating the adhesion and/or internalization of Dr or Afa-III fimbria-positive bacteria. We also demonstrated that (i) whether or not the AfaD or DraD invasin subunits were present, there was no difference in the cell association and entry of bacteria and that (ii) DraE or AfaE-III adhesin subunits are necessary and sufficient to promote the receptor-mediated bacterial internalization into epithelial cells expressing human decay-accelerating factor (DAF), CEACAM1, CEA, or CEACAM6. Internalization of Dr fimbria-positive E. coli within CHO-DAF, CHO-CEACAM1, CHO-CEA, or CHO-CEACAM6 cells occurs through a microfilament-independent, microtubule-dependent, and lipid raft-dependent mechanism. Wild-type Dr fimbria-positive bacteria survived better within cells expressing DAF than bacteria internalized within CHO-CEACAM1, CHO-CEA, or CHO-CEACAM6 cells. In DAF-positive cells, internalized Dr fimbria-positive bacteria were located in vacuoles that contained more than one bacterium, displaying some of the features of late endosomes, including the presence of Lamp-1 and Lamp-2, and some of the features of CD63 proteins, but not of cathepsin D, and were acidic. No interaction between Dr fimbria-positive-bacterium-containing vacuoles and the autophagic pathway was observed.

The N-terminal domain of Nogo-A inhibits cell adhesion and axonal outgrowth by an integrin-specific mechanism

Myelin-derived Nogo-A protein limits axonal growth after CNS injury. One domain binds to the Nogo-66 receptor to inhibit axonal outgrowth, whereas a second domain, Amino-Nogo, inhibits axonal outgrowth and cell adhesion through unknown mechanisms. Here, we show that Amino-Nogo inhibition depends strictly on the composition of the extracellular matrix, suggesting that Amino-Nogo inhibits the function of certain integrins. Amino-Nogo inhibition can be partially overcome by antibodies that activate integrin beta1 or by the addition of Mn2+, an integrin activator. Furthermore, Amino-Nogo reduces focal adhesion kinase activation by fibronectin. Analysis of various cell lines reveals that alpha(v)beta3, alpha5, and alpha4 integrins are sensitive to Amino-Nogo, but alpha6 integrin is not. Both alpha(v) and alpha5 integrins have widespread expression in adult brain and are found in axonal growth cones. Thus, inhibition of integrin signaling by Amino-Nogo contributes to the failure of CNS axon regeneration.

Tenascin-W is a novel marker for activated tumor stroma in low-grade human breast cancer and influences cell behavior

This is the first report about human tenascin-W, the fourth and final member of the extracellular matrix protein family of tenascins. Sixty-three human breast tumor extracts were analyzed by Western blotting for the presence of tenascin-W and compared with tenascin-C, an established marker of tumor stroma. Interestingly, we found tenascin-W expression in the majority of the tumor tissues, but no detectable expression in the normal mammary parenchyma. Eighty-one percent of the breast tumor samples were tenascin-W positive and 86% showed expression of tenascin-C. However, tenascin-W and tenascin-C amounts varied greatly between tumors and some contained either tenascin-W or tenascin-C exclusively, indicating independent mechanisms regulating their expression. Although there was no difference between high- or low-grade tumors with respect to the presence of tenascin-C, tenascin-W was more prominent in low-grade tumors. For 42 of the breast cancer tissues, a frozen tumor microarray was available to confirm the Western blot data by immunohistochemistry. Similar to tenascin-C, tenascin-W was detected in the tumor stroma. Fibroblasts adhered to tenascin-W in a beta(1) integrin-dependent manner and spread with a distinctive morphology under conditions where they remained round on tenascin-C. CHOB2 cells expressing alpha(v)beta(1) or alpha4beta(1) integrins were able to spread on tenascin-W. Furthermore, addition of tenascin-W to the culture medium increased migration of breast cancer cells toward a fibronectin substratum in vitro. These data imply that tenascin-W expression in the activated tumor stroma facilitates tumorigenesis by supporting the migratory behavior of breast cancer cells.

N-Glycosylation of the β-Propeller Domain of the Integrin α5 Subunit Is Essential for α5β1 Heterodimerization, Expression on the Cell Surface, and Its Biological Function

The N-glycosylation of integrin alpha5beta1 is thought to play crucial roles in cell spreading, cell migration, ligand binding, and dimer formation, but the underlying mechanism remains unclear. To investigate the importance of the N-glycans of this integrin in detail, sequential site-directed mutagenesis was carried out to remove single or combined putative N-glycosylation sites on the alpha5 integrin. Removal of the putative N-glycosylation sites on the beta-propeller, Thigh, Calf-1, or Calf-2 domains of the alpha5 subunit resulted in a decrease in molecular weight compared with the wild type, suggesting that all of these domains contain attached N-glycans. Importantly, the absence of N-glycosylation sites (sites 1-5) on the beta-propeller resulted in the persistent association of integrin subunit with calnexin in the endoplasmic reticulum, which subsequently blocked heterodimerization and its expression on the cell surface. Interestingly, the activities for cell spreading and migration for the alpha5 subunit carrying only three potential N-glycosylation sites (3-5 sites) on the beta-propeller were comparable with those of the wild type. In contrast, mutation of these three sites resulted in a significant decrease in cell spreading as well as functional expression, although the total expression level of the Delta3-5 mutant on the cell surface was comparable with that of wild type. Furthermore, we found that site 5 is a most important site for its expression on the cell surface, whereas the S5 mutant did not show any biological functions. Taken together, this study reveals for the first time that the N-glycosylation on the beta-propeller domain of the alpha5 subunit is essential for heterodimerization and biological functions of alpha5beta1 integrin and might also be useful for studies of the molecular structure.

Adeno-associated virus type 2 contains an integrin alpha5beta1 binding domain essential for viral cell entry

Integrins have been implicated as coreceptors in the infectious pathways of several nonenveloped viruses. For example, adenoviruses are known to interact with alphaV integrins by virtue of a high-affinity arginine-glycine-aspartate (RGD) domain present in the penton bases of the capsids. In the case of adeno-associated virus type 2 (AAV2), which lacks this RGD motif, integrin alphaVbeta5 has been identified as a coreceptor for cellular entry. However, the molecular determinants of AAV2 capsid-integrin interactions and the potential exploitation of alternative integrins as coreceptors by AAV2 have not been established thus far. In this report, we demonstrate that integrin alpha5beta1 serves as an alternative coreceptor for AAV2 infection in human embryonic kidney 293 cells. Such interactions appear to be mediated by a highly conserved domain that contains an asparagine-glycine-arginine (NGR) motif known to bind alpha5beta1 integrin with moderate affinity. The mutation of this domain reduces transduction efficiency by an order of magnitude relative to that of wild-type AAV2 vectors in vitro and in vivo. Further characterization of mutant and wild-type AAV2 capsids through transduction assays in cell lines lacking specific integrins, cell adhesion studies, and cell surface/solid-phase binding assays confirmed the role of the NGR domain in promoting AAV2-integrin interactions. Molecular modeling studies suggest that NGR residues form a surface loop close to the threefold axis of symmetry adjacent to residues previously implicated in binding heparan sulfate, the primary receptor for AAV2. The aforementioned results suggest that the internalization of AAV2 in 293 cells might follow a "click-to-fit" mechanism that involves the cooperative binding of heparan sulfate and alpha5beta1 integrin by the AAV2 capsids.

Ligand density and integrin repertoire regulate cellular response to LPA

Engagement of integrin receptors by the extracellular matrix (ECM) protein fibronectin (FN) activates intracellular signaling, cytoskeletal reorganization and cellular tension. The soluble factor lysophosphatidic acid (LPA) acts through Rho GTPase and its effector Rho kinase (ROCK) to enhance alpha5beta1 integrin-mediated cell spreading on the Arg-Gly-Asp (RGD) cell-binding domain of FN. A second cell-binding site for alpha4 integrins resides in the CS1 segment of the alternatively spliced V region of FN. We show here that LPA treatment of alpha4beta1-expressing CHOalpha4 cells on FN induced a significant decrease in spread cell area. LPA also decreased apoptosis induced by serum-deprivation in CHOalpha4 and human A375 melanoma cells in an alpha4beta1-dependent manner. Improvement in cell viability and changes in cell morphology were dependent on ROCK and on the number of substrate binding sites for alpha4beta1. LPA signaling combined with alpha4beta1-mediated adhesion appears to sustain cell viability in situations where FN matrix is limiting. Such cooperation may impact dynamic cellular events such as wound healing, fibrosis, and metastasis.

alpha5beta1 integrin stimulates Bcl-2 expression and cell survival through Akt, focal adhesion kinase, and Ca2+/calmodulin-dependent protein kinase IV

CHO cells expressing alpha5beta1 integrin are more resistant to apoptosis and express more Bcl-2 than the same cells engineered to express alphavbeta1 or cytoplasmically truncated alpha5Deltacbeta1 integrin as their main fibronectin receptor. The Bcl-2 up-regulation by alpha5beta1 is mediated, at least in part, by the focal adhesion kinase (FAK) and phosphatidylinositol-3 kinase (PI3K)/Akt pathways. Here, we show that integrin-mediated activation of Ca2+/calmodulin-dependent protein kinase (CaMK) IV, and the NF-kappaB and CREB transcription factors also enhance the integrin-dependent regulation of Bcl-2 expression in the alpha5beta1cells. A forkhead transcription factor, which is inactivated by Akt, blocked Bcl-2 expression. The FAK pathway was found to be defective in both the alphavbeta1 and alpha5Deltacbeta1 cells. These cell lines differed from one another in two Bcl-2-regulating pathways: adhesion through alphavbeta1 failed to activate Akt, allowing forkhead to suppress Bcl-2 transcription, whereas alpha5Deltacbeta1 did not activate NF-kappaB and CREB, presumably because CaMK IV was not activated. Our results indicate that three pathways, the FAK, PI3K/Akt, and CaMK IV mediate the survival-supporting activity of alpha5beta1 integrin.

Rotaviruses interact with alpha4beta7 and alpha4beta1 integrins by binding the same integrin domains as natural ligands

Group A rotaviruses are major intestinal pathogens that express potential alpha4beta1 and alpha4beta7 integrin ligand sequences Leu-Asp-Val and Leu-Asp-Ile in their outer capsid protein VP7, and Ile-Asp-Ala in their spike protein VP4. Monkey rotavirus SA11 can use recombinant alpha4beta1 as a cellular receptor. In this study a new potential alpha4beta1, alpha4beta7 and alpha9beta1 integrin ligand sequence, Tyr-Gly-Leu, was identified in VP4. It was shown that several human and monkey rotaviruses bound alpha4beta1 and alpha4beta7, but not alpha9beta1. Binding to alpha4beta1 mediated the infectivity and growth of monkey rotaviruses, and binding to alpha4beta7 mediated their infectivity. A porcine rotavirus interacted with alpha4 integrins at a post-binding stage to facilitate infection. Activation of alpha4beta1 increased rotavirus infectivity. Cellular treatment with peptides containing the alpha4 integrin ligand sequences Tyr-Gly-Leu and Ile-Asp-Ala eliminated virus binding to alpha4 integrins and infectivity. In contrast, rotavirus recognition of alpha4 integrins was unaffected by a peptide containing the sequence Leu-Asp-Val or by a mutation in the VP7 Leu-Asp-Val sequence. VP4 involvement in rotavirus recognition of alpha4beta1 was demonstrated with rotavirus reassortants. Swapping and point mutagenesis of alpha4 surface loops showed that rotaviruses required the same alpha4 residues and domains for binding as the natural alpha4 integrin ligands: mucosal addressin cell adhesion molecule-1, fibronectin and vascular cell adhesion molecule-1. Several rotaviruses are able to use alpha4beta7 and alpha4beta1 for cell binding or entry, through the recognition of the same alpha4-subunit domains as natural alpha4 ligands.

Stimulatory effects of a three-dimensional microenvironment on cell-mediated fibronectin fibrillogenesis

The assembly of fibronectin into a fibrillar matrix is a regulated step-wise process that involves binding to integrin receptors and interactions between fibronectin molecules. This process has been studied extensively using cells in two-dimensional (2D) monolayer culture. In most situations in vivo, however, matrix assembly occurs within existing three-dimensional (3D) extracellular matrix networks. In an attempt to mimic this environment, we analyzed matrix assembly by fibroblasts cultured on a pre-assembled 3D fibronectin matrix and found significant stimulation of fibronectin fibril assembly compared to cells in 2D culture. Lower amounts of fibronectin were needed to initiate the assembly process, fibrils accumulated to higher density, and the 3D fibril organization played a key role in the stimulatory effect. Moreover, cells expressing activation-dependent integrins were able to assemble fibronectin matrix without exogenous stimulation, suggesting regulatory effects of the 3D fibronectin matrix on integrin activity. These results provide evidence for an additional level of control of fibronectin deposition through cell interactions with the local microenvironment.

Quantitatively distinct requirements for signaling-competent cell spreading on engineered versus natural adhesion ligands

To design synthetic microenvironments that elicit desired cell behaviors, we must better understand the molecular mechanisms by which cells interact with candidate biomaterials. Using cell lines with distinct alpha5beta1 integrin expression profiles, we demonstrate that this integrin mediates cell spreading on substrata coated with genetically engineered artificial extracellular matrix (aECM) proteins containing the RGD sequence (RGD-containing aECM protein [aRGD]) but lacking the PHSRN synergy site. Furthermore, aRGD-mediated adhesion stimulates an intracellular focal adhesion kinase (FAK) signal that is indicative of integrin tethering. Although both aRGD and the natural ECM protein fibronectin (FN) support alpha5beta1 integrin-mediated cell spreading, quantitative single-cell analysis revealed that aRGD-mediated spreading requires ten-fold greater threshold amount of integrin expression than FN-mediated spreading. Our analysis demonstrates that aRGD-based substrata mediate both biophysical (cell spreading) and biochemical (FAK signaling) events via the alpha5beta1 integrin, albeit with efficacy quantitatively distinct from that of natural ECM proteins that possess the full spectrum of adhesion and synergy domains.

Cross-talk of integrin alpha3beta1 and tissue factor in cell migration

In cancer and angiogenesis, coagulation-independent roles of tissue factor (TF) in cell migration are incompletely understood. Immobilized anti-TF extracellular domain antibodies induce cell spreading, but this phenomenon is epitope specific and is not induced by anti-TF 5G9. Spreading on anti-TF is beta1 integrin-dependent, indicating functional interactions of the TF extracellular domain 5G9 epitope (a presumed integrin-binding site) and integrins. Recombinant TF extracellular domain supports adhesion of cells expressing alphavbeta3 or certain beta1 integrin heterodimers (alpha3beta1, alpha4beta1, alpha5beta1, alpha6beta1, alpha9beta1) and adhesion is blocked by specific anti-integrin antibodies or mutations in the integrin ligand-binding site. Although several studies have linked TF to cell migration, we here demonstrate that TF specifically regulates alpha3beta1-dependent migration on laminin 5. Expression of TF suppresses alpha3beta1-dependent migration, but only when the TF cytoplasmic domain is not phosphorylated. Suppression of migration can be reversed by 5G9, presumably by disrupting integrin interaction, or by the protease ligand VIIa, known to induce PAR-2-dependent phosphorylation of TF. In both cases, release of alpha3beta1 inhibition is prevented by mutation of critical phosphorylation sites in the TF cytoplasmic domain. Thus, TF influences integrin-mediated migration through cooperative intra- and extracellular interactions and phosphorylation regulates TF's function in cell motility.

A mitochondrial protein, Bit1, mediates apoptosis regulated by integrins and Groucho/TLE corepressors

A delicate balance of signals regulates cell survival. One set of these signals is derived from integrin-mediated cell adhesion to the extracellular matrix (ECM). Loss of cell attachment to the ECM causes apoptosis, a process known as anoikis. In searching for proteins involved in cell adhesion-dependent regulation of anoikis, we identified Bit1, a mitochondrial protein that is released into the cytoplasm during apoptosis. Cytoplasmic Bit1 forms a complex with AES, a small Groucho/transducin-like enhancer of split (TLE) protein, and induces cell death with characteristics of caspase-independent apoptosis. Cell attachment to fibronectin counteracts the apoptotic effect of Bit1 and AES. Increasing Bit1 expression enhances anoikis, while suppressing the expression reduces it. Thus, we have elucidated an integrin-controlled pathway that is, at least in part, responsible for the cell survival effects of cell-ECM interactions.

Fibronectin matrix assembly regulates alpha5beta1-mediated cell cohesion

Integrin-extracellular matrix (ECM) interactions in two-dimensional (2D) culture systems are widely studied (Goldstein and DiMilla, 2002. J Biomed. Mater. Res. 59, 665-675; Koo et al., 2002. J. Cell Sci. 115, 1423-1433). Less understood is the role of the ECM in promoting intercellular cohesion in three-dimensional (3D) environments. We have demonstrated that the alpha5beta1-integrin mediates strong intercellular cohesion of 3D cellular aggregates (Robinson et al., 2003. J. Cell Sci. 116, 377-386). To further investigate the mechanism of alpha5beta1-mediated cohesivity, we used a series of chimeric alpha5beta1-integrin-expressing cells cultured as multilayer cellular aggregates. In these cell lines, the alpha5 subunit cytoplasmic domain distal to the GFFKR sequence was truncated, replaced with that of the integrin alpha4, the integrin alpha2, or maintained intact. Using these cells, alpha5beta1-integrin-mediated cell aggregation, compaction and cohesion were determined and correlated with FN matrix assembly. The data presented demonstrate that cells cultured in the absence of external mechanical support can assemble a FN matrix that promotes integrin-mediated aggregate compaction and cohesion. Further, inhibition of FN matrix assembly blocks the intercellular associations required for compaction, resulting in cell dispersal. These results demonstrate that FN matrix assembly contributes significantly to tissue cohesion and represents an alternative mechanism for regulating tissue architecture.

De novo expression of the integrin alpha5beta1 regulates alphavbeta3-mediated adhesion and migration on fibrinogen

Recent evidence demonstrates that interactions between different integrins that are present on the cell surface can strongly influence the adhesive function of individual receptors. In this report, we show that Chinese hamster ovary cells that express the integrin alphavbeta3 in the absence of alpha5beta1 demonstrate increased adhesion and migration on fibrinogen. Furthermore, alphavbeta3-mediated adhesion to fibrinogen is not augmented by the soluble agonist, MnCl2, suggesting that alphavbeta3 exists in a higher affinity state in these cells. De novo expression of wild-type alpha5beta1 negatively regulates alphavbeta3-mediated adhesion and migration. This effect is not seen with expression of a chimeric alpha5beta1 integrin in which the cytoplasmic portion of the alpha5 integrin subunit is replaced by the cytoplasmic portion of the alpha4 integrin. In addition, it does not require ligation of alpha5beta1 by fibronectin. Cells that express a constitutively active beta3 integrin that contains a point mutation in the conserved membrane proximal region of the cytoplasmic tail, D723R, are resistant to the effect of alpha5beta1 expression. These data provide additional evidence of "cross-talk" between the integrins alpha5beta1 and alphavbeta3, and support the idea that alpha5beta1 regulates alphavbeta3-mediated ligand binding. This provides a relevant biological mechanism whereby variations in alpha5beta1 expression in vivo may modulate activation of alphavbeta3 to influence its adhesive function.

Expression level of integrin alpha 5 on tumour cells affects the rate of metastasis to the kidney

Tumour metastasis is known clinically to have organ specificity. We hypothesised that integrins might be involved in determining the organ specificity of tumour metastasis. Here, we report the results of spontaneous metastasis tested in nude mice that were inoculated with Chinese hamster ovary (CHO) cells expressing integrin alpha 5 beta 1 at various levels. The growth of the primary tumour inversely correlated with the alpha 5 expression level on CHO cells, which is consistent with a previous report (Schreiner et al, 1991). The rates of pulmonary, lymph node, and adrenal metastases that developed in nude mice were not related to changes of the alpha 5 expression level on CHO cells. Kidney metastasis developed in 40% of nude mice inoculated with alpha 5B2 cells (CHO cells overexpressing alpha 5) and in 20% of mice with CHO-K1 cells (CHO cells expressing native alpha 5), whereas inoculation with CHO-B2 cells (alpha 5-defective mutants) and alpha 5CHO cells with the highest expression of alpha 5 did not lead to development of kidney metastasis. Furthermore, alpha 5CHO, which shows the slowest growth of these cell types, did not lead to primary tumours in nude mice. These findings suggest that there is an appropriate level of alpha 5 expression on tumour cells that leads to metastasis. Microscopic observations revealed that micrometastasis in the kidney was formed in glomeruli. An adhesion assay using frozen sections of the kidney demonstrated that alpha 5B2 cells, but not CHO-B2 cells, effectively adhered to glomeruli. Kidney metastasis in vivo and the adhesion of alpha 5B2 to glomeruli shown ex vivo were significantly suppressed by the administration of GRGDS peptide. Finally, we conclude that the interaction of alpha 5 beta 1 on tumour cells with fibronectin in kidney glomeruli is involved in kidney metastasis and that the tumour has appropriate levels of integrins crucial for metastasis.

Identification of CD9 extracellular domains important in regulation of CHO cell adhesion to fibronectin and fibronectin pericellular matrix assembly

CD9, a 24-kDa member of the tetraspanin family, influences cellular growth and development, activation, adhesion, and motility. Our investigation focuses on the hypothesis that the CD9 second extracellular loop (EC2) is important in modulating cell adhesive events. Using a Chinese hamster ovary (CHO) cell expression system, we previously reported that CD9 expression inhibited cell adhesion to fibronectin and fibronectin matrix assembly. For the first time, a functional epitope on CD9 EC2 that regulates these processes is described. Binding of mAb7, an EC2-specific anti-CD9 monoclonal antibody, reversed the CD9 inhibitory activity on CHO cell adhesion and fibronectin matrix assembly. This reversal of cell phenotype also was observed in CHO cells expressing CD9 EC2 truncations. Furthermore, our data showed that the EC2 sequence (173)LETFTVKSCPDAIKEVFDNK(192) was largely responsible for the CD9-mediated CHO cell phenotype. Two peptides, (135)K-V(172) (peptide 5b) and (168)P-I(185) (peptide 6a), selectively blocked mAb7 binding to soluble CD9 and to CD9 on intact cells. These active peptides reversed the influence of CD9 expression on CHO cell adhesion to fibronectin. In addition, confocal microscopy revealed that CD9 colocalized with the integrin alpha(5)beta(1) and cytoskeletal F-actin in punctate clusters on the cell surface, particularly at the cell margins. Immunoprecipitation studies confirmed CD9 association with beta(1) integrin. The cellular distribution and colocalization of focal adhesion kinase and alpha-actinin with cytoskeletal actin was also influenced by CD9 expression. Thus, CD9 may exhibit its effect by modulating the composition of adhesive complexes important in facilitating cell adhesion and matrix assembly.

Chinese hamster ovary cell motility to fibronectin is modulated by the second extracellular loop of CD9. Identification of a putative fibronectin binding site

CD9, a member of the tetraspanin family of proteins, is characterized by four transmembrane domains and two extracellular loops. Surface expression of CD9 on Chinese hamster ovary (CHO) cells dramatically enhances spreading and motility on fibronectin. To elucidate the mechanistic basis of CD9-fibronectin interaction, binding to fibronectin was investigated using purified and recombinant forms of CD9. The affinity of fibronectin for CD9 in enzyme-linked immunosorbent assay was 81 +/- 25 nm. The binding of fibronectin to immobilized CD9 was enhanced by Ca(2+) ions. Protein binding and peptide competition studies demonstrated that peptide 6 derived from CD9 extracellular loop 2 (amino acids 168-192) contained part of the fibronectin-binding domain. Additionally, enhanced adhesion of CD9-CHO-B2 cells to fibronectin was significantly reduced by peptide 6. CD9-CHO cells had a 5-fold increase in motility to fibronectin as compared with mock-transfected controls, an effect that correlated with CD9 cell surface density. Truncation of CD9 extracellular loop 2 and peptide 6 caused inhibition of CD9-CHO cell motility to fibronectin. Deletion of CD9 extracellular loop 1 had no significant effect on CHO cell motility. These findings demonstrate a critical role for CD9 extracellular loop 2 in cell motility to fibronectin and clarify the mechanism by which CD9-fibronectin interaction modulates cell adhesion and motility.

alpha4beta1 integrin regulates lamellipodia protrusion via a focal complex/focal adhesion-independent mechanism

alpha4beta1 integrin plays an important role in cell migration. We show that when ectopically expressed in Chinese hamster ovary cells, alpha4beta1 is sufficient and required for promoting protrusion of broad lamellipodia in response to scratch-wounding, whereas alpha5beta1 does not have this effect. By time-lapse microscopy of cells expressing an alpha4/green fluorescent protein fusion protein, we show that alpha4beta1 forms transient puncta at the leading edge of cells that begin to protrude lamellipodia in response to scratch-wounding. The cells expressing a mutant alpha4/green fluorescent protein that binds paxillin at a reduced level had a faster response to scratch-wounding, forming alpha4-positive puncta and protruding lamellipodia much earlier. While enhancing lamellipodia protrusion, this mutation reduces random motility of the cells in Transwell assays, indicating that lamellipodia protrusion and random motility are distinct types of motile activities that are differentially regulated by interactions between alpha4beta1 and paxillin. Finally, we show that, at the leading edge, alpha4-positive puncta and paxillin-positive focal complexes/adhesions do not colocalize, but alpha4beta1 and paxillin colocalize partially in ruffles. These findings provide evidence for a specific role of alpha4beta1 in lamellipodia protrusion that is distinct from the motility-promoting functions of alpha5beta1 and other integrins that mediate cell adhesion and signaling events through focal complexes and focal adhesions.

Reduced focal adhesion kinase and paxillin phosphorylation in BCR-ABL-transfected cells

BACKGROUND

BCR-ABL formation is critical to oncogenic transformation in chronic myelogenous leukemia and has been implicated as a key event leading to alterations in cytoskeletal structures and adhesion in the leukemic cells. The authors therefore investigated the effect of p210(BCR-ABL) on actin polymerization as well as on the expression and phosphorylation state of the adhesion proteins paxillin and focal adhesion kinase (FAK).

METHODS

Transfection with BCR-ABL constructs abrogated the ability of NIH 3T3 fibroblasts to adhere and the cells underwent striking morphologic changes.

RESULTS

Scanning electron microscopy revealed that the cells lost their elongated appearance and became rounded. This alteration was associated with significantly reduced actin polymerization. In addition, steady-state levels of paxillin and FAK protein were increased. However, while the overall level of phosphotyrosines was also increased, the amount of tyrosine phosphorylated paxillin and FAK was reduced in the BCR-ABL-transfected cells as compared to the parental cells. Culture on extracellular fibronectin matrix partially reversed the morphologic changes and resulted in a return, albeit incomplete, of filamentous actin in BCR-ABL-transfected 3T3 fibroblasts. In addition, phosphorylation of paxillin and FAK in the BCR-ABL-transfected NIH 3T3 cells was restored.

CONCLUSIONS

The authors conclude that, in the current system, transfection of BCR-ABL attenuates FAK and paxillin phosphorylation and reduces actin polymerization, events accompanied by significant alterations in cellular morphology. The observation that exposure of the cells to fibronectin partially reverses all these changes suggests that the focal adhesion proteins and actin structures nevertheless remain responsive to signaling from the outside.