Structural basis of integrin-mediated signal transduction

Molecular basis for the recognition of 24-(S)-hydroxycholesterol by integrin αvβ3

A growing body of evidence suggests that oxysterols such as 25-hydroxycholesterol (25HC) are biologically active and involved in many physiological and pathological processes. Our previous study demonstrated that 25HC induces an innate immune response during viral infections by activating the integrin-focal adhesion kinase (FAK) pathway. 25HC produced the proinflammatory response by binding directly to integrins at a novel binding site (site II) and triggering the production of proinflammatory mediators such as tumor necrosis factor-α (TNF) and interleukin-6 (IL-6). 24-(S)-hydroxycholesterol (24HC), a structural isomer of 25HC, plays a critical role in cholesterol homeostasis in the human brain and is implicated in multiple inflammatory conditions, including Alzheimer's disease. However, whether 24HC can induce a proinflammatory response like 25HC in non-neuronal cells has not been studied and remains unknown. The aim of this study was to examine whether 24HC produces such an immune response using in silico and in vitro experiments. Our results indicate that despite being a structural isomer of 25HC, 24HC binds at site II in a distinct binding mode, engages in varied residue interactions, and produces significant conformational changes in the specificity-determining loop (SDL). In addition, our surface plasmon resonance (SPR) study reveals that 24HC could directly bind to integrin αvβ3, with a binding affinity three-fold lower than 25HC. Furthermore, our in vitro studies with macrophages support the involvement of FAK and NFκB signaling pathways in triggering 24HC-mediated production of TNF. Thus, we have identified 24HC as another oxysterol that binds to integrin αvβ3 and promotes a proinflammatory response via the integrin-FAK-NFκB pathway.

Integrin activation by the lipid molecule 25-hydroxycholesterol induces a proinflammatory response

Integrins are components of cell-matrix adhesions, and function as scaffolds for various signal transduction pathways. So far no lipid ligand for integrin has been reported. Here we show that a lipid, oxysterol 25-hydroxycholesterol (25HC), directly binds to α5β1 and αvβ3 integrins to activate integrin-focal adhesion kinase (FAK) signaling. Treatment of macrophages and epithelial cells with 25HC results in an increase in activated αvβ3 integrin in podosome and focal adhesion matrix adhesion sites. Moreover, activation of pattern recognition receptor on macrophages induces secretion of 25HC, triggering integrin signaling and the production of proinflammatory cytokines such as TNF and IL-6. Thus, the lipid molecule 25HC is a physiologically relevant activator of integrins and is involved in positively regulating proinflammatory responses. Our data suggest that extracellular 25HC links innate immune inflammatory response with integrin signaling.

Switch from αvβ5 to αvβ6 integrin is required for CD9-regulated keratinocyte migration and MMP-9 activation

Our previous research found that tetraspanin CD9 is downregulated in migrating epidermis during wound healing, and CD9 downregulation contributes to keratinocyte migration via matrix metalloproteinase-9 (MMP-9) activation. However, little is known about the mechanisms involved in CD9-regulated keratinocyte migration and MMP-9 activation. In this study, we revealed that the expressions of integrin subunits β5 and β6 were regulated by CD9. Furthermore, CD9 silencing triggered the switch from αvβ5 to αvβ6 integrin in HaCaT keratinocytes and CD9 overexpression reversed the switch. Importantly, integrin αvβ6 functional blocking antibody 10D5 significantly inhibited CD9 silencing-induced keratinocyte migration and MMP-9 activation, suggesting that the switch from αvβ5 to αvβ6 integrin plays a key role in CD9-regulated cell migration and MMP-9 activation in keratinocytes.

Eukaryotic translation initiation factor 4E binding protein 1 (4EBP-1) function is suppressed by Src and protein phosphatase 2A (PP2A) on extracellular matrix

Human lung fibroblasts utilize integrins to attach and proliferate on type I collagen. β1 integrin is the major integrin subunit for this attachment. Integrins coordinate cellular responses to cell-cell and cell-extracellular matrix interactions that regulate a variety of biological processes. Although β1 integrin-mediated signaling pathways in lung fibroblasts have been studied, a detailed molecular mechanism regulating translational control of gene expression by 4EBP-1 is not understood. 4EBP-1 inhibits cap-dependent translation by binding to the eIF4E translation initiation factor. We found that when lung fibroblasts attach to collagen via β1 integrin, high Src activity suppresses 4EBP-1 expression via PP2A, and the decrease of 4EBP-1 is due to protein degradation. The inhibition of Src activity dramatically increases PP2A and 4EBP-1 expression. Furthermore ectopic expression of PP2A, or PP2A silencing using PP2A siRNA confirmed that 4EBP-1 is regulated by PP2A. In addition, we found that 4EBP-1 inhibition by fibroblast attachment to collagen increases cap-dependent translation. Our study showed that when lung fibroblasts are attached to collagen matrix, the β1 integrin/Src/PP2A-mediated 4EBP-1 regulatory pathway is activated. We suggest that β1 integrin-mediated signaling pathway may be a crucial event in regulating fibroblast translational control machinery on collagen matrix.

Identification and characterization of a novel alpha-kinase with a von Willebrand factor A-like motif localized to the contractile vacuole and Golgi complex in Dictyostelium discoideum

We have identified a new protein kinase in Dictyostelium discoideum that carries the same conserved class of "alpha-kinase" catalytic domain as reported previously in myosin heavy chain kinases (MHCKs) in this amoeba but that has a completely novel domain organization. The protein contains an N-terminal von Willebrand factor A (vWFA)-like motif and is therefore named VwkA. Manipulation of VwkA expression level via high copy number plasmids (VwkA++ cells) or gene disruption (vwkA null cells) results in an array of cellular defects, including impaired growth and multinucleation in suspension culture, impaired development, and alterations in myosin II abundance and assembly. Despite sequence similarity to MHCKs, the purified protein failed to phosphorylate myosin II in vitro. Autophosphorylation activity, however, was enhanced by calcium/calmodulin, and the enzyme can be precipitated from cellular lysates with calmodulin-agarose, suggesting that VwkA may directly bind calmodulin. VwkA is cytosolic in distribution but enriched on the membranes of the contractile vacuole and Golgi-like structures in the cell. We propose that VwkA likely acts indirectly to influence myosin II abundance and assembly behavior and possibly has broader roles than previously characterized alpha kinases in this organism, which all seem to be MHCKs.

Integrins as linker proteins between osteoblasts and bone replacing materials. A critical review

The adhesion of osteoblasts to substrates is mediated through proteins that have adsorbed to the substrate, providing integrins on the cell membrane with ligands to connect to. The integrins regulate cell behavior through bi-directional signaling pathways. This critical review has the purpose to consider the research that has been performed with osteoblasts, integrins, and bone replacing materials. Until now, most research has been done to investigate the integrin expression of osteoblasts in culture during cellular adhesion. However, it remains difficult to draw general conclusions from this research. Nevertheless, it can be concluded that the used substrates and protein or peptide coatings can influence the integrin expression and cellular behavior. Additional research has to be done to fully understand all the parameters involved in integrin expression, the adhesion of cells to substrates, and the subsequent cellular behavior. For this purpose, model substrates are under development. The signaling pathway is receiving more and more attention, but for biomaterial purposes, too little consideration is paid to the translation of the in vitro results to the in vivo situation, and to practical applications.

Model of the alphaLbeta2 integrin I-domain/ICAM-1 DI interface suggests that subtle changes in loop orientation determine ligand specificity

The interaction of the alphaLbeta2 integrin with its cellular ligand the intercellular adhesion molecule-1 (ICAM-1) is critical for the tight binding interaction between most leukocytes and the vascular endothelium before transendothelial migration to the sites of inflammation. In this article we have modeled the alphaL subunit I-domain in its active form, which was computationally docked with the D1 domain of the ICAM-1 to probe potential protein-protein interactions. The experimentally observed key interaction between the carboxylate of Glu 34 in the ICAM-1 D1 domain and the metal ion-dependent adhesion site (MIDAS) in the open alphaL I-domain was consistently reproduced by our calculations. The calculations reveal the nature of the alphaLbeta2/ICAM-1 interaction and suggest an explanation for the increased ligand-binding affinity in the "open" versus the "closed" conformation of the alphaL I-domain. A mechanism for substrate selectivity among alphaL, alphaM, and alpha2 I-domains is suggested whereby the orientation of the loops within the I-domain is critical in mediating the interaction of the Glu 34 carboxylate of ICAM-1 D1 with the MIDAS.

Conformational regulation of integrin structure and function

Integrins are a structurally elaborate family of heterodimers that mediate divalent cation-dependent cell adhesion in a wide range of biological contexts. The inserted (I) domain binds ligand in the subset of integrins in which it is present. Its structure has been determined in two alternative conformations, termed open and closed. In striking similarity to signaling G proteins, rearrangement of a Mg(2+)-binding site is linked to large conformational movements in distant backbone regions. Mutations have been used to stabilize either the closed or open structures. These show that the snapshots of the open conformation seen only in the presence of a ligand or a ligand mimetic represent a high-affinity, ligand-binding conformation, whereas those of the closed conformation correspond to a low-affinity conformation. The C-terminal alpha-helix moves 10 A down the side of the domain in the open conformation. Locking in the conformation of the preceding loop is sufficient to increase affinity for ligand 9000-fold. This C-terminal "bell-rope" provides a mechanism for linkage to conformational movements in other domains. The transition from the closed to open conformation has been implicated in fast (<1 s) regulation of integrin affinity in response to activation signals from inside the cell. Recent integrin structures and functional studies reveal interactions between beta-propeller, I, and I-like domains in the headpiece, and a critical role for integrin EGF domains in the stalk region. These studies suggest that the headpiece of the integrin faces down toward the membrane in the inactive conformation and extends upward in a "switchblade"-like opening motion upon activation. These long-range structural rearrangements of the entire integrin molecule involving multiple interdomain contacts appear closely linked to conformational changes in the I domain, which result in increased affinity and competence for ligand binding.

Differential regulation of Rho GTPases by β1 and β3 integrins: the role of an extracellular domain of integrin in intracellular signaling

Integrins mediate cell adhesion and signal transduction at focal adhesions. Here we investigate the roles of integrin β subunits in the regulation of actin cytoskeletal structure and the activities of Rho and Rac. The overexpression of β3 integrin in Chinese hamster ovary cells enhances Rho activity and stress fiber formation, whereas the overexpression of β1 integrin increases Rac activity and lamellipodia formation. The overexpression of a mutant β1-3-1 integrin, in which the extracellular I-domain-like sequence of β1 integrin has been replaced with the corresponding sequence of β3 integrin, also enhances Rho activity and the formation of stress fibers. Our results demonstrate that β1 and β3 integrins differentially regulate the activities of Rho family GTPases and that the extracellular domains of integrin β subunits play a critical role in transducing the extracellular ligand-binding information into specific intracellular signaling events.

Generation of a minimal alpha5beta1 integrin-Fc fragment

The tertiary structure of the integrin heterodimer is currently unknown, although several predictive models have been generated. Detailed structural studies of integrins have been consistently hampered for several reasons, including the small amounts of purified protein available, the large size and conformational flexibility of integrins, and the presence of transmembrane domains and N-linked glycosylation sites in both receptor subunits. As a first step toward obtaining crystals of an integrin receptor, we have expressed a minimized dimer. By using the Fc dimerization and mammalian cell expression system designed and optimized by Stephens et al. (Stephens, P. E., Ortlepp, S., Perkins, V. C., Robinson, M. K., and Kirby, H. (2000) Cell. Adhes. Commun. 7, 377-390), a series of recombinant soluble human alpha(5)beta(1) integrin truncations have been expressed as Fc fusion proteins. These proteins were examined for their ligand-binding properties and for their expression of anti-integrin antibody epitopes. The shortest functional alpha(5)-subunit truncation contained the N-terminal 613 residues, whereas the shortest beta(1)-subunit was a fragment containing residues 121-455. Each of these minimally truncated integrins displayed the antibody binding characteristics of alpha(5)beta(1) purified from human placenta and bound ligand with the same apparent affinity as the native receptor.

Urokinase-type plasminogen activator receptor (CD87) is a ligand for integrins and mediates cell-cell interaction

Binding of urokinase-type plasminogen activator (uPA) to its receptor (uPAR/CD87) regulates cellular adhesion, migration, and tumor cell invasion. However, it is unclear how glycosyl phosphatidylinositol-anchored uPAR, which lacks a transmembrane structure, mediates signal transduction. It has been proposed that uPAR forms cis-interactions with integrins as an associated protein and thereby transduces proliferative or migratory signals to cells upon binding of uPA. We provide evidence that soluble uPAR (suPAR) specifically binds to integrins alpha4beta1, alpha6beta1, alpha9beta1, and alphavbeta3 on Chinese hamster ovary cells in a cation-dependent manner. Anti-integrin and anti-uPAR antibodies effectively block binding of suPAR to these integrins. Binding of suPAR to alpha4beta1 and alphavbeta3 is blocked by known soluble ligands and by the integrin mutations that inhibit ligand binding. These results suggest that uPAR is an integrin ligand rather than, or in addition to, an integrin-associated protein. In addition, we demonstrate that glycosyl phosphatidylinositol-anchored uPAR on the cell surface specifically binds to integrins on the apposing cells, suggesting that uPAR-integrin interaction may mediate cell-cell interaction (trans-interaction). These previously unrecognized uPAR-integrin interactions may allow uPAR to transduce signals through the engaged integrin without a hypothetical transmembrane adapter and may provide a potential therapeutic target for control of inflammation and cancer.

Can we produce a human corneal equivalent by tissue engineering?

Tissue engineering is progressing rapidly. Bioengineered substitutes are already available for experimental applications and some clinical purposes such as skin replacement. This review focuses on the development of reconstructed human cornea in vitro by tissue engineering. Key elements to consider in the corneal reconstruction, such as the source for epithelial cells and keratocytes, are discussed and the various steps of production are presented. Since one application of this human model is to obtain a better understanding of corneal wound healing, the mechanisms of this phenomenon as well as the function played both by membrane-bound integrins and components from the extracellular matrix have also been addressed. The analysis of integrins by immunohistofluorescence labelling of our reconstructed human cornea revealed that beta(1), alpha(3), alpha(5), and alpha(6) integrin subunits were expressed but alpha(4) was not. Laminin, type VII collagen and fibronectin were also detected. Finally, the future challenges of corneal reconstruction by tissue engineering are discussed and the tremendous applications of such tissue produced in vitro for experimental as well as clinical purposes are considered.

Molecular basis of ligand recognition by integrin alpha 5beta 1. I. Specificity of ligand binding is determined by amino acid sequences in the second and third NH2-terminal repeats of the alpha subunit

The NH(2)-terminal portion (putative ligand-binding domain) of alpha subunits contains 7 homologous repeats, the last 3 or 4 of which possess divalent cation binding sequences. These repeats are predicted to form a seven-bladed beta-propeller structure. To map ligand recognition sites on the alpha(5) subunit we have taken the approach of constructing and expressing alpha(V)/alpha(5) chimeras. Although the NH(2)-terminal repeats of alpha(5) and alpha(V) are >50% identical at the amino acid level, alpha(5)beta(1) and alpha(V)beta(1) show marked differences in their ligand binding specificities. Thus: (i) although both integrins recognize the Arg-Gly-Asp (RGD) sequence in fibronectin, the interaction of alpha(5)beta(1) but not of alpha(V)beta(1) with fibronectin is strongly dependent on the "synergy" sequence Pro-His-Ser-Arg-Asn; (ii) alpha(5)beta(1) binds preferentially to RGD peptides in which RGD is followed by Gly-Trp (GW) whereas alpha(V)beta(1) has a broader specificity; (iii) only alpha(5)beta(1) recognizes peptides containing the sequence Arg-Arg-Glu-Thr-Ala-Trp-Ala (RRETAWA). Therefore, amino acid residues involved in ligand recognition by alpha(5)beta(1) can potentially be identified in gain-of-function experiments by their ability to switch the ligand binding properties of alpha(V)beta(1) to those of alpha(5)beta(1). By introducing appropriate restriction enzyme sites, or using site-directed mutagenesis, parts of the NH(2)-terminal repeats of alpha(V) were replaced with the corresponding regions of the alpha(5) subunit. Chimeric subunits were expressed on the surface of Chinese hamster ovary-B2 cells (which lack endogenous alpha(5)) as heterodimers with hamster beta(1). Stable cell lines were generated and tested for their ability to attach to alpha(5)beta(1)-selective ligands. Our results demonstrate that: (a) the first three NH(2)-terminal repeats contain the amino acid sequences that determine ligand binding specificity and the same repeats include the epitopes of function blocking anti-alpha subunit mAbs; (b) the divalent cation-binding sites (in repeats 4-7) do not confer alpha(5)beta(1)- or alpha(V)beta(1)-specific ligand recognition; (c) amino acid residues Ala(107)-Tyr(226) of alpha(5) (corresponding approximately to repeats 2 and 3) are sufficient to change all the ligand binding properties of alpha(V)beta(1) to those of alpha(5)beta(1); (d) swapping a small part of a predicted loop region of alpha(V) with the corresponding region of alpha(5) (Asp(154)-Ala(159)) is sufficient to confer selectivity for RGDGW and the ability to recognize RRETAWA.

Molecular basis of ligand recognition by integrin alpha5beta 1. II. Specificity of arg-gly-Asp binding is determined by Trp157 OF THE alpha subunit

Different beta(1) integrins bind Arg-Gly-Asp (RGD) peptides with differing specificities, suggesting a role for residues in the alpha subunit in determining ligand specificity. Integrin alpha(5)beta(1) has been shown to bind with high affinity to peptides containing an Arg-Gly-Asp-Gly-Trp (RGDGW) sequence but with relatively low affinity to other RGD peptides. The residues within the ligand-binding pocket that determine this specificity are currently unknown. A cyclic peptide containing the RGDGW sequence was found to strongly perturb the binding of the anti-alpha(5) monoclonal antibody (mAb) 16 to alpha(5)beta(1). In contrast, RGD peptides lacking the tryptophan residue acted as weak inhibitors of mAb 16 binding. The epitope of mAb 16 has previously been localized to a region of the alpha(5) subunit that contains Ser(156)-Trp(157). Mutation of Trp(157) (but not of Ser(156) or surrounding residues) to alanine blocked recognition of mAb 16 and perturbed the high affinity binding of RGDGW-containing peptides to alpha(5)beta(1). The same mutation also abrogated recognition of the alpha(5)beta(1)-specific ligand peptide Arg-Arg-Glu-Thr-Ala-Trp-Ala (RRETAWA). Based on these findings, we propose that Trp(157) of alpha(5) participates in a hydrophobic interaction with the tryptophan residue in RGDGW, and that this interaction determines the specificity of alpha(5)beta(1) for RGDGW-containing peptides. Since the RGD sequence is recognized predominantly by amino acid residues on the beta(1) subunit, our results suggest that Trp(157) of alpha(5) must lie very close to these residues. Our findings therefore provide new insights into the structure of the ligand-binding pocket of alpha(5)beta(1).

Effects of increased ambient pressure on colon cancer cell adhesion

Forces such as strain modulate intestinal epithelial biology. Shear and pressure influence other cells. The effects of pressure on human colon cancer cells are poorly understood. Increasing ambient pressure for 30 min by 15 mm Hg over atmospheric stimulated adhesion to matrix proteins of four human colon cancer cell lines and primary cells from three human colon cancers, but not bovine aortic smooth-muscle cells. This effect was energy dependent and cation dependent (blocked by azide and chelation), accompanied by tyrosine phosphorylation of intracellular proteins including focal adhesion kinase, and blocked by tyrosine kinase inhibition (genistein, tyrphostin, and erbstatin) and a functional antibody to the beta1 integrin subunit. Although pressure stimulated adhesion even in a balanced salt solution, baseline and pressure-stimulated adhesion were each substantially diminished in the absence of serum. These data suggest that relatively low levels of increased pressure may stimulate malignant colonocyte adhesion by a cation-dependent beta1-integrin-mediated mechanism, perhaps via focal adhesion kinase-related tyrosine phosphorylation. In addition to elucidating another aspect of physical force regulation of colonocyte biology, these findings may be relevant to the effects of increased pressure engendered by colonic peristalsis, surgical manipulation, or laparoscopic surgery on colon cancer cell adhesion.

Keratinocyte growth factor (KGF) promotes keratinocyte cell attachment and migration on collagen and fibronectin

Keratinocyte growth factor (KGF) induction of keratinocyte attachment and migration on provisional and basement membrane proteins was examined. KGF-treated keratinocytes showed increased attachment to collagen types I and IV and fibronectin, but, not to laminin-1, vitronectin, or tenascin. This increase was time- and dose-dependent. Increase in attachment occurred with 2 10 microg/ml of ECM proteins. This KGF-stimulated cell attachment was beta1 integrin-dependent but was not associated with stimulation of the cell surface expression nor affinity (activity) of the collagen integrin receptor (alpha2beta1) nor the fibronectin integrin receptors (alpha5beta1 or alphav). At the basal layer of KGF-treated cells significant accumulation of beta1 integrins was found at the leading edges, and actin stress fibers colocalized with beta1. KGF also induced migratory phenotype and stimulated keratinocyte migration on both fibronectin and collagen types I and IV but not on laminin-1, vitronectin nor tenascin. The results suggest that in addition to its proliferation promoting activity. KGF is able to modulate keratinocyte adhesion and migration on collagen and fibronectin. Our data suggest that KGF induced integrin avidity (clustering), a signaling event, which is not dependent on the alteration of cell surface integrin numbers.

Alpha 3 beta 1 adhesion to laminin-5 and invasin: critical and differential role of integrin residues clustered at the boundary between alpha 3 N-terminal repeats 2 and 3

Integrin/ligand interaction is a therapeutic target for many diseases. We previously reported that residues critical for ligand binding are clustered in N-terminal repeat 3 (in the predicted 2-3 loop) of alpha 4, alpha 5 and alpha IIb. Here we have localized residues critical for ligand binding in the alpha 3 subunit of integrin alpha 3 beta 1 with distinct ligand specificity (laminin-5). We identified an alpha 3 epitope common to several function-blocking anti-alpha 3 antibodies at the boundary between repeats 1 and 2 (residues 75-80). We found that swapping the predicted 4-1 loop (residues 153-165) at the boundary between repeats 2 and 3 with the corresponding alpha 4 sequence and mutating Thr-162 and Gly-163 residues in this predicted loop block laminin-5 binding. Thr-162 and Gly-163 and the antibody epitope are separated in the primary structure; however, they are close to each other in the proposed beta-propeller model. Mutating residues recently reported to block (Tyr-186 and Trp-188) or enhance (Asp-122) laminin-5 binding to alpha 3 beta 1 [Krukonis, E. S., Dersch, P., Eble, J. A., and Isberg, R. R.(1998) J. Biol. Chem. 273, 31837-31843] did not affect laminin-5 binding under the assay conditions used. Thr-162 and Gly-163 are not critical for adhesion to invasin, indicating that laminin-5 and invasin may use different recognition mechanisms, and that mutation of Thr-162 and Gly-163 does not drastically affect the integrity of alpha 3 beta 1. These results suggest that residues critical for ligand binding may be similarly (but not identically) located in repeat 3 of the alpha subunit regardless of ligand specificity.

Structural basis for LFA-1 inhibition upon lovastatin binding to the CD11a I-domain

The lymphocyte function-associated antigen (LFA-1) belongs to the family of beta2-integrins and plays an important role in T-cell activation and leukocyte migration to sites of inflammation. We report here that lovastatin, a drug clinically used for lowering cholesterol levels, inhibits the interaction of human LFA-1 with its counter-receptor intercellular adhesion molecule-1. Using nuclear magnetic resonance spectroscopy and X-ray crystallography we show that the inhibitor binds to a highly conserved domain of the LFA-1 alpha-chain called the I-domain. The first three-dimensional structure of an integrin inhibitor bound to its receptor reveals atomic details for a hitherto unknown mode of LFA-1 inhibition. It also sheds light into possible mechanisms of LFA-1 mediated signalling and will support the design of novel anti-adhesive and immunosuppressive drugs.

cDNA cloning and chromosomal localization of human alpha(11) integrin. A collagen-binding, I domain-containing, beta(1)-associated integrin alpha-chain present in muscle tissues

We previously identified a novel integrin alpha-chain in human fetal muscle cells (Gullberg, D., Velling, T., Sjöberg, G., and Sejersen, T. (1995) Dev. Dyn. 204, 57-65). We have now isolated the full-length cDNA for this integrin subunit, alpha(11). The open reading frame of the cDNA encodes a precursor of 1188 amino acids. The predicted mature protein of 1166 amino acids contains seven conserved FG-GAP repeats, an I domain with a metal ion-dependent adhesion site motif, a short transmembrane region, and a unique cytoplasmic domain of 24 amino acids containing the sequence GFFRS. alpha(11), like other I domain integrins, lacks a dibasic cleavage site for generation of a heavy chain and a light chain, and it contains three potential divalent cation binding sites in repeats 5-7. The presence of 22 inserted amino acids in the extracellular stalk portion (amino acids 804-826) distinguishes the alpha(11) integrin sequence from other integrin alpha-chains. Amino acid sequence comparisons reveal the highest identity of 42% with the alpha(10) integrin chain. Immunoprecipitation with antibodies to alpha(11) integrin captures a 145-kDa protein distinctly larger than the 140-kDa alpha(2) integrin chain when analyzed by SDS-polyacrylamide gel electrophoresis under nonreducing conditions. Fluorescence in situ hybridization maps the integrin alpha(11) gene to chromosome 15q23, in the vicinity of an identified locus for Bardet-Biedl syndrome. Based on Northern blotting, integrin alpha(11) mRNA levels are high in the adult human uterus and in the heart and intermediate in skeletal muscle and some other tissues tested. During in vitro myogenic differentiation, alpha(11) mRNA and protein are up-regulated. Studies of ligand binding properties show that alpha(11)beta(1) binds collagen type I-Sepharose, and cultured muscle cells localize alpha(11)beta(1) into focal contacts on collagen type I. Future studies will reveal the importance of alpha(11)beta(1) for muscle development and integrity in adult muscle and other tissues.

"...those left behind." Biology and oncology of invasive glioma cells

Although significant technical advances in surgical and radiation treatment for brain tumors have emerged in recent years, their impact on clinical outcome for patients has been disappointing. A fundamental source of the management challenge presented by glioma patients is the insidious propensity of the malignant cells to invade into adjacent normal brain. Invasive tumor cells escape surgical removal and geographically dodge lethal radiation exposure. Recent improved understanding of the biochemistry and molecular determinants of glioma cell invasion provide valuable insight to the underlying biological features of the disease, as well as illuminating possible new therapeutic targets. Heightened commitment to migrate and invade is accompanied by a glioma cell's reduced proliferative activity. The microenvironmental manipulations coincident to invasion and migration may also impact the glioma cell's response to cytotoxic treatments. These collateral aspects of the glioma cell invasive phenotype should be further explored and exploited as novel antiglioma therapies.

Identification of amino acid residues that form part of the ligand-binding pocket of integrin alpha5 beta1

Arg-Arg-Glu-Thr-Ala-Trp-Ala (RRETAWA) is a novel ligand peptide for integrin alpha5 beta1, which blocks alpha5 beta1-mediated cell adhesion to fibronectin (Koivunen, E., Wang, B., and Ruoslahti, E. (1994) J. Cell Biol. 124, 373-380). Here we have localized the binding site for RRETAWA on alpha5 beta1 using inhibitory monoclonal antibodies (mAbs) and site-directed mutagenesis. A cyclic peptide containing this sequence (*CRRETAWAC*) had little effect on the binding of most anti-alpha5 and anti-beta1 mAbs to alpha5 beta1 but completely blocked binding of the anti-alpha5 mAb 16 in a directly competitive manner. Hence, the binding site of RRETAWA appears to closely overlap with the epitope of mAb 16. *CRRETAWAC* also acted as a direct competitive inhibitor of the binding of Arg-Gly-Asp (RGD)-containing fibronectin fragments to alpha5 beta1, suggesting that the binding site for RRETAWA is also closely overlapping with that for RGD. However, differences between the binding sites of RRETAWA and RGD were apparent in that (i) RGD peptides allosterically inhibited the binding of mAb 16 to alpha5 beta1, and (ii) several mAbs that perturbed binding of alpha5 beta1 to RGD had little effect on binding of alpha5 beta1 to RRETAWA. A double mutation in alpha5 (S156G/W157S) blocked the interaction of both RRETAWA and mAb 16 with alpha5 beta1 but had no effect on fibronectin binding or on the binding of other anti-alpha5 mAbs. Ser156-Trp157 is located near the apex of a putative loop region on the upper surface of a predicted beta-propeller structure formed by the NH2-terminal repeats of alpha5. Our findings suggest that this sequence forms part of the ligand-binding pocket of alpha5 beta1. Furthermore, as Ser156-Trp157 is unique to the alpha5 subunit, it may be responsible for the specific recognition of RRETAWA by alpha5 beta1.