Talin Binding to Integrin Tails: A Final Common Step in Integrin Activation

Control of integrin affinity for ligands (integrin activation) is essential for normal cell adhesion, migration, and assembly of an extracellular matrix. Integrin activation is usually mediated through the integrin beta subunit cytoplasmic tail and can be regulated by many different biochemical signaling pathways. We report that specific binding of the cytoskeletal protein talin to integrin beta subunit cytoplasmic tails leads to the conformational rearrangements of integrin extracellular domains that increase their affinity. Thus, regulated binding of talin to integrin beta tails is a final common element of cellular signaling cascades that control integrin activation.

CD9, CD63, CD81, and CD82 are components of a surface tetraspan network connected to HLA-DR and VLA integrins

CD9, CD63, CD81, and CD82 are glycoproteins of unknown function which belong to the tetraspan superfamily. These molecules have short cytoplasmic sequences, four transmembrane domains and two unequal extracellular regions. Here, we show that these molecules are associated with each other on cell surface and with other glycoproteins such as very late antigen (VLA) integrins and HLA-DR antigens. Moreover, the VLA integrins and HLA-DR antigens were also found to be associated. The interactions of these molecules were analyzed by transfection experiments. It is demonstrated that overexpression of CD9 antigen in Raji cells leads to a lower efficiency of precipitation of CD81 and CD82, suggesting a direct interaction between these molecules. In these cells, the co-precipitation of CD81 and CD82 was not modified, suggesting that these tetraspans did not compete for association. However, in COS-7 cells, transfection of both CD81 and CD82 led to a marked reduction of the number of CD9/CD81 or CD9/CD82 complexes compared to single-transfected cells, and this was associated with the appearance of CD81/CD82 complexes. Therefore, in this cellular system, CD9 competes with CD81 and CD82 for association with the other tetraspan proteins. Finally, the tetraspans do not compete for the association with integrins or HLA-DR. Indeed, when CD9 was expressed in Raji cells, it was incorporated into the pre-existing complexes of these molecules with CD81 and CD82. These data suggest the existence of a tetraspan network which, by connecting several molecules, may organize the positioning of cell surface proteins and play a role in signal transduction, cell adhesion, and motility.

Inhibition of pp125FAK in cultured fibroblasts results in apoptosis

The tyrosine kinase called pp125FAK is believed to play an important role in integrin-mediated signal transduction. pp125FAK is associated both functionally and spatially with integrins, which are the cell surface receptors for extracellular matrix components. Although the precise function of pp125FAK is not known, two possibilities have been proposed: pp125FAK may regulate the assembly of focal adhesions in spreading or migrating cells, or pp125FAK may participate in a signal transduction cascade to inform the nucleus that the cell is anchored. To test these models in living cells, a peptide representing the focal adhesion kinase (FAK)-binding site of the beta 1 tail was coupled to carrier protein and injected into cultured cells to competitively inhibit the binding of pp125FAK to endogenous integrin, thus inhibiting activation of pp125FAK on a cell-by-cell basis. In addition, an antibody directed against an epitope adjacent to the focal adhesion targeting sequence on pp125FAK was microinjected, as an alternative means of inhibiting pp125FAK activation. It was observed that when rounded cells were injected with either the integrin peptide or the anti-FAK antibody, the cells rapidly began to apoptose, within 4 h after injection. These results indicate that pp125FAK may play a critical role in suppressing apoptosis in fibroblasts.

Activated conformations of very late activation integrins detected by a group of antibodies (HUTS) specific for a novel regulatory region (355-425) of the common beta 1 chain

The very late activation antigens (VLA) or beta 1 integrins mediate cell attachment to different extracellular matrix proteins and intercellular adhesions. The ligand binding activity of these adhesion receptors is not constitutive and can be regulated by temperature, presence of extracellular divalent cations, stimulatory monoclonal antibodies (mAbs), and cellular activation. We have generated three novel mAbs, HUTS-4, HUTS-7, and HUTS-21, recognizing specific epitopes on the common beta 1 subunit (CD29) of VLA integrins whose expression correlates with the ligand binding activity of these heterodimeric glycoproteins. This correlation has been demonstrated for several integrin heterodimers in different cell systems using a variety of extracellular and intracellular stimuli for integrin activation. Thus, the presence of micromolar concentrations of extracellular Mn2+, preincubation with the activating anti-beta 1 mAb TS2/16, and cell treatment with phorbol esters or calcium ionophores, induced the expression of the HUTS beta 1 epitopes on T lymphoblasts. Using a panel of human-mouse beta 1 chimeric molecules, we have mapped these epitopes to the 355-425 sequence of the beta 1 polypeptide. This segment represents therefore a novel regulatory region of beta 1 that is exposed upon integrin activation. Interestingly, binding of HUTS mAbs to partially activated VLA integrins results in maximal activation of these adhesion receptors and enhancement of cell adhesion to beta 1 integrin ligands collagen, laminin, and fibronectin.

Ephrin-A5 modulates cell adhesion and morphology in an integrin-dependent manner

The ephrins are membrane-tethered ligands for the Eph receptor tyrosine kinases, which play important roles in patterning of the nervous and vascular systems. It is now clear that ephrins are more than just ligands and can also act as signalling-competent receptors, participating in bidirectional signalling. We have recently shown that ephrin-A5 signals within caveola-like domains of the plasma membrane upon engagement with its cognate Eph receptor, leading to increased adhesion of the cells to fibronectin. Here we show that ephrin-A5 controls sequential biological events that are consistent with its role in neuronal guidance. Activation of ephrin-A5 induces an initial change in cell adhesion followed by changes in cell morphology. Both effects are dependent on the activation of beta1 integrin involving members of the Src family of protein tyrosine kinases. The prolonged activation of ERK-1 and ERK-2 is required for the change in cell morphology. Our work suggests a new role for class A ephrins in specifying the affinity of the cells towards various extracellular substrates by regulating integrin function.

Isolation, cloning, and sequence analysis of the integrin subunit alpha10, a beta1-associated collagen binding integrin expressed on chondrocytes

We have found that chondrocytes express a novel collagen type II-binding integrin, a new member of the beta1-integrin family. The integrin alpha subunit, which has a Mr of 160 kDa reduced, was isolated from bovine chondrocytes by collagen type II affinity purification. The human homologue was obtained by screening a human chondrocyte library with a bovine cDNA probe. Cloning and cDNA sequence analysis of the human integrin alpha subunit designated alpha10 show that it shares the general structure of other integrin alpha subunits. The predicted amino acid sequence consists of a 1167-amino acid mature protein, including a signal peptide (22 amino acids), a long extracellular domain (1098 amino acids), a transmembrane domain (25 amino acids), and a short cytoplasmic domain (22 amino acids). The extracellular part contains a 7-fold repeated sequence, an I-domain (199 amino acids) and three putative divalent cation-binding sites. The deduced amino acid sequence of alpha10 is 35% identical to the integrin subunit alpha2 and 37% identical to the integrin subunit alpha1. Northern blot analysis shows a single mRNA of 5.4 kilobases in chondrocytes. A peptide antibody against the predicted sequence of the cytoplasmic domain of alpha10 immunoprecipitated two proteins with masses of 125 and 160 kDa from chondrocyte lysates under reducing conditions. The peptide antibody specifically stained chondrocytes in tissue sections of human articular cartilage, showing that alpha10 beta1 is expressed in cartilage tissue.

ICAP-1, a novel beta1 integrin cytoplasmic domain-associated protein, binds to a conserved and functionally important NPXY sequence motif of beta1 integrin

The cytoplasmic domains of integrins are essential for cell adhesion. We report identification of a novel protein, ICAP-1 (integrin cytoplasmic domain- associated protein-1), which binds to the 1 integrin cytoplasmic domain. The interaction between ICAP-1 and beta1 integrins is highly specific, as demonstrated by the lack of interaction between ICAP-1 and the cytoplasmic domains of other beta integrins, and requires a conserved and functionally important NPXY sequence motif found in the COOH-terminal region of the beta1 integrin cytoplasmic domain. Mutational studies reveal that Asn and Tyr of the NPXY motif and a Val residue located NH2-terminal to this motif are critical for the ICAP-1 binding. Two isoforms of ICAP-1, a 200-amino acid protein (ICAP-1alpha) and a shorter 150-amino acid protein (ICAP-1beta), derived from alternatively spliced mRNA, are expressed in most cells. ICAP-1alpha is a phosphoprotein and the extent of its phosphorylation is regulated by the cell-matrix interaction. First, an enhancement of ICAP-1alpha phosphorylation is observed when cells were plated on fibronectin-coated but not on nonspecific poly-L-lysine-coated surface. Second, the expression of a constitutively activated RhoA protein that disrupts the cell-matrix interaction results in dephosphorylation of ICAP-1alpha. The regulation of ICAP-1alpha phosphorylation by the cell-matrix interaction suggests an important role of ICAP-1 during integrin-dependent cell adhesion.

Rolling adhesion through an extended conformation of integrin alphaLbeta2 and relation to alpha I and beta I-like domain interaction

In vivo, beta(2) integrins and particularly alpha(L)beta(2) (LFA-1) robustly support firm adhesion of leukocytes, but can also cooperate with other molecules in supporting rolling adhesion. Strikingly, a small molecule alpha/beta I-like allosteric antagonist, XVA143, inhibits LFA-1-dependent firm adhesion, while at the same time it enhances adhesion in shear flow and rolling both in vitro and in vivo. XVA143 appears to induce the extended conformation of integrins as shown by increased activation epitope exposure. Fab to the beta(2) I-like domain converts firm adhesion to rolling adhesion, but does not enhance adhesion. Residue alpha(L)-Glu-310 in the linker following the I domain is critical for communication to the beta(2) I-like domain, rolling, integrin extension, and activation by Mn(2+) of firm adhesion. The results demonstrate the importance of integrin extension in rolling, and suggest that rolling and firm adhesion are mediated by extended conformations of alpha(L)beta(2) that differ in the affinity of the alpha(L) I domain for ICAM-1.

Regulation of focal adhesion kinase by its amino-terminal domain through an autoinhibitory interaction

We have investigated a role for the amino-terminal FERM-like domain of the focal adhesion kinase (FAK) as a negative regulator of its own activity and phosphorylation state. Deletion of the first 375 amino acids from the amino terminus of FAK increases its catalytic activity in vitro, its phosphorylation when expressed in mammalian cells, and the phosphorylation of a FAK substrate, paxillin. Deletion mutants are phosphorylated in suspension, suggesting that they are no longer regulated by adhesion. The amino terminus of FAK can interact with the kinase domain of FAK in vitro and in vivo, suggesting that it might act as an autoinhibitor of FAK activity. The amino terminus of FAK can act in trans to inhibit FAK phosphorylation when expressed in mammalian cells or to directly inhibit FAK activity in vitro. Expression of the amino terminus of FAK inhibits cell cycle progression in CHO cells, consistent with its inhibition of FAK phosphorylation and function in trans. A glutathione S-transferase fusion protein containing the cytoplasmic tail of the beta1 integrin stimulates FAK activity in vitro, suggesting that FAK could be regulated by molecular interactions with the amino terminus. Based on these and previous data, we propose a working model for activation of FAK in cell adhesion.

The Kindler syndrome protein is regulated by transforming growth factor-beta and involved in integrin-mediated adhesion

Transforming growth factor-beta1 (TGF-beta1) contributes to tumor invasion and cancer progression by increasing the motility of tumor cells. To identify genes involved in TGF-beta-mediated cell migration, the transcriptional profiles of human mammary epithelial cells (HMEC) treated with TGF-beta were compared with untreated cells by cDNA microarray analysis. One gene up-regulated by TGF-beta was recently named kindlerin (Jobard, F., Bouadjar, B., Caux, F., Hadj-Rabia, S., Has, C., Matsuda, F., Weissenbach, J., Lathrop, M., Prud'homme, J. F., and Fischer, J. (2003) Hum. Mol. Genet. 12, 925-935). This gene is significantly overexpressed in some cancers (Weinstein, E. J., Bourner, M., Head, R., Zakeri, H., Bauer, C., and Mazzarella, R. (2003) Biochim. Biophys. Acta 1637, 207-216), and mutations in this gene lead to Kindler syndrome, an autosomal-recessive genodermatosis. TGF-beta stimulation of HMEC resulted in a marked induction of kindlerin RNA, and Western blotting demonstrated a corresponding increase in protein abundance. Kindlerin displays a putative FERM (four point one ezrin radixin moesin) domain that is closely related to the sequences in talin that interact with integrin beta subunit cytoplasmic domains. The critical residues in the talin FERM domain that mediate integrin binding show a high degree of conservation in kindlerin. Furthermore, kindlerin is recruited into a molecular complex with the beta1A and beta3 integrin cytoplasmic domains. Consistent with these biochemical findings, kindlerin is present at focal adhesions, sites of integrin-rich, membrane-substratum adhesion. Additionally, kindlerin is required for normal cell spreading. Taken together, these data suggest a role for kindlerin in mediating cell processes that depend on integrins.

Impaired integrin-mediated signal transduction, altered cytoskeletal structure and reduced motility in Hck/Fgr deficient macrophages

Integrin-mediated adhesion of monocytes and macrophages initiates a signal transduction pathway that leads to actin cytoskeletal reorganization, cell migration and immunologic activation. This signaling pathway is critically dependent on tyrosine kinases. To investigate the role of the Src-family of tyrosine kinases in integrin signal transduction, we have examined the adhesive properties of macrophages isolated from hck-/-fgr-/- double knockout mice which lack two of the three predominant Src-family kinases expressed in myeloid cells. Previous examination of polymorphonuclear leukocytes from these animals indicated that these kinases were critical in initiating the actin cytoskeletal rearrangements that lead to respiratory burst and granule secretion following integrin ligation. Double mutant peritoneal exudate macrophages demonstrated markedly reduced tyrosine phosphorylation responses compared to wild-type cells following plating on fibronectin, collagen or vitronectin-coated surfaces. Tyrosine phosphorylation of several actin-associated proteins (cortactin, paxillin, and tensin), as well as the Syk and Pyk2 tyrosine kinases, were all significantly reduced in double mutant cells. The subcellular localization of focal-adhesion associated proteins was also dramatically altered in mutant macrophages cultured on fibronectin-coated surfaces. In wild-type cells, filamentous actin, paxillin, and talin were concentrated along leading edges of the plasma membrane, suggesting that these proteins contribute to cellular polarization during migration in culture. Double mutant cells failed to show the polarized subcellular localization of these proteins. Likewise, double mutant macrophages failed to form normal filopodia under standard culture conditions. Together, these signaling and cytoskeletal defects may contribute to the reduced motility observed in in vitro assays. These data provide biochemical and morphological evidence that the Src-family kinases Hck and Fgr are required for normal integrin-mediated signal transduction in murine macrophages.

Activated R-ras, Rac1, PI 3-kinase and PKCepsilon can each restore cell spreading inhibited by isolated integrin beta1 cytoplasmic domains

Attachment of many cell types to extracellular matrix proteins triggers cell spreading, a process that strengthens cell adhesion and is a prerequisite for many adhesion-dependent processes including cell migration, survival, and proliferation. Cell spreading requires integrins with intact beta cytoplasmic domains, presumably to connect integrins with the actin cytoskeleton and to activate signaling pathways that promote cell spreading. Several signaling proteins are known to regulate cell spreading, including R-Ras, PI 3-kinase, PKCepsilon and Rac1; however, it is not known whether they do so through a mechanism involving integrin beta cytoplasmic domains. To study the mechanisms whereby cell spreading is regulated by integrin beta cytoplasmic domains, we inhibited cell spreading on collagen I or fibrinogen by expressing tac-beta1, a dominant-negative inhibitor of integrin function, and examined whether cell spreading could be restored by the coexpression of either V38R-Ras, p110alpha-CAAX, myr-PKCepsilon, or L61Rac1. Each of these activated signaling proteins was able to restore cell spreading as assayed by an increase in the area of cells expressing tac-beta1. R-Ras and Rac1 rescued cell spreading in a GTP-dependent manner, whereas PKCstraightepsilon required an intact kinase domain. Importantly, each of these signaling proteins required intact beta cytoplasmic domains on the integrins mediating adhesion in order to restore cell spreading. In addition, the rescue of cell spreading by V38R-Ras was inhibited by LY294002, suggesting that PI 3-kinase activity is required for V38R-Ras to restore cell spreading. In contrast, L61Rac1 and myr-PKCstraightepsilon each increased cell spreading independent of PI 3-kinase activity. Additionally, the dominant-negative mutant of Rac1, N17Rac1, abrogated cell spreading and inhibited the ability of p110alpha-CAAX and myr-PKCstraightepsilon to increase cell spreading. These studies suggest that R-Ras, PI 3-kinase, Rac1 and PKCepsilon require the function of integrin beta cytoplasmic domains to regulate cell spreading and that Rac1 is downstream of PI 3-kinase and PKCepsilon in a pathway involving integrin beta cytoplasmic domain function in cell spreading.

Beta1 integrins participate in the hypertrophic response of rat ventricular myocytes

Multiple signaling pathways have been implicated in the hypertrophic response of ventricular myocytes, yet the importance of cell-matrix interactions has not been extensively examined. Integrins are cell-surface molecules that link the extracellular matrix to the cellular cytoskeleton. They can function as cell signaling molecules and transducers of mechanical information in noncardiac cells. Given these properties and their abundance in cardiac cells, we evaluated the hypothesis that beta1 integrin function is involved in the alpha1-adrenergic mediated hypertrophic response of neonatal rat ventricular myocytes. The hypertrophic response of this model required interaction with extracellular matrix proteins. Specificity of these results was confirmed by demonstrating that ventricular myocytes plated onto an anti-beta1 integrin antibody supported the hypertrophic gene response. Adenovirus-mediated overexpression of beta1 integrin augmented the myocyte hypertrophic response when assessed by protein synthesis and atrial natriuretic factor production, a marker gene of hypertrophic induction. DNA synthesis was not altered by integrin overexpression. Transfection of cultured cardiac myocytes with either the ubiquitously expressed beta1A integrin or the cardiac/skeletal muscle-specific beta1 isoform (beta1D) activated reporter expression from both the atrial natriuretic factor and myosin light chain-2 ventricular promoters, genetic markers of ventricular cell hypertrophy. Finally, suppression of integrin signaling by overexpression of free beta1 integrin cytoplasmic domains inhibited the adrenergically mediated atrial natriuretic factor response. These findings show that integrin ligation and signaling are involved in the cardiac hypertrophic response pathway.

beta1 integrins regulate keratinocyte adhesion and differentiation by distinct mechanisms

In keratinocytes, the beta1 integrins mediate adhesion to the extracellular matrix and also regulate the initiation of terminal differentiation. To explore the relationship between these functions, we stably infected primary human epidermal keratinocytes and an undifferentiated squamous cell carcinoma line, SCC4, with retroviruses encoding wild-type and mutant chick beta1 integrin subunits. We examined the ability of adhesion-blocking chick beta1-specific antibodies to inhibit suspension-induced terminal differentiation of primary human keratinocytes and the ability of the chick beta1 subunit to promote spontaneous differentiation of SCC4. A D154A point mutant clustered in focal adhesions but was inactive in the differentiation assays, showing that differentiation regulation required a functional ligand-binding domain. The signal transduced by beta1 integrins in normal keratinocytes was "do not differentiate" (transduced by ligand-occupied receptors) as opposed to "do differentiate" (transduced by unoccupied receptors), and the signal depended on the absolute number, rather than on the proportion, of occupied receptors. Single and double point mutations in cyto-2 and -3, the NPXY motifs, prevented focal adhesion targeting without inhibiting differentiation control. However, deletions in the proximal part of the cytoplasmic domain, affecting cyto-1, abolished the differentiation-regulatory ability of the beta1 subunit. We conclude that distinct signaling pathways are involved in beta1 integrin-mediated adhesion and differentiation control in keratinocytes.

Integrin activation involves a conformational change in the alpha 1 helix of the beta subunit A-domain

The ligand-binding region of integrin beta subunits contains a von Willebrand factor type A-domain: an alpha/beta "Rossmann" fold containing a metal ion-dependent adhesion site (MIDAS) on its top face. Although there is evidence to suggest that the betaA-domain undergoes changes in tertiary structure during receptor activation, the identity of the secondary structure elements that change position is unknown. The mAb 12G10 recognizes a unique cation-regulated epitope on the beta(1) A-domain, induction of which parallels the activation state of the integrin (i.e. competency for ligand recognition). The ability of Mn(2+) and Mg(2+) to stimulate 12G10 binding is abrogated by mutation of the MIDAS motif, demonstrating that the MIDAS is a Mn(2+)/Mg(2+) binding site and that occupancy of this site induces conformational changes in the A-domain. The cation-regulated region of the 12G10 epitope maps to Arg(154)/Arg(155) in the alpha1 helix. Our results demonstrate that the alpha1 helix undergoes conformational alterations during integrin activation and suggest that Mn(2+) acts as a potent activator of beta(1) integrins because it can promote a shift in the position of this helix. The mechanism of beta subunit A-domain activation appears to be distinct from that of the A-domains found in some integrin alpha subunits.

Galectin-1 interacts with β-1 subunit of integrin

Galectin-1, a beta-galactoside-binding dimeric lectin, is involved in adhesion, migration, and proliferation of vascular smooth muscle cells (SMC), the key steps in the development of atherosclerosis and restenosis. Here we investigated the molecular basis of the interactions between galectin-1 and SMCs. Galectin-1 modulated SMC attachment in a dose- and beta-galactoside-dependent manner. Direct binding of galectin-1 to beta1 integrin was detected by the immune precipitation of beta1 integrin after chemical cross-linking of 125I-labelled galectin-1 to the cell surface proteins. Galectin-1 transiently increased availability of beta1 integrins on the cell surface to antibodies against beta1 integrin. Incubation of SMCs with galectin-1 transiently increased the amount of the active form of beta1 integrin and tyrosine phosphorylation of two cytoskeleton-associated proteins; one of them coincided with focal adhesion kinase (FAK). Galectin-1 is likely to affect SMC adhesion by interacting with beta1 integrin on the cell surface of SMCs and inducing outside-in signalling.

Integrin activation by dithiothreitol or Mn2+ induces a ligand-occupied conformation and exposure of a novel NH2-terminal regulatory site on the beta1 integrin chain

Integrins can be expressed in at least three functional states (i.e. latent, active, and ligand-occupied). However, the molecular bases for the transitions between these states are unknown. In the present study, changes in the accessibility of several beta1 epitopes (e.g. N29, B44, and B3B11) were used to probe activation-related conformational changes. Dithiothreitol or Mn2+ activation of integrin-mediated adhesion in the human B cell line, IM9, resulted in a marked increase in the exposure of the B44 epitope, while N29 expression levels were most sensitive to dithiothreitol treatment. These results contrasted with the epitope expression patterns of spontaneously adherent K562 cells, where N29 was almost fully accessible and B44 was low. Addition of a soluble ligand resulted in a marked increase in B44 levels, suggesting that this antibody detected a ligand-induced binding site. The N29 epitope was mapped to a cysteine-rich region near the NH2 terminus of the integrin chain, thus defining a novel regulatory site. These studies indicate that the activation of integrin function by different stimuli may involve related but nonidentical conformations. Both Mn2+ and dithiothreitol appear to induce localized conformational changes that mimic a ligand-occupied receptor. This differs from the "physiologically" activated integrins on K562 cells that display a marked increase in overall epitope accessibility without exposure of the ligand-induced binding site epitopes. The increased exposure of the N29 site on K562 cells may indicate a role for this region in the regulation of integrin function.

Different splice variants of filamin-B affect myogenesis, subcellular distribution, and determine binding to integrin [beta] subunits

Integrins connect the extracellular matrix with the cell interior, and transduce signals through interactions of their cytoplasmic tails with cytoskeletal and signaling proteins. Using the yeast two-hybrid system, we isolated a novel splice variant (filamin-Bvar-1) of the filamentous actin cross-linking protein, filamin-B, that interacts with the cytoplasmic domain of the integrin beta1A and beta1D subunits. RT-PCR analysis showed weak, but wide, expression of filamin-Bvar-1 and a similar splice variant of filamin-A (filamin-Avar-1) in human tissues. Furthermore, alternative splice variants of filamin-B and filamin-C, from which the flexible hinge-1 region is deleted (DeltaH1), were induced during in vitro differentiation of C2C12 mouse myoblasts. We show that both filamin-Avar-1 and filamin-Bvar-1 bind more strongly than their wild-type isoforms to different integrin beta subunits. The mere presence of the high-affinity binding site for beta1A is not sufficient for targeting the filamin-Bvar-1 construct to focal contacts. Interestingly, the simultaneous deletion of the H1 region is required for the localization of filamin-B at the tips of actin stress fibers. When expressed in C2C12 cells, filamin-Bvar-1(DeltaH1) accelerates their differentiation into myotubes. Furthermore, filamin-B variants lacking the H1 region induce the formation of thinner myotubes than those in cells containing variants with this region. These findings suggest that specific combinations of filamin mRNA splicing events modulate the organization of the actin cytoskeleton and the binding affinity for integrins.

Mechanical strain regulates osteoblast proliferation through integrin-mediated ERK activation

Mechanical strain plays a critical role in the proliferation, differentiation and maturation of bone cells. As mechanical receptor cells, osteoblasts perceive and respond to stress force, such as those associated with compression, strain and shear stress. However, the underlying molecular mechanisms of this process remain unclear. Using a four-point bending device, mouse MC3T3-E1 cells was exposed to mechanical tensile strain. Cell proliferation was determined to be most efficient when stimulated once a day by mechanical strain at a frequency of 0.5 Hz and intensities of 2500 µε with once a day, and a periodicity of 1 h/day for 3 days. The applied mechanical strain resulted in the altered expression of 1992 genes, 41 of which are involved in the mitogen-activated protein kinase (MAPK) signaling pathway. Activation of ERK by mechanical strain promoted cell proliferation and inactivation of ERK by PD98059 suppressed proliferation, confirming that ERK plays an important role in the response to mechanical strain. Furthermore, the membrane-associated receptors integrin β1 and integrin β5 were determined to regulate ERK activity and the proliferation of mechanical strain-treated MC3T3-E1 cells in opposite ways. The knockdown of integrin β1 led to the inhibition of ERK activity and cell proliferation, whereas the knockdown of integrin β5 led to the enhancement of both processes. This study proposes a novel mechanism by which mechanical strain regulates bone growth and remodeling.

Melusin is a new muscle-specific interactor for beta(1) integrin cytoplasmic domain

Here we describe the isolation and partial characterization of a new muscle-specific protein (Melusin) which interacts with the integrin cytoplasmic domain. The cDNA encoding Melusin was isolated in a two-hybrid screening of a rat neonatal heart library using beta(1)A and beta(1)D integrin cytoplasmic regions as baits. Melusin is a cysteine-rich cytoplasmic protein of 38 kDa, with a stretch of acidic amino acid residues at the extreme carboxyl-terminal end. In addition, putative binding sites for SH3 and SH2 domains are present in the amino-terminal half of the molecule. Chromosomic analysis showed that melusin gene maps at Xq12.1/13 in man and in the synthenic region X band D in mouse. Melusin is expressed in skeletal and cardiac muscles but not in smooth muscles or other tissues. Immunofluorescence analysis showed that Melusin is present in a costamere-like pattern consisting of two rows flanking alpha-actinin at Z line. Its expression is up-regulated during in vitro differentiation of the C2C12 murine myogenic cell line, and it is regulated during in vivo skeletal muscle development. A fragment corresponding to the tail region of Melusin interacted strongly and specifically with beta(1) integrin cytoplasmic domain in a two-hybrid test, but the full-length protein did not. Because the tail region of Melusin contains an acidic amino acid stretch resembling high capacity and low affinity calcium binding domains, we tested the possibility that Ca(2+) regulates Melusin-integrin association. In vitro binding experiments demonstrated that interaction of full-length Melusin with detergent-solubilized integrin heterodimers occurred only in absence of cations, suggesting that it can be regulated by intracellular signals affecting Ca(2+) concentration.

A structure prediction for the ligand-binding region of the integrin beta subunit: evidence for the presence of a von Willebrand factor A domain

The integrins are a family of cell surface receptors that mediate biologically important adhesive interactions. Integrin-ligand binding has been extensively studied because of the potential for the development of anti-adhesive therapies, but the molecular basis of this interaction is still poorly understood. A conserved region near the N-terminus of the beta subunit appears to be of particular importance in ligand binding, but to date this domain has not been expressed in isolation. As a prelude to expression and potential structure determination, we have performed a detailed structure prediction for this region. Primary, secondary and tertiary structure analyses indicate that the region folds into a von Willebrand factor A-domain, thereby potentially placing a previously characterised module at the centre of a key functional region.

Striated muscle-specific beta(1D)-integrin and FAK are involved in cardiac myocyte hypertrophic response pathway

Alterations in the extracellular matrix occur during the cardiac hypertrophic process. Because integrins mediate cell-matrix adhesion and beta(1D)-integrin (beta1D) is expressed exclusively in cardiac and skeletal muscle, we hypothesized that beta1D and focal adhesion kinase (FAK), a proximal integrin-signaling molecule, are involved in cardiac growth. With the use of cultured ventricular myocytes and myocardial tissue, we found the following: 1) beta1D protein expression was upregulated perinatally; 2) alpha(1)-adrenergic stimulation of cardiac myocytes increased beta1D protein levels 350% and altered its cellular distribution; 3) adenovirally mediated overexpression of beta1D stimulated cellular reorganization, increased cell size by 250%, and induced molecular markers of the hypertrophic response; and 4) overexpression of free beta1D cytoplasmic domains inhibited alpha(1)-adrenergic cellular organization and atrial natriuretic factor (ANF) expression. Additionally, FAK was linked to the hypertrophic response as follows: 1) coimmunoprecipitation of beta1D and FAK was detected; 2) FAK overexpression induced ANF-luciferase; 3) rapid and sustained phosphorylation of FAK was induced by alpha(1)-adrenergic stimulation; and 4) blunting of the alpha(1)-adrenergically modulated hypertrophic response was caused by FAK mutants, which alter Grb2 or Src binding, as well as by FAK-related nonkinase, a dominant interfering FAK mutant. We conclude that beta1D and FAK are both components of the hypertrophic response pathway of cardiac myocytes.

Identification of a functionally important sequence in the cytoplasmic tail of integrin beta 3 by using cell-permeable peptide analogs

Integrins are major two-way signaling receptors responsible for the attachment of cells to the extracellular matrix and for cell-cell interactions that underlie immune responses, tumor metastasis, and progression of atherosclerosis and thrombosis. We report the structure-function analysis of the cytoplasmic tail of integrin beta 3 (glycoprotein IIla) based on the cellular import of synthetic peptide analogs of this region. Among the four overlapping cell-permeable peptides, only the peptide carrying residues 747-762 of the carboxyl-terminal segment of integrin beta 3 inhibited adhesion of human erythroleukemia (HEL) cells and of human endothelial cells (ECV) 304 to immobilized fibrinogen mediated by integrin beta 3 heterodimers, alpha IIb beta 3, and alpha v beta 3, respectively. Inhibition of adhesion was integrin-specific because the cell-permeable beta 3 peptide (residues 747-762) did not inhibit adhesion of human fibroblasts mediated by integrin beta 1 heterodimers. Conversely, a cell-permeable peptide representing homologous portion of the integrin beta 1 cytoplasmic tail (residues 788-803) inhibited adhesion of human fibroblasts, whereas it was without effect on adhesion of HEL or ECV 304 cells. The cell-permeable integrin beta 3 peptide (residues 747-762) carrying a known loss-of-function mutation (Ser752Pro) responsible for the genetic disorder Glanzmann thrombasthenia Paris I did not inhibit cell adhesion of HEL or ECV 304 cells, whereas the beta 3 peptide carrying a Ser752Ala mutation was inhibitory. Although Ser752 is not essential, Tyr747 and Tyr759 form a functionally active tandem because conservative mutations Tyr747Phe or Tyr759Phe resulted in a nonfunctional cell permeable integrin beta 3 peptide. We propose that the carboxyl-terminal segment of the integrin beta 3 cytoplasmic tail spanning residues 747-762 constitutes a major intracellular cell adhesion regulatory domain (CARD) that modulates the interaction of integrin beta 3-expressing cells with immobilized fibrinogen. Import of cell-permeable peptides carrying this domain results in inhibition "from within" of the adhesive function of these integrins.

Genetic analysis of beta1 integrin "activation motifs" in mice

Akey feature of integrins is their ability to regulate the affinity for ligands, a process termed integrin activation. The final step in integrin activation is talin binding to the NPXY motif of the integrin beta cytoplasmic domains. Talin binding disrupts the salt bridge between the alpha/beta tails, leading to tail separation and integrin activation. We analyzed mice in which we mutated the tyrosines of the beta1 tail and the membrane-proximal aspartic acid required for the salt bridge. Tyrosine-to-alanine substitutions abolished beta1 integrin functions and led to a beta1 integrin-null phenotype in vivo. Surprisingly, neither the substitution of the tyrosines with phenylalanine nor the aspartic acid with alanine resulted in an obvious defect. These data suggest that the NPXY motifs of the beta1 integrin tail are essential for beta1 integrin function, whereas tyrosine phosphorylation and the membrane-proximal salt bridge between alpha and beta1 tails have no apparent function under physiological conditions in vivo.

Human pancreatic tumor cells are sensitized to ionizing radiation by knockdown of caveolin-1

Caveolin-1 (Cav-1) is an integral transmembrane protein and a critical component in interactions of integrin receptors with cytoskeleton-associated and signaling molecules. Since integrin-mediated cell adhesion generates signals conferring radiation resistance, we examined the effects of small interfering RNA-mediated knockdown of Cav-1 alone or in combination with beta1-integrin or focal adhesion kinase (FAK) on radiation survival and proliferation of pancreatic carcinoma cell lines. Irradiation induced Cav-1 expression in PATU8902, MiaPaCa2 and Panc1 cell lines. The cell lines showed significant radiosensitization after knockdown of Cav-1, beta1-integrin or FAK and cholesterol depletion by beta-cyclodextrin relative to nonspecific controls. Under knockdown conditions, proliferation of non-irradiated and irradiated cells was significantly attenuated relative to controls. These findings correlated with changes in expression or phosphorylation of Akt, glycogen synthase kinase 3beta, Paxillin, Src, c-Jun N-terminal kinase and mitogen-activated protein kinase. Analysis of DNA microarray data revealed a Cav-1 overexpression in a subset of pancreatic ductal adenocarcinoma samples. The data presented show, for the first time, that disruption of interactions of Cav-1 with beta1-integrin or FAK affects radiation survival and proliferation of pancreatic carcinoma cells and suggest that Cav-1 is critical to these processes. These results indicate that strategies targeting Cav-1 may be useful as an approach to improve conventional therapies, including radiotherapy, for pancreatic cancer.