Integrin beta 1 cytoplasmic domain dominant negative effects revealed by lysophosphatidic acid treatment

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.

The conformational state of hERG1 channels determines integrin association, downstream signaling, and cancer progression

Ion channels regulate cell proliferation, differentiation, and migration in normal and neoplastic cells through cell-cell and cell-extracellular matrix (ECM) transmembrane receptors called integrins. K⁺ flux through the human ether-à-go-go-related gene 1 (hERG1) channel shapes action potential firing in excitable cells such as cardiomyocytes. Its abundance is often aberrantly high in tumors, where it modulates integrin-mediated signaling. We found that hERG1 interacted with the β₁ integrin subunit at the plasma membrane of human cancer cells. This interaction was not detected in cardiomyocytes because of the presence of the hERG1 auxiliary subunit KCNE1 (potassium voltage-gated channel subfamily E regulatory subunit 1), which blocked the β₁ integrin-hERG1 interaction. Although open hERG1 channels did not interact as strongly with β₁ integrins as did closed channels, current flow through hERG1 channels was necessary to activate the integrin-dependent phosphorylation of Tyr³⁹⁷ in focal adhesion kinase (FAK) in both normal and cancer cells. In immunodeficient mice, proliferation was inhibited in breast cancer cells expressing forms of hERG1 with impaired K⁺ flow, whereas metastasis of breast cancer cells was reduced when the hERG1/β₁ integrin interaction was disrupted. We conclude that the interaction of β₁ integrins with hERG1 channels in cancer cells stimulated distinct signaling pathways that depended on the conformational state of hERG1 and affected different aspects of tumor progression.

Mechanisms of talin-dependent integrin signaling and crosstalk

Cells undergo dynamic remodeling of the cytoskeleton during adhesion and migration on various extracellular matrix (ECM) substrates in response to physiological and pathological cues. The major mediators of such cellular responses are the heterodimeric adhesion receptors, the integrins. Extracellular or intracellular signals emanating from different signaling cascades cause inside-out signaling of integrins via talin, a cystokeletal protein that links integrins to the actin cytoskeleton. Various integrin subfamilies communicate with each other and growth factor receptors under diverse cellular contexts to facilitate or inhibit various integrin-mediated functions. Since talin is an essential mediator of integrin activation, much of the integrin crosstalk would therefore be influenced by talin. However, despite the existence of an extensive body of knowledge on the role of talin in integrin activation and as a stabilizer of ECM-actin linkage, information on its role in regulating inter-integrin communication is limited. This review will focus on the structure of talin, its regulation of integrin activation and discuss its potential role in integrin crosstalk. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Integrin-induced PIP5K1C kinase polarization regulates neutrophil polarization, directionality, and in vivo infiltration

Neutrophils are important in innate immunity and acute inflammatory responses. However, the regulation of their recruitment to sites of inflammation has not been well characterized. Here, we investigated the kinase PIP5K1C and showed that PIP5K1C deficiency impaired neutrophil recruitment because of an adhesion defect. PIP5K1C regulated the adhesion through facilitating RhoA GTPase and integrin activation by chemoattractants. Integrins could induce polarization of an isoform of PIP5K1C, PIP5K1C-90, in neutrophils through intracellular vesicle transport independently of exogenous chemoattractant. PIP5K1C-90 polarization was required for polarized RhoA activation at uropods and provided an initial directional cue for neutrophil polarization on the endothelium. Importantly, the polarization was also required for circumventing the inhibition of lamellipodium formation by RhoA so that neutrophils could form leading edges required for transendothelial migration. Because integrins are not known to regulate neutrophil polarization, our study revealed a previously underappreciated role of integrin signaling in neutrophil regulation.

Vascular Integrins in Tumor Angiogenesis: Mediators and Therapeutic Targets

The notion that tumor angiogenesis may have therapeutic implications in the control of tumor growth was introduced by Dr. Judah Folkman in 1971. The approval of Avastin in 2004 as the first antiangiogenic systemic drug to treat cancer patients came as a validation of this visionary concept and opened new perspectives to the treatment of cancer. In addition, this success boosted the field to the quest for new therapeutic targets and antiangiogenic drugs. Preclinical and clinical evidence indicate that vascular integrins may be valid therapeutic targets. In preclinical studies, pharmacological inhibition of integrin function efficiently suppressed angiogenesis and inhibited tumor progression. alphaVbeta3 and alphaVbeta5 were the first vascular integrins targeted to suppress tumor angiogenesis. Subsequent experiments revealed that at least four additional integrins (i.e., alpha1beta1, alpha2beta1, alpha5beta1, and alpha6beta4) might be potential therapeutic targets. In clinical studies low-molecular-weight integrin inhibitors and anti-integrin function-blocking antibodies demonstrated low toxicity and good tolerability and are now being tested in combination with radiotherapy and chemotherapy for anticancer activity in patients. In this article the authors review the role of integrins in angiogenesis, present recent development in the use of alphaVbeta3 and alpha5beta1 integrin antagonists as potential therapeutics in cancer, and discuss future perspectives.

Multiple factors contribute to integrin-talin interactions in vivo

The cytoplasmic protein talin is an essential part of the integrin-cytoskeleton link. We characterized the interaction between integrin and two conserved regions of talin, the N-terminal ;head' domain and the C-terminus, which includes the I/LWEQ domain, within the living organism. Green-fluorescent-protein-tagged head and C-terminal domains were recruited to integrin adhesion sites. Both required integrins for recruitment, but the C-terminal domain also required endogenous talin, showing it was not recruited directly by integrins. We used chimeric transmembrane proteins containing the cytoplasmic domain of the integrin beta subunit to examine the integrin-talin head interaction. Monomeric chimeric proteins did not recruit talin head, whereas dimeric chimeras efficiently recruited it and caused a strong inhibition of integrin-mediated adhesion. These chimeras recruited surprisingly few integrin-associated proteins, indicating that recruitment of talin did not initiate a cascade of recruitment. Mutagenesis of the integrin cytoplasmic domain, within the chimera, showed the dominant-negative inhibition was not due to talin sequestration alone and that additional interactions are required.

Antiangiogenic peptides and proteins: from experimental tools to clinical drugs

The formation of a 'tumor-associated vasculature', a process referred to as tumor angiogenesis, is a stromal reaction essential for tumor progression. Inhibition of tumor angiogenesis suppresses tumor growth in many experimental models, thereby indicating that tumor-associated vasculature may be a relevant target to inhibit tumor progression. Among the antiangiogenic molecules reported to date many are peptides and proteins. They include cytokines, chemokines, antibodies to vascular growth factors and growth factor receptors, soluble receptors, fragments derived from extracellular matrix proteins and small synthetic peptides. The polypeptide tumor necrosis factor (TNF, Beromun) was the first drug registered for the regional treatment of human cancer, whose mechanisms of action involved selective disruption of the tumor vasculature. More recently, bevacizumab (Avastin), an antibody against vascular endothelial growth factor (VEGF)-A, was approved as the first systemic antiangiogenic drug that had a significant impact on the survival of patients with advanced colorectal cancer, in combination with chemotherapy. Several additional peptides and antibodies with antiangiogenic activity are currently tested in clinical trials for their therapeutic efficacy. Thus, peptides, polypeptides and antibodies are emerging as leading molecules among the plethora of compounds with antiangiogenic activity. In this article, we will review some of these molecules and discuss their mechanism of action and their potential therapeutic use as anticancer agents in humans.

Integrin α4β1 function is required for cell survival in developing retina

In the retina, integrins in the beta1 family have been shown to be important in many phases of neuronal development, particularly neuroblast migration and axon outgrowth. However, the functions of specific integrin heterodimers are not well defined. In this study, we investigated the functions of beta1 integrins in developing chicken retina by expression of a dominant-negative beta1A construct using a replication-competent retrovirus. Inhibition of integrins using this approach resulted in alteration of cell morphology and increased apoptosis, but did not preclude migration and axon elongation. In an attempt to identify which specific beta1 heterodimer was important, expression and function of the alpha4beta1 heterodimer were also investigated. At early developmental stages, alpha4 protein and mRNA were detected in undifferentiated neuroblasts throughout the retina. At later stages, expression was confined to retinal ganglion cells (RGCs) and amacrine cells. A small molecule antagonist of alpha4 integrins was shown to inhibit neurite outgrowth on recombinant soluble vascular cell adhesion molecule-1 (VCAM-1), a known ligand of alpha4beta1. Introduction of alpha4 antagonist in vivo gave rise to increased apoptosis and led to a thinning of the retina and reduced numbers of retinal ganglion cells (RGCs). We conclude that the integrin alpha4beta1 is important for survival of developing retinal neurons, including RGCs.

Integrin functions play a key role in the differentiation of thymocytes in vivo

T cells express a variety of surface proteins as they develop to maturity in the thymus. In addition to the TCR-CD3 complex and the two major coreceptors, CD4 and CD8, other surface proteins expressed include receptors for cytokines, growth factors, counterreceptors, and extracellular matrix molecules. To determine the role of integrin adhesion receptors in T cell development, we have expressed a trans-dominant inhibitor of integrin function in the thymus. This inhibitor leads to a block of adhesion to fibronectin due to reduced activation of integrin receptors. This reduced adhesion leads to a partial block in differentiation from CD4-CD8- cells to CD4+CD8+ cells, after the CD25+ stage, suggesting that integrins are important during Lck-mediated differentiation. Furthermore, the overall production of CD4+ cells is reduced compared with that of CD8+ cells without changes in negative selection, suggesting that integrins may be involved in the determination of the fate of the cell as well. These results demonstrate that integrin receptor function is required for proper thymocyte development in vivo.

A functional comparison of mutations in integrin β cytoplasmic domains

Cell adhesion is a multistep process that requires the interaction of integrins with their ligands in cell attachment, the activation of integrin-triggered signals, and cell spreading. Integrin β subunit cytoplasmic domains (β tails) participate in regulating each of these steps; however, it is not known whether the same or different regions within β tails are required. We generated a panel of amino acid substitutions within the β1 and β3 cytoplasmic domains to determine whether distinct regions within β tails regulate different steps in adhesion. We expressed these β cytoplasmic domains in the context of interleukin 2 (IL-2) receptor (tac) chimeras and tested their ability to activate tyrosine phosphorylation, to regulate β1 integrin conformation and to inhibit β1 integrin function in cell attachment and spreading. We found that many of the mutant β1 and β3 chimeras either had no effect on these parameters or dramatically inhibited the function of the β tail in most assays. However, one set of analogous Ala substitutions in the β1 and β3 tails differentially affected the ability of the tac-β1 and tac-β3 chimeras to activate tyrosine phosphorylation. The tac-β1 mutant containing Ala substitutions for the VTT motif did not signal, whereas the analogous tac-β3 mutant was able to activate tyrosine phosphorylation, albeit not to wild-type levels. We also identified a few mutations that inhibited β tail function in only a subset of assays. Ala substitutions for the Val residue in the VTT motif of the β1 tail or for the conserved Asp and Glu residues in the membrane-proximal region of the β3 tail greatly diminished the ability of tac-β1 and tac-β3 to inhibit cell spreading, but had minimal effects in other assays. Ala substitutions for the Trp and Asp residues in the conserved WDT motif in the β1 tail had dramatic effects on the ability of tac-β1 to regulate integrin conformation and function in cell spreading, but had no or intermediate effects in other assays. The identification of mutations in the β1 and β3 tails that specifically abrogated the ability of these β tails to regulate β1 integrin conformation and function in cell spreading suggests that distinct protein interactions with β tails regulate β cytoplasmic domain function in these processes.

Disruption of integrin function in the murine myocardium leads to perinatal lethality, fibrosis, and abnormal cardiac performance

The molecular mechanisms that regulate the cardiac hypertrophic response and the progression from compensated hypertrophy to decompensated heart failure have not been thoroughly defined. Alteration in cardiac extracellular matrix is a distinguishing characteristic of these pathological processes. Integrins, cell surface receptors that mediate cellular adhesion to the extracellular matrix, are signaling molecules that possess mechanotransduction properties. Therefore, we hypothesized that integrins are likely candidates to play an important role in cardiac function. To test this hypothesis, transgenic mice were constructed in which normal integrin function was disrupted by expression of a chimeric molecule encoding the transmembrane and extracellular domains of the Tac subunit of the IL-2 receptor, fused to the cytoplasmic domain of beta(1A) integrin (Tacbeta(1A)). Using the alpha myosin heavy chain promoter to target expression of this chimera to the cardiac myocyte, transgenic mice were generated that had varied levels of transgene expression. Multiple transgenic founders that expressed the transgene at high levels, died perinatally and exhibited replacement fibrosis. Lines that survived showed 1) hypertrophic changes concordant with reduction in endogenous beta(1) integrin levels, or 2) reduced basal contractility and relaxation as well as alterations in components of integrin signaling pathways. These data support an important role for beta(1) integrin in normal cardiac function.

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.

Disruption of integrin-dependent adhesion and survival of endothelial cells by recombinant adenovirus expressing isolated beta integrin cytoplasmic domains

We explored the possibility of using a genetic approach to inhibit integrin-mediated endothelial cell adhesion and survival. We constructed recombinant adenoviruses (Ads) expressing chimeric proteins consisting of the cytoplasmic and transmembrane domains of integrin beta1 (CH1), beta3 (CH3) or the beta1 transmembrane domain alone (CH2) connected to the extracellular domain of L3T4 placed under the control of the CMV promoter (AdCMV) or the endothelial cell specific Tie-1 promoter (AdTie). All constructs were expressed in a dose- and time-dependent manner with over 90% of cells expressing the constructs within 24 h (AdCMVs) or 72 h (AdTies) after infection. Confluent monolayers of HUVEC infected with AdCMVCH1 or AdCMVCH3 detached from the substrate in a time- and dose-dependent manner with over 95% of the cells being detached 2 days (AdCMVs) or 3 to 4 days (AdTies) after infection. Cell detachment was preceded by the disruption of focal adhesions and reorganization of the actin cytoskeleton and was associated with a reduced ligand-binding activity of beta1, while cell surface density of beta1 integrins remained unchanged. Detached cells failed to re-adhere to different matrix proteins, without, however, any specificity toward beta1 or beta3 integrin-mediated adhesion. Upon detachment, HUVEC rapidly died by apoptosis. These results demonstrate that dominant negative inhibition of integrin function is an effective approach to disrupt endothelial cell adhesion and survival in vitro.

Impaired bone formation in transgenic mice resulting from altered integrin function in osteoblasts

To determine the role of integrins in mature osteoblasts in vivo, we expressed in transgenic mice a dominant-negative integrin subunit (beta1-DN) consisting of the beta1 subunit cytoplasmic and transmembrane domains, driven by the osteoblast-specific osteocalcin promoter. Immature osteoblasts isolated from transgenic animals differentiated normally in vitro until the osteocalcin promoter became active; thereafter they detached from the substratum, suggesting that beta1-DN was impairing adhesion in mature osteoblasts. Transgenic animals had reduced bone mass, with increased cortical porosity in long bones and thinner flat bones in the skull. At 35 days, the rate of bone formation was reduced in cortical bone, and the parietal bones were 45% thinner than in wild-type animals. Active osteoblasts were less polar and had larger areas of cytoplasm with intracellular stores of matrix molecules. Osteocyte lacunae appeared normal around the cell body but did not have normal canilicular structures. At 90 days, the parietal bone of transgenic males was of normal width, suggesting that the original defect in matrix deposition had been repaired or compensated for. In contrast, transgenic females still had decreased bone mass in the parietal bone at 90 days. The decreased bone mass in TG females was accompanied by increased staining for osteoclast activity, suggesting that there was a sex-specific defect in mature animals.

Focal adhesion motility revealed in stationary fibroblasts

Focal adhesions (FAs) are clustered integrins and associated proteins that mediate cell adhesion and signaling. A green fluorescent protein-beta1 integrin chimera was used to label FAs in living cells. In stationary cells, FAs were highly motile, moving linearly for several plaque lengths toward the cell center. FA motility was independent of cell density and resulted from contraction of associated actin fibers. In migrating cells, FAs were stationary and only moved in the tail. FA motility in stationary cells suggests that cell movement may be regulated by a clutch-like mechanism by which the affinity of integrins to substrate may be altered in response to migratory cues.

Integrins interact with focal adhesions through multiple distinct pathways

Integrin signaling involves oligomerization and a transmembrane conformational change induced by receptor occupancy. Previous work has shown that subsets of focal adhesion-associated proteins are recruited to integrins as a result of clustering, ligand binding, or both. However, it is unclear whether these discrete subsets reflect the differential binding of cytoplasmic proteins to the integrin or whether a single protein or set of proteins binds the integrin and is differentially activated by receptor occupancy or clustering. To address this question, we made mutations of the beta1 integrin cytoplasmic domain in the context of a single subunit chimera and studied their activation of various known integrin-mediated signaling pathways. We show here that the indirect association of the integrin with actin is distinct from its interactions with both preformed focal adhesions and FAK. Therefore, multiple independent signaling pathways exist from the integrin to the focal adhesion, which may reflect the association of independent factors with the integrin beta1 cytoplasmic domain.

Signaling via beta1 integrins and mitogen-activated protein kinase determines human epidermal stem cell fate in vitro

Human epidermal stem cells express higher levels of beta1 integrins and are more adhesive than keratinocytes that are destined to differentiate. To investigate whether high beta1 integrin expression and adhesiveness are essential for maintaining keratinocytes in the stem cell compartment, we introduced a dominant-negative beta1 integrin mutant, CD8beta1, into cultured human keratinocytes, thereby interfering with beta1 integrin function. Surface beta1 integrin levels, adhesiveness, and mitogen-activated protein (MAP) kinase activation on fibronectin were reduced, and exit from the stem cell compartment was stimulated. Adhesiveness and proliferative potential were restored by overexpressing wild-type beta1 integrin or by constitutive MAP kinase activation. Conversely, a dominant-negative MAP kinase kinase 1 mutant decreased adhesiveness and stem cell number in the absence of CD8beta1. MAP kinase activation by alpha6beta4-mediated adhesion and mitogens was normal in CD8beta1 cells, and constitutive MAP kinase activation did not affect adhesion and proliferation of control keratinocytes. We conclude that beta1 integrins and MAP kinase cooperate to maintain the epidermal stem cell compartment in vitro.

Mutational analysis of cell cycle inhibition by integrin beta1C

Integrin beta1C is an alternatively spliced cytoplasmic variant of the beta1 subunit that potently inhibits cell cycle progression. In this study, we analyzed the requirements for growth suppression by beta1C. A chimera containing the extracellular/transmembrane domain of the Tac subunit of the human interleukin 2 receptor (gp55) fused to the cytoplasmic domain of beta1C (residues 732-805) strongly inhibited growth in mouse 10T1/2 cells even at low expression levels, whereas chimeras containing the beta1A, beta1B, beta1D, beta3, and beta5 cytoplasmic domains had weak and variable effects. The beta1C cytoplasmic domain is composed of a membrane proximal region (732-757) common to all beta1 variants and a COOH-terminal 48-amino acid domain (758-805) unique to beta1C. The beta1C-specific domain (758-805) was sufficient to block cell growth even when expressed as a soluble cytoplasmic green fluorescent protein fusion protein. These results indicate that growth inhibition by beta1C does not require the intact receptor and can function in the absence of membrane targeting. Analysis of deletions within the beta1C-specific domain showed that the 18-amino acid sequence 775-792 is both necessary and sufficient for maximal growth inhibition, although the 13 COOH-terminal residues (793-805) also had weak activity. Finally, beta1C is known to be induced in endothelial cells in response to tumor necrosis factor and is down-regulated in prostate epithelial cells after transformation. The green fluorescent protein/beta1C (758-805) chimera blocked growth in the human endothelial cell line EV304 and in the transformed prostate epithelial cell line DU145, consistent with a role for beta1C as a growth inhibitor in vivo.

Amino acid motifs required for isolated beta cytoplasmic domains to regulate ‘in trans’ beta1 integrin conformation and function in cell attachment

The role of beta cytoplasmic domains in regulating beta1 integrin conformation and function in cell attachment is not fully understood. In this study, we tested the ability of transiently expressed beta cytoplasmic domains connected to an extracellular reporter domain to regulate ‘in trans’ the conformation of endogenous beta1 integrins, and compared these effects on cell attachment. We found that chimeric receptors containing either the beta1, beta3 or beta5 cytoplasmic domains inhibited the expression of the conformationally dependent 9EG7 and 12G10 epitopes on endogenous beta1 integrins. In contrast, chimeric receptors containing the beta4 or alpha5 cytoplasmic domain, or a control receptor lacking a cytoplasmic domain, had no effect. This inhibition occurred in a dose-dependent manner that required high levels of expression of the chimeric receptor. These results suggest that beta1 integrin conformation can be regulated by conserved cytosolic interactions involving beta cytoplasmic domains. This is further supported by our findings that mutations within amino acid motifs conserved among these beta cytoplasmic domains, specifically the NXXY, NPXY and TST-like motifs, reduced the ability of these chimeric receptors to regulate beta1 integrin conformation. Interestingly, the chimeric receptors inhibited cell attachment in a similar dose-dependent manner and required intact NXXY, NPXY, and TST-like motifs. The beta1 chimera also inhibited the binding of soluble fibronectin to endogenous beta1 integrins. Thus, the concomitant inhibition in the expression of conformation-dependent integrin epitopes, cell attachment and ligand binding by the chimeras, suggests that the expression of the 9EG7 and 12G10 epitopes correlates with integrin function. However, Mn2+, which is an extracellular activator of integrin function, increased 9EG7 expression to basal levels in the presence of the beta1 chimera, but did not rescue cell attachment to the same extent. Thus, although the beta1 integrin conformation recognized by mAb 9EG7 may be required for cell attachment, it is not sufficient, suggesting that the beta chimeras may be inhibiting both ligand binding and post-ligand binding events required for cell attachment. In addition, the inhibitory effects of the chimeric receptors on cell attachment were not reversed by the addition of the pharmacological agents that inhibit intracellular signals previously shown to inhibit integrin function. This finding, together with the requirement for high levels of the chimeric receptors and the fact that mutations in the same conserved motifs in heterodimeric beta1 integrins have been reported to regulate beta1 integrin conformation and function in cell attachment, suggest that beta cytoplasmic domains regulate these processes by interacting with cytosolic factors and that the regulatory effect of the chimeras may be due to their ability to titrate proteins from endogenous integrins.

Interaction of the integrin beta1 cytoplasmic domain with ICAP-1 protein

In a yeast two-hybrid screen, a protein named ICAP-1 (beta1 integrin cytoplasmic domain associated protein) associated with the integrin beta1 cytoplasmic tail but not with tails from three other integrin beta subunits (beta2, beta3, and beta5) or from seven different alpha subunits. Likewise in human cells, ICAP-1 associated specifically with the beta1 but not beta2, beta3, or beta5 tails. The carboxyl-terminal 14 amino acids of beta1 were critical for ICAP-1 interaction. ICAP-1 is a ubiquitously expressed protein of 27 and 31 kDa, with the smaller form being preferentially solubilized by Triton X-100. Phosphorylation of both 27- and 31-kDa forms was constitutive but was increased by 1.5-2-fold upon cell spreading on fibronectin, compared with poly-L-lysine. Also, ICAP-1 contributes to beta1 integrin-dependent migration because (i) ICAP-1 transfection markedly increased chemotactic migration of COS7 cells through fibronectin-coated but not vitronectin-coated porous filters, and (ii) support of beta1-dependent cell migration (in Chinese hamster ovary cells transfected with various wild type and mutant beta1 forms) correlated with ICAP-1 association. In summary, ICAP-1 (i) associates specifically with beta1 integrins, (ii) is phosphorylated upon beta1 integrin-mediated adhesion, and (iii) may regulate beta1-dependent cell migration.

The cytoplasmic domains of a beta1 integrin mediate polarization in Madin-Darby canine kidney cells by selective basolateral stabilization

In Madin-Darby canine kidney cells, newly synthesized apical and basolateral membrane proteins are generally transported directly to their respective cell surface domain due to targeting determinants that mediate sorting in the Golgi complex. In several basolateral membrane proteins, these targeting determinants reside in the cytoplasmic domains. We show here that basolateral expression of the human alpha5beta1 integrin in stably transfected Madin-Darby canine kidney cells is also mediated by the cytoplasmic domains. Distinct regions in both cytoplasmic domains were found to be sufficient to mediate basolateral expression independently from one another. Unexpectedly, newly synthesized wild-type alpha5beta1 and basolaterally expressed chimeras containing the cytoplasmic domain of either alpha5 or beta1 were integrated into both cell surface domains, preferentially apically, during biosynthesis. The apical pools of wild-type integrin and chimeric subunits were found to become quickly degraded, whereas the basolateral pools were stabilized. Thus, the cytoplasmic domains of the alpha5beta1 integrin are independently sufficient to mediate sorting by selective basolateral stabilization.

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.

Perturbation of beta1-integrin function alters the development of murine mammary gland

The expression of a transgene coding for a chimeric molecule, containing the cytoplasmic and transmembrane domains of the beta1-integrin chain and the extracellular domain of the T-cell differentiation antigen CD4, was targeted to the mouse mammary gland by the mouse mammary tumor virus (MMTV) promoter. The chimera does not interact with the extracellular ligands; however, its expression in cultured cells was shown to interfere with focal adhesion kinase (FAK) phosphorylation following ligation of endogenous beta1-integrin. Therefore, expression of the transgenic protein on the cell surface should uncouple adhesion from intracellular events associated with the beta1-cytoplasmic domain and thus perturb beta1-integrin functions. Although most of the transgenic females were able to lactate, their mammary glands had a phenotype clearly distinct from that of wild-type mice. At mid-pregnancy and the beginning of lactation, transgenic glands were underdeveloped and the epithelial cell proliferation rates were decreased, while the apoptosis levels were higher than in wild-type glands. In lactation, the amounts of the whey acidic protein (WAP) and beta-casein gene transcripts were diminished, and the basement membrane component, laminin and the beta4-integrin chain accumulated at the lateral surface of luminal epithelial cells, revealing defects in polarization. Our observations prove that in vivo, beta1-integrins are involved in control of proliferation, apoptosis, differentiation and maintenance of baso-apical polarity of mammary epithelial cells, and therefore are essential for normal mammary gland development and function.

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

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

The role of conserved amino acid motifs within the integrin beta3 cytoplasmic domain in triggering focal adhesion kinase phosphorylation

Integrin-mediated adhesion of cells to extracellular matrix proteins triggers a variety of intracellular signaling pathways including a cascade of tyrosine phosphorylations. In many cell types, the cytoplasmic focal adhesion tyrosine kinase, FAK, appears to be the initial protein that becomes tyrosine-phosphorylated in response to adhesion; however, the molecular mechanisms regulating integrin-triggered FAK phosphorylation are not understood. Previous studies have shown that the integrin beta1, beta3, and beta5 subunit cytoplasmic domains all contain sufficient information to trigger FAK phosphorylation when expressed in single-subunit chimeric receptors connected to an extracellular reporter. In the present study, beta3 cytoplasmic domain deletion and substitution mutants were constructed to identify amino acids within the integrin beta3 cytoplasmic domain that regulate its ability to trigger FAK phosphorylation. Cells transiently expressing chimeric receptors containing these mutant cytoplasmic domains were magnetically sorted and assayed for the tyrosine phosphorylation of FAK. Analysis of these mutants indicated that structural information in both the membrane-proximal and C-terminal segments of the beta3 cytoplasmic domain is important for triggering FAK phosphorylation. In the C-terminal segment of the beta3 cytoplasmic domain, the highly conserved NPXY motif was found to be required for the beta3 cytoplasmic domain to trigger FAK phosphorylation. However, the putative FAK binding domain within the N-terminal segment of the beta3 cytoplasmic domain was found to be neither required nor sufficient for this signaling event. We also demonstrate that the serine 752 to proline mutation, known to cause a variant of Glanzmann's thrombasthenia, inhibits the ability of the beta3 cytoplasmic domain to signal FAK phosphorylation, suggesting that a single mutation in the beta3 cytoplasmic domain can inhibit both "inside-out" and "outside-in" integrin signaling.

The membrane-cytoplasm interface of integrin alpha subunits is critical for receptor latency

Localization of integrin receptors to focal contact sites occurs upon ligand binding. This activity is latent, since unoccupied integrin receptors do not localize to focal contacts. Deletion analysis has revealed that the alpha cytoplasmic domains is required for the maintenance of integrin receptor latency. Our current hypothesis for the mechanism of integrin post-ligand binding events is that there is a change in relationship of alpha and beta cytoplasmic domains, which overcomes receptor latency. One possible mechanism for such a change would involve the amino acid residues at the membrane-cytoplasm interface. To test this hypothesis, we have produced point mutations in the human integrin alpha 1 subunit. These mutations had no effect on the adhesion via alpha 1 beta 1 to its ligand, collagen IV. However, receptor latency is lost in one of these mutants, leading to constitutive focal contact localization. This effect did not occur in receptors with an exchange of intracellular domains, suggesting that the mechanism of loss of latency involves a relative motion of the integrin chains. These results suggest a model in which post-ligand binding events in integrin receptors are associated with changes in the position of the alpha and beta cytoplasmic domains.

Dominant-negative effect on adhesion by myelin Po protein truncated in its cytoplasmic domain

The myelin Po protein is believed to hold myelin together via interactions of both its extracellular and cytoplasmic domains. We have already shown that the extracellular domains of Po can interact in a homophilic manner (Filbin, M.T., F.S. Walsh, B.D. Trapp, J.A. Pizzey, and G.I. Tennekoon. 1990. Nature (Lond.). 344:871-872). In addition, we have shown that for this homophilic adhesion to take place, the cytoplasmic domain of Po must be intact and most likely interacting with the cytoskeleton; Po proteins truncated in their cytoplasmic domains are not adhesive (Wong, M.H., and M.T. Filbin, 1994. J. Cell Biol. 126:1089-1097). To determine if the presence of these truncated forms of Po could have an effect on the functioning of the full-length Po, we coexpressed both molecules in CHO cells. The adhesiveness of CHO cells expressing both full-length Po and truncated Po was then compared to cells expressing only full-length Po. In these coexpressors, both the full-length and the truncated Po proteins were glycosylated. They reached the surface of the cell in approximately equal amounts as shown by an ELISA and surface labeling, followed by immunoprecipitation. Furthermore, the amount of full-length Po at the cell surface was equivalent to other cell lines expressing only full-length Po that we had already shown to be adhesive. Therefore, there should be sufficient levels of full-length Po at the surface of these coexpressors to measure adhesion of Po. However, as assessed by an aggregation assay, the coexpressors were not adhesive. By 60 min they had not formed large aggregates and were indistinguishable from the control transfected cells not expressing Po. In contrast, in the same time, the cells expressing only the full-length Po had formed large aggregates. This indicates that the truncated forms of Po have a dominant-negative effect on the adhesiveness of the full-length Po. Furthermore, from cross-linking studies, full-length Po, when expressed alone but not when coexpressed with truncated Po, appears to cluster in the membrane. We suggest that truncated Po exerts its dominant-negative effect by preventing clustering of full-length Po. We also show that colchicine, which disrupts microtubules, prevents adhesion of cells expressing only the full-length Po. This strengthens our suggestion that an interaction of Po with the cytoskeleton, either directly or indirectly, is required for adhesion to take place.

Beta 1D integrin displaces the beta 1A isoform in striated muscles: localization at junctional structures and signaling potential in nonmuscle cells

The cytoplasmic domains of integrins provide attachment of these extracellular matrix receptors to the cytoskeleton and play a critical role in integrin-mediated signal transduction. In this report we describe the identification, expression, localization, and initial functional characterization of a novel form of beta 1 integrin, termed beta 1D. This isoform contains a unique alternatively spliced cytoplasmic domain of 50 amino acids, with the last 24 amino acids encoded by an additional exon. Of these 24 amino acids, 11 are conserved when compared to the beta 1A isoform, but 13 are unique (Zhidkova, N. I., A. M. Belkin, and R. Mayne. 1995. Biochem. Biophys. Res. Commun. 214:279-285; van der Flier, A., I. Kuikman, C. Baudoin, R, van der Neuf, and A. Sonnenberg. 1995. FEBS Lett. 369:340-344). Using an anti-peptide antibody against the beta 1D integrin subunit, we demonstrated that the beta 1D isoform is synthesized only in skeletal and cardiac muscles, while very low amounts of beta 1A were detected by immunoblot in striated muscles. Whereas beta 1A could not be detected in adult skeletal muscle fibers and cardiomyocytes by immunofluorescence, beta 1D was localized to the sarcolemma of both cell types. In skeletal muscle, beta 1D was concentrated in costameres, myotendinous, and neuromuscular junctions. In cardiac muscle this beta 1 isoform was found in costamers and intercalated discs. beta 1D was associated with alpha 7A and alpha 7B in adult skeletal muscle. In cardiomyocytes of adult heart, alpha 7B was the major partner for the beta 1D isoform. beta 1D could not be detected in proliferating C2C12 myoblasts, but it appeared immediately after myoblast fusion and its amount continued to rise during myotube growth and maturation. In contrast, expression of the beta 1A isoform was downregulated during myodifferentiation in culture and it was completely displaced by beta 1D in mature differentiated myotubes. We also analyzed some functional properties of the beta 1D integrin subunit. Expression of human beta 1D in CHO cells led to its localization at focal adhesions. Clustering of this integrin isoform on the cell surface stimulated tyrosine phosphorylation of pp125FAK (focal adhesion kinase) and caused transient activation of mitogen-activated protein (MAP) kinases. These data indicate that beta 1D and beta 1A integrin isoforms are functionally similar with regard to integrin-mediated signaling.

Integrin beta 3 cytoplasmic tail is necessary and sufficient for regulation of alpha 5 beta 1 phagocytosis by alpha v beta 3 and integrin-associated protein

Using a K562 cell transfection model, we have previously described a novel relationship between the integrins alpha v beta 3 and alpha 5 beta 1. alpha v beta 3 ligation was able to inhibit alpha 5 beta 1-mediated phagocytosis without effect on alpha 5 beta 1-mediated adhesion. The alpha v beta 3-dependent inhibition apparently required a signal transduction cascade as it was reversed by inhibitors of serine/threonine kinases. Now, we have studied the mechanisms of signal transduction in this system and have found that the beta 3 cytoplasmic tail is both necessary and sufficient for initiation of the signal leading to inhibition of alpha 5 beta 1 phagocytosis. Ligation of integrin-associated protein (IAP), which has been implicated in alpha v beta 3 signal transduction, mimics the effects of alpha v beta 3 ligation only when the beta 3 integrin with an intact cytoplasmic tail is present. Although fibronectin-mediated phagocytosis requires the high affinity conformation of alpha 5 beta 1, ligation of alpha v beta 3/IAP does not prevent acquisition of this high affinity state. We conclude that alpha v beta 3/IAP ligation initates a signal transduction cascade, dependent upon the beta 3 cytoplasmic tail, which inhibits the phagocytic function of alpha 5 beta 1 at a step subsequent to modulation of integrin affinity.

Specialized functional properties of the integrin alpha 4 cytoplasmic domain

For functional studies of the integrin alpha 4 cytoplasmic domain, we have expressed the following in K562 and Chinese hamster ovary (CHO) cells: 1) wild-type alpha 4 (called X4C4), 2) two chimeric forms of alpha 4 (called X4C2 and X4C5) that contain the cytoplasmic domains of alpha 2 and alpha 5, respectively, and 3) alpha 4 with no cytoplasmic domain (X4C0). Cytoplasmic domain exchange had no effect on VLA-4-dependent static cell adhesion or tethering to VCAM-1 in conditions of shear flow. However, the presence of the alpha 2 or alpha 5 tails markedly enhanced VLA-4-dependent K562 cells spreading (X4C2 > X4C5 > X4C4 > X4C0), increased localization of VLA-4 into focal adhesion-like complexes in CHO cells (X4C2 > X4C5 > X4C4), and strengthened CHO and K562 cell resistance to detachment from VCAM-1 in conditions of shear flow (X4C2 > X4C5 > X4C4 > X4C0). Conversely, the alpha 4 tail supported greater VLA-4-dependent haptotactic and chemotactic cell migration. In the absence of any alpha tail (i.e., X4C0), robust focal adhesions were observed, even though cell spreading and adhesion strengthening were minimal. Thus, such focal adhesions may have relatively little functional importance, and should not be compared with focal adhesions formed when alpha tails are present. Together, these results indicate that all three alpha-chain tails exert defined positive effects (compared with no tail at all), but suggest that the alpha 4 cytoplasmic domain may be specialized to engage in weaker cytoskeletal interactions, leading to diminished focal adhesion formation, cell spreading, and adhesion strengthening, while augmenting cell migration and facilitating rolling under shear flow. These properties of the alpha 4 tail are consistent with the role of alpha 4 integrins on highly motile lymphocytes, monocytes, and eosinophils.