Corrigendum to: "Task syndromes: linking personality and task allocation in social animal groups" by J. C. Loftus, A. Perez, and A. Sih. 32:1-17

[This corrects the article DOI: 10.1093/beheco/araa083.].

Task syndromes: linking personality and task allocation in social animal groups

Studies of eusocial insects have extensively investigated two components of task allocation: how individuals distribute themselves among different tasks in a colony and how the distribution of labor changes to meet fluctuating task demand. While discrete age- and morphologically-based task allocation systems explain much of the social order in these colonies, the basis for task allocation in non-eusocial organisms and within eusocial castes remains unknown. Building from recent advances in the study of among-individual variation in behavior (i.e., animal personalities), we explore a potential mechanism by which individuality in behaviors unrelated to tasks can guide the developmental trajectories that lead to task specialization. We refer to the task-based behavioral syndrome that results from the correlation between the antecedent behavioral tendencies and task participation as a task syndrome. In this review, we present a framework that integrates concepts from a long history of task allocation research in eusocial organisms with recent findings from animal personality research to elucidate how task syndromes and resulting task allocation might manifest in animal groups. By drawing upon an extensive and diverse literature to evaluate the hypothesized framework, this review identifies future areas for study at the intersection of social behavior and animal personality.

Inhibition of focal adhesion kinase expression or activity disrupts epidermal growth factor-stimulated signaling promoting the migration of invasive human carcinoma cells

Elevated focal adhesion kinase (FAK) expression in human tumor cells has been correlated with an increased cell invasion potential. In cell culture, studies with FAK-null fibroblasts have shown that FAK function is required for cell migration. To determine the role of elevated FAK expression in facilitating epidermal growth factor (EGF)-stimulated human adenocarcinoma (A549) cell motility, antisense oligonucleotides were used to reduce FAK protein expression >75%. Treatment of A549 cells with FAK antisense (ISIS 15421) but not a mismatched control (ISIS 17636) oligonucleotide resulted in reduced EGF-stimulated p130(Cas)-Src complex formation, c-Jun NH(2)-terminal kinase (JNK) activation, directed cell motility, and serum-stimulated cell invasion through Matrigel. Because residual FAK protein in ISIS 15421-treated A549 cells was highly phosphorylated at the Tyr-397/Src homology (SH)2 binding site, expression of the FAK COOH-terminal domain (FRNK) was also used as an inhibitor of FAK function. Adenoviral-mediated infection and expression of FRNK promoted FAK dephosphorylation at Tyr-397, resulted in reduced EGF-stimulated JNK as well as extracellular-regulated kinase 2 (ERK2) kinase activation, inhibited matrix metalloproteinase-9 (MMP-9) secretion, and potently blocked both random and EGF-stimulated A549 cell motility. Equivalent expression of a FRNK (S-1034) point-mutant that did not promote FAK dephosphorylation also did not affect EGF-stimulated signaling or cell motility. Dose-dependent reduction in EGF-stimulated A549 motility was observed with the PD98059 MEK1 inhibitor and the batimastat (BB-94) inhibitor of MMP activity, but not with the SB203580 inhibitor of p38 kinase. Finally, comparisons between normal, FAK-null, and FAK-reconstituted fibroblasts revealed that FAK enhanced EGF-stimulated JNK and ERK2 kinase activation that was required for cell motility. These data indicate that FAK functions as an important signaling platform to coordinate EGF-stimulated cell migration in human tumor cells and support a role for inhibitors of FAK expression or activity in the control of neoplastic cell invasion.

Autoantibodies to alpha(IIb)beta(3) in patients with chronic immune thrombocytopenic purpura bind primarily to epitopes on alpha(IIb)

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disease caused by platelet destruction resulting from autoantibodies against platelet surface proteins, particularly platelet glycoprotein IIb/IIIa (alpha(IIb)beta(3)). To localize the auto-epitopes on platelet alpha(IIb)beta(3), the binding of autoantibodies to Chinese hamster ovary (CHO) cells expressing either alpha(IIb)beta(3) or alpha(v)beta(3) was studied. Thirteen of 14 ITP autoantibodies bound only to CHO cells expressing alpha(IIb)beta(3). Because these 2 integrins have the same beta chain (beta(3)), these results show that most epitopes in chronic ITP are dependent on the presence of glycoprotein alpha(IIb.) (Blood. 2001;97:2171-2172)

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.

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.

Differential recognition of snake venom proteins expressing specific Arg-Gly-Asp (RGD) sequence motifs by wild-type and variant integrin alphaIIbbeta3: further evidence for distinct sites of RGD ligand recognition exhibiting negative allostery

Several studies have demonstrated that the amino acid residues flanking the Arg-Gly-Asp (RGD) sequence of high-affinity ligands modulate their specificity of interaction with integrin complexes. Because of the absence of structural data for integrin complexes with bound ligand, the molecular basis for this specificity modulation remains obscure. In a previous paper [Rahman, Lu, Kakkar and Authi (1995) Biochem. J. 312, 223-232] we demonstrated that two genetically distinct venom-derived RGD proteins, kistrin and dendroaspin (both containing the sequence PRGDMP), were simple competitors, indicating the recognition of an identical binding site on the alpha(IIb)beta(3) complex. Furthermore, both kistrin and dendroaspin inhibited the binding of the disintegrin elegantin (containing the sequence ARGDNP) via a non-competitive mechanism, suggesting that the binding of elegantin to the alpha(IIb)beta(3) complex was at a remote site and down-regulated via an allosteric mechanism. Here we present further evidence for distinct RGD ligand recognition sites on the alpha(IIb)beta(3) complex that exhibit a negative allosteric relationship. A panel of well-characterized recombinant dendroaspin and elegantin derivatives were employed for this study. These recombinant molecules were constructed as glutathione S-transferase fusion proteins with either an Ala or Pro residue N-terminal to the RGD sequence in combination with either a Met or an Asn residue immediately C-terminal. Equilibrium competition experiments showed that elegantin binding to ADP-treated platelets was inhibited by derivatives Eleg. AM (ARGDMP) and Eleg. PM (PRGDMP) via an allosteric competitive mechanism, providing direct evidence that modulation of the RGD motif can alter competitive behaviour. In addition, recombinant kistrin and dendroaspin both inhibited elegantin binding via a non-competitive mechanism, confirming our previous observations. Further evidence for distinct binding sites employing an independent approach was obtained by analysing the binding of the panel of venom proteins to the functionally defective heterodimer alpha(IIb)beta(3) Ser(123)-->Ala expressed on Chinese hamster ovary cells. These studies demonstrated that simple competitors kistrin and dendroaspin bound with high affinity to the variant integrin complex. In contrast, the binding of elegantin and most significantly, recombinant Dendro. PN (PRGDNP) and Dendro. AN (ARGDNP) were abolished. These observations, taken together, are consistent with a model depicting the presence of distinct sites of RGD ligand recognition on the alpha(IIb)beta(3) complex that show the preferential recognition of specific RGD motifs. Competition experiments demonstrate a negative allosteric relationship between these RGD recognition sites.

A mutation in the alpha subunit of the platelet integrin alphaIIbbeta3 identifies a novel region important for ligand binding

An unbiased genetic approach was used to identify a specific amino acid residue in the alphaIIb subunit important for the ligand binding function of the integrin alphaIIbbeta. Chemically mutagenized cells were selected by flow cytometry based on their inability to bind the ligand mimetic antibody PAC1 and a cell line containing a single amino acid substitution in alphaIIb at position 224 (D-->V) was identified. Although well expressed on the surface of transfected cells, alphaIIbD224Vbeta3 as well as alphaIIbD224Abeta3 did not bind alphaIIbbeta3-specific ligands or a RGD peptide, a ligand shared in common with alphavbeta3. Insertion of exon 5 of alphaIIb, residues G193-W235, into the backbone of the alphav subunit did not enable the chimeric receptor to bind alphaIIbbeta3-specific ligands. However, the chimeric receptor was still capable of binding to a RGD affinity matrix. alphaIIbD224 is not well conserved among other integrin alpha subunits and is located in a region of significant variability. In addition, amino acid D224 lies within a predicted loop of the recently proposed beta-propeller model for integrin alpha subunits and is adjacent to a loop containing amino acid residues previously implicated in receptor function. These data support a role for this region in ligand binding function of the alphaIIbbeta3 receptor.

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.

A genetic analysis of integrin function: Glanzmann thrombasthenia in vitro

Glanzmann thrombasthenia, an inherited bleeding disorder, can be caused by a defect or deficiency in platelet integrin alphaIIb beta3 (GPIIb-IIIa). Studies of thrombasthenia variants have facilitated identification of sites involved in the functions of alphaIIb beta3 and other integrins. Such sites include those that bind ligand and those that participate in the "activation" of alphaIIb beta3 required for high affinity binding of ligands such as fibrinogen or PAC1, a monoclonal antibody. Here we describe the isolation of such variants, created in vitro with Chinese hamster ovary cells that express an activated form of alphaIIb beta3. These cells were exposed to a mutagen, ethyl methane sulfonate, and variants that lost the capacity to bind PAC1 were isolated by fluorescence-activated cell sorting. These variants were grouped into three phenotypic classes. One comprised integrin mutations that disrupt ligand binding function; a second comprised mutations that interfere with the capacity of cells to activate the integrin. Most of these activation-defective mutations were in the integrin cytoplasmic domain, but surprisingly, several were caused by mutations affecting three closely spaced residues in the beta3 extracellular domain. A third class of mutants exhibited a defect in integrin activation not ascribable to changes in the integrin sequence. Thus, these may represent mutated signaling molecules required for integrin activation. This unbiased genetic approach provides new insights into the structural basis of integrin function and may assist in identifying the cellular events that regulate integrin function.

Integrin-ligand binding properties govern cell migration speed through cell-substratum adhesiveness

Migration of cells in higher organisms is mediated by adhesion receptors, such as integrins, that link the cell to extracellular-matrix ligands, transmitting forces and signals necessary for locomotion. Whether cells will migrate or not on a given substratum, and also their speed, depends on several variables related to integrin-ligand interactions, including ligand levels, integrin levels, and integrin-ligand binding affinities. These and other factors affect the way molecular systems integrate to effect and regulate cell migration. Here we show that changes in cell migration speed resulting from three separate variables-substratum ligand level, cell integrin expression level, and integrin-ligand binding affinity-are all quantitatively predictable through the changes they cause in a single unifying parameter: short-term cell-substratum adhesion strength. This finding is consistent with predictions of a mathematical model for cell migration. The ligand concentration promoting maximum migration speed decreases reciprocally as integrin expression increases. Increases in integrin-ligand affinity similarly result in maximal migration at reciprocally lower ligand concentrations. The maximum speed attainable, however, remains unchanged as ligand concentration, integrin expression, or integrin-ligand affinity vary, suggesting that integrin coupling with intracellular motors remains unaltered.

Ligand binding to integrin alphaIIbbeta3 is dependent on a MIDAS-like domain in the beta3 subunit

Substitution of beta3 residue Asp119, Ser121, or Ser123 results in a loss of the ligand binding function of integrin alphaIIbbeta3. Homologous residues in other integrin beta subunits are similarly critical for ligand binding function. This DXSXS motif is also present in the I domain of certain integrin alpha subunits, where it constitutes a portion of the unique metal ion-dependent adhesion site (MIDAS). In this report, we have utilized the crystal structure of the recombinant alphaM I domain to produce a three-dimensional model of the homologous region in the integrin beta3 subunit. We performed mutagenesis of candidate amino acid residues predicted from this model to be involved in cation coordination and ligand binding. We report the identification of Asp217 and Glu220 as residues essential for the ligand binding function of alphaIIbbeta3. Alanine substitution of these residues did not affect receptor expression but abolished the binding of activation-dependent (PAC1) and -independent (OPG2) ligand mimetic antibodies. In our proposed model, beta3 Asp217 is analogous to a metal-coordinating residue in the alphaM MIDAS domain, while Glu220 does not correspond to a functional MIDAS domain residue. Substitution of the highly conserved beta3 residue Thr197 corresponding to a critical MIDAS metal-coordinating Thr residue did not affect ligand binding function, suggesting that this region of beta3 adopts a structure that is very similar to but not identical to that of the MIDAS domain. These data support a functional linkage between these two sequences and further define a common feature of ligand binding to integrins.

A molecular basis for affinity modulation of Fab ligand binding to integrin alphaIIb beta3

The Arg-Gly-Asp (RGD) sequence within the third complementarity-determining region (CDR3) of the heavy chain (H3) is responsible for the binding of the recombinant murine Fab molecules, AP7 and PAC1.1, to the platelet integrin alphaIIbbeta3. AP7 binding is minimally influenced by the conformational state of this receptor, whereas PAC1.1 binds preferentially to the activated state of the receptor induced by platelet agonists. To study the molecular basis for this functional difference, we replaced the AP7 H3 loop (HPFYRGDGGN) with all or segments of the analogous sequence from PAC1.1 (RSPSYYRGDGAGP). AP7 Fd (VH domain + Cgamma1 domain) segments containing these H3 loop sequences were expressed as active Fab molecules by coinfection of Spodoptera frugiperda cell lines with recombinant baculoviruses containing Fd and AP7 kappa chain cDNA. Replacement of the entire AP7 H3 loop with that from PAC1.1 generated the mutant AP7.3 Fab molecule, which bound selectively to either activated, gel-filtered platelets or to purified alphaIIbbeta3 in a manner identical to that of PAC1.1. Identical results were obtained when solely the sequences flanking the amino side of RGD within the respective H3 loops were exchanged. AP7.3 and PAC1.1 exhibited saturable but submaximal binding to activated gel-filtered platelets. Relative to AP7, the number of AP7.3 or PAC1. 1 Fab molecules bound per platelet was 17% in the presence of 1 m Ca2+ + 1 mM Mg2+ or 40% in the presence of 10 microM Mn2+. The ratio of Fab molecules bound after versus before activation (mean =/- S.D.; n = 3) was: for AP7.3, 9.8 =/- 0.6; for PAC1.1, 8.8 +/- 0.3; and for AP7, 1.4 =/- 0.2. In addition, AP7 bound to the stably expressed integrin mutant alphaIIbbeta3(S123A), whereas AP7.3 and PAC1 did not. Because AP7.3 behaves in every respect like PAC1.1, we conclude that the ability of RGD-based ligands to distinguish activated from resting conformations of the integrin alphaIIbbeta3 can be regulated by limited amino acid sequences immediately adjacent to the RGD tripeptide. Furthermore, those Fab molecules that exhibit increased selectivity for the activated conformation of alphaIIbbeta3 bind to a subpopulation of this integrin on platelets that is modulated by divalent cations.

The amino-terminal one-third of alpha IIb defines the ligand recognition specificity of integrin alpha IIb beta 3

The integrin alpha subunits play a major role in the regulation of ligand binding specificity. To gain further insight into the regions of the alpha subunits that regulate ligand specificity, we have utilized alpha v / alpha IIb chimeras to identify regions of alpha IIb that when substituted for the homologous regions of alpha v switched the ligand binding phenotype of alpha v beta 3 to that of alpha IIb beta 3. We report that the ligand recognition specificity of beta 3 integrins is regulated by the amino-terminal one-third of the alpha subunit. Substitution of the amino-terminal portion of alpha v with the corresponding 334 residues of alpha IIb reconstituted reactivity with both alpha IIb beta 3-specific activation-dependent (PAC1) and -independent (OPG2) ligand mimetic antibodies in addition to small highly specific activation-independent ligands. In contrast, substitution of the amino-terminal portion alone or the divalent cation repeats alone were not sufficient to change ligand binding specificity. These data in combination with previous studies demonstrate that integrin ligand recognition requires cooperation between elements in both the alpha and beta subunits and indicate that the ligand binding pocket is a structure assembled from elements of both the alpha and beta subunits.

Ligand binding and affinity modulation of integrins

Integrins are cell adhesion receptors that mediate cell-cell and cell-extracellular matrix interactions. The extracellular domains of these receptors possess binding sites for a diverse range of protein ligands. Ligand binding is divalent cation dependent and involves well-defined motifs in the ligand. Integrins can dynamically regulate their affinity for ligands (inside-out signaling). This ability to rapidly modulate their affinity state is key to their involvement in such processes as cell migration and platelet aggregation. This review will focus on two aspects of integrin function: first, on the molecular basis of ligand-integrin interactions and, second, on the underlying mechanisms controlling the affinity state of integrins for their ligands.

Platelet adhesion receptors

The response to vascular injury involves attachment and aggregation of platelets, coupled with initiation of the coagulation cascade. These inter-related processes ensure that the vessel injury is rapidly blocked with an aggregated clump of platelets that is then stabilized by a crosslinked fibrin matrix. Initial adherence relies on the ability of the platelet adhesion receptors to bind subendothelial matrix molecules. Platelet activation then results in the expression of high affinity integrin receptors on the cell surface that bind soluble ligands, allowing platelets to aggregate and form a haemostatic plug.

Mutation of a ligand binding domain of beta 3 integrin. Integral role of oxygenated residues in alpha IIb beta 3 (GPIIb-IIIa) receptor function

A single amino acid substitution in beta 3 (Asp119 --> Tyr) abrogates the ligand binding function of beta 3 integrins and alters the divalent cation conformation of the platelet integrin alpha IIb beta 3 (GPIIb-IIIa). This aspartic acid residue resides within a conserved cluster of oxygenated residues that may provide ligands for the coordination of divalent cations. To assign function to the other oxygenated residues in this group (Ser121, Ser123, Asp126, Asp127, and Ser130), each of these amino acids in beta 3 was individually substituted by alanine. None of these amino acid substitutions altered heterodimer formation or surface expression. However, the substitutions had differential effects on receptor function. Substitution at positions Asp119 or Ser121 produced a complete loss of receptor function. Cells expressing these mutants failed to adhere to fibrinogen, failed to bind activation-independent ligand-mimetic peptides, and did not bind the ligand-mimetic mAb PAC1 following activation of the receptor. Similarly, cells expressing beta 3 with a substitution at Ser123 also failed to adhere to fibrinogen and did not bind RGD peptide or mAb PAC1. These cells did retain the capacity to bind an alpha IIb beta 3-specific, high affinity peptidomimetic, but occupancy did not induce the conformational change from resting to activated state observed following occupancy of the wild type receptor. Substitution at positions Asp126, Asp127, or Ser130 had no effect on ligand binding function. These data indicate that Asp119, along with Ser121 and Ser123, plays an integral role in the ligand binding function of alpha IIb beta 3.

Integrin cytoplasmic domains mediate inside-out signal transduction

We analyzed the binding of fibronectin to integrin alpha 5 beta 1 in various cells; in some cells fibronectin bound with low affinity (e.g., K562 cells) whereas in others (e.g., CHO), it bound with high affinity (Kd approximately 100 nM) in an energy-dependent manner. We constructed chimeras of the extracellular and transmembrane domains of alpha IIb beta 3 joined to the cytoplasmic domains of alpha 5 beta 1. The affinity state of these chimeras was assessed by binding of fibrinogen or the monoclonal antibody, PAC1. The cytoplasmic domains of alpha 5 beta 1 conferred an energy-dependent high affinity state on alpha IIb beta 3 in CHO but not K562 cells. Three additional alpha cytoplasmic domains (alpha 2, alpha 6A, alpha 6B) conferred PAC1 binding in CHO cells, while three others (alpha M, alpha L, alpha v) did not. In the high affinity alpha chimeras, cotransfection with a truncated (beta 3 delta 724) or mutated (beta 3(S752-->P)) beta 3 subunit abolished high affinity binding. Thus, both cytoplasmic domains are required for energy-dependent, cell type-specific affinity modulation. In addition, mutations that disrupted a highly conserved alpha subunit GFFKR motif, resulted in high affinity binding of ligands to alpha IIb beta 3. In contrast to the chimeras, the high affinity state of these mutants was independent of cellular metabolism, cell type, and the bulk of the beta subunit cytoplasmic domain. Thus, integrin cytoplasmic domains mediate inside-out signaling. Furthermore, the highly conserved GFFKR motif of the alpha subunit cytoplasmic domain maintains the default low affinity state.

Distinct functions of integrin alpha and beta subunit cytoplasmic domains in cell spreading and formation of focal adhesions

Integrin-mediated cell adhesion often results in cell spreading and the formation of focal adhesions. We exploited the capacity of recombinant human alpha IIb beta 3 integrin to endow heterologous cells with the ability to adhere and spread on fibrinogen to study the role of integrin cytoplasmic domains in initiation of cell spreading and focal adhesions. The same constructs were also used to analyze the role of the cytoplasmic domains in maintenance of the fidelity of the integrin repertoire at focal adhesions. Truncation mutants of the cytoplasmic domain of alpha IIb did not interfere with the ability of alpha IIb beta 3 to initiate cell spreading and form focal adhesions. Nevertheless, deletion of the alpha IIb cytoplasmic domain allowed indiscriminate recruitment of alpha IIb beta 3 to focal adhesions formed by other integrins. Truncation of the beta 3 subunit cytoplasmic domain abolished cell spreading mediated by alpha IIb beta 3 and also abrogated recruitment of alpha IIb beta 3 to focal adhesions. This truncation also dramatically impaired the ability of alpha IIb beta 3 to mediate the contraction of fibrin gels. In contrast, the beta 3 subunit cytoplasmic truncation did not reduce the fibrinogen binding affinity of alpha IIb beta 3. Thus, the integrin beta 3 subunit cytoplasmic domain is necessary and sufficient for initiation of cell spreading and focal adhesion formation. Further, the beta 3 cytoplasmic domain is required for the transmission of intracellular contractile forces to fibrin gels. The alpha subunit cytoplasmic domain maintains the fidelity of recruitment of the integrins to focal adhesions and thus regulates their repertoire of integrins.

Characterization of a gain of function mutation of integrin alpha IIb beta 3 (platelet glycoprotein IIb-IIIa)

Integrin alpha IIb beta 3 (platelet glycoprotein IIb-IIIa) is a prototype of integrins involved in cellular adhesive functions. As part of a structure-function analysis of this molecule, we constructed a mutant, designated alpha IIb beta 3 (beta 1-2), by replacing 6 amino acids within a putative ligand binding domain of the beta 3 subunit with sequences derived from beta 1. The alteration did not affect the capacity of beta 3(beta 1-2) to combine with transfected alpha IIb, nor did it cause it to combine with endogenous alpha 5. Integrin alpha IIb beta 3(beta 1-2) was in a "resting" state on Chinese hamster ovary cells as judged by minimal binding of an activation-specific anti-alpha IIb beta 3, PAC1. Nevertheless, cells expressing alpha IIb beta 3(beta 1-2) spontaneously bound fibrinogen with low affinity (Ka = (4.85 +/- 0.84) x 10(6) M-1). Activation with an anti-beta 3 antibody (monoclonal antibody 62) resulted in a 10-fold increase in fibrinogen binding affinity (Ka = (4.55 +/- 0.77) x 10(7) M-1), which was 3-fold greater than fibrinogen binding to activated wild type alpha IIb beta 3 (Ka = (1.66 +/- 0.33) x 10(7) M-1, F = 7.46, p = 0.008). The mutant receptor also bound fibrinogen mimetic peptide ligands with enhanced affinity as measured by the conformation-specific antibody, anti-LIBS1. This indicates that the increased affinity for fibrinogen was caused by enhanced interaction of alpha IIb beta 3(beta 1-2) with known recognition sequences in fibrinogen. Thus, this gain of function mutant augments ligand binding function, supporting a role for this region of the beta subunit in ligand binding to integrins.

Molecular cloning and preliminary characterization of a novel cytoplasmic antigen recognized by myasthenia gravis sera

A cDNA clone was isolated by screening of a lambda gt11 endothelial expression library with serum from a patient with myasthenia gravis (MG). Rabbit antisera raised against the recombinant protein and human MG sera reactive with the clone immunoblotted an M(r) integral of 250,000 polypeptide (gravin) present in endothelial cells and several adherent cells. Gravin was not detected in platelets, leukocytes, U937, or human erythroleukemic (HEL) cell lines, but was expressed in HEL cells after induction with phorbol myristate acetate. Northern blot analysis showed two transcripts of approximately 6.7 and 8.4 kb in endothelial cells but not U937 or HEL cells. Indirect immunofluorescence of permeabilized cells revealed a trabecular network of gravin staining with a distinct linear component. Antibodies to gravin, were present in sera from 22:72 (31%) of MG patients. In contrast 0:50 normal sera and 1:72 sera from patients with other autoimmune diseases contained antigravin antibodies. Gravin is not likely to be a nonerythroid spectrin, talin, myosin, or actin-binding protein based on the lack of reactivity of antigravin with these polypeptides in immunoblots. The nucleotide sequence of the immunoreactive clone indicated that it encodes a highly acidic polypeptide fragment that contains the carboxyl terminus of the protein. Neither amino acid nor nucleotide sequences were present in Genbank, EMBL, or Swissprot databases as of March, 1992. These data indicate that gravin is an inducible, cell type-specific cytoplasmic protein and that auto-antibodies to gravin may be highly specific for MG.

The Arg-Gly-Asp (RGD) recognition site of platelet glycoprotein IIb-IIIa on nonactivated platelets is accessible to high-affinity macromolecules

We have characterized a murine IgG monoclonal antibody, OP-G2, specific for platelet glycoprotein (GP) IIb-IIIa (alpha IIb beta 3). OP-G2 Fab fragments inhibit fibrinogen-mediated platelet aggregation and competitively inhibit adenosine diphosphate-induced binding of 125I-fibrinogen to washed platelets. OP-G2 binding to GPIIb-IIIa is specifically inhibited by RGD-containing peptides but not the fibrinogen gamma-chain carboxy-terminal peptide, and OP-G2 Fab fragments, like RGD-containing peptides, alter the conformation of GPIIb-IIIa resulting in the expression of a ligand-induced binding site (LIBS) recognized by PMI-1. OP-G2 fails to bind to the recombinant Cam variant of GPIIb-IIIa (alpha III beta 3Cam) wherein an Asp119 to Tyr119 substitution in GPIIIa abrogates the ability to recognize RGD. These data indicate that OP-G2 recognizes an epitope at or in very close proximity to the RGD recognition site of GPIIb-IIIa and that, in every aspect tested, OP-G2 behaves like a macromolecular RGD ligand. Interestingly, two-color flow cytometry shows that OP-G2 IgG can bind to nonactivated platelets. Quantitative binding assays indicate that nonactivated platelets bind approximately 50,000 125I-OP-G2 molecules/platelet. Furthermore, the affinity of OP-G2 for platelets activated with thrombin is roughly fivefold higher (nonactivated, kd = 24.8 nmol/L; activated, kd = 4.9 nmol/L). These results suggest that the RGD recognition site of GPIIb-IIIa is available to macromolecules that contain RGD even on nonactivated platelets, provided that the affinity of the ligand is adequate.

A spontaneous mutation of integrin alpha IIb beta 3 (platelet glycoprotein IIb-IIIa) helps define a ligand binding site

This work characterizes a mutant integrin alpha IIb beta 3 (glycoprotein (GP) IIb-IIIa) from a thrombasthenic patient, ET, whose platelets fail to aggregate in response to stimuli. The nature of defect was defined by the reduced ability of synthetic peptide ligands, corresponding to the carboxyl terminus of the fibrinogen gamma chain (gamma 402-411) and Arg-Gly-Asp (RGD), to increase the binding of the occupancy-dependent anti-LIBS1 antibody to mutant alpha IIb beta 3 and the reduced binding of mutant alpha IIb beta 3 to an immobilized RGD peptide. In addition, ET's platelets failed to bind the ligand-mimetic monoclonal anti-alpha IIb beta 3, PAC1. DNA sequence analysis of amplified ET genomic DNA revealed a single G----A base change which encoded substitution of R214 by Q in mature beta 3. Introduction of this point mutation into recombinant wild type alpha IIb beta 3 expressed in Chinese hamster ovary cells reproduced the ET platelet alpha IIb beta 3 deficits in binding of fibrinogen, mAb PAC1, and synthetic peptide ligands. Furthermore, substitution of R214 by Q in the synthetic peptide containing the sequence of beta 3(211-222) resulted in decreased ability of this peptide to block fibrinogen binding to purified alpha IIb beta 3. These findings suggest that substitution of beta 3 R214 by Q is responsible for the functional defect in alpha IIb beta 3 and that R214 is proximal to or part of a ligand binding domain in alpha IIb beta 3.

Ligand bridging mediates integrin alpha IIb beta 3 (platelet GPIIB-IIIA) dependent homotypic and heterotypic cell-cell interactions

The aggregation of cells bearing recombinant integrin alpha IIb beta 3 (platelet GPIIb-IIIa) has been analyzed by two-color flow cytometry. As in normal platelets, aggregation requires functional alpha IIb beta 3, "activation" of alpha IIb beta 3, and fibrinogen (fg) binding to alpha IIb beta 3. Cellular aggregation required that both interacting cells express functional alpha IIb beta 3, because a binding defective mutant, alpha IIb beta 3 (D119----Y), failed to support interaction with wild type alpha IIb beta 3-bearing cells. In addition, cells bearing resting alpha IIb beta 3 were incorporated into aggregates formed by cells bearing a constitutively active mutant, alpha IIb beta 3 (beta 1-2), indicating that only one of the cells in an interacting pair must be activated. Finally, heterotypic interactions occurred between cells bearing activated alpha IIb beta 3 and cells bearing alpha V beta 3, a fg-binding integrin present on endothelial and tumor cells. Thus, ligand bridging between fg-binding integrins represents a mechanism of cell-cell interaction, cells bearing resting alpha IIb beta 3 (e.g., resting platelets) may be incorporated into aggregates formed by cells bearing activated alpha IIb beta 3, and alpha IIb beta 3 mediates heterotypic interactions with cells bearing other fg receptors.

Platelet glycoprotein IIb-IIIa (alpha IIb beta 3 integrin) confers fibrinogen- and activation-dependent aggregation on heterologous cells

To analyze molecular mechanisms of platelet aggregation, we have studied the aggregation of Chinese hamster ovary (CHO) cells expressing between 1 and 4 x 10(5) recombinant human glycoprotein (GP) IIb-IIIa molecules per cell (A5 cells). These cells aggregated as measured by the disappearance of single cells during rotary agitation. Aggregation was dependent on the presence of extracellular fibrinogen (approximately 500 nmol/L) and divalent cations, and required prior activation of the GPIIb-IIIa. A synthetic peptide (GRGDSP) and monoclonal anti-GPIIb-IIIa antibody (2G12) that block platelet aggregation also blocked aggregation of these cells. Parent CHO cells or those expressing recombinant GPIIb-IIIa containing a point mutation that causes variant thrombasthenia both failed to aggregate when stimulated in the presence of fibrinogen. These data show that GPIIb-IIIa is the only unique platelet surface component required for aggregation.

Autoantibodies to the presumptive cytoplasmic domain of platelet glycoprotein IIIa in patients with chronic immune thrombocytopenic purpura

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder due to autoantibodies against platelets that result in their destruction. In some patients, these autoantibodies bind to platelet glycoprotein (GP) IIIa. With the aim of better defining the antigenic epitopes, plasma from 13 selected patients with chronic ITP known to have anti-GPIIb/IIIa autoantibodies was tested for reactivity with nine synthetic peptides corresponding to different regions of the GP IIIa molecule. Of these plasmas, five bound significantly (P less than .001) with either peptide 8 (amino acids 721-744) or peptide 9 (amino acids 742-762), which together form most of the carboxyterminal region presumed to be the cytoplasmic domain. Three of these positive plasmas, were tested further. In two of these positive plasmas, the anti-peptide antibodies represented greater than 80% of the detectable circulating autoantibody. To further evaluate the importance of the carboxyterminal region as an antigenic site, the chronic ITP plasmas were tested against Chinese hamster ovary cells transfected with GPIIb and either whole GPIIIa or GPIIIa lacking amino acids 728 to 762. Ten of the 13 plasmas required the presence of this region for significant autoantibody binding. We conclude that the carboxyterminal region is an important area for stimulating antiplatelet autoantibody formation in some patients with chronic ITP. It is not known whether these autoantibodies to the presumed cytoplasmic domain play an important role in the pathogenesis of the disease or occur as a secondary phenomenon during the course of platelet destruction.

Ligands "activate" integrin alpha IIb beta 3 (platelet GPIIb-IIIa)

Integrin alpha IIb beta 3 (platelet GPIIb-IIIa) binds fibrinogen via recognition sequences such as Arg-Gly-Asp (RGD). Fibrinogen binding requires agonist activation of platelets, whereas the binding of short synthetic RGD peptides does not. We now find that RGD peptide binding leads to changes in alpha IIb beta 3 that are associated with acquisition of high affinity fibrinogen-binding function (activation) and subsequent platelet aggregation. The structural specificities for peptide activation and for inhibition of ligand binding are similar, indicating that both are consequences of occupancy of the same site(s) on alpha IIb beta 3. Thus, the RGD sequence is a trigger of high affinity ligand binding to alpha IIb beta 3, and certain RGD-mimetics are partial agonists as well as competitive antagonists of integrin function.

Affinity modulation of the alpha IIb beta 3 integrin (platelet GPIIb-IIIa) is an intrinsic property of the receptor

To analyze the basis of affinity modulation of integrin function, we studied cloned stable Chinese hamster ovary cell lines expressing recombinant integrins of the beta 3 family (alpha IIb beta 3 and alpha v beta 3). Antigenic and peptide recognition specificities of the recombinant receptors resembled those of the native receptors found in platelets or endothelial cells. The alpha IIb beta 3-expressing cell line (A5) bound RGD peptides and immobilized fibrinogen (Fg) but not soluble fibrinogen or the activation-specific monoclonal anti-alpha IIb beta 3 (PAC1), indicating that it was in the affinity state found on resting platelets. Several platelet agonists failed to alter the affinity state of ("activate") recombinant alpha IIb beta 3. The binding of soluble Fg and PAC1, however, was stimulated in both platelets and A5 cells by addition of IgG papain-digestion products (Fab) fragments of certain beta 3-specific monoclonal antibodies. These antibodies stimulated PAC1 binding to platelets fixed under conditions rendering them unresponsive to other agonists. Addition of these antibodies to detergent-solubilized alpha IIb beta 3 also stimulated specific Fg binding. These data demonstrate that certain anti-beta 3 antibodies activate alpha IIb beta 3 by acting directly on the receptor, possibly by altering its conformation. Furthermore, they indicate that the activation state of alpha IIb beta 3 is a property of the receptor itself rather than of the surrounding cell membrane microenvironment.

A beta 3 integrin mutation abolishes ligand binding and alters divalent cation-dependent conformation

The ligand-binding function of integrin adhesion receptors depends on divalent cations. A mutant alpha IIb beta 3 integrin (platelet gpIIb/IIIa) that lacks ligand recognition shows immunologic evidence of a perturbed interaction with divalent cations. This was found to be caused by a G----T mutation that resulted in an Asp119----Tyr119 substitution in the beta 3 subunit. This residue is proximal to bound ligand and is in a conserved region among integrins that are enriched in oxygenated residues. The spacing of these residues aligns with the calcium-binding residues in EF hand proteins, suggesting interaction with receptor-bound divalent cation as a mechanism of ligand binding common to all integrins.

Efficient surface expression of platelet GPIIb-IIIa requires both subunits

Platelet membrane GPIIb-IIIa is a member of the integrin family of heterodimeric adhesion receptors. Processing and export of certain leukocyte and melanoma integrins is disrupted in cells lacking one subunit. We found that surface expression of GPIIb-IIIa, measured by fluorescent activated cell sorting or by surface labeling, required cotransfection of both subunits. In contrast, surface expression was not detected when the subunits were transfected individually. Immunoprecipitation of metabolically labeled transfected cells confirmed the presence of comparable levels of intracellular protein in all cases. When both subunits were transfected, post-translational cleavage of Pro-GPIIb to yield GPIIb heavy chain was also seen, while transfection with GPIIb alone resulted in coprecipitation of Pro-GPIIb with a second band that may be an endogenous beta subunit. Pro-GPIIb in these transfectants was not processed to yield GPIIb heavy chain. When transfected into COS cells alone, transiently expressed GPIIIa remained intracellular and did not appear to complex with any endogenous proteins. Thus, surface expression of processed GPIIb-IIIa depends on the presence of both subunits; the coordinate reduction of both subunits observed in some cases of Glanzmann's thrombasthenia may result from mutation affecting only one.

Occupancy of an adhesive glycoprotein receptor modulates expression of an antigenic site involved in cell adhesion

Binding of ligands that contain Arg-Gly-Asp to adhesion receptors induces cell spreading and aggregation and alters gene expression, possibly due to conformational changes within occupied adhesion receptors. PMI-1 is a monoclonal antibody which reacts with the platelet fibrinogen receptor, glycoprotein IIb-IIIa, and reports such a conformational change. ADP stimulation of platelets results in a fibrinogen-dependent increase in binding of the PMI-1 antibody. Peptides containing Arg-Gly-Asp also reversibly increase the binding of this antibody to cells and to purified glycoprotein IIb-IIIa. The PMI-1 antibody inhibits platelet adhesion and spreading on certain substrata (Shadle, P. J., Ginsberg, M. H., Plow, E. F., and Barondes, S. H. (1984) J. Cell Biol. 99, 2056-2060); thus this occupancy-modulated site may participate in adhesive function.

Alternative proteolytic processing of platelet membrane glycoprotein IIb

Platelet membrane glycoprotein (GP) IIb-IIIa is a component of a receptor for the adhesive proteins fibrinogen, fibronectin, and von Willebrand factor. GPIIb is initially synthesized as a single-chain polypeptide that is proteolytically processed to yield the two chains of mature GPIIb present on the cell surface. Analysis of the amino acid sequence surrounding the proposed light-heavy chain junction of GPIIb suggests a second potential site following a pair of basic residues 12-15 residues upstream from the reported amino terminus of the light chain. We have utilized anti-peptide antibodies to examine the possibility of alternative cleavage at these two potential sites. Peptide V43 precedes the dibasic sequence and is known to reside in the heavy chain. Peptide V41 contains the sequence between the two potential sites. In immunoblots, anti-V43 reacted only with the heavy chain while anti-V41 reacted only with the light chain. Immunoprecipitation of surface-labeled platelets indicated 97% of the GPIIb light chain contains the V41 sequence while approximately 3% of GPIIb molecules lack the V41 sequence on both the light and heavy chains. These data indicate that GPIIb is primarily cleaved 12-15 amino acids upstream from the reported amino terminus of the light chain while in a minor proportion of GPIIb molecules cleavage occurs at both sites.

Molecular cloning and chemical synthesis of a region of platelet glycoprotein IIb involved in adhesive function

Membrane glycoprotein (GP) IIb-IIIa is a component of a platelet adhesive protein receptor. A region of the heavy chain of GPIIb, defined by the monoclonal antibody PMI-1, is involved in adhesion receptor function. We have localized and chemically synthesized this region of GPIIb. A cDNA clone that directs the synthesis of a fusion protein reactive with the PMI-1 antibody was isolated from a phage lambda gt11 expression library constructed with mRNA from an erythroleukemia (HEL) cell line. The deduced amino acid sequence of this clone indicates that it spans the light-heavy chain junction of GPIIb and contains a portion of the carboxyl terminus of the heavy chain and the amino terminus of the light chain. The PMI-1 epitope was found to be contained within a 9-kDa staphylococcal V8 protease fragment of GPIIb, and such a fragment was predicted within the putative heavy-chain sequence. A computerized antigen prediction program identified a single sequence with a high probability of containing a continuous epitope. A synthetic 17-residue peptide containing this sequence binds PMI-1 and inhibits PMI-1 binding to GPIIb-IIIa. The peptide-antibody complex has an approximate Kd of 1.2 microM, which compares to a Kd of 0.95 microM for PMI-1 binding to GPIIb. The region containing the PMI-1 epitope shows no similarity to corresponding regions of two other adhesion receptors, indicating that this portion of GPIIb may function in activities unique to the platelet receptor.

Immunologic relationship between platelet membrane glycoprotein GPIIb/IIIa and cell surface molecules expressed by a variety of cells

A polyclonal antiserum to platelet membrane glycoprotein GPIIb/IIIa was used to detect antigenically related molecules on a diverse panel of human cells. Umbilical vein endothelial cells, erythroleukemic HEL cells, and diploid fetal lung GM1380 fibroblasts expressed GPIIb/IIIa-related molecules, as judged by immunofluorescence and immunoprecipitation of surface-labeled proteins. The GPIIb and GPIIIa subunits were both present and were of similar molecular weight in these cell types. These molecules were synthetic products of the cells, as shown by immunoprecipitation of intrinsically labeled proteins. Promyeloid U937 cells could be induced by 4 beta-phorbol 12-myristate 13-acetate to synthesize and express GPIIb/IIIa-related molecules on their cell surface. The GPIIb/IIIa-related molecules were not precisely identical in the various cell types, based on slight differences in electrophoretic mobility and their failure to react with monoclonal antibodies specific for each subunit of platelet GPIIb/IIIa. These results suggest the existence of a widely distributed family of GPIIb/IIIa-related molecules. This family of "cytoadhesins" may share a common function in cellular adhesive reactions.

Redistribution of the fibrinogen receptor of human platelets after surface activation

We investigated the whole cell distribution of the platelet membrane receptor for fibrinogen in surface-activated human platelets. Fibrinogen-labeled colloidal gold was used in conjunction with platelet whole mount preparations to visualize directly the fibrinogen receptor. Unstimulated platelets fail to bind fibrinogen, and binding was minimal in the stages of activation immediately following adhesion. The amount of fibrinogen bound per platelet increased rapidly during the shape changes associated with surface activation until 7,600 +/- 500 labels were present at saturation. Maximal binding of fibrinogen was followed by receptor redistribution. During the early stages of spreading, fibrinogen labels were uniformly distributed over the entire platelet surface, including pseudopodia, but the labels become progressively centralized as the spreading process continued. In well spread platelets, labels were found over the central regions, whereas peripheral areas were cleared of receptors. Receptor redistribution during spreading was accompanied by cytoskeletal reorganization such that a direct correlation was seen between the development of specific ultrastructural zones and the distribution of surface receptor sites suggesting a link between the surface receptors and the cytoskeleton. The association of fibrinogen receptors with contractile elements of the cytoskeleton, which permits coordinated receptor centralization, is important to the understanding of the role of fibrinogen in normal platelet aggregation and clot retraction.

Platelet activation and cytoskeletal reorganization: high voltage electron microscopic examination of intact and Triton-extracted whole mounts

The sequential changes in the three-dimensional organization of the filamentous components of human platelets following surface activation were investigated in whole-mount preparations. Examination of intact and Triton-extracted platelets by high voltage electron microscopy provides morphological evidence of increased polymerization of actin into the filamentous form and an increased organization of the cytoskeletal elements after activation. The structure of resting platelets consists of the circumferential band of microtubules and a small number of microfilaments randomly arranged throughout a dense cytoplasmic matrix. Increased spreading is accompanied by cytoskeletal reorganization resulting in the development of distinct ultrastructural zones including the peripheral web, the outer filamentous zone, the "trabecular-like" inner filamentous zone, and the granulomere . These zones are present only in well-spread platelets during the late stages of surface activation and are retained following Triton extraction. Extraction of the less stable cytoplasmic components provides additional information about the underlying structure and filament interactions within each zone.