Proteolytic degradation of the RGD-binding and non-RGD-binding conformers of human platelet integrin glycoprotein IIb/IIIa: clues for identification of regions involved in the receptor's activation

Bioinspired thermoresponsive nanoscaled coatings: Tailor-made polymer brushes with bioconjugated arginine-glycine-aspartic acid-peptides

The development of bioengineered surface coatings with stimuli-responsive properties is beneficial for a number of biomedical applications. Environmentally responsive and switchable polymer brush systems have a great potential to create such smart biointerfaces. This study focuses on the bioconjugation of cell-instructive peptides, containing the arginine-glycine-aspartic acid tripeptide sequence (RGD motif), onto well-defined polymer brush films. Herein, the highly tailored end-grafted homo polymer brushes are either composed of the polyelectrolyte poly(acrylic) acid (PAA), providing the reactive carboxyl functionalities, or of the temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm). Of particular interest is the preparation of grafted-to binary brushes using both polymers and their subsequent conversion to RGD-biofunctionalized PNIPAAm-PAA binary brushes by a carbodiimide conjugation method. The bioconjugation process of two linear RGD-peptides Gly-Arg-Gly-Asp-Ser and Gly-Arg-Gly-Asp-Ser-Pro-Lys and one cyclic RGD-peptide cyclo(Arg-Gly-Asp-D-Tyr-Lys) is comparatively investigated by complementary analysis methods. Both techniques, in situ attenuated total reflectance Fourier transform infrared spectroscopy measurements and the in situ spectroscopic ellipsometric analysis, describe changes of the brush surface properties due to biofunctionalization. Besides, the bound RGD-peptide amount is quantitatively evaluated by ellipsometry in comparison to high performance liquid chromatography analysis data. Additionally, molecular dynamic simulations of the RGD-peptides themselves allow a better understanding of the bioconjugation process depending on the peptide properties. The significant influence on the bioconjugation result can be derived, on the one hand, of the polymer brush composition, especially from the PNIPAAm content, and, on the other hand, of the peptide dimension and its reactivity.

Large-scale purification of active platelet integrin glycoprotein IIb-IIIa

Glycoprotein IIb-IIIa is an abundant platelet receptor of the integrin family that plays a primary role in platelet aggregation. It exists on the platelet surface predominantly in a resting or inactive conformation that is converted to an active binding competent conformation upon platelet activation. There is much interest in studying the difference between active and inactive GP IIb-IIIa, developing therapeutic agents targeted towards GP IIb-IIIa and developing diagnostic assays for antibodies that recognize epitopes on GP IIb-IIIa. We present here the development of a large-scale process for purifying active GP IIb-IIIa from human platelets. The procedure results in 25mg batch sizes of high purity and activity. Additionally, the effects of detergent concentration and impurities such as IgG on ELISA assays are examined.

Organization of the glycoprotein (GP) IIb/IIIa heterodimer on resting human platelets studied by flow cytometric energy transfer

Glycoprotein IIb/IIIa is a heterodimer of glycoproteins IIb and IIIa which serves as the inducible receptor for fibrinogen and other adhesive proteins at the surface of platelets. Although a model of the quaternary structure of the GPIIb/IIIa molecule has been constructed in solution by Calvete et al. [Biochem. J. 282 (1992) 523], a corresponding model at the surface of intact platelets is still missing. In the present work conformation and lateral distribution of the GPIIb/IIIa heterodimer were studied at a nanometer resolution on the surface of resting human platelets under physiological conditions. The experiments were based on dual wavelength flow cytometric detection of fluorescence resonance energy transfer and application of a panel of monoclonal antibodies raised against well described binding sites. Monodisperse distribution of the GPIIb/IIIa heterodimer has been observed and a detailed three-dimensional proximity map of antibody binding sites was constructed on the platelet membrane, under physiological conditions, for the first time. Our data support the view that the GPIIb subunit is in a bent conformation. A detailed analysis of the K(d)-values and the number of binding sites for a set of monoclonal antibodies was also carried out giving supplementary data for the topology of the binding sites. Our results provide a refinement of the membrane-topology of the GPIIb/IIIa heterodimer.

Unusually stable and long-lived ligand-induced conformations of integrins

Integrins are a large family of cell surface receptors that are involved in a wide range of biological processes. The integrin alpha(IIb)beta3 (glycoprotein IIb-IIIa) is a major platelet glycoprotein heterodimeric receptor that mediates platelet aggregation and is currently a target for pharmaceutical intervention. Ligand binding to the receptor has been shown to induce conformational changes by physical methods and the exposure of neoepitopes (the ligand-induced binding sites). Here we show that the antagonist XP280 induces a conformation that is stable to treatment with SDS and that the protein retains this conformation for several days even after dissociation of the inhibitor. These ligand-induced conformational changes take place with purified protein and on intact platelets. They are competable with an RGDS peptide and are stable to reduction but not boiling or treatment with EDTA. The retention of an altered conformation in the absence of the ligand implies the possibility of ligand-induced alteration of biological function even in the absence of ligand. Finally, similar behavior is observed with the integrin alpha(v)beta3, suggesting that access to SDS stable conformations may be conserved throughout the integrin superfamily. The unusual stability, long-lived nature, and potential generality of these conformations could have profound implications for integrin biology.

Conformational changes in tertiary structure near the ligand binding site of an integrin I domain

For efficient ligand binding, integrins must be activated. Specifically, a conformational change has been proposed in a ligand binding domain present within some integrins, the inserted (I) domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340]. This proposal remains controversial, however, despite extensive crystal structure studies on the I domain [Lee, J., Bankston, L., Arnaout, M. & Liddington, R. C. (1995) Structure (London) 3, 1333-1340; Liddington, R. & Bankston, L. (1998) Structure (London) 6, 937-938; Qu, A. & Leahy, D. J. (1996) Structure (London) 4, 931-942; and Baldwin, E. T., Sarver, R. W., Bryant, G. L., Jr., Curry, K. A., Fairbanks, M. B., Finzel, B. C. , Garlick, R. L., Heinrikson, R. L., Horton, N. C. & Kelly, L. L. (1998) Structure (London) 6, 923-935]. By defining the residues present in the epitope of a mAb against the human Mac-1 integrin (alphaMbeta2, CD11b/CD18) that binds only the active receptor, we provide biochemical evidence that the I domain itself undergoes a conformational change with activation. This mAb, CBRM1/5, binds the I domain very close to the ligand binding site in a region that is widely exposed regardless of activation as judged by reactivity with other antibodies. The conformation of the epitope differs in two crystal forms of the I domain, previously suggested to represent active and inactive receptor. Our data suggests that conformational differences in the I domain are physiologically relevant and not merely a consequence of different crystal lattice interactions. We also demonstrate that the transition between the two conformational states depends on species-specific residues at the bottom of the I domain, which are proposed to be in an interface with another integrin domain, and that this transition correlates with functional activity.

The structure of the beta-propeller domain and C-terminal region of the integrin alphaM subunit. Dependence on beta subunit association and prediction of domains

The alphaM subunit of integrin Mac-1 contains several distinct regions in its extracellular segment. The N-terminal region has been predicted to fold into a beta-propeller domain composed of seven beta-sheets each about 60 amino acid residues long, with the I-domain inserted between beta-sheets 2 and 3. The structure of the C-terminal region is unknown. We have used monoclonal antibodies (mAbs) as probes to study the dependence of the structure of different regions of the alphaM subunit on association with the beta2 subunit in the alphaM/beta2 heterodimer. All of the mAbs to the I-domain immunoprecipitated the unassociated alphaM precursor and reacted with the alphaM subunit expressed alone on the surface of COS cells. By contrast, four mAbs to the beta-propeller domain did not react with the unassociated alphaM precursor nor with the uncomplexed alphaM subunit expressed on COS cell surface. The four mAbs were mapped to three subregions in three different beta-sheets, making it unlikely that each recognized an interface between the alpha and beta subunits. These results suggest that folding of different beta-propeller subregions is coordinate and is dependent on association with the beta2 subunit. The segment C-terminal to the beta-propeller domain, residues 599-1092, was studied with nine mAbs. A subset of four mAbs that reacted with the alphaM/beta2 complex but not with the unassociated alphaM subunit were mapped to one subregion, residues 718-759, and five other mAbs that recognized both the unassociated and the complexed alphaM subunit were localized to three other subregions, residues 599-679, 820-882, and 943-1047. This suggests that much of the region C-terminal to the beta-propeller domain folds independently of association with the beta2 subunit. Our data provide new insights into how different domains in the integrin alpha and beta subunits may interact.

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

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

Interaction of beta 3 integrin-derived peptides 214-218 and 217-231 with alpha IIb beta 3 complex and with fibrinogen A alpha-chain

beta 3 integrin-derived peptides 214-218 and 217-231 have been shown previously to inhibit platelet aggregation and fibrinogen binding to platelets and to purified receptor. In this paper we study the activity of both peptides in inhibition of binding of biotinylated fibrinogen to activated platelets and to immobilized alpha IIb beta 3 receptor. We found that the mechanism of this inhibition by both peptides is different 125I-labeled 214-218 peptide binds to alpha IIb beta 3 but in contrast, 125I-labeled 217-231 peptide binds to the A alpha-chain of native and gamma' fibrinogen, as judged by the cross-linking study. In solid phase assay both purified alpha IIb beta 3 and 217-231 peptide bound extensively to native and recombinant fibrinogen, and to fibrinogen with either D574E or D97E mutations in the A alpha-chain. Binding of purified alpha IIb beta 3 to gamma' fibrinogen was markedly impaired whereas binding of 217-231 was only slightly impaired in comparison with native fibrinogen. Binding of 217-231 to fibrinogen fragment X was also reduced suggesting that sequences other than RGDS and RGDF may represent binding sites for this peptide. We hypothesize that the close vicinity of fibrinogen binding site (217-231) and of the site participating in conformational changes of the alpha IIb beta 3 receptor (214-218) may facilitate fibrinogen interaction with its receptor.

Two conformations of the integrin A-domain (I-domain): a pathway for activation?

BACKGROUND

Integrins are plasma membrane proteins that mediate adhesion to other cells and to components of the extracellular matrix. Most integrins are constitutively inactive in resting cells, but are rapidly and reversibly activated in response to agonists, leading to highly regulated cell adhesion. This activation is associated with conformational changes in their extracellular portions, but the nature of the structural changes that lead to a change in adhesiveness is not understood. The interactions of several integrins with their extracellular ligands are mediated by an A-type domain (generally called the I-domain in integrins). Binding of the I-domain to protein ligands is dependent on divalent cations. We have described previously the structure of the I-domain from complement receptor 3 with bound Mg2+, in which the glutamate side chain from a second I-domain completes the octahedral coordination sphere of the metal, acting as a ligand mimetic.

RESULTS

We now describe a new crystal form of the I-domain with bound Mn2+, in which water completes the metal coordination sphere and there is no equivalent of the glutamate ligand. Comparison of the two crystal forms reveals a change in metal coordination which is linked to a large (10 A) shift of the C-terminal helix and the burial of two phenylalanine residues into the hydrophobic core of the Mn2+ form. These structural changes, analogous to those seen in the signal-transducing G-proteins, alter the electrophilicity of the metal, reducing its ability to bind ligand-associated acidic residues, and dramatically alter the surface of the protein implicated in binding ligand.

CONCLUSIONS

Our observations provide the first atomic resolution view of conformational changes in an integrin domain, and suggest how these changes are linked to a change in integrin adhesiveness. We propose that the Mg2+ form represents the conformation of the domain in the active state and the Mn2+ form the conformation in the inactive state of the integrin.

Chemical cross-linking leads to two high molecular mass aggregates of rat alpha 1 beta 1 integrin differing in their conformation but not in their composition

In order to detect protein interactions of the collagen/laminin receptor alpha 1 beta 1 integrin, covalent chemical cross-linking was performed with the homo-bifunctional, amine reactive reagents DSS (disuccinimidylsuberate) and DSP (dithiobis(succinimidylpropionate)). After cross-linking of the 190 kDa rat alpha 1 integrin subunit, immunoblotting revealed two additional, immunoreactive, high molecular mass complexes (M(r) 240/290 k). Generation of the 240/290 kDa aggregates depended on the presence of the intact tertiary protein structure. As shown with immunoaffinity purified proteins, the 240/290 kDa aggregates consist exclusively of alpha 1 and beta 1 integrin subunits. No other cross-linked proteins associated with the alpha 1 or beta 1 subunit were detected. In contrast to the non-cross-linkable alpha 1 beta 1 integrin, the 240/290 kDa aggregates presumably represent active forms of the adhesion receptor, because both bound in vitro to collagen I and IV. This ability of alpha 1 beta 1 integrin to cross-link and produce two additional high molecular mass forms is shared by rat alpha 9 beta 1 integrin. Thus, the cross-linking approach directly indicates that beta 1 integrins occur in different conformations caused by variations in the folding and/or spatial arrangement of their subunits.

The sticking point: how integrins bind to their ligands

The integrin adhesion receptors are alpha beta heterodimers that exist in different ligand-binding states. Because of their large size and conformational lability, it has been difficult to determine how they interact with their ligands. Ligand-binding sites have been identified in the beta subunit, and now more recently in the 'I' domain and EF-hand-like domains V and VI of the alpha subunit. We speculate here about how these various sites might operate together to bind ligand in a stable manner.