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Drug-induced activation of integrin alpha IIb beta 3 leads to minor localized structural changes.

ABSTRACT: Integrins are transmembrane proteins involved in hemostasis, wound healing, immunity and cancer. In response to intracellular signals and ligand binding, integrins adopt different conformations: the bent (resting) form; the intermediate extended form; and the ligand-occupied active form. An integrin undergoing such conformational dynamics is the heterodimeric platelet receptor ?IIb?3. Although the dramatic rearrangement of the overall structure of ?IIb?3 during the activation process is potentially related to changes in the protein secondary structure, this has not been investigated so far in a membrane environment. Here we examine the Mn2+- and drug-induced activation of ?IIb?3 and the impact on the structure of this protein reconstituted into liposomes. By quartz crystal microbalance with dissipation monitoring and activation assays we show that Mn2+ induces binding of the conformation-specific antibody PAC-1, which only recognizes the extended, active integrin. Circular dichroism spectroscopy reveals, however, that Mn2+-treatment does not induce major secondary structural changes of ?IIb?3. Similarly, we found that treatment with clinically relevant drugs (e.g. quinine) led to the activation of ?IIb?3 without significant changes in protein secondary structure. Molecular dynamics simulation studies revealed minor local changes in the beta-sheet probability of several extracellular domains of the integrin. Our experimental setup represents a new approach to study transmembrane proteins, especially integrins, in a membrane environment and opens a new way for testing drug binding to integrins under clinically relevant conditions.

SUBMITTER: Janke U

PROVIDER: S-EPMC6461286 | biostudies-literature | 2019

REPOSITORIES: biostudies-literature

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Drug-induced activation of integrin alpha IIb beta 3 leads to minor localized structural changes.

Janke Una U Kulke Martin M Buchholz Ina I Geist Norman N Langel Walter W Delcea Mihaela M

PloS one 20190412 4

Integrins are transmembrane proteins involved in hemostasis, wound healing, immunity and cancer. In response to intracellular signals and ligand binding, integrins adopt different conformations: the bent (resting) form; the intermediate extended form; and the ligand-occupied active form. An integrin undergoing such conformational dynamics is the heterodimeric platelet receptor αIIbβ3. Although the dramatic rearrangement of the overall structure of αIIbβ3 during the activation process is potentia ...[more]

PMID: 30978226

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Project description:Binding sites on glycoprotein (GP) IIb/IIIa exposed by 0.5 unit/ml alpha-thrombin are insensitive to prostaglandin I2 (PGI2), in contrast with sites exposed by ADP or platelet-activating factor. Here we show that the thrombin receptor agonist peptide (TRAP) (SFLLRN; 15 microM) opens almost the same number of GPIIb/IIIa molecules as 0.5 unit/ml alpha-thrombin (64840 +/- 8920 compared with 81050 +/- 6030 molecules of fibronectin bound/platelet), but these sites rapidly close on addition of PGI2. To investigate whether alpha-thrombin and TRAP initiate different signalling pathways, we measured phospholipase C (PLC)-mediated control of GPIIb/IIIa and its sensitivity to cyclic AMP. Optimal concentrations of alpha-thrombin and TRAP activated PLC maximally, but TRAP induced only about 50% protein kinase C PKC) activation after 10 min stimulation compared with alpha-thrombin. These concentrations also suppressed PGI2-induced cyclic AMP accumulation, with alpha-thrombin inducing complete inhibition and TRAP about 10% less. Direct activation of PKC by phorbol 12-myristate 13-acetate confirmed earlier observations that PGI2-induced cyclic AMP accumulation is partly inhibited via PKC. Applying different concentration of alpha-thrombin, TRAP or a combination of alpha-thrombin and the thrombin receptor inhibitory peptide (TRIP) (Mpr-F-Cha-Cha-RKPNDK-NH2; 800 microM) (Mpr, 3-mercaptopropionic acid; Cha, cyclohexylalanine), we show that the different means of stimulating the thrombin receptor all suppressed PGI2-induced cyclic AMP accumulation via (i) activation of PKC and (ii) activation of the heterotrimeric G-protein, Gi. We conclude that complete inhibition of cyclic AMP accumulation requires activation of both PKC and Gi, as observed with 0.5 unit/ml alpha-thrombin. Although TRAP almost fully exposes GPIIb/IIIa, its activation of PKC is incomplete, enabling PGI2 to raise cyclic AMP concentration from 1.4 +/- 0.7 to 4.1 +/- 1.3 nmol/10(11) platelets (P < 0.005) which is sufficient to close exposed GPIIb/IIIa molecules.

Project description:The major platelet integrin, glycoprotein IIb-IIIa, binds soluble fibrinogen only after platelet activation. To investigate the mechanism by which platelets convert glycoprotein IIb-IIIa into a functional fibrinogen receptor, we characterized the opening and closing of fibrinogen-binding sites in isolated platelet membranes and compared the regulatory properties of membrane-bound glycoprotein IIb-IIIa with those of the detergent-solubilized receptor. Basal fibrinogen binding to the membranes possessed many of the properties of fibrinogen binding to activated platelets; however, less than 10% of glycoprotein IIb-IIIa in the membranes was capable of binding fibrinogen. Preincubating the membranes with either an activating glycoprotein IIb-IIIa antibody or alpha-chymotrypsin increased fibrinogen binding. In contrast, agents that require intracellular mediators, such as platelet agonists, guanine-nucleotide-binding-protein activators and purified protein kinase C, did not stimulate fibrinogen binding to the membranes, suggesting that cytosolic factor(s) may be required for activation of the receptor in platelets. Occupancy of glycoprotein IIb-IIIa in the membranes with RGD (Arg-Gly-Asp)-containing peptides reversibly exposed neoantigenic epitopes and fibrinogen-binding sites in the receptor. These conformational changes required membrane fixation to be maintained following peptide removal. Similar results were obtained with purified glycoprotein IIb-IIIa incorporated into phospholipid vesicles, indicating that the resting state of the receptor is favoured in these environments. In contrast, when the conformation of detergent-solubilized glycoprotein IIb-IIIa was altered by exposure to RGD-containing peptides, the receptor remained active even after incorporation into phospholipid vesicles. These results demonstrate that platelet membranes are a useful model in which to study the regulation of glycoprotein IIb-IIIa and suggest that the environment surrounding the receptor may have a profound influence on this process.

Project description:Integrin alpha(IIb)beta(3) plays a pivotal role in hemostasis and thrombosis by mediating adhesive interactions of platelets. Binding of alpha(IIb)beta(3) to its physiological ligands, immobilized fibrinogen and fibrin, induces outside-in signaling in platelets, leading to their adhesion and spreading even without prior stimulation by agonists. Implicit in these phenomena is a requirement for the linkage between integrins' cytoplasmic tails and intracellular proteins. However, the nature of the initiating signal has not been established. In this study, we examined whether binding of alpha(IIb)beta(3) to immobilized fibrin(ogen), per se, triggers interaction of the integrin with cytoplasmic proteins. Using the integrin-binding skelemin fragment as a marker of exposure of residues involved in the clasp between alpha(IIb) and beta(3) cytoplasmic tails, we showed that its binding site in the membrane-proximal beta(3) 715-730 segment is cryptic and becomes exposed as a result of binding of isolated alpha(IIb)beta(3) to immobilized ligands. Furthermore, the skelemin-like protein present in platelets and CHO cells does not associate with alpha(IIb)beta(3) in resting platelets or suspended alpha(IIb)beta(3)-expressing CHO cells but is recruited to integrin during cell adhesion. In addition, not only beta(3) but also the membrane-proximal 989-1000 segment of the alpha(IIb) cytoplasmic tail binds the skelemin fragment. Finally, the same residues, alpha(IIb) Val(990), alpha(IIb) Arg(995), and beta(3) His(722), involved in the formation of the clasp between the tails are also required for skelemin binding. These studies suggest that ligation of alpha(IIb)beta(3) by immobilized ligands during platelet adhesion induces a transmembrane conformation change in the integrin, resulting in unclasping of the complex between the membrane-proximal parts of cytoplasmic tails, thereby unmasking residues involved in binding the skelemin-like protein. Thus, the junction between alpha(IIb) and beta(3) cytoplasmic tails may contain the critical structural information for the initiation of outside-in signaling.

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Drug-induced activation of integrin alpha IIb beta 3 leads to minor localized structural changes.

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Drug-induced activation of integrin alpha IIb beta 3 leads to minor localized structural changes.

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