Evaluation of the role of platelet integrins in fibronectin-dependent spreading and adhesion

Collagen promotes sustained glycoprotein VI signaling in platelets and cell lines

BACKGROUND

Glycoprotein (GP)VI is the major signaling receptor for collagen on platelets and signals via the associated FcRgamma-chain, which has an immunoreceptor tyrosine-containing activation motif (ITAM).

OBJECTIVE

To determine why GPVI-FcRgamma signals poorly, or not at all, in response to collagen in hematopoietic cell lines, despite robust responses to the GPVI-reactive snake venom toxin convulxin.

METHODS AND RESULTS

Using a nuclear factor of activated T-cells (NFAT) transcriptional reporter assay, a sensitive readout for sustained ITAM signaling, we demonstrate collagen-induced GPVI-FcRgamma signaling in hematopoietic cell lines. This is accompanied by relatively weak but sustained protein tyrosine phosphorylation, in contrast to the stronger but transient response to convulxin. Sustained signaling by collagen is also observed in platelets and is necessary for the maintenance of spreading on collagen. Finally, in cell lines, the inhibitory collagen receptor leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), which is not expressed on platelets but is present on most hematopoietic cells, inhibits GPVI responses to collagen but not convulxin.

CONCLUSION

The inability of previous studies to readily detect GPVI collagen signaling in cell lines is probably because of the weak but sustained nature of the signal and the presence of the inhibitory collagen receptor LAIR-1. In platelets, we propose that GPVI-FcRgamma has evolved to transmit sustained signals in order to maintain spreading over several hours, as well as facilitating rapid activation through release of feedback agonists and integrin activation. The establishment of a cell line NFAT assay will facilitate the molecular dissection of GPVI signaling and the identification of GPVI antagonists in drug discovery.

MyosinIIa contractility is required for maintenance of platelet structure during spreading on collagen and contributes to thrombus stability

BACKGROUND

MyosinIIs are adenosine triphosphate-driven molecular motors that form part of a cell's contractile machinery. They are activated by phosphorylation of their light chains, by either activation of myosin light chain (MLC) kinase or inhibition of MLC phosphatase via Rho kinase (ROCK). MyosinIIa phosphorylation underlies platelet rounding and stress fiber formation.

OBJECTIVE

To identify the functional significance of myosinIIa in platelet spreading and thrombus formation on collagen using inhibitors of ROCK (Y27632) and myosinII (blebbistatin).

RESULTS

Stress fiber formation on collagen is inhibited by both Y27632 and blebbistatin. A substantial proportion of spread platelets generate internal holes or splits on collagen, presumably because of a reduction in contractile strength. Platelet integrity, however, is maintained. In an in vitro model, thrombus embolization on collagen is increased in the presence of Y27632 and blebbistatin at intermediate shear, leading to a reduction in platelet aggregate growth. Moreover, Y27632 causes a marked reduction in thrombus formation in an in vivo laser-injury model.

CONCLUSIONS

MyosinIIa contractility is required for maintenance of platelet structure during spreading on collagen and contributes to thrombus stability.

Platelet integrins and immunoreceptors

Stable platelet adhesion to extracellular matrices and the formation of a hemostatic or pathological thrombus are dependent on integrin alphaIIbbeta3, also known as GPIIb-IIIa. However, maximal platelet responses to vascular injury may involve the participation of other integrins expressed in platelets (alphaVbeta3, alpha2beta1, alpha5beta1, and alpha6beta1). Platelet membrane 'immunoreceptors' contain at least one subunit with an extracellular immunoglobulin superfamily domain and/or an intracellular stimulatory immunoreceptor tyrosine-based activation motif (ITAM) or immunoreceptor tyrosine-based inhibitory motif (ITIM). Platelet ITAM receptors, such as FcgammaRIIA and the GPVI-FcRgamma complex, promote activation of integrins, while ITIM receptors, such as platelet-endothelial cell adhesion molecule-1, may promote their inhibition. This review summarizes the structure and function of platelet integrins and immunoreceptors, the emerging functional relationships between these receptor classes, and the consequences of their interaction for platelet function in hemostasis and thrombosis.

A major role for Scar/WAVE-1 downstream of GPVI in platelets

BACKGROUND

The small GTPase Rac1 plays a critical role in lamellipodia assembly in platelets on matrix proteins in the absence or presence of G protein-coupled receptor (GPCR) agonists. Rac mediates actin assembly via Scar/WAVE, a family of scaffolding proteins that direct actin reorganization by relaying signals from Rac to the Arp2/3 complex.

OBJECTIVE

To evaluate the role of Scar/WAVE-1 in mediating platelet activation and cytoskeletal reorganization.

METHODS AND RESULTS

Using specific antibodies, we demonstrate that murine platelets, like human platelets, express Scar/WAVE-1 and Scar/WAVE-2. Lamellipodia formation in Scar/WAVE-1(-/-) platelets is markedly inhibited on immobilized collagen-related peptide (CRP) and on laminin, both of which signal through the collagen receptor GPVI. In contrast, lamellipodia formation on collagen, which requires release of the GPCR agonists ADP and thromboxane A(2), is not altered. Immobilized fibrinogen supports limited formation of lamellipodia in murine platelets, which is not altered in Scar/WAVE-1(-/-) platelets. As with Rac1(-/-) platelets, Scar/WAVE-1(-/-) platelets exhibit a marked inhibition of aggregation in response to CRP, whereas the response to the GPCR agonist thrombin is not altered. Platelet aggregation on immobilized collagen under shear, which is dependent on signaling by matrix and GPCR agonists, was unaltered in the absence of Scar/WAVE-1.

CONCLUSION

This study demonstrates a major role for Scar/WAVE-1 in mediating platelet cytoskeletal reorganization and aggregate formation downstream of activation by GPVI but not by GPCR agonists.

The leech product saratin is a potent inhibitor of platelet integrin α2β1 and von Willebrand factor binding to collagen

Subendothelial collagen plays an important role, via both direct and indirect mechanisms, in the initiation of thrombus formation at sites of vascular injury. Collagen binds plasma von Willebrand factor, which mediates platelet recruitment to collagen under high shear. Subsequently, the direct binding of the platelet receptors glycoprotein VI and alpha2beta1 to collagen is critical for platelet activation and stable adhesion. Leeches, have evolved a number of inhibitors directed towards platelet-collagen interactions so as to prevent hemostasis in the host during hematophagy. In this article, we describe the molecular mechanisms underlying the ability of the leech product saratin to inhibit platelet binding to collagen. In the presence of inhibitors of ADP and thromboxane A2, both saratin and 6F1, a blocking alpha2beta1 mAb, abrogated platelet adhesion to fibrillar and soluble collagen. Additionally, saratin eliminated alpha2beta1-dependent platelet adhesion to soluble collagen in the presence of an Src kinase inhibitor. Moreover, saratin prevented platelet-rich plasma adhesion to fibrillar collagen, a process dependent upon both alpha2beta1 and von Willebrand factor binding to collagen. Furthermore, saratin specifically inhibited the binding of the alpha2 integrin subunit I domain to collagen, and prevented platelet adhesion to collagen under flow to the same extent as observed in the presence of a combination of mAbs to glycoprotein Ib and alpha2beta1. These results demonstrate that saratin interferes with integrin alpha2beta1 binding to collagen in addition to inhibiting von Willebrand factor-collagen binding, presumably by binding to an overlapping epitope on collagen. This has significant implications for the use of saratin as a tool to inhibit platelet-collagen interactions.

Trophoblast adhesion of the peri-implantation mouse blastocyst is regulated by integrin signaling that targets phospholipase C

Integrin signaling modulates trophoblast adhesion to extracellular matrices during blastocyst implantation. Fibronectin (FN)-binding activity on the apical surface of trophoblast cells is strengthened after elevation of intracellular Ca(2+) downstream of integrin ligation by FN. We report here that phosphoinositide-specific phospholipase C (PLC) mediates Ca(2+) signaling in response to FN. Pharmacological agents used to antagonize PLC (U73122) or the inositol phosphate receptor (Xestospongin C) inhibited FN-induced elevation of intracellular Ca(2+) and prevented the upregulation of FN-binding activity. In contrast, inhibitors of Ca(2+) influx through either voltage-gated or non-voltage-gated Ca(2+) channels were without effect. Inhibition of protein tyrosine kinase activity by genistein, but not G-protein inhibition by suramin, blocked FN-induced intracellular Ca(2+) signaling and upregulation of adhesion, consistent with involvement of PLC-gamma. Confocal immunofluorescence imaging of peri-implantation blastocysts demonstrated that PLC-gamma2, but not PLC-gamma1 nor PLC-beta1, accumulated near the outer surface of the embryo. Phosphotyrosine site-directed antibodies revealed phosphorylation of PLC-gamma2, but not PLC-gamma1, upon integrin ligation by FN. These data suggest that integrin-mediated activation of PLC-gamma to initiate phosphoinositide signaling and intracellular Ca(2+) mobilization is required for blastocyst adhesion to FN. Signaling cascades regulating PLC-gamma could, therefore, control a critical feature of trophoblast differentiation during peri-implantation development.

Differential regulation of adapter proteins Dok2 and Dok1 in platelets, leading to an association of Dok2 with integrin alphaIIbbeta3

BACKGROUND

We previously demonstrated that Dok2 is rapidly phosphorylated on tyrosine residues in platelets in response to thrombin, the immunoreceptor tyrosine-based activation motif-coupled collagen receptor glycoprotein (GP) VI, and by integrin alphaIIbbeta3.

OBJECTIVES AND METHODS

In this study we further delineate the regulation of phosphorylation of Dok2 and compare this to the related adapter Dok1.

RESULTS

We demonstrate expression of Dok1 in platelets and the unexpected observation that the adapter protein undergoes tyrosine phosphorylation in response to thrombin but not to GPVI or integrin alphaIIbbeta3. Furthermore, Dok1 phosphorylation is transient, peaking at 30 s and returning to basal by 5 min, whereas Dok2 phosphorylation is delayed but sustained. Dok2 phosphorylation, but not that of Dok1, is inhibited by Src kinase inhibitors and by chelation of intracellular calcium. Further, phosphorylation of Dok2 by thrombin and integrin alphaIIbbeta3 in mouse platelets is independent of Syk and phospholipase Cgamma2. Additionally, Dok2 coimmunoprecipitates with integrin alphaIIbbeta3 downstream of Src kinases.

CONCLUSIONS

These results demonstrate differential modes of regulation of Dok1 and Dok2 in platelets. Further, they raise the interesting possibility that Dok2 plays an important role in integrin outside-in signaling through a physical and functional interaction with integrin alphaIIbbeta3.

N-WASP involvement in dorsal ruffle formation in mouse embryonic fibroblasts

The Wiskott-Aldrich syndrome protein (WASP) family activates the Arp2/3 complex leading to the formation of new actin filaments. Here, we study the involvement of Scar1, Scar2, N-WASP, and Arp2/3 complex in dorsal ruffle formation in mouse embryonic fibroblasts (MEFs). Using platelet-derived growth factor to stimulate circular dorsal ruffle assembly in primary E13 and immortalized E9 Scar1(+/+) and Scar1 null MEFs, we establish that Scar1 loss does not impair the formation of dorsal ruffles. Reduction of Scar2 protein levels via small interfering RNA (siRNA) also did not affect dorsal ruffle production. In contrast, wiskostatin, a chemical inhibitor of N-WASP, potently suppressed dorsal ruffle formation in a dose-dependent manner. Furthermore, N-WASP and Arp2 siRNA treatment significantly decreased the formation of dorsal ruffles in MEFs. In addition, the expression of an N-WASP truncation mutant that cannot bind Arp2/3 complex blocked the formation of these structures. Finally, N-WASP(-/-) fibroblast-like cells generated aberrant dorsal ruffles. These ruffles were highly unstable, severely depleted of Arp2/3 complex, and diminished in size. We hypothesize that N-WASP and Arp2/3 complex are part of a multiprotein assembly important for the generation of dorsal ruffles and that Scar1 and Scar2 are dispensable for this process.

Distinct but critical roles for integrin ?IIb?3in platelet lamellipodia formation on fibrinogen, collagen-related peptide and thrombin

Integrins are the major receptor type known to facilitate cell adhesion and lamellipodia formation on extracellular matrix proteins. However, collagen-related peptide and thrombin have recently been shown to mediate platelet lamellipodia formation when presented as immobilized surfaces. The aims of this study were to establish if there exists a role for the platelet integrin alpha(IIb)beta(3) in this response; and if so, whether signalling from the integrin is required for lamellipodia formation on these surfaces. Real-time analysis was used to compare platelet morphological changes on surfaces of fibrinogen, collagen-related peptide or thrombin in the presence of various pharmacological inhibitors and platelets from 'knockout' mice. We demonstrate that collagen-related peptide and thrombin stimulate distinct patterns of platelet lamellipodia formation and elevation of intracellular Ca(2+) to that induced by the integrin alpha(IIb)beta(3) ligand, fibrinogen. Nevertheless, lamellipodia formation on collagen-related peptide and thrombin is dependent upon engagement of alpha(IIb)beta(3), consistent with release of alpha(IIb)beta(3) ligand(s) from platelet granules. However, the requirement for signalling by the integrin on fibrinogen can be bypassed by the addition of thrombin to the solution. These observations reveal a critical role for alpha(IIb)beta(3) in forming lamellipodia on collagen-related peptide and thrombin which is dependent on its ability to function as an adhesive receptor but not necessarily on its ability to signal. These results suggest that integrins may play an important role in lamellipodia formation triggered by nonintegrin ligands in platelets and possibly in other cell types.

The synergy site of fibronectin is required for strong interaction with the platelet integrin alphaIIbbeta3

Integrins are a class of cell adhesion molecules that bind to ligands containing the RGD peptide sequence. There is increasing evidence that peptide sites other than the RGD site are required for optimal binding of integrins with their ligands. We have examined the sites on the protein fibronectin that are needed for optimal binding to the platelet integrin alphaIIbbeta3 using a strategy of site directed mutagenesis. Single amino acids near the RGD site or near the synergy site of fibronectin were mutated and the resultant proteins were expressed in a bacterial expression system. The purified protein was coated onto glass cover slips. Platelets, expressing alphaIIbbeta3 were perfused over the surface at physiologically relevant shear rates and the extent of adhesion was quantified. We found that the single amino acid substitution of the aspartic acid in the RGD sequence, D1495A, completely abolished adhesion. Surprisingly, the mutants R1445A and R1448Q that are near the RGD site also abolished adhesion of platelets under flow. Additionally, the synergy site mutants R1371A, R1374Q, or R1379A displayed only minimal adhesion of platelets. These results show that the binding site for alphaIIbbeta3 on fibronectin extends over a considerable distance from the RGD site and that these distant sites are required for optimal attachment of cells in the presence of physiologically relevant shear stress.

Analysis of intact phosphoinositides in biological samples

It is now apparent that each of the known, naturally occurring polyphosphoinositides, the phosphatidylinositol monophosphates (PtdIns3P, PtdIns4P, PtdIns5P), phosphatidylinositol bisphosphates [PtdIns(3,4)P(2), PtdIns(3,5)P(2), PtdIns(4,5)P(2)], and phosphatidylinositol trisphosphate [PtdIns(3,4,5)P(3)], have distinct roles in regulating many cellular events, including intracellular signaling, migration, and vesicular trafficking. Traditional identification techniques require [(32)P]inorganic phosphate or [(3)H]inositol radiolabeling, acidified lipid extraction, deacylation, and ion-exchange head group separation, which are time-consuming and not suitable for samples in which radiolabeling is impractical, thus greatly restricting the study of these lipids in many physiologically relevant systems. Thus, we have developed a novel, high-efficiency, buffered citrate extraction methodology to minimize acid-induced phosphoinositide degradation, together with a high-sensitivity liquid chromatography-mass spectrometry (LC-MS) protocol using an acetonitrile-chloroform-methanol-water-ethylamine gradient with a microbore silica column that enables the identification and quantification of all phosphoinositides in a sample. The liquid chromatograph is sufficient to resolve PtdInsP(3) and PtdInsP(2) regioisomers; however, the PtdInsP regioisomers require a combination of LC and diagnostic fragmentation to MS(3). Data are presented using this approach for the analysis of phosphoinositides in human platelet and yeast samples.

Decreased Plasma Fibronectin Leads to Delayed Thrombus Growth in Injured Arterioles

Objective— Plasma fibronectin (FN) is decreased in several clinical conditions. We were interested to study the thrombotic and hemostatic consequences of the decrease in plasma FN (pFN), the role of FN splice variants in thrombosis, and to examine whether pFN incorporates into thrombi in vivo.

Methods and Results— We compared the thrombotic response to a vessel injury in FN heterozygous ( FN +/−) mice and corresponding FN +/+ mice. Although normal thrombosis in venules was observed, a decrease to half in the pFN concentration in FN +/− mice caused a delay in the appearance of thrombi in arterioles and consequently a delay in their occlusion. We were able to rescue the thrombotic defect in the FN +/− mice by infusion of rat pFN. Additionally, we could show intense incorporation of fluorescent pFN-coated microspheres into the developing thrombi. Moreover, we found that mice expressing FN without the EIIIA or EIIIB domains specific to cellular FN including platelet FN had no thrombotic defect.

Conclusions— Mice heterozygous for FN have a striking defect in thrombus initiation and growth in arterioles attributable to the decrease of pFN. Our study is an example of haploid insufficiency for FN, and it emphasizes the fundamental role of this plasma protein in thrombosis in the arterial system.

von Willebrand factor mediates platelet spreading through glycoprotein Ib and alpha(IIb)beta3 in the presence of botrocetin and ristocetin, respectively

BACKGROUND

von Willebrand factor (VWF) plays a critical role in the process of hemostasis by mediating flow-dependent adhesion and spreading of platelets on exposed extracellular matrix proteins following vascular injury. To accomplish this, VWF binds to two distinct platelet receptors: glycoprotein (GP)Ib-IX-V and integrin alpha(IIb)beta3.

OBJECTIVE

To evaluate the ability of GPIb and alpha(IIb)beta3 to mediate platelet adhesion and lamellipodia formation on immobilized VWF in the presence of the biochemical modulators, ristocetin and botrocetin.

RESULTS

In the presence of botrocetin and inhibitors of adenosine diphosphate (ADP) and thromboxane A2 (TxA2), VWF is able to support formation of lamellipodia through a GPIb-dependent mechanism that is independent of alpha(IIb)beta3 and PI3-kinase. Lamellipodia formation under these conditions is incomplete. In marked contrast, in the presence of ristocetin, VWF stimulates formation of fully spread lamellipodia through a pathway that is dependent upon alpha(IIb)beta3 and PI3-kinase. Furthermore, alpha(IIb)beta3 also supports platelet spreading on VWF alone, but only in the absence of inhibitors of ADP and TxA2. The localization of filamentous actin and the Arp2/3 complex in platelets on VWF in the presence of botrocetin and ristocetin are distinct, yielding disparate lamellipodium kinetic signatures. Interestingly, botrocetin significantly enhances platelet adhesion to VWF under flow in whole blood in an alpha(IIb)beta3-independent manner, while ristocetin augments washed platelet adhesion and spreading to VWF under flow in an alpha(IIb)beta3-dependent manner.

CONCLUSIONS

These observations demonstrate that VWF is able to induce lamellipodia formation through distinct receptors, and has important consequences for investigation of the role of VWF-GPIb interactions in the context of platelet regulation.

Photocoupling of fibronectin to titanium surfaces influences keratinocyte adhesion, pellicle formation and thrombogenicity

OBJECTIVES

Coating of implant surfaces with biomolecules can influence basic host responses and enhance subsequent tissue integration. The biological factors have to be immobilized on the implant material. Human fibronectin (Fn) was used as a model protein and covalently coupled to titanium (Ti) surfaces via silanization and an anthraquinone linker. The impact on several aspects of initial host/biomaterial interactions (keratinocyte adhesion, platelet interactions and pellicle formation) was studied.

METHODS

Coupling efficiency was characterized by immunological techniques. The effects of coupled Fn on initial host/biomaterial interactions were assessed. Cell adhesion and spreading were investigated by fluorescent staining, pellicle formation by an acoustic sensor system (quartz crystal microbalance with dissipation, QCM-D), and platelet adhesion as one parameter mediating the inflammatory response by scanning electron microscopy (SEM) and immunological assays.

RESULTS

Coupling efficiency was related to irradiation time used for photochemical coupling of the UV-activated anthraquinone to the silanized Ti surface. With an optimized protocol, the amount of Fn coupled to the surface could be almost doubled compared to standard dip-coating methods. On the anthraquinone-coupled Fn coatings, cell adhesion and spreading of human keratinocytes was significantly enhanced. Online detection of pellicle formation revealed strong reversibility of saliva protein adhesion on Fn coated surfaces compared to the pure Ti surface. Furthermore, the Fn coated Ti showed a low thrombogenicity.

SIGNIFICANCE

This study suggests that anthraquinone-coupled biological coatings may be useful for biofunctionalization of Ti dental implants by enhancement of soft tissue re-integration (restoration of the epithelial seal) combined with diminished pellicle formation.

Rac1 is essential for platelet lamellipodia formation and aggregate stability under flow

The role of Rac family proteins in platelet spreading on matrix proteins under static and flow conditions has been investigated by using Rac-deficient platelets. Murine platelets form filopodia and undergo limited spreading on fibrinogen independent of Rac1 and Rac2. In the presence of thrombin, marked lamellipodia formation is observed on fibrinogen, which is abrogated in the absence of Rac1. However, Rac1 is not required for thrombin-induced aggregation or elevation of F-actin levels. Formation of lamellipodia on collagen and laminin is also Rac1-dependent. Analysis of platelet adhesion dynamics on collagen under flow conditions in vitro revealed that Rac1 is required for platelet aggregate stability at arterial rates of shear, as evidenced by a dramatic increase in platelet embolization. Furthermore, studies employing intravital microscopy demonstrated that Rac1 plays a critical role in the development of stable thrombi at sites of vascular injury in vivo. Thus, our data demonstrated that Rac1 is essential for lamellipodia formation in platelets and indicated that Rac1 is required for aggregate integrity leading to thrombus formation under physiologically relevant levels of shear both in vitro and in vivo.

Laminin stimulates spreading of platelets through integrin alpha6beta1-dependent activation of GPVI

The extracellular matrix protein, laminin, supports platelet adhesion through binding to integrin alpha6beta1 In the present study, we demonstrate that human laminin, purified from placenta, also stimulates formation of filopodia and lamellipodia in human and mouse platelets through a pathway that is dependent on alpha6beta1 and the collagen receptor GPVI. The integrin alpha6beta1 is essential for adhesion to laminin, as demonstrated using an alpha6-blocking antibody, whereas GPVI is dispensable for this response, as shown using "knockout" mouse platelets. On the other hand, lamellipodia formation on laminin is completely inhibited in the absence of GPVI, although filopodia formation remains and is presumably mediated via alpha6beta1 Lamellipodia and filopodia formation are inhibited in Syk-deficient platelets, demonstrating a key role for the kinase in signaling downstream of GPVI and integrin alpha6beta1 GPVI was confirmed as a receptor for laminin using surface plasmon resonance spectroscopy and by demonstration of lamellipodia formation on laminin in the presence of collagenase. These results identify GPVI as a novel receptor for laminin and support a model in which integrin alpha6beta1 brings laminin to GPVI, which in turn mediates lamellipodia formation. We speculate that laminin contributes to platelet spreading in vivo through a direct interaction with GPVI.

GPVI and integrin alphaIIb beta3 signaling in platelets

This review summarizes recent developments in our understanding of the molecular basis of platelet activation by two distinct types of surface receptor, the immunoglobulin GPVI, and the integrin alphaIIb beta3 (also known as GPIIbIIIa). These two classes of receptor signal through similar yet distinct tyrosine kinase-based signaling cascades leading to activation of phospholipase C gamma2. The significance of these signaling cascades in platelet adhesion and platelet aggregation at arterial rates of shear is discussed.