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

Differential dynamics of early stages of platelet adhesion and spreading on collagen IV- and fibrinogen-coated surfaces

Background: Upon wound formation, platelets adhere to the neighboring extracellular matrix and spread on it, a process which is critical for physiological wound healing. Multiple external factors, such as the molecular composition of the environment and its mechanical properties, play a key role in this process and direct its speed and outcome. Methods: We combined live cell imaging, quantitative interference reflection microscopy and cryo-electron tomography to characterize, at a single platelet level, the differential spatiotemporal dynamics of the adhesion process to fibrinogen- and collagen IV-functionalized surfaces. Results: Initially, platelets sense both substrates by transient rapid extensions of filopodia. On collagen IV, a short-term phase of filopodial extension is followed by lamellipodia-based spreading. This transition is preceded by the extension of a single or couple of microtubules into the platelet's periphery and their apparent insertion into the core of the filopodia. On fibrinogen surfaces, the filopodia-to-lamellipodia transition was partial and microtubule extension was not observed leading to limited spreading, which could be restored by manganese or thrombin. Conclusions: Based on these results, we propose that interaction with collagen IV stimulate platelets to extend microtubules to peripheral filopodia, which in turn, enhances filopodial-to-lamellipodial transition and overall lamellipodia-based spreading. Fibrinogen, on the other hand, fails to induce these early microtubule extensions, leading to full lamellipodia spreading in only a fraction of the seeded platelets. We further suggest that activation of integrin αIIbβ3 is essential for filopodial-to-lamellipodial transition, based on the capacity of integrin activators to enhance lamellipodia spreading on fibrinogen.

The basement membrane protein nidogen-1 supports platelet adhesion and activation

The core structure of the extracellular basement membrane is made up of self-assembling networks of collagen and laminin which associate with each other through the bridging adapter proteins including the sulfated monomeric glycoprotein nidogen. While collagen and laminin are known to support platelet adhesion and activation via β1 integrins and glycoprotein (GP) VI, respectively, whether nidogen contributes to platelet activation and hemostasis is unknown. In this study, we demonstrate that recombinant human nidogen-1 supports platelet adhesion and stimulates platelet activation in a phospholipase-C γ-2 (PLCγ2), Src and Syk kinase-dependent manner downstream. Platetet adhesion to nidogen-1 was inhibited by blocking the platelet receptors GPVI and β1 integrins. Platelet adhesion to nidogen-1 activated the IκB kinase (IKK) complex, while pharmacological inhibition of IKK blocked platelet spreading on nidogen. Taken together our results suggest that nidogen may play a redundant role in hemostasis by activating platelets downstream of GPVI.

Recent advances in microfluidic platelet function assays: Moving microfluidics into clinical applications

The analysis of platelet aggregation and thrombosis kinetics has significantly advanced with progress in microfluidic technology. However, the results of platelet aggregation tests do not fully reflect the observed clinical outcomes. To address the present unmet clinical needs, the basic but essential biology of platelets should be reconsidered in relation to the characteristics of microfluidic systems employed for platelet tests. To this end, the present article provides an overview of commercially available point of care devices and focuses on recent microfluidic studies, describing their measurement principles. We critically discuss the characteristics of the microfluidics systems used to evaluate the complex processes underlying platelet aggregation, and that are specifically designed to mimic the pathophysiological environment of blood vessels, including hemodynamic factors as well as blood vessel injury. To this end, we summarize unsolved issues related to the application of platelet function tests based on microfluidics. Overall, we confirm that platelet function tests based on microfluidics provide a versatile platform that encompasses a variety of basic research, as well as clinical diagnostic applications.

Differential Modulation of Platelet Adhesion and Spreading by Adhesive Ligand Density

Platelets play a major role in hemostasis and thrombosis, by binding to the underlying extracellular matrix around injured blood vessels, via integrin receptors. In this study, we investigated the effects of adhesive ligand spacing on the stability of platelets' adhesion and the mode of their spreading on extracellular surfaces. Toward this end, we have examined the differential adhesion and spreading of human platelets onto nanogold-patterned surfaces, functionalized with the αIIbβ3 integrin ligand, SN528. Combining light- and scanning electron-microscopy, we found that interaction of platelets with surfaces coated with SN528 at spacing of 30-60 nm induces the extension of filopodia through which the platelets stably attach to the nanopatterned surface and spread on it. Increasing the nanopattern-gold spacing to 80-100 nm resulted in a dramatic reduction (>95%) in the number of adhering platelets. Surprisingly, a further increase in ligand spacing to 120 nm resulted in platelet binding to the surface at substantially larger numbers, yet these platelets remained discoid and were essentially devoid of filopodia and lamellipodia. These results indicate that the stimulation of filopodia extension by adhering platelets, and the consequent spreading on these surfaces depend on different ligand densities. Thus, the extension of filopodia occurs on surfaces with a ligand spacing of 100 nm or less, while the sustainability and growth of these initial adhesions and induction of extensive platelet adhesion and spreading requires lower ligand-to-ligand spacing (≤60 nm). The mechanisms underlying this differential ligand-density sensing by platelets, as well as the unexpected retention of discoid platelets on surfaces with even larger spacing (120 nm) are discussed.

Mucor circinelloides induces platelet aggregation through integrin αIIbβ3 and FcγRIIA

Thrombosis is a hallmark of the fatal fungal infection mucormycosis. Yet, the platelet activation pathway in response to mucormycetes is unknown. In this study we determined the platelet aggregation potential of Mucor circinelloides (M. circinelloides) NRRL3631, characterized the signaling pathway facilitating aggregation in response to fungal spores, and identified the influence of the spore developmental stage upon platelet aggregation potential. Using impedance and light-transmission aggregometry, we showed that M. circinelloides induced platelet aggregation in whole blood and in platelet-rich plasma, respectively. The formation of large spore-platelet aggregates was confirmed by light-sheet microscopy, which showed spores dispersed throughout the aggregate. Aggregation potential was dependent on the spore's developmental stage, with the strongest platelet aggregation by spores in mid-germination. Inhibitor studies revealed platelet aggregation was mediated by the low affinity IgG receptor FcγRIIA and integrin αIIbβ3; Src and Syk tyrosine kinase signaling; and the secondary mediators TxA₂ and ADP. Flow cytometry of antibody stained platelets showed that interaction with spores increased expression of platelet surface integrin αIIbβ3 and the platelet activation marker CD62P. Together, this is the first elucidation of the signaling pathways underlying thrombosis formation during a fungal infection, highlighting targets for therapeutic intervention.

Thrombin-induced cytoskeleton dynamics in spread human platelets observed with fast scanning ion conductance microscopy

Platelets are small anucleate blood cells involved in haemostasis. Platelet activation, caused by agonists such as thrombin or by contact with the extracellular matrix, leads to platelet adhesion, aggregation, and coagulation. Activated platelets undergo shape changes, adhere, and spread at the site of injury to form a blood clot. We investigated the morphology and morphological dynamics of human platelets after complete spreading using fast scanning ion conductance microscopy (SICM). In contrast to unstimulated platelets, thrombin-stimulated platelets showed increased morphological activity after spreading and exhibited dynamic morphological changes in the form of wave-like movements of the lamellipodium and dynamic protrusions on the platelet body. The increase in morphological activity was dependent on thrombin concentration. No increase in activity was observed following exposure to other activation agonists or during contact-induced activation. Inhibition of actin polymerization and inhibition of dynein significantly decreased the activity of thrombin-stimulated platelets. Our data suggest that these morphological dynamics after spreading are thrombin-specific and might play a role in coagulation and blood clot formation.

Oral administration of Bruton's tyrosine kinase inhibitors impairs GPVI-mediated platelet function

The Tec family kinase Bruton's tyrosine kinase (Btk) plays an important signaling role downstream of immunoreceptor tyrosine-based activation motifs in hematopoietic cells. Mutations in Btk are involved in impaired B-cell maturation in X-linked agammaglobulinemia, and Btk has been investigated for its role in platelet activation via activation of the effector protein phospholipase Cγ2 downstream of the platelet membrane glycoprotein VI (GPVI). Because of its role in hematopoietic cell signaling, Btk has become a target in the treatment of chronic lymphocytic leukemia and mantle cell lymphoma; the covalent Btk inhibitor ibrutinib was recently approved by the US Food and Drug Administration for treatment of these conditions. Antihemostatic events have been reported in some patients taking ibrutinib, although the mechanism of these events remains unknown. We sought to determine the effects of Btk inhibition on platelet function in a series of in vitro studies of platelet activation, spreading, and aggregation. Our results show that irreversible inhibition of Btk with two ibrutinib analogs in vitro decreased human platelet activation, phosphorylation of Btk, P-selectin exposure, spreading on fibrinogen, and aggregation under shear flow conditions. Short-term studies of ibrutinib analogs administered in vivo also showed abrogation of platelet aggregation in vitro, but without measurable effects on plasma clotting times or on bleeding in vivo. Taken together, our results suggest that inhibition of Btk significantly decreased GPVI-mediated platelet activation, spreading, and aggregation in vitro; however, prolonged bleeding was not observed in a model of bleeding.

Protein kinase Cε and protein kinase Cθ double-deficient mice have a bleeding diathesis

BACKGROUND

In comparison to the classical isoforms of protein kinase C (PKC), the novel isoforms are thought to play minor or inhibitory roles in the regulation of platelet activation and thrombosis.

OBJECTIVES

To measure the levels of PKCθ and PKCε and to investigate the phenotype of mice deficient in both novel PKC isoforms.

METHODS

Tail bleeding and platelet activation assays were monitored in mice and platelets from mice deficient in both PKCθ and PKCε.

RESULTS

PKCε plays a minor role in supporting aggregation and secretion following stimulation of the collagen receptor GPVI in mouse platelets but has no apparent role in spreading on fibrinogen. PKCθ, in contrast, plays a minor role in supporting adhesion and filopodial generation on fibrinogen but has no apparent role in aggregation and secretion induced by GPVI despite being expressed at over 10 times the level of PKCε. Platelets deficient in both novel isoforms have a similar pattern of aggregation downstream of GPVI and spreading on fibrinogen as the single null mutants. Strikingly, a marked reduction in aggregation on collagen under arteriolar shear conditions is observed in blood from the double but not single-deficient mice along with a significant increase in tail bleeding.

CONCLUSIONS

These results reveal a greater than additive role for PKCθ and PKCε in supporting platelet activation under shear conditions and demonstrate that, in combination, the two novel PKCs support platelet activation.

Distinct and overlapping functional roles of Src family kinases in mouse platelets

BACKGROUND AND OBJECTIVES

Src family kinases (SFKs) play a critical role in initiating and propagating signals in platelets. The aims of this study were to quantitate SFK members present in platelets and to analyze their contribution to platelet regulation using glycoprotein VI (GPVI) and intregrin αIIbβ3, and in vivo.

METHODS AND RESULTS

Mouse platelets express four SFKs, Fgr, Fyn, Lyn and Src, with Lyn expressed at a considerably higher level than the others. Using mutant mouse models, we demonstrate that platelet activation by collagen-related peptide (CRP) is delayed and then potentiated in the absence of Lyn, but only marginally reduced in the absence of Fyn or Fgr, and unaltered in the absence of Src. Compound deletions of Lyn/Src or Fyn/Lyn, but not of Fyn/Src or Fgr/Lyn, exhibit a greater delay in activation relative to Lyn-deficient platelets. Fibrinogen-adherent platelets show reduced spreading in the absence of Src, potentiation in the absence of Lyn, but no change in the absence of Fyn or Fgr. In mice double-deficient in Lyn/Src or Fgr/Lyn, the inhibitory role of Lyn on spreading on fibrinogen is lost. Lyn is the major SFK-mediating platelet aggregation on collagen at arterial shear and its absence leads to a reduction in thrombus size in a laser injury model.

CONCLUSION

These results demonstrate that SFKs share individual and overlapping roles in regulating platelet activation, with Lyn having a dual role in regulating GPVI signaling and an inhibitory role downstream of αIIbβ3, which requires prior signaling through Src.

A role for adhesion and degranulation-promoting adapter protein in collagen-induced platelet activation mediated via integrin α(2) β(1)

BACKGROUND

Collagen-induced platelet activation is a key step in the development of arterial thrombosis via its interaction with the receptors glycoprotein (GP)VI and integrin α(2) β(1) . Adhesion and degranulation-promoting adapter protein (ADAP) regulates α(IIb) β(3) in platelets and α(L) β(2) in T cells, and is phosphorylated in GPVI-deficient platelets activated by collagen.

OBJECTIVES

To determine whether ADAP plays a role in collagen-induced platelet activation and in the regulation and function of α(2) β(1).

METHODS

Using ADAP(-/-) mice and synthetic collagen peptides, we investigated the role of ADAP in platelet aggregation, adhesion, spreading, thromboxane synthesis, and tyrosine phosphorylation.

RESULTS AND CONCLUSIONS

Platelet aggregation and phosphorylation of phospholipase Cγ2 induced by collagen were attenuated in ADAP(-/-) platelets. However, aggregation and signaling induced by collagen-related peptide (CRP), a GPVI-selective agonist, were largely unaffected. Platelet adhesion to CRP was also unaffected by ADAP deficiency. Adhesion to the α(2) β(1) -selective ligand GFOGER and to a peptide (III-04), which supports adhesion that is dependent on both GPVI and α(2) β(1), was reduced in ADAP(-/-) platelets. An impedance-based label-free detection technique, which measures adhesion and spreading of platelets, indicated that, in the absence of ADAP, spreading on GFOGER was also reduced. This was confirmed with non-fluorescent differential-interference contrast microscopy, which revealed reduced filpodia formation in ADAP(-/-) platelets adherent to GFOGER. This indicates that ADAP plays a role in mediating platelet activation via the collagen-binding integrin α(2) β(1). In addition, we found that ADAP(-/-) mice, which are mildly thrombocytopenic, have enlarged spleens as compared with wild-type animals. This may reflect increased removal of platelets from the circulation.

pH-controlled delivery of luminescent europium coated nanoparticles into platelets

Water soluble, luminescent gold nanoparticles are delivered into human platelets via a rapid, pH-controlled mechanism using a pH low insertion peptide, pHLIP. The approach introduces cocoating of gold nanoparticles with a europium luminescent complex, EuL and the pHLIP peptide to give pHLIP•EuL•Au. The 13-nm diameter gold nanoparticles act as a scaffold for the attachment of both the luminescent probe and the peptide to target delivery. Their size allows delivery of approximately 640 lanthanide probes per nanoparticle to be internalized in human platelets, which are not susceptible to transfection or microinjection. The internalization of pHLIP•EuL•Au in platelets, which takes just minutes, was studied with a variety of imaging modalities including luminescence, confocal reflection, and transmission electron microscopy. The results show that pHLIP•EuL•Au only enters the platelets in low pH conditions, pH 6.5, mediated by the pHLIP translocation across the membrane, and not at pH 7.4. Luminescence microscopy images of the treated platelets show clearly the red luminescence signal from the europium probe and confocal reflection microscopy confirms the presence of the gold particles. Furthermore, transmission electron microscopy gives a detailed insight of the internalization and spatial localization of the gold nanoparticles in the platelets. Thus, we demonstrate the potential of the design to translocate multimodal nanoparticle probes into cells in a pH dependent manner.

Preparation and characterization of chitosan-heparin composite matrices for blood contacting tissue engineering

Chitosan has been widely used for biomaterial scaffolds in tissue engineering because of its good mechanical properties and cytocompatibility. However, the poor blood compatibility of chitosan has greatly limited its biomedical utilization, especially for blood contacting tissue engineering. In this study, we exploited a polymer blending procedure to heparinize the chitosan material under simple and mild conditions to improve its antithrombogenic property. By an optimized procedure, a macroscopically homogeneous chitosan-heparin (Chi-Hep) blended suspension was obtained, with which Chi-Hep composite films and porous scaffolds were fabricated. X-ray photoelectron spectroscopy and sulfur elemental analysis confirmed the successful immobilization of heparin in the composite matrices (i.e. films and porous scaffolds). Toluidine blue staining indicated that heparin was distributed homogeneously in the composite matrices. Only a small amount of heparin was released from the matrices during incubation in normal saline for 10 days. The composite matrices showed improved blood compatibility, as well as good mechanical properties and endothelial cell compatibility. These results suggest that the Chi-Hep composite matrices are promising candidates for blood contacting tissue engineering.

Platelet and endothelial adhesion on fluorosurfactant polymers designed for vascular graft modification

A prominent failure mechanism of small diameter expanded polytetrafluoroethylene (ePTFE) vascular grafts is platelet-mediated thrombosis. We have designed a surface modification for ePTFE consisting of a self-assembling fluorosurfactant polymer (FSP) bearing biologically active ligands, including adhesive peptides and polysaccharide moieties. The goal of this biomimetic construct is to improve graft hemocompatibility by promoting rapid surface endothelialization, whereas minimizing platelet adhesion. Here we present a direct comparison of platelet and endothelial cell (EC) adhesion to FSPs containing one of three cell-adhesion peptides: cyclic Arg-Gly-Asp-D-Phe-Glu (cRGD), cyclic *Cys-Arg-Arg-Glu-Thr-Ala-Trp-Ala-Cys* (cRRE, *denotes disulfide bond cyclization), linear Gly-Arg-Gly-Asp-Ser-Pro-Ala (RGD), or a polysaccharide moiety: oligomaltose (M-7), later designed to prevent nonspecific protein adhesion. Measurements of soluble peptide-integrin binding indicated that cRRE exhibits very low affinity for the alpha(IIb)beta(3) platelet fibrinogen receptor. Static and dynamic adhesion of washed, activated platelets on FSP-modified surfaces revealed that M-7 and cRRE promote significantly less platelet adhesion compared to RGD and cRGD FSPs, whereas EC adhesion was similar on all peptide FSPs and minimal on M-7 FSP. These results illustrate the potential for ligands presented in a FSP surface modification to selectively adhere ECs with limited platelet attachment.

Differential roles of the PKC novel isoforms, PKCdelta and PKCepsilon, in mouse and human platelets

BACKGROUND

Increasing evidence suggests that individual isoforms of protein kinase C (PKC) play distinct roles in regulating platelet activation.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we focus on the role of two novel PKC isoforms, PKCdelta and PKCepsilon, in both mouse and human platelets. PKCdelta is robustly expressed in human platelets and undergoes transient tyrosine phosphorylation upon stimulation by thrombin or the collagen receptor, GPVI, which becomes sustained in the presence of the pan-PKC inhibitor, Ro 31-8220. In mouse platelets, however, PKCdelta undergoes sustained tyrosine phosphorylation upon activation. In contrast the related isoform, PKCepsilon, is expressed at high levels in mouse but not human platelets. There is a marked inhibition in aggregation and dense granule secretion to low concentrations of GPVI agonists in mouse platelets lacking PKCepsilon in contrast to a minor inhibition in response to G protein-coupled receptor agonists. This reduction is mediated by inhibition of tyrosine phosphorylation of the FcRgamma-chain and downstream proteins, an effect also observed in wild-type mouse platelets in the presence of a PKC inhibitor.

CONCLUSIONS

These results demonstrate a reciprocal relationship in levels of the novel PKC isoforms delta and epsilon in human and mouse platelets and a selective role for PKCepsilon in signalling through GPVI.

Genetic analysis of the role of protein kinase Ctheta in platelet function and thrombus formation

Background

PKCθ is a novel protein kinase C isozyme, predominately expressed in T cells and platelets. PKCθ^−/− T cells exhibit reduced activation and PKCθ^−/− mice are resistant to autoimmune disease, making PKCθ an attractive therapeutic target for immune modulation. Collagen is a major agonist for platelets, operating through an immunoreceptor-like signalling pathway from its receptor GPVI. Although it has recently been shown that PKCθ positively regulates outside-in signalling through integrin α_IIbβ₃ in platelets, the role of PKCθ in GPVI-dependent signalling and functional activation of platelets has not been assessed.

Methodology/Principal Findings

In the present study we assessed static adhesion, cell spreading, granule secretion, integrin α_IIbβ₃ activation and platelet aggregation in washed mouse platelets lacking PKCθ. Thrombus formation on a collagen-coated surface was assessed in vitro under flow. PKCθ^−/− platelets exhibited reduced static adhesion and filopodia generation on fibrinogen, suggesting that PKCθ positively regulates outside-in signalling, in agreement with a previous report. In contrast, PKCθ^−/− platelets also exhibited markedly enhanced GPVI-dependent α-granule secretion, although dense granule secretion was unaffected, suggesting that PKCθ differentially regulates these two granules. Inside-out regulation of α_IIbβ₃ activation was also enhanced downstream of GPVI stimulation. Although this did not result in increased aggregation, importantly thrombus formation on collagen under high shear (1000 s⁻¹) was enhanced.

Conclusions/Significance

These data suggest that PKCθ is an important negative regulator of thrombus formation on collagen, potentially mediated by α-granule secretion and α_IIbβ₃ activation. PKCθ therefore may act to restrict thrombus growth, a finding that has important implications for the development and safe clinical use of PKCθ inhibitors.

A Mechanistic Model for Paradoxical Platelet Activation by Ligand-Mimetic α IIb β 3 (GPIIb/IIIa) Antagonists

Objective— Integrins are attractive therapeutic targets. Inhibition of integrin α IIb β 3 effectively blocks platelet aggregation. However, limitations with intravenous α IIb β 3 antagonists and failure of oral α IIb β 3 antagonists prompted doubts on the current concept of ligand-mimetic integrin blockade.

Methods and Results— Evaluating P-selectin expression on platelets by flow cytometry, we report a mechanism of paradoxical platelet activation by ligand-mimetic α IIb β 3 antagonists and define three requirements: (1) Induction of ligand-bound conformation of α IIb β 3 , (2) receptor clustering, (3) prestimulation of platelets. Conformational change is inducible by clinically used ligand-mimetic α IIb β 3 antagonists, RGD-peptides, and anti-LIBS antibodies. In a mechanistic experimental model, clustering is achieved by crosslinking integrins via antibodies, and preactivation is induced by low-dose ADP. Finally, we demonstrate that platelet adhesion on collagen represents an in vivo correlate of platelet prestimulation and receptor clustering, in which the presence of ligand-mimetic α IIb β 3 antagonists results in platelet activation as detected by P-selectin, CD63, and CD40L expression as well as by measuring Ca 2+ -signaling. Blockade of the ADP receptor P2Y 12 by AR-C69931MX and clopidogrel inhibits α IIb β 3 antagonist-induced platelet activation.

Conclusion— These findings can explain limitations of ligand-mimetic anti-α IIb β 3 therapy. They describe potential benefits of concomitant ADP receptor blockade and support a shift in drug development from ligand-mimetic toward allosteric or activation-specific integrin antagonists.

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.