Differential integrin activity mediated by platelet collagen receptor engagement under flow conditions

Modelling arterial thrombus formation in vitro

PURPOSE OF REVIEW

Models of arterial thrombus formation represent a vital experimental tool to investigate platelet function and test novel antithrombotic drugs. This review highlights some of the recent advances in modelling thrombus formation in vitro and suggests potential future directions.

RECENT FINDINGS

Microfluidic devices and the availability of commercial chips in addition to enhanced accessibility of 3D printing has facilitated a rapid surge in the development of novel in-vitro thrombosis models. These include progression towards more sophisticated, 'vessel on a chip' models which incorporate vascular endothelial cells and smooth muscle cells. Other approaches include the addition of branches to the traditional single channel to yield an occlusive model; and developments in the adhesive coating of microfluidic chambers to better mimic the thrombogenic surface exposed following plaque rupture. Future developments in the drive to create more biologically relevant chambers could see a move towards the use of human placental vessels, perfused ex-vivo. However, further work is required to determine the feasibility and validity of this approach.

SUMMARY

Recent advances in thrombus formation models have significantly improved the pathophysiological relevance of in-vitro flow chambers to better reflect the in-vivo environment and provide a more translational platform to test novel antithrombotics.

G protein-coupled P2Y12 receptor is involved in the progression of neuropathic pain

The pathological mechanism of neuropathic pain is complex, which seriously affects the physical and mental health of patients, and its treatment is also difficult. The role of G protein-coupled P2Y12 receptor in pain has been widely recognized and affirmed. After nerve injury, stimulated cells can release large amounts of nucleotides into the extracellular matrix, act on P2Y12 receptor. Activated P2Y12 receptor activates intracellular signal transduction and is involved in the development of pain. P2Y12 receptor activation can sensitize primary sensory neurons and receive sensory information. By transmitting the integrated information through the dorsal root of the spinal cord to the secondary neurons of the posterior horn of the spinal cord. The integrated information is then transmitted to the higher center through the ascending conduction tract to produce pain. Moreover, activation of P2Y12 receptor can mediate immune cells to release pro-inflammatory factors, increase damage to nerve cells, and aggravate pain. While inhibits the activation of P2Y12 receptor can effectively relieve pain. Therefore, in this article, we described P2Y12 receptor antagonists and their pharmacological properties. In addition, we explored the potential link between P2Y12 receptor and the nervous system, discussed the intrinsic link of P2Y12 receptor and neuropathic pain and as a potential pharmacological target for pain suppression.

Pharmacological Inhibition of Glycoprotein VI- and Integrin α2β1-Induced Thrombus Formation Modulated by the Collagen Type

BACKGROUND

 In secondary cardiovascular disease prevention, treatments blocking platelet-derived secondary mediators pose a risk of bleeding. Pharmacological interference of the interaction of platelets with exposed vascular collagens is an attractive alternative, with clinical trials ongoing. Antagonists of the collagen receptors, glycoprotein VI (GPVI), and integrin α2β1, include recombinant GPVI-Fc dimer construct Revacept, 9O12 mAb based on the GPVI-blocking reagent Glenzocimab, Syk tyrosine-kinase inhibitor PRT-060318, and anti-α2β1 mAb 6F1. No direct comparison has been made of the antithrombic potential of these drugs.

METHODS

 Using a multiparameter whole-blood microfluidic assay, we compared the effects of Revacept, 9O12-Fab, PRT-060318, or 6F1 mAb intervention with vascular collagens and collagen-related substrates with varying dependencies on GPVI and α2β1. To inform on Revacept binding to collagen, we used fluorescent-labelled anti-GPVI nanobody-28.

RESULTS AND CONCLUSION

 In this first comparison of four inhibitors of platelet-collagen interactions with antithrombotic potential, we find that at arterial shear rate: (1) the thrombus-inhibiting effect of Revacept was restricted to highly GPVI-activating surfaces; (2) 9O12-Fab consistently but partly inhibited thrombus size on all surfaces; (3) effects of GPVI-directed interventions were surpassed by Syk inhibition; and (4) α2β1-directed intervention with 6F1 mAb was strongest for collagens where Revacept and 9O12-Fab were limitedly effective. Our data hence reveal a distinct pharmacological profile for GPVI-binding competition (Revacept), GPVI receptor blockage (9O12-Fab), GPVI signaling (PRT-060318), and α2β1 blockage (6F1 mAb) in flow-dependent thrombus formation, depending on the platelet-activating potential of the collagen substrate. This work thus points to additive antithrombotic action mechanisms of the investigated drugs.

Modulation of Glycoprotein VI and Its Downstream Signaling Pathways as an Antiplatelet Target

Antiplatelet therapy aims to reduce the risk of thrombotic events while maintaining hemostasis. A promising current approach is the inhibition of platelet glycoprotein GPVI-mediated adhesion pathways; pathways that do not involve coagulation. GPVI is a signaling receptor integral for collagen-induced platelet activation and participates in the thrombus consolidation process, being a suitable target for thrombosis prevention. Considering this, the blocking or antibody-mediated depletion of GPVI is a promising antiplatelet therapy for the effective and safe treatment of thrombotic diseases without a significant risk of bleeding and impaired hemostatic plug formation. This review describes the current knowledge concerning pharmaceutical approaches to platelet GPVI modulation and its downstream signaling pathways in this context.

Cucurbitacins Elicit Anti-Platelet Activity via Perturbation of the Cytoskeleton and Integrin Function

Cucurbitacins are dietary compounds that have been shown to elicit a range of anti-tumour, anti-inflammatory and anti-atherosclerotic activities. Originally identified as signal transducer and activator of transcription, STAT, inhibitors, a variety of mechanisms of action have since been described, including dysregulation of the actin cytoskeleton and disruption of integrin function. Integrin outside-in signalling and cytoskeletal rearrangements are critical for the propagation of stable thrombus formation and clot retraction following platelet adhesion at the site of vessel damage. The effects of cucurbitacins on platelet function and thrombus formation are unknown. We report for the first time anti-platelet and anti-thrombotic effects of cucurbitacins B, E and I in human platelets. Treatment of platelets with cucurbitacins resulted in attenuation of platelet aggregation, secretion and fibrinogen binding following stimulation by platelet agonists. Cucurbitacins were also found to potently inhibit other integrin- and cytoskeleton-mediated events, including adhesion, spreading and clot retraction. Further investigation of cytoskeletal dynamics found treatment with cucurbitacins altered cofilin phosphorylation, enhanced activation and increased F actin polymerisation and microtubule assembly. Disruption to cytoskeletal dynamics has been previously shown to impair integrin activation, platelet spreading and clot retraction. Anti-platelet properties of cucurbitacins were found to extend to a disruption of stable thrombus formation, with an increase in thrombi instability and de-aggregation under flow. Our research identifies novel, anti-platelet and anti-thrombotic actions of cucurbitacins that appear to be linked to dysregulation of cytoskeletal dynamics and integrin function.

The GPIbα intracellular tail - role in transducing VWF- and collagen/GPVI-mediated signaling

The GPIbT-VWF A1 domain interaction is essential for platelet tethering under high shear. Synergy between GPIbα and GPVI signaling machineries has been suggested previously, however its molecular mechanism remains unclear. We generated a novel GPIbα transgenic mouse (GpIbαΔsig/Δsig) by CRISPR-Cas9 technology to delete the last 24 residues of the GPIbα intracellular tail that harbors the 14-3-3 and phosphoinositide-3 kinase binding sites. GPIbαΔsig/Δsig platelets bound VWF normally under flow. However, they formed fewer filopodia on VWF/botrocetin in the presence of a oIIbI3 blocker, demonstrating that despite normal ligand binding, VWF-dependent signaling is diminished. Activation of GpIbαΔsig/Δsig platelets with ADP and thrombin was normal, but GpIbαΔsig/Δsig platelets stimulated with collagen-related-peptide (CRP) exhibited markedly decreased P-selectin exposure and eIIbI3 activation, suggesting a role for the GpIbaaintracellular tail in GPVI-mediated signaling. Consistent with this, while haemostasis was normal in GPIbαΔsig/Δsig mice, diminished tyrosine-phosphorylation, (particularly pSYK) was detected in CRP-stimulated GpIbαΔsig/Δsig platelets as well as reduced platelet spreading on CRP. Platelet responses to rhodocytin were also affected in GpIbαΔsig/Δsig platelets but to a lesser extent than those with CRP. GpIbαΔsig/Δsig platelets formed smaller aggregates than wild-type platelets on collagen-coated microchannels at low, medium and high shear. In response to both VWF and collagen binding, flow assays performed with plasma-free blood or in the presence of bIIbI3- or GPVI-blockers suggested reduced bIIbI3 activation contributes to the phenotype of the GpIbαΔsig/Δsig platelets. Together, these results reveal a new role for the intracellular tail of GPIbiiin transducing both VWF-GPIbGGand collagen-GPVI signaling events in platelets.

Protein C or Protein S deficiency associates with paradoxically impaired platelet‐dependent thrombus and fibrin formation under flow

Background

Low plasma levels of protein C or protein S are associated with venous thromboembolism rather than myocardial infarction. The high coagulant activity in patients with thrombophilia with a (familial) defect in protein C or S is explained by defective protein C activation, involving thrombomodulin and protein S. This causes increased plasmatic thrombin generation.

Objective

Assess the role of platelets in the thrombus‐ and fibrin‐forming potential in patients with familial protein C or protein S deficiency under high‐shear flow conditions.

Patients/Methods

Whole blood from 23 patients and 15 control subjects was perfused over six glycoprotein VI–dependent microspot surfaces. By real‐time multicolor microscopic imaging, kinetics of platelet thrombus and fibrin formation were characterized in 49 parameters.

Results and Conclusion

Whole‐blood flow perfusion over collagen, collagen‐like peptide, and fibrin surfaces with low or high GPVI dependency indicated an unexpected impairment of platelet activation, thrombus phenotype, and fibrin formation but unchanged platelet adhesion, observed in patients with protein C deficiency and to a lesser extent protein S deficiency, when compared to controls. The defect extended from diminished phosphatidylserine exposure and thrombus contraction to delayed and suppressed fibrin formation. The mechanism was thrombomodulin independent, and may involve negative platelet priming by plasma components.

Immobilized collagen prevents shedding and induces sustained GPVI clustering and signaling in platelets

Collagen, the most thrombogenic constituent of blood vessel walls, activates platelets through glycoprotein VI (GPVI). In suspension, following platelet activation by collagen, GPVI is cleaved by A Disintegrin And Metalloproteinase (ADAM)10 and ADAM17. In this study, we use single-molecule localization microscopy and a 2-level DBSCAN-based clustering tool to show that GPVI remains clustered along immobilized collagen fibers for at least 3 hours in the absence of significant shedding. Tyrosine phosphorylation of spleen tyrosine kinase (Syk) and Linker of Activated T cells (LAT), and elevation of intracellular Ca²⁺, are sustained over this period. Syk, but not Src kinase-dependent signaling is required to maintain clustering of the collagen integrin α2β1, whilst neither is required for GPVI. We propose that clustering of GPVI on immobilized collagen protects GPVI from shedding in order to maintain sustained Src and Syk-kinases dependent signaling, activation of integrin α2β1, and continued adhesion.

Effect of Learning Dataset for Identification of Active Molecules: A Case Study of Integrin αIIbβ3 Inhibitors

Efficient in silico approaches are needed to identify strong integrin αIIbβ3 inhibitors through a small number of measurements. To address the challenge, we investigated the effect of learning dataset on the classification performance of machine learning models focusing on weak and inactive compounds. The structure and activity information of the compounds were obtained from ChEMBL, and pCHEMBL values were used to classify them as active, inactive, or weak. Datasets with various imbalance levels from active:inactive=1 : 1 to 1 : 1000 were used for the machine learning. The prediction scores of the weak samples were found to lie between the predictive values of active and inactive compounds. In addition, another dataset that consists of 149 actives and 6.9 million inactives was screened; the results indicated that the number of positive predictions decreased for models trained with a higher number of inactives. Although there is a trade-off between false positives and false negatives, for determination of compounds with strong activity using a reduced number of measurements, it is better to use a large number of inactives for learning and identifying compounds that score higher than the weak samples.

The role of phospho-tyrosine signaling in platelet biology and hemostasis

Platelets are small enucleated cell fragments specialized in the control of hemostasis, but also playing a role in angiogenesis, inflammation and immunity. This plasticity demands a broad range of physiological processes. Platelet functions are mediated through a variety of receptors, the concerted action of which must be tightly regulated, in order to allow specific and timely responses to different stimuli. Protein phosphorylation is one of the main key regulatory mechanisms by which extracellular signals are conveyed. Despite the importance of platelets in health and disease, the molecular pathways underlying the activation of these cells are still under investigation. Here, we review current literature on signaling platelet biology and in particular emphasize the newly emerging role of phosphatases in these processes.

Nonredundant Roles of Platelet Glycoprotein VI and Integrin αIIbβ3 in Fibrin-Mediated Microthrombus Formation

OBJECTIVE

Fibrin is considered to strengthen thrombus formation via integrin αIIbβ3, but recent findings indicate that fibrin can also act as ligand for platelet glycoprotein VI. Approach and Results: To investigate the thrombus-forming potential of fibrin and the roles of platelet receptors herein, we generated a range of immobilized fibrin surfaces, some of which were cross-linked with factor XIIIa and contained VWF-BP (von Willebrand factor-binding peptide). Multicolor microfluidics assays with whole-blood flowed at high shear rate (1000 s^-1) indicated that the fibrin surfaces, regardless of the presence of factor XIIIa or VWF-BP, supported platelet adhesion and activation (P-selectin expression), but only microthrombi were formed consisting of bilayers of platelets. Fibrinogen surfaces produced similar microthrombi. Markedly, tiggering of coagulation with tissue factor or blocking of thrombin no more than moderately affected the fibrin-induced microthrombus formation. Absence of αIIbβ3 in Glanzmann thrombasthenia annulled platelet adhesion. Blocking of glycoprotein VI with Fab 9O12 substantially, but incompletely reduced platelet secretion, Ca²⁺ signaling and aggregation, while inhibition of Syk further reduced these responses. In platelet suspension, glycoprotein VI blockage or Syk inhibition prevented fibrin-induced platelet aggregation. Microthrombi on fibrin surfaces triggered only minimal thrombin generation, in spite of thrombin binding to the fibrin fibers.

CONCLUSIONS

Together, these results indicate that fibrin fibers, regardless of their way of formation, act as a consolidating surface in microthrombus formation via nonredundant roles of platelet glycoprotein VI and integrin αIIbβ3 through signaling via Syk and low-level Ca²⁺ rises.

Ca2+ waves coordinate purinergic receptor-evoked integrin activation and polarization

Cells sense extracellular nucleotides through the P2Y class of purinergic G protein-coupled receptors (GPCRs), which stimulate integrin activation through signaling events, including intracellular Ca²⁺ mobilization. We investigated the relationship between P2Y-stimulated repetitive Ca²⁺ waves and fibrinogen binding to the platelet integrin α_IIbβ₃ (GPIIb/IIIa) through confocal fluorescence imaging of primary rat megakaryocytes. Costimulation of the receptors P2Y₁ and P2Y₁₂ generated a series of Ca²⁺ transients that each induced a rapid, discrete increase in fibrinogen binding. The peak and net increase of individual fibrinogen binding events correlated with the Ca²⁺ transient amplitude and frequency, respectively. Using BAPTA loading and selective receptor antagonists, we found that Ca²⁺ mobilization downstream of P2Y₁ was essential for ADP-evoked fibrinogen binding, whereas P2Y₁₂ and the kinase PI3K were also required for α_IIbβ₃ activation and enhanced the number of Ca²⁺ transients. ADP-evoked fibrinogen binding was initially uniform over the cell periphery but subsequently redistributed with a polarity that correlated with the direction of the Ca²⁺ waves. Polarization of α_IIbβ₃ may be mediated by the actin cytoskeleton, because surface-bound fibrinogen is highly immobile, and its motility was enhanced by cytoskeletal disruption. In conclusion, spatial and temporal patterns of Ca²⁺ increase enable fine control of α_IIbβ₃ activation after cellular stimulation. P2Y₁-stimulated Ca²⁺ transients coupled to α_IIbβ₃ activation only in the context of P2Y₁₂ coactivation, thereby providing an additional temporal mechanism of synergy between these Gq- and Gi-coupled GPCRs.

Adsorption and Absorption of Collagen Peptides to Polydimethlysiloxane and Its Influence on Platelet Adhesion Flow Assays

Collagen peptides are an alternative to animal derived collagens for platelet function studies under flow. The purpose of this study was to examine the use of collagen peptides in polydimethylsiloxane (PDMS) devices. Three collagen peptides with amino acid sequences and structures that capture von Willebrand factor and bind it with the platelet receptors integrin α₂β₁ and glycoprotein VI were patterned on glass, silicon, and PDMS. Each of these surfaces was also functionalized with tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS). Surfaces were characterized by their ability to support platelet adhesion, topology by atomic force microscopy, contact angle, and peptides absorption. PDMS readily absorbs collagen peptides, depleting them from solution, thus reducing their adsorption to glass and silicon substrates when used for micropatterning. Treatment of PDMS with FOTS, but not bovine serum albumin or poloxamer 407, inhibits collagen peptide absorption and supports adsorption and platelet adhesion at venous and arterial shear rates. Similarly, FOTS treatment of glass or silicon supports collagen peptide adsorption even in the presence of untreated PDMS. In conclusion, PDMS acts as an absorptive sink for collagen peptides, rendering a non-adhesive surface for platelet adhesion and competing for peptides when used for micropatterning. The absorption of collagen peptides can be overcome by functionalization of PDMS with a fluorinated alkyl silane, thus allowing its use as a material for micropatterning or as a surface for platelet adhesion flow assays.