Method article: an in vitro blood flow model to advance the study of platelet adhesion utilizing a damaged endothelium

In vitro flow assays utilizing microfluidic devices are often used to study human platelets as an alternative to the costly animal models of hemostasis and thrombosis that may not accurately represent human platelet behavior in vivo. Here, we present a tunable in vitro model to study platelet behavior in human whole blood flow that includes both an inflamed, damaged endothelium and exposed extracellular matrix. We demonstrate that the model is adaptable across various anticoagulants, shear rates, and proteins for endothelial cell culture without the need for a complicated, custom-designed device. Furthermore, we verified the ability of this 'damaged endothelium' model as a screening method for potential anti-platelet or anti-thrombotic compounds using a P2Y₁₂ receptor antagonist (ticagrelor), a pan-selectin inhibitor (Bimosiamose), and a histamine receptor antagonist (Cimetidine). These compounds significantly decreased platelet adhesion to the damaged endothelium, highlighting that this model can successfully screen anti-platelet compounds that target platelets directly or the endothelium indirectly.

Multimerin 1 supports platelet function in vivo and binds to specific GPAGPOGPX motifs in fibrillar collagens that enhance platelet adhesion

BACKGROUND

Multimerin 1 (human: MMRN1, mouse: Mmrn1) is a homopolymeric, adhesive, platelet and endothelial protein that binds to von Willebrand factor and enhances platelet adhesion to fibrillar collagen ex vivo.

OBJECTIVES

To examine the impact of Mmrn1 deficiency on platelet adhesive function, and the molecular motifs in fibrillar collagen that bind MMRN1 to enhance platelet adhesion.

METHODS

Mmrn1-deficient mice were generated and assessed for altered platelet adhesive function. Collagen Toolkit peptides, and other triple-helical collagen peptides, were used to identify multimerin 1 binding motifs and their contribution to platelet adhesion.

RESULTS

MMRN1 bound to conserved GPAGPOGPX sequences in collagens I, II, and III (including GPAGPOGPI, GPAGPOGPV, and GPAGPOGPQ) that enhanced activated human platelet adhesion to collagen synergistically with other triple-helical collagen peptides (P < .05). Mmrn1-/- and Mmrn1+/- mice were viable and fertile, with complete and partial platelet Mmrn1 deficiency, respectively. Relative to wild-type mice, Mmrn1-/- and Mmrn1+/- mice did not have overt bleeding, increased median bleeding times, or increased wound blood loss (P ≥ .07); however, they both showed significantly impaired platelet adhesion and thrombus formation in the ferric chloride injury model (P ≤ .0003). Mmrn1-/- platelets had impaired adhesion to GPAGPOGPX peptides and fibrillar collagen (P ≤ .03) and formed smaller aggregates than wild-type platelets when captured onto collagen, triple-helical collagen mimetic peptides, von Willebrand factor, or fibrinogen (P ≤ .008), despite preserved, low shear, and high shear aggregation responses.

CONCLUSIONS

Multimerin 1 supports platelet adhesion and thrombus formation and binds to highly conserved, GPAGPOGPX motifs in fibrillar collagens that synergistically enhance platelet adhesion.

Good Platelets Gone Bad: The Effects of Trauma Patient Plasma on Healthy Platelet Aggregation

BACKGROUND

Altered postinjury platelet behavior is recognized in the pathophysiology of trauma-induced coagulopathy (TIC), but the mechanisms remain largely undefined. Studies suggest that soluble factors released by injury may inhibit signaling pathways and induce structural changes in circulating platelets. Given this, we sought to examine the impact of treating healthy platelets with plasma from injured patients. We hypothesized that healthy platelets treated ex-vivo with plasma from injured patients with shock would impair platelet aggregation, while treatment with plasma from injured patients with significant injury burden, but without shock, would enhance platelet aggregation.

METHODS

Plasma samples were isolated from injured patients (pretransfusion) and healthy donors at a Level I trauma center and stored at -80°C. Plasma samples from four separate patients in each of the following stratified clinical groups were used: mild injury/no shock (injury severity score [ISS] 2-15, base excess [BE]>-6), mild injury/with shock (ISS 2-15, BE≤-6), severe injury/no shock (ISS>25, BE>-6), severe injury/with shock (ISS>25, BE≤-6), minimal injury (ISS 0/1, BE>-6), and healthy. Platelets were isolated from three healthy adult males and were treated with plasma for 30 min. Aggregation was stimulated with a thrombin receptor agonist and measured via multiple-electrode platelet aggregometry. Data were normalized to HEPES Tyrode's (HT) buffer-only treated platelets. Associations of plasma treatment groups with platelet aggregation measures were tested with Mann-Whitney U tests.

RESULTS

Platelets treated with plasma from patients with shock (regardless of degree of injury) had significantly impaired thrombin-stimulated aggregation compared with platelets treated with plasma from patients without shock (P = 0.002). Conversely, platelets treated with plasma from patients with severe injury, but without shock, had amplified thrombin-stimulated aggregation (P = 0.030).

CONCLUSION

Shock-mediated soluble factors impair platelet aggregation, and tissue injury-mediated soluble factors amplify platelet aggregation. Future characterization of these soluble factors will support development of novel treatments of TIC.

Validated model of platelet slip at stenosis and device surfaces

Platelets are central to thrombosis. However, it is unknown whether platelets slip at vascular or device surfaces. The presence of platelet slip at a surface would interrupt physical contact between the platelet and that surface, and therefore diminish adhesion and thrombosis. Unfortunately, no existing technology can directly measure platelet slip in a biological environment. The objective of this study was to explore whether microspheres-modeling platelets-slip at different vascular and device surfaces in an acrylic scaled-up model coronary artery. The microspheres (3.12 µm diameter) were suspended in a transparent glycerol/water experimental fluid, which flowed continuously at Reynolds numbers typical of coronary flow (200-400) through the model artery. We placed a series of axisymmetric acrylic stenoses (cross-sectional area reduction [CSAr], 20-90%) into the model artery, both without and with a central cylinder present (modeling a percutaneous interventional guide wire, and with a scaled-up Doppler catheter mounted upstream). We used laser Doppler velocimetry (LDV) to measure microsphere velocities within, proximal and distal to each stenosis, and compared to computer simulations of fluid flow with no-slip. For validation, we replaced the acrylic with paraffin stenoses (more biologically relevant from a surface roughness perspective) and then analyzed the signal recorded by the scaled-up Doppler catheter. Using the LDV, we identified progressive microsphere slip proportional to CSAr inside entrances for stenoses ≥60% and ≥40% without and with cylinder present, respectively. Additionally, microsphere slip occurred universally along the cylinder surface. Computer simulations indicated increased fluid shear rates (velocity gradients) at these particular locations, and logistic regression analysis comparing microsphere slip with fluid shear rate resulted in a c-index of 0.989 at a cut-point fluid shear rate of (10.61 [cm^-1]×mean velocity [cm×sec^-1]). Moreover, the presence of the cylinder caused disordering of microsphere shear rates distal to higher grade stenoses, indicating a disturbance in their flow. Finally, despite lower precision, the signal recorded by the scaled-up Doppler catheter nonetheless indicated slip at the entry into and at most locations distal to the 90% stenosis. Our validated model establishes proof of concept for platelet slip, and platelet slip explains several important basic and clinical observations. If technological advances allow confirmation in a true biologic environment, then our results will likely influence the development of shear-dependent antiplatelet drugs. Also, adding shear rate information, our results provide a direct experimental fluid dynamic foundation for antiplatelet-focused antithrombotic therapy during coronary interventions directed towards higher grade atherosclerotic stenoses.

Clustering of glycoprotein VI (GPVI) dimers upon adhesion to collagen as a mechanism to regulate GPVI signaling in platelets

Essentials Dimeric high-affinity collagen receptor glycoprotein VI (GPVI) is present on resting platelets. Spatio-temporal organization of platelet GPVI-dimers was evaluated using advanced microscopy. Upon platelet adhesion to collagenous substrates, GPVI-dimers coalesce to form clusters. Clustering of GPVI-dimers may increase avidity and facilitate platelet activation SUMMARY: Background Platelet glycoprotein VI (GPVI) binding to subendothelial collagen exposed upon blood vessel injury initiates thrombus formation. Dimeric GPVI has high affinity for collagen, and occurs constitutively on resting platelets. Objective To identify higher-order oligomerization (clustering) of pre-existing GPVI dimers upon interaction with collagen as a mechanism to initiate GPVI-mediated signaling. Methods GPVI was located by use of fluorophore-conjugated GPVI dimer-specific Fab (antigen-binding fragment). The tested substrates include Horm collagen I fibers, soluble collagen III, GPVI-specific collagen peptides, and fibrinogen. GPVI dimer clusters on the platelet surface interacting with these substrates were visualized with complementary imaging techniques: total internal reflection fluorescence microscopy to monitor real-time interactions, and direct stochastic optical reconstruction microscopy (dSTORM), providing relative quantification of GPVI cluster size and density. Confocal microscopy was used to locate GPVI dimer clusters, glycoprotein Ib, integrin α2 β1 , and phosphotyrosine. Results Upon platelet adhesion to all collagenous substrates, GPVI dimers coalesced to form clusters; notably clusters formed along the fibers of Horm collagen. dSTORM revealed that GPVI density within clusters depended on the substrate, collagen III being the most effective. Clusters on fibrinogen-adhered platelets were much smaller and more numerous; whether these are pre-existing oligomers of GPVI dimers or fibrinogen-induced is not clear. Some GPVI dimer clusters colocalized with areas of phosphotyrosine, indicative of signaling activity. Integrin α2 β1 was localized to collagen fibers close to GPVI dimer clusters. GPVI clustering depends on a dynamic actin cytoskeleton. Conclusions Platelet adhesion to collagen induces GPVI dimer clustering. GPVI clustering increases both avidity for collagen and the proximity of GPVI-associated signaling molecules, which may be crucial for the initiation and persistence of signaling.

Assessment of neonatal platelet adhesion, activation, and aggregation

BACKGROUND

Acquired and inherited bleeding disorders may present in the neonatal period with devastating lifelong effects. Diagnosing bleeding disorders in the neonatal population could aid in preventing and treating the associated complications. However, currently available platelet function testing is limited in neonates, owing to difficulties in obtaining an adequate blood volume, a lack of normal reference ranges, and an incomplete understanding of the neonatal platelet functional phenotype.

OBJECTIVE

To develop small-volume, whole blood platelet function assays in order to quantify and compare neonatal and adult platelet function.

METHODS AND RESULTS

Peripheral blood was obtained from healthy, full-term neonates at 24 h of life. Platelet activation, secretion and aggregation were measured via flow cytometry. Platelet adhesion and aggregation were assessed under static and flow conditions. As compared with adult platelets, peripheral neonatal platelet P-selectin expression and integrin glycoprotein IIbIIIa activation were significantly reduced in response to the G-protein-coupled receptor (GPCR) agonists thrombin receptor activator peptide-6 (TRAP-6), ADP, and U46619, and the immunoreceptor tyrosine-based activation motif (ITAM) signaling pathway agonists collagen-related peptide (CRP) and rhodocytin. Neonatal platelet aggregation was markedly reduced in response to TRAP-6, ADP, U46619, CRP and rhodocytin as compared with adult platelets. The extents of neonatal and adult platelet adhesion and aggregate formation under static and shear conditions on collagen and von Willebrand factor were similar.

CONCLUSIONS

As compared with adult platelets, we found that neonatal platelet activation and secretion were blunted in response to GPCR or ITAM agonists, whereas the extent of neonatal platelet adhesion and aggregate formation was similar to that of adult platelets.

A molten globule intermediate of the von Willebrand factor A1 domain firmly tethers platelets under shear flow

Clinical mutations in patients diagnosed with Type 2A von Willebrand disease (VWD) have been identified that break the single disulfide bond linking N- and C-termini in the vWF A1 domain. We have modeled the effect of these mutations on the disulfide-bonded structure of A1 by reducing and carboxy-amidating these cysteines. Solution biophysical studies show that loss of this disulfide bond induces a molten globule conformational state lacking global tertiary structure but retaining residual secondary structure. The conformational dependence of platelet adhesion to these native and molten globule states of A1 is quantitatively compared using real-time high-speed video microscopy analysis of platelet translocation dynamics under shear flow in a parallel plate microfluidic flow chamber. While normal platelets translocating on surface-captured native A1 domain retain the catch-bond character of pause times that increase as a function of shear rate at low shear and decrease as a function of shear rate at high shear, platelets that interact with A1 lacking the disulfide bond remain stably attached and do not translocate. Based on these findings, we propose that the shear stress-sensitive regulation of the A1-GPIb interaction is due to folding the tertiary structure of this domain. Removal of the tertiary structure by disrupting the disulfide bond destroys this regulatory mechanism resulting in high-strength interactions between platelets and vWF A1 that are dependent only on residual secondary structure elements present in the molten globule conformation.

The type 2B p.R1306W natural mutation of von Willebrand factor dramatically enhances the multimer sensitivity to shear stress

BACKGROUND

Shear stress triggers conformational stretching of von Willebrand factor (VWF), which is responsible for its self-association and binding to the platelet receptor glycoprotein (GP)Ibα. This phenomenon supports primary hemostasis under flow. Type 2B VWF natural mutants are considered to have increased affinity for platelet GPIbα.

OBJECTIVES

To assess the mechanism responsible for the enhanced interaction of the p.R1306W VWF mutant with the platelet receptor.

METHODS

The interaction of GPIbα with wild-type (WT) and p.R1306W VWF multimers and A1-A2-A3 constructs was investigated with surface plasmon resonance spectroscopy. Analysis of the static VWF conformation in solution was performed with dynamic light scattering spectroscopy. The shear stress-induced self-association of VWF multimers was investigated with atomic force microscopy (AFM) over a 0-60 dyn cm(-2) range.

RESULTS

WT VWF did not interact with GPIbα under static conditions, whereas the mutant at ~ 2 μg mL(-1) already bound to the receptor. By contrast, the WT and p.R1306W-A1-A2-A3 constructs showed comparable affinities for GPIbα (Kd  ~ 20 nm). The hydrodynamic diameter of resting R1306W VWF multimers was significantly greater than that of the wild type (210 ± 60 nm vs. 87 ± 22 nm). At shear forces of < 14 dyn cm(-2) , the p.R1306W multimers rapidly changed conformation, entering a regime of self-aggregation, which, in contrast, was induced for WT VWF by shear forces of > 30 dyn cm(-2) . Mechanical stretching AFM experiments showed that p.R1306W multimers needed less energy per length unit (~ 10 pN) to be stretched than the WT protein.

CONCLUSIONS

The increased affinity of p.R1306W VWF for GPIbα arises mostly from higher sensitivity to shear stress, which facilitates exposure of GPIbα binding sites.

Platelet accumulation on an artificial surface in vivo: effect of intravascular hemolysis

Studies of thromboembolic complications after implantation of cardiovascular prosthetic devices have suggested that there is a correlation between the occurrence of hemolysis and the way in which platelets respond to an artificial surface. To test this hypothesis, we investigated the effect of low-level hemolysis (involving approximately 1% of the circulating red blood cells [RBCs] per hour) on the number of platelets that accumulated on woven Dacron arterial grafts used to bypass the carotid artery in six dogs. Each dog was used in one control and one test procedure, which were performed in random order. In the test procedures, hemolyzed autologous erythrocytes were infused (1 ml/kg/hr) for 10 minutes before, and during, blood flow through the graft. In the control procedures, physiologic saline solution was substituted for the hemolysate. Autologous indium-111 labeled platelets were used to estimate the number of platelets present on the Dacron surface one hour after the initiation of blood flow through the graft. More platelets accumulated on the Dacron surface in the presence of the hemolysate than in the presence of the saline. The increase caused by hemolysis ranged from 2% to 96%, with a mean of 43.7% +/- 35.9% (standard deviation).