Bernard-Soulier syndrome caused by a novel GP1BB variant and 22q11.2 deletion

Bernard-Soulier syndrome (BSS) is caused by defects in GP1BA, GP1BB, or GP9 genes. Patients with 22q11.2 deletion syndrome (22q11.2DS) are obligate carriers of BSS because GP1BB resides on chromosome 22q11.2. A 15-month-old girl without bleeding symptoms had giant platelets and thrombocytopenia. Physical findings and macrothrombocytopenia suggested 22q11.2DS, which was confirmed by fluorescence in situ hybridization. Flow cytometry showed decreased GPIbα on the platelets. Gene panel testing revealed a novel variant in GP1BB, p.(Val169_Leu172del). These findings confirmed that the patient had BSS. This case suggests that any patient with 22q11.2DS and macrothrombocytopenia should be further tested for BSS.

Platelet Adhesion Mediated by von Willebrand Factor at High Shear Rates Is Associated with Premature Coronary Artery Disease

This study investigated von Willebrand factor (VWF)-mediated platelet adhesion at high shear rates in patients with premature coronary artery disease (CAD). The study included 84 patients with stable premature CAD and 64 patients without CAD. Whole blood samples were perfused through a microfluidic cell over a collagen-coated surface at a shear rate of 1300 s^-1. Measurements were performed before and after the inhibition of VWF-specific platelet GPIb receptors with an anti-GPIb monoclonal antibody (mAb). Platelet adhesion decreased by 77.0% (55.9; 84.7) in patients with premature CAD and by 29.6% (0.0; 59.7) in control patients after the inhibition of VWF-platelet interaction with anti-GPIb mAb (p < 0.001). After adjusting for traditional risk factors, the odds ratio for premature CAD per 1% decrease in GPIb-mediated platelet adhesion was 1.03 (95% CI, 1.02-1.05; p < 0.001). The optimal cut-off level value of GPIb-mediated platelet adhesion was 62.8%, with 70.2% sensitivity and 81.2% specificity for CAD. The plasma levels of VWF or antiplatelet therapy did not affect the GPIb-mediated component of platelet adhesion. Thus, the GPIb-mediated component of platelet adhesion was more pronounced in patients with premature CAD. This may indicate the possible role of excessive VWF-platelet interactions in the development of premature CAD.

[The diagnosis and management of von Willebrand disease]

Von Willebrand disease (VWD) is a congenital bleeding disorder caused by quantitative and qualitative abnormalities in von Willebrand factor (VWF). VWD was first reported in 1926 by Dr. Erik von Willebrand, a Swedish physician, who reported a bleeding disorder in a family. VWD manifests as bleeding from the skin and mucous membranes (abnormal primary hemostasis). The symptoms are less severe than those of hemophilia and may be undiagnosed in many cases. Evidence-based guidelines are required for selection and patient management. Since ristocetin cofactor activity (vWF:RCo) and VWF antigen levels (vWF:Ag) are not routinely measured in clinical laboratories, the actual diagnosis is often determined by a mild prolongation of the activated partial thromboplastic time (APTT) associated with a relative decrease in factor VIII activity. Particularly, it is desirable to provide standard treatment measures on how to introduce the recently proposed bleeding score (BS; clinically scored for various bleeding symptoms of VWD) and how to deal with gynecological symptoms, such as excessive menstruation and irregular vaginal bleeding. In terms of treatment options, recombinant vWF concentrates were introduced in 2020, which has led to the expansion in the range of treatment options for patients.

Activation-induced changes in platelet surface receptor expression and the contribution of the large-platelet subpopulation to activation

OBJECTIVE

Platelet surface receptors are also present subcellularly in organelle membranes and can be expressed on the surface upon platelet activation. However, some receptors were reported to be decreased after activation. We analyzed the mechanism of activation-dependent expression for different receptors.

METHODS

Flow cytometry using platelet-rich plasma or washed platelets was used to analyze receptor-expression changes after platelet activation by glycoprotein (GP) VI-specific agonists, crosslinked collagen-related peptide (CRP-XL) and convulxin (Cvx), and thrombin. Platelets prelabeled with fluorescent antibody specific for a receptor were allowed to adhere on immobilized collagen or fibrinogen and post-stained with antibody against the same receptor labeled with another fluorophore, allowing us to differentiate preexisting receptors from newly expressed receptors.

RESULTS

Surface expression of αIIbβ3 increased in CRP-XL-, Cvx-, or thrombin-stimulated platelets, but GPIb decreased due to shedding and internalization. Both total and dimeric GPVI increased in thrombin-induced platelets, but decreased in platelets stimulated by Cvx, as a result of internalization. The larger platelets showed a greater increase in surface receptor (α2β1, αIIbβ3, GPVI, GPIb) expression upon activation compared to the smaller ones. Pre- and postlabeling with antibody specific for the same receptor, but conjugated with different fluorophores, allowed us to differentiate the receptors expressed on the surface of resting platelets from receptors newly exposed to the surface upon platelet activation.

CONCLUSIONS

Increased receptor expressions after activation are mainly manifested in the larger platelets. On platelets adhered on fibrinogen, the newly expressed receptors, especially GPVI, are localized in the lamellipodia of the spread platelets.

Shear stress and platelet-induced tensile forces regulate ADAMTS13-localization within the platelet thrombus

BACKGROUND

The multimeric glycoprotein von Willebrand factor (VWF) mediates platelet adhesion and aggregation at the site of vessel injury. The adhesive activity of VWF is influenced by its multimer length which is regulated by the metalloprotease ADAMTS13. The ability of ADAMTS13 to regulate platelet thrombus growth in a shear-dependent manner has been described, however, the mechanistic basis of this action has not been well characterized.

METHODS

We developed an mCherry-tagged murine ADAMTS13 protein and utilized an ex vivo flow chamber system to visualize the localization of ADAMTS13 within the platelet thrombus under different conditions of shear. Using this system, we also assessed the influence of platelet-mediated tensile force on ADAMTS13 localization within the thrombus using gain-of-function GPIb binding and loss-of-function GPIIbIIIa binding mutants in VWF/ADAMTS13 DKO mice.

RESULTS

ADAMTS13 was visualized on the growing platelet thrombus under very high shear using ADAMTS13-mcherry. ADAMTS13-mCherry localized particularly at the top portion of the thrombus and reduced thrombus size as it grew to occlusion. At the pathological high shear of 7500 s-1, platelet-mediated tensile force, involving GPIb but not GPIIbIIIa receptors, influenced localization of ADAMTS13 to the thrombus under conditions of shear.

CONCLUSIONS

Tensile force applied on VWF produced by shear stress and platelet GPIb binding has a crucial role in ADAMTS13 activity at the site of thrombus formation. These results suggest that ADAMTS13 activity at the site of platelet thrombus formation is regulated by a shear stress and platelet-dependent feedback mechanism to prevent vessel occlusion and pathological thrombosis.

Two-Dimensional Analysis of Cross-Junctional Molecular Interaction by Force Probes

Upon engagement with a specific ligand, a cell surface receptor transduces intracellular signals to activate various cellular functions. This chapter describes a set of biomechanical methods for analyzing the characteristics of cross-junctional receptor-ligand interactions at the surface of living cells. These methods combine the characterization of kinetics of receptor-ligand binding with real-time imaging of intracellular calcium fluxes, which allow researchers to assess how the signal initiated from single receptor-ligand engagement is transduced across the cell membrane. A major application of these methods is the analysis of antigen recognition by triggering of the T cell receptor (TCR). Three related methods are described in this chapter: (1) the micropipette adhesion assay, (2) the biomembrane force probe (BFP) assay, and (3) combining BFP with fluorescence microscopy (fBFP). In all cases, an ultrasoft human red blood cell (RBC) is used as an ultrasensitive mechanical force probe. The micropipette assay detects binding events visually. The BFP uses a high-speed camera and real-time image tracking techniques to measure mechanical variables on a single molecular bond with up to ~1 pN (10^-12 Newton), ~3 nm (10^-9 m), and ~0.5 ms (10^-3 s) in force, spatial, and temporal resolution, respectively. As an upgrade to the BFP, the fBFP simultaneously images binding-triggered intracellular calcium signals on a single live cell. These technologies can be widely used to study other membrane receptor-ligand interactions and signaling under mechanical regulation.

Cooperative unfolding of distinctive mechanoreceptor domains transduces force into signals

How cells sense their mechanical environment and transduce forces into biochemical signals is a crucial yet unresolved question in mechanobiology. Platelets use receptor glycoprotein Ib (GPIb), specifically its α subunit (GPIbα), to signal as they tether and translocate on von Willebrand factor (VWF) of injured arterial surfaces against blood flow. Force elicits catch bonds to slow VWF-GPIbα dissociation and unfolds the GPIbα leucine-rich repeat domain (LRRD) and juxtamembrane mechanosensitive domain (MSD). How these mechanical processes trigger biochemical signals remains unknown. Here we analyze these extracellular events and the resulting intracellular Ca(2+) on a single platelet in real time, revealing that LRRD unfolding intensifies Ca(2+) signal whereas MSD unfolding affects the type of Ca(2+) signal. Therefore, LRRD and MSD are analog and digital force transducers, respectively. The >30 nm macroglycopeptide separating the two domains transmits force on the VWF-GPIbα bond (whose lifetime is prolonged by LRRD unfolding) to the MSD to enhance its unfolding, resulting in unfolding cooperativity at an optimal force. These elements may provide design principles for a generic mechanosensory protein machine.

Platelet sequestration and activation during GalTKO.hCD46 pig lung perfusion by human blood is primarily mediated by GPIb, GPIIb/IIIa, and von Willebrand Factor

BACKGROUND

Here, we ask whether platelet GPIb and GPIIb/IIIa receptors modulate platelet sequestration and activation during GalTKO.hCD46 pig lung xenograft perfusion.

METHODS

GalTKO.hCD46 transgenic pig lungs were perfused with heparinized fresh human blood. Results from perfusions in which αGPIb Fab (6B4, 10 mg/l blood, n = 6), αGPIIb/IIIa Fab (ReoPro, 3.5 mg/l blood, n = 6), or both drugs (n = 4) were administered to the perfusate were compared to two additional groups in which the donor pig received 1-desamino-8-d-arginine vasopressin (DDAVP), 3 μg/kg (to pre-deplete von Willebrand Factor (pVWF), the main GPIb ligand), with or without αGPIb (n = 6 each).

RESULTS

Platelet sequestration was significantly delayed in αGPIb, αGPIb+DDAVP, and αGPIb+αGPIIb/IIIa groups. Median lung "survival" was significantly longer (>240 vs. 162 min reference, p = 0.016), and platelet activation (as CD62P and βTG) were significantly inhibited, when pigs were pre-treated with DDAVP, with or without αGPIb Fab treatment. Pulmonary vascular resistance rise was not significantly attenuated in any group, and was associated with residual thromboxane and histamine elaboration.

CONCLUSIONS

The GPIb-VWF and GPIIb/IIIa axes play important roles in platelet sequestration and coagulation cascade activation during GalTKO.hCD46 lung xenograft injury. GPIb blockade significantly reduces platelet activation and delays platelet sequestration in this xenolung rejection model, an effect amplified by adding αGPIIb/IIIa blockade or depletion of VWF from pig lung.

Developmental endothelial locus-1 modulates platelet-monocyte interactions and instant blood-mediated inflammatory reaction in islet transplantation

Platelet-monocyte interactions are strongly implicated in thrombo-inflammatory injury by actively contributing to intravascular inflammation, leukocyte recruitment to inflamed sites, and the amplification of the procoagulant response. Instant blood-mediated inflammatory reaction (IBMIR) represents thrombo-inflammatory injury elicited upon pancreatic islet transplantation (islet-Tx), thereby dramatically affecting transplant survival and function. Developmental endothelial locus-1 (Del-1) is a functionally versatile endothelial cell-derived homeostatic factor with anti-inflammatory properties, but its potential role in IBMIR has not been previously addressed. Here, we establish Del-1 as a novel inhibitor of IBMIR using a whole blood-islet model and a syngeneic murine transplantation model. Indeed, Del-1 pre-treatment of blood before addition of islets diminished coagulation activation and islet damage as assessed by C-peptide release. Consistently, intraportal islet-Tx in transgenic mice with endothelial cell-specific overexpression of Del-1 resulted in a marked decrease of monocytes and platelet-monocyte aggregates in the transplanted tissues, relative to those in wild-type recipients. Mechanistically, Del-1 decreased platelet-monocyte aggregate formation, by specifically blocking the interaction between monocyte Mac-1-integrin and platelet GPIb. Our findings reveal a hitherto unknown role of Del-1 in the regulation of platelet-monocyte interplay and the subsequent heterotypic aggregate formation in the context of IBMIR. Therefore, Del-1 may represent a novel approach to prevent or mitigate the adverse reactions mediated through thrombo-inflammatory pathways in islet-Tx and perhaps other inflammatory disorders involving platelet-leukocyte aggregate formation.

The influence of intermittent hypoxemia on platelet activation in obese patients with obstructive sleep apnea

OBJECTIVES

Literature regarding platelet function in obstructive sleep apnea (OSA) has considerable limitations. Given the central role of platelets in atherothrombosis and the known cardiovascular risk of OSA, we hypothesized that OSA severity is predictive of platelet function, independent of known comorbidities.

DESIGN

Obese subjects, without comorbidities, underwent overnight, in-lab polysomnography. The following morning, 5 biomarkers of platelet activation were measured by whole-blood flow cytometry at baseline and in response to agonists (no stimulation, stimulation with 5 μM ADP agonist, and stimulation with 20 μM ADP agonist): platelet surface P-selectin, activated glycoprotein (GP) IIb/IIIa, and GPIb receptor expression, platelet-monocyte aggregation (PMA) and platelet-neutrophil aggregation (PNA).

RESULTS

Of the 77 subjects, 47 were diagnosed with OSA (median apnea-hypopnea index [AHI] of 24.7 ± 28.1/h in subjects with OSA and 3.0 ± 3.9/h in subjects without OSA, p < 0.001). The groups were matched for body mass index, with a mean body mass index of 40.3 ± 9.6 kg/m(2) in subjects with OSA and 38.9 ± 6.0 kg/m(2) in subjects without OSA (p = 0.48). A comparison of time spent with an oxygen saturation of less than 90% showed that subjects who had 1 minute or more of desaturation time per hour of sleep had lower GPIb fluorescence in circulating platelets, as compared with those subjects who had less than 1 minute of desaturation time per hour of sleep; similar findings were observed following 5 μM and 20 μM of ADP stimulation, as compared with control vehicle, suggesting higher levels of circulating platelet activity. In multivariate analyses, only nocturnal hypoxemia and female sex predicted agonist response. Platelet surface P-selectin, platelet surface-activated GPIIb/IIIa, PMA, and PNA were not significantly correlated with markers of OSA.

CONCLUSIONS

In obese patients with OSA, platelet activation is associated with greater levels of oxygen desaturation, compared with matched control subjects. Metrics other than AHI (e.g., hypoxemia) may determine OSA-related thrombotic risk.