Role of Purinergic Receptor Expression and Function for Reduced Responsiveness to Adenosine Diphosphate in Washed Human Platelets

Platelet toll-like-receptor 2 and 4 mediate different immune-related responses to bacterial ligands

Background  Like immune cells, platelets express toll-like receptors (TLRs) on their surface membrane. TLR2 and TLR4 are able to recognize bacterial antigens and have the potential to influence hemostatic functions and classical intracellular signaling pathways. This study investigated the role of TLR2 and TLR4 for immune-related functions in human platelets.

Materials and Methods  Washed platelets and neutrophils were prepared from fresh human peripheral blood. Basal-, Pam3CSK4- (as TLR2 agonist) and Lipopolysaccharides (LPS; as TLR4 agonist) -induced CD62P expression, fibrinogen binding and TLR2 or TLR4 expression, intracellular reactive oxygen species (ROS) production in H ₂ DCFDA-loaded platelets and uptake of fluorescence-labeled TLR ligands, and fluorophore-conjugated fibrinogen were evaluated by flow cytometry. Analysis of platelet–neutrophil complexes was performed after coincubation of washed platelets and neutrophils in the presence and absence of TLR2 or TLR4 agonists on poly-L-lysine coated surfaces, followed by immunostaining and immunofluorescence imaging.

Results  Pam3CSK4 rapidly and transiently increased TLR2 and TLR4 expression. Over the course of 30 minutes after activation with Pam3CSK4 and LPS, the expression of both receptors decreased. Pam3CSK4-stimulated intracellular ROS production and the uptake of TLR ligands or fibrinogen much stronger than LPS. Besides, TLR4 activation led to a significant increase of platelet–neutrophil contacts.

Conclusion  Stimulation leads to rapid mobilization of TLR2 or TLR4 to the platelet surface, presumably followed by receptor internalization along with bound TLR ligands. After activation, platelet TLR2 and TLR4 mediate different immune-related reactions. In particular, TLR2 induces intracellular responses in platelets, whereas TLR4 initiates interactions with other immune cells such as neutrophils.

S1R agonist modulates rat platelet eicosanoid synthesis and aggregation

Sigma-1 receptor (S1R) is detected in different cell types and can regulate intracellular signaling pathways. S1R plays a role in the pathomechanism of diseases and the regulation of neurotransmitters. Fluvoxamine can bind to S1R and reduce the serotonin uptake of neurons and platelets. We therefore hypothesized that platelets express S1R, which can modify platelet function. The expression of the SIGMAR1 gene in rat platelets was examined with a reverse transcription polymerase chain reaction and a quantitative polymerase chain reaction. The receptor was also visualized by immunostaining and confocal laser scanning microscopy. The effect of S1R agonist PRE-084 on the eicosanoid synthesis of isolated rat platelets and ADP- and AA-induced platelet aggregation was examined. S1R was detected in rat platelets both at gene and protein levels. Pretreatment with PRE-084 of resting platelets induced elevation of eicosanoid synthesis. The rate of elevation in thromboxane B₂ and prostaglandin D₂ synthesis was similar, but the production of prostaglandin E₂ was higher. The concentration-response curve showed a sigmoidal form. The most effective concentration of the agonist was 2 µM. PRE-084 increased the quantity of cyclooxygenase-1 as detected by ELISA. PRE-084 also elevated the ADP- and AA-induced platelet aggregation. S1R of platelets might regulate physiological or pathological functions.

Influence of long-term proteasome inhibition on platelet responsiveness mediated by bortezomib

INTRODUCTION

Although platelets contain a full proteasome system, its role in platelet function is not completely understood yet. Since the proteasome system may be involved in time-delayed processes, platelet responsiveness was investigated after long-term, bortezomib-mediated proteasome inhibition.

MATERIALS AND METHODS

Citrate-anticoagulated whole blood was stored with 5 nM and 1 μM bortezomib for 24 h. Consecutively, aggregation was measured by light transmission in platelet-rich-plasma (PRP). Flow cytometry was performed to determine phosphorylation levels of the vasodilator-stimulated phosphoprotein (VASP), fibrinogen binding, PAC1-antibody binding and purinergic receptor expression in PRP, P2Y12 activity or glycoprotein (GP) Ib and IIb expression in whole blood. P2Y1 and P2X1 activities were assessed by calcium flux-induced fluorescence in washed platelets. Using PRP, adherent platelets on fibrinogen-, collagen- and ristocetin-coated surfaces were visualized and quantified by immunostaining.

RESULTS

Under bortezomib, VASP phosphorylation was less inducible and nitric oxide-induced inhibition of fibrinogen binding was slightly reduced. Proteasome inhibition did not tamper adenosine diphosphate-mediated aggregation or purinergic receptor expression and activity. Induced expression of activated fibrinogen receptors and fibrinogen binding were not significantly influenced by incubation with bortezomib for 24 h. Aggregation values with threshold agonist concentrations were increased under bortezomib. Despite unchanged GPIb expression, bortezomib-treated platelets showed enhanced adhesion on coated surfaces.

CONCLUSIONS

In platelets incubated for 24 h, bortezomib mediates a slight attenuation of inhibitory signaling, associated with facilitated platelet aggregation using threshold agonist concentrations and enhanced adhesion on agonist-coated surfaces.

Low-dose Btk inhibitors selectively block platelet activation by CLEC-2

Inhibitors of the tyrosine kinase Btk have been proposed as novel antiplatelet agents. In this study we show that low concentrations of the Btk inhibitor ibrutinib block CLEC-2-mediated activation and tyrosine phosphorylation including Syk and PLCγ2 in human platelets. Activation is also blocked in patients with X-linked agammaglobulinemia (XLA) caused by a deficiency or absence of Btk. In contrast, the response to GPVI is delayed in the presence of low concentrations of ibrutinib or in patients with XLA, and tyrosine phosphorylation of Syk is preserved. A similar set of results is seen with the second-generation inhibitor, acalabrutinib. The differential effect of Btk inhibition in CLEC-2 relative to GPVI signalling is explained by the positive feedback role involving Btk itself, as well as ADP and thromboxane A2 mediated activation of P2Y12 and TP receptors, respectively. This feedback role is not seen in mouse platelets and, consistent with this, CLEC-2-mediated activation is blocked by high but not by low concentrations of ibrutinib. Nevertheless, thrombosis was absent in 8 out of 13 mice treated with ibrutinib. These results show that Btk inhibitors selectively block activation of human platelets by CLEC-2 relative to GPVI suggesting that they can be used at 'low dose' in patients to target CLEC-2 in thrombo-inflammatory disease.

The Role of Human Platelet Preparation for Toll-Like Receptors 2 and 4 Related Platelet Responsiveness

Background  Like immune cells, platelets express the repertoire of toll-like receptors (TLR), among them TLR2 and TLR4, which are important for the recognition of bacterial patterns. Receptor-mediated functional effects in platelets have been investigated, but reliable conclusions are tampered due to heterogeneous study designs with variable platelet preparation methods. This study compares TLR2- and TLR4-dependent platelet responsiveness in platelet-rich plasma (PRP) and in washed platelets (WPs).

Material and Methods  Fresh peripheral blood samples from healthy donors served for the preparation of PRP and WP. Basal and agonist-stimulated TLR2 and TLR4 expression levels were evaluated by flow cytometry. Light transmission aggregometry was used to investigate functional effects of TLR2 and TLR4 stimulation with Pam3CSK4 or LPS (lipopolysaccharides from Escherichia coli ) as ligands. The capacity of chemokine release was determined by immunoassays.

Results  Pam3CSK4 and LPS (in combination with thrombin) were able to induce aggregation in WP, but not in PRP, with threshold concentrations of 15 µg/mL. Basal expression levels of TLR2 and TLR4 were higher in WP than in PRP, increasing several-fold rapidly and persistently upon platelet activation with potent agonists. Pam3CSK4 (15 µg/mL) or LPS led to the submaximal release of RANTES, PF4, PDGF, NAP-2, and sCD40L from WP. In PRP, secretory effects are less pronounced for RANTES, PDGF, or PF4, and not detectable for NAP-2 or sCD40L.

Conclusion  The effects mediated by TLR2 and TLR4 stimulation are dependent on platelet preparation, an important issue for experimental designs and for manufacturing of platelet concentrates in transfusion medicine.

The role of adenosine diphosphate mediated platelet responsiveness for the stability of platelet integrity in citrated whole blood under ex vivo conditions

Background

Platelets are important for effective hemostasis and considered to be involved in pathophysiological processes, e.g. in cardiovascular diseases. Platelets provided for research or for therapeutic use are frequently separated from citrated whole blood (WB) stored for different periods of time. Although functionally intact platelets are required, the stability of platelet integrity, e.g. adenosine diphosphate (ADP) mediated responsiveness, has never been thoroughly investigated in citrated WB under ex vivo conditions.

Objectives

Platelet integrity was evaluated at different time points in citrated WB units, collected from healthy donors and stored for 5 days at ambient temperature. The analysis included the measurement of activation markers, of induced light transmission aggregometry and of purinergic receptor expression or function. Inhibitory pathways were explored by determination of basal vasodilator-stimulated phosphoprotein (VASP)-phosphorylation, intracellular cyclic nucleotide levels and the content of phosphodiesterase 5A. Fresh peripheral blood (PB) samples served as controls.

Results

On day 5 of storage, thrombin receptor activating peptide-6 (TRAP-6) stimulated CD62P expression and fibrinogen binding were comparable to PB samples. ADP induced aggregation continuously decreased during storage. Purinergic receptor expression remained unchanged, whereas the P2Y1 activity progressively declined in contrast to preserved P2Y12 and P2X1 function. Inhibitory pathways were unaffected except for a slight elevation of VASP phosphorylation at Ser²³⁹ on day 5.

Conclusion

After 5 days of storage in citrated WB, platelet responsiveness to TRAP-6 is sufficiently maintained. However, ADP-mediated platelet integrity is more sensitive to deterioration, especially after storage for more than 2 days. Decreasing ADP-induced aggregation is particularly caused by the impairment of the purinergic receptor P2Y1 activity. These characteristics should be considered in the use of platelets from stored citrated WB for experimental or therapeutic issues.

Plasma kallikrein enhances platelet aggregation response by subthreshold doses of ADP

Human plasma kallikrein (huPK) potentiates platelet responses to subthreshold doses of ADP, although huPK itself, does not induce platelet aggregation. In the present investigation, we observe that huPK pretreatment of platelets potentiates ADP-induced platelet activation by prior proteolysis of the G-protein-coupled receptor PAR-1. The potentiation of ADP-induced platelet activation by huPK is mediated by the integrin α_IIbβ₃ through interactions with the KGD/KGE sequence motif in huPK. Integrin α_IIbβ₃ is a cofactor for huPK binding to platelets to support PAR-1 hydrolysis that contributes to activation of the ADP signaling pathway. This activation pathway leads to phosphorylation of Src, AktS⁴⁷³, ERK1/2, and p38 MAPK, and to Ca²⁺ release. The effect of huPK is blocked by specific antagonists of PAR-1 (SCH 19197) and α_IIbβ₃ (abciximab) and by synthetic peptides comprising the KGD and KGE sequence motifs of huPK. Further, recombinant plasma kallikrein inhibitor, rBbKI, also blocks this entire mechanism. These results suggest a new function for huPK. Formation of plasma kallikrein lowers the threshold for ADP-induced platelet activation. The present observations are consistent with the notion that plasma kallikrein promotes vascular disease and thrombosis in the intravascular compartment and its inhibition may ameliorate cardiovascular disease and thrombosis.