Background: Activation of the immune system strongly influences blood coagulation and pathological thrombus formation. Antimicrobial agents that are released during leukocyte degranulation can directly activate platelets and induce the coagulation cascade. Myeloperoxidase (MPO) is an enzyme that is contained in azurophilic granules of neutrophils. Recently MPO was shown to potentiate platelet aggregation, so it may contribute to pathological thrombosis during inflammation, but the mechanism is poorly understood.

Aims: To investigate MPO binding to platelet membrane and to assess MPO effect on primary platelet signaling (cytosolic Ca²⁺ concentration, membrane potential and ionic currents, phosphatidylserine (PS) exposure on platelet surface).

Methods: Platelets were isolated from citrate-stabilized blood from healthy volunteers. Single channel ionic currents were registered using patch-clamp technique in a voltage-clamp mode. Platelet calcium responses were measured by TIRF miscroscopy using CalBryte 590. PS exposure was measured by flow cytometry using annexin V-Alexa Fluor 647.

Results: MPO (100 nM) increased frequency (2-fold) and duration of calcium oscillations compared to the basal level. This effect was accompanied by plasma membrane hyperpolarization by 10-15 mV. To inactivate voltage-gated potassium channels patch was hyperpolarized and registration of single channel events was performed. It revealed MPO-induced currents from 7 to 20 nSm (outward Cl^– or inward K⁺ movement) in Ca²⁺-containing KCl and K-aspartate solutions, indicating the involvement of Ca²⁺-dependent chloride and potassium channels. Since platelet procoagulant activity closely relates to its calcium response, we next examined the effect of MPO on PS exposure on platelet surface. At MPO concentrations higher than 100 nM there was a significant increase in number of PS-positive platelets.

Conclusions: Obtained results indicate that MPO binding to platelets increases the rate of calcium oscillations as well as Ca²⁺-dependent ionic permeability that may lead to exposure of a procoagulant platelet surface. This suggests possible targets for regulation of platelet response at sites of inflammation.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/effect-of-human-myeloperoxidase-on-ca2-response-membrane-conductivity-and-phosphatidylserine-exposure-on-platelet-surface/