2 research outputs found
Chronic Immune Platelet Activation Is Followed by Platelet Refractoriness and Impaired Contractility
Autoimmune diseases, including systemic lupus erythematosus (SLE), have a high risk of thrombotic and hemorrhagic complications associated with altered platelet functionality. We studied platelets from the blood of SLE patients and their reactivity. The surface expression of phosphatidylserine, P-selectin, and active integrin αIIbβ3 were measured using flow cytometry before and after platelet stimulation. Soluble P-selectin was measured in plasma. The kinetics of platelet-driven clot contraction was studied, as well as scanning and transmission electron microscopy of unstimulated platelets. Elevated levels of membrane-associated phosphatidylserine and platelet-attached and soluble P-selectin correlated directly with the titers of IgG, anti-dsDNA-antibodies, and circulating immune complexes. Morphologically, platelets in SLE lost their resting discoid shape, formed membrane protrusions and aggregates, and had a rough plasma membrane. The signs of platelet activation were associated paradoxically with reduced reactivity to a physiological stimulus and impaired contractility that revealed platelet exhaustion and refractoriness. Platelet activation has multiple pro-coagulant effects, and the inability to fully contract (retract) blood clots can be either a hemorrhagic or pro-thrombotic mechanism related to altered clot permeability, sensitivity of clots to fibrinolysis, obstructiveness, and embologenicity. Therefore, chronic immune platelet activation followed by secondary platelet dysfunction comprise an understudied pathogenic mechanism that supports hemostatic disorders in autoimmune diseases, such as SLE
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Fatal dysfunction and disintegration of thrombin-stimulated platelets.
Platelets play a key role in the formation of hemostatic clots and obstructive thrombi as well as in other biological processes. In response to physiological stimulants, including thrombin, platelets change shape, express adhesive molecules, aggregate, and secrete bioactive substances, but their subsequent fate is largely unknown. Here we examined late-stage structural, metabolic, and functional consequences of thrombin-induced platelet activation. Using a combination of confocal microscopy, scanning and transmission electron microscopy, flow cytometry, biochemical and biomechanical measurements, we showed that thrombin-induced activation is followed by time-dependent platelet dysfunction and disintegration. After ~30 minutes of incubation with thrombin, unlike with collagen or ADP, human platelets disintegrated into cellular fragments containing organelles, such as mitochondria, glycogen granules, and vacuoles. This platelet fragmentation was preceded by Ca2+ influx, integrin αIIbβ3 activation and phosphatidylserine exposure (activation phase), followed by mitochondrial depolarization, generation of reactive oxygen species, metabolic ATP depletion and impairment of platelet contractility along with dramatic cytoskeletal rearrangements, concomitant with platelet disintegration (death phase). Coincidentally with the platelet fragmentation, thrombin caused calpain activation but not activation of caspases 3 and 7. Our findings indicate that the late functional and structural damage of thrombin-activated platelets comprise a calpain-dependent platelet death pathway that shares some similarities with the programmed death of nucleated cells, but is unique to platelets, therefore representing a special form of cellular destruction. Fragmentation of activated platelets suggests that there is an underappreciated pathway of enhanced elimination of platelets from the circulation in (pro)thrombotic conditions once these cells have performed their functions