Mitochondrial Uncoupling Protein 2 Is Present in Human Platelets and Regulates Platelet Activation

It has long been recognized that platelets contain functional mitochondria and accumulating data suggest that several aspects of mitochondrial function, including reactive oxygen species production, modulate platelet thrombotic function. We have recently shown that platelet mitochondria isolated from patients with Sickle Cell Disease generate significantly greater concentrations of reactive oxygen species than healthy African American subjects and that this mitochondrial oxidant generation contributes to higher levels of basal platelet activation in these patients (Blood. 2014 May 1;123(18):2864-72). Based on these data, we have now investigated the factors that regulate platelet mitochondrial superoxide generation in healthy and Sickle Cell Disease platelets. Here we demonstrate that human platelets express mitochondrial uncoupling protein-2 (UCP2), a protein that is known to decrease the efficiency of oxidative phosphorylation and oxidant generation in other cell types but has previously not been identified in platelets. In this study we show that UCP2 protein is expressed in healthy human platelets and is fully functional as it facilitates proton leak across the inner mitochondrial membrane, leading to decreased mitochondrial membrane potential. Further, we demonstrate that the expression of this protein attenuates platelet mitochondrial superoxide generation, as treatment of platelets with Genipin (2-10µM), a pharmacological inhibitor of UCPs, concentration-dependently increases mitochondrial membrane potential and reactive oxygen species production. Further, an approximately 70% inhibition of UCP activity results in platelet activation demonstrated by increased membrane p-selectin expression (65±7% versus 6±3% in untreated controls) and augmented glycoprotein IIb/IIIa activation (57±9% versus 9±4% in untreated controls). The use of the mitochondrial-targeted antioxidant mitoTEMPO (10µM) decreases genipin-induced superoxide generation and significantly attenuates platelet activation. Notably, preliminary data presented here also suggest that UCP expression is decreased in platelets isolated from Sickle Cell Disease patients (3-fold) compared to healthy African American subjects. Additionally, ongoing studies are investigating platelet function in mice deficient in UCP2. Taken together, these data demonstrate a novel mechanism of regulation of platelet thrombotic function whose physiological relevance is apparent in the context of Sickle Cell Disease. More broadly this study advances the understanding of the role of the mitochondrion in platelet biology and thrombotic disease.