Investigations into the contributions of mitochondrial dynamics and function to platelet ageing and reactivity

Abstract

Platelets are essential for the physiological process of haemostasis, but also drive pathological thrombosis. Platelet lifespan is a tightly controlled process through which platelets exist for approximately 10 days within the circulation of healthy individuals. However, in a number of disease states this process is dysregulated leading to an accelerated platelet turnover. Indeed, there are a number of reports suggesting that newly formed platelets are hyper-reactive and their presence has been associated with a higher risk of thrombosis. Whilst there are these indications of hyper-reactivity in young platelets, there are few systematic studies. Here I have used proteomics coupled with functional studies and immunofluorescence to show that there is a progressive decline in mitochondrial and cytoskeletal proteins as platelets age and an increase in apoptotic pathways. Given the apparent importance of mitochondria in supporting the predetermined platelet lifespan, it raised the question as to whether mitochondria are important for other platelet functional processes. Therefore, I sought to elucidate the impact of platelet activation on mitochondrial function and dynamics. Physiological stimulation causes an increase in mitochondrial respiration, consistent with an increase in energy demand. Interestingly, P2Y12 receptor inhibition causes a reduction in basal oxygen consumption, suggesting a dysregulation in mitochondrial function. Furthermore, this work highlights a role for mitochondria beyond energy production, with indications that stimulation causes platelets to package and release their mitochondria into microvesicles. Interestingly, these mitochondria-containing microvesicles have high P-selectin expression suggesting they may be more likely to interact with neutrophils than the rest of the microvesicle population. Indeed, incubation of neutrophils with mitochondria-positive microvesicles but not mitochondria-negative microvesicles causes alterations in the expression of surface markers; CD11b, CD66b and CXCR2, indicative of neutrophil activation potentially as a result of phagocytosis. This work highlights an important role of mitochondria in both platelet ageing and activation.