Rapid kinetics of changes in oxygen consumption rate in thrombin-stimulated platelets measured by high-resolution respirometry

Modulatory effects of dietary saturated fatty acids on platelet mitochondrial function following short-term exposure to ambient Particulate Matter (PM2.5)

Exposure to ambient fine particulate matter (PM2.5) was found to produce vascular injury, possibly by activating platelets within days after exposure. The aim of this study was to investigate the modulatory effects of dietary saturated fatty acids on platelet mitochondrial respiratory parameters following short-term inhalational exposure to PM2.5. A total of 22 healthy male volunteers were recruited from the Research Triangle area of North Carolina. Platelets were isolated from fresh whole blood samples and mitochondrial respiratory parameters were measured using an extracellular flux analyzer. Intake of saturated fat was averaged from multiple 24-hr dietary recalls. Daily ambient PM2.5 concentrations were obtained from ambient air quality monitoring stations. Correlation and ANOVA were used in data analyses, along with the pick-a-point method and the Johnson-Neyman technique for probing moderation. After controlling for age and omega-3 index, the intake of dietary saturated fatty acids after reaching 9.3% or higher of the total caloric intake significantly moderated the associations between PM2.5 exposure and several platelet mitochondrial respiratory parameters. In conclusion, dietary saturated fatty acids above 9.3% of total caloric intake influenced the relationship between short-term PM2.5 exposure and platelet mitochondrial respiration. Further research is needed to understand these associations and their implications for cardiovascular health.

Mitofusin-2 Regulates Platelet Mitochondria and Function

BACKGROUND

Single-nucleotide polymorphisms linked with the rs1474868 T allele (MFN2 [mitofusin-2] T/T) in the human mitochondrial fusion protein MFN2 gene are associated with reduced platelet MFN2 RNA expression and platelet counts. This study investigates the impact of MFN2 on megakaryocyte and platelet biology.

METHODS

Mice with megakaryocyte/platelet deletion of Mfn2 (Mfn2-/- [Mfn2 conditional knockout]) were generated using Pf4-Cre crossed with floxed Mfn2 mice. Human megakaryocytes were generated from cord blood and platelets isolated from healthy subjects genotyped for rs1474868. Ex vivo approaches assessed mitochondrial morphology, function, and platelet activation responses. In vivo measurements included endogenous/transfused platelet life span, tail bleed time, transient middle cerebral artery occlusion, and pulmonary vascular permeability/hemorrhage following lipopolysaccharide-induced acute lung injury.

RESULTS

Mitochondria was more fragmented in megakaryocytes derived from Mfn2-/- mice and from human cord blood with MFN2 T/T genotype compared with control megakaryocytes. Human resting platelets of MFN2 T/T genotype had reduced MFN2 protein, diminished mitochondrial membrane potential, and an increased rate of phosphatidylserine exposure during ex vivo culture. Platelet counts and platelet life span were reduced in Mfn2-/- mice accompanied by an increased rate of phosphatidylserine exposure in resting platelets, especially aged platelets, during ex vivo culture. Mfn2-/- also decreased platelet mitochondrial membrane potential (basal) and activated mitochondrial oxygen consumption rate, reactive oxygen species generation, calcium flux, platelet-neutrophil aggregate formation, and phosphatidylserine exposure following dual agonist activation. Ultimately, Mfn2-/- mice showed prolonged tail bleed times, decreased ischemic stroke infarct size after cerebral ischemia-reperfusion, and exacerbated pulmonary inflammatory hemorrhage following lipopolysaccharide-induced acute lung injury. Analysis of MFN2 SNPs in the iSPAAR study (Identification of SNPs Predisposing to Altered ALI Risk) identified a significant association between MFN2 and 28-day mortality in patients with acute respiratory distress syndrome.

CONCLUSIONS

Mfn2 preserves mitochondrial phenotypes in megakaryocytes and platelets and influences platelet life span, function, and outcomes of stroke and lung injury.

Metabolic targeting of platelets to combat thrombosis: dawn of a new paradigm?

Current antithrombotic therapies used in clinical settings target either the coagulation pathways or platelet activation receptors (P2Y12 or GPIIb/IIIa), as well as the cyclooxygenase (COX) enzyme through aspirin. However, they are associated with bleeding risk and are not suitable for long-term use. Thus, novel strategies which provide broad protection against platelet activation with minimal bleeding risks are required. Regardless of the nature of agonist stimulation, platelet activation is an energy-intensive and ATP-driven process characterized by metabolic switching toward a high rate of aerobic glycolysis, relative to oxidative phosphorylation (OXPHOS). Consequently, there has been considerable interest in recent years in investigating whether targeting metabolic pathways in platelets, especially aerobic glycolysis and OXPHOS, can modulate their activation, thereby preventing thrombosis. This review briefly discusses the choices of metabolic substrates available to platelets that drive their metabolic flexibility. We have comprehensively elucidated the relevance of aerobic glycolysis in facilitating platelet activation and the underlying molecular mechanisms that trigger this switch from OXPHOS. We have provided a detailed account of the antiplatelet effects of targeting vital metabolic checkpoints such as pyruvate dehydrogenase kinases (PDKs) and pyruvate kinase M2 (PKM2) that preferentially drive the pyruvate flux to aerobic glycolysis. Furthermore, we discuss the role of fatty acids and glutamine oxidation in mitochondria and their subsequent role in driving OXPHOS and platelet activation. While the approach of targeting metabolic regulatory mechanisms in platelets to prevent their activation is still in a nascent stage, accumulating evidence highlights its beneficial effects as a potentially novel antithrombotic strategy.

Energy metabolism in platelets fuels thrombus formation: Halting the thrombosis engine with small-molecule modulators of platelet metabolism

Platelets are circulating cells central to haemostasis that follows vessel injury, as well as thrombosis that ensues as a consequence of pathological stasis or plaque rupture. Platelet responses to various stimuli that mediate these processes are all energy-intensive. Hence, platelets need to adapt their energy metabolism to fulfil the requirements of clot formation while overcoming the adversities of the thrombus niche such as restricted access to oxygen and nutrient. In the present review, we describe the changes in energy metabolism of platelets upon agonist challenge and their underlying molecular mechanisms. We briefly discuss the metabolic flexibility and dependency of stimulated platelets in terms of choice of energy substrates. Finally, we discuss how targeting the metabolic vulnerabilities of stimulated platelets such as aerobic glycolysis and/or beta oxidation of fatty acids could forestall platelet activation and thrombus formation. Thus, we present a case for modulating platelet energy metabolism using small-molecules as a novel anti-platelet strategy in the management of vaso-occlusive disorders like acute myocardial infarction, ischemic stroke, deep vein thrombosis and pulmonary embolism.

Fatty acid oxidation fuels agonist-induced platelet activation and thrombus formation: Targeting β-oxidation of fatty acids as an effective anti-platelet strategy

Platelet mitochondria possess remarkable plasticity for oxidation of energy substrates, where metabolic dependency on glucose or fatty acids is higher than glutamine. Since platelets metabolize nearly the entire pool of glucose to lactate rather than fluxing through mitochondrial tricarboxylic acid cycle, we posit that majority of mitochondrial ATP, which is essential for platelet granule secretion and thrombus formation, is sourced from oxidation of fatty acids. We performed a comprehensive analysis of bioenergetics and function of stimulated platelets in the presence of etomoxir, trimetazidine and oxfenicine, three pharmacologically distinct inhibitors of β-oxidation. Each of them significantly impaired oxidative phosphorylation in unstimulated as well as thrombin-stimulated platelets leading to a small but consistent drop in ATP level in activated cells due to a lack of compensation from glycolytic ATP. Trimetazidine and oxfenicine attenuated platelet aggregation, P-selectin externalization and integrin α_IIb β₃ activation. Both etomoxir and trimetazidine impeded agonist-induced dense granule release and platelet thrombus formation on collagen under arterial shear. The effect of inhibitors on platelet aggregation and dense granule release was dose- and incubation time- dependent with significant inhibition at higher doses and prolonged incubation times. Neither of the inhibitors could protect mice from collagen-epinephrine-induced pulmonary embolism or prolong mouse tail bleeding times. However, mice pre-administered with etomoxir, trimetazidine and oxfenicine were protected from ferric chloride-induced mesenteric thrombosis. In conclusion, β-oxidation of fatty acids sustains ATP level in stimulated platelets and is therefore essential for energy-intensive agonist-induced platelet responses. Thus, fatty acid oxidation may constitute an attractive therapeutic target for novel antiplatelet agents.

Platelet extracellular vesicles and the secretory interactome join forces in health and disease

Extracellular vesicles (EVs) are small membrane-bound vesicles released by cells under various conditions. They are found in the extracellular milieu in all biological fluids. As the concentrations, contents, and origin of EVs can change during inflammation, the assessment of EVs can be used as a proxy of cellular activation. Here, we review the literature regarding EVs, more particularly those released by platelets and their mother cells, the megakaryocytes. Their cargo includes cytokines, growth factors, organelles (mitochondria and proteasomes), nucleic acids (messenger and non-coding RNA), transcription factors, and autoantigens. EVs may thus contribute to intercellular communication by facilitating exchange of material between cells. EVs also interact with other molecules secreted by cells. In autoimmune diseases, EVs are associated with antibodies secreted by B cells. By definition, EVs necessarily comprise a phospholipid moiety, which is thus the target of secreted phospholipases also abundantly expressed in the extracellular milieu. We discuss how platelet-derived EVs, which represent the majority of the circulating EVs, may contribute to immunity through the activity of their cargo or in combination with the secretory interactome.

Isotopically nonstationary 13C metabolic flux analysis in resting and activated human platelets

Platelet metabolism is linked to platelet hyper- and hypoactivity in numerous human diseases. Developing a detailed understanding of the link between metabolic shifts and platelet activation state is integral to improving human health. Here, we show the first application of isotopically nonstationary ¹³C metabolic flux analysis to quantitatively measure carbon fluxes in both resting and thrombin activated platelets. Metabolic flux analysis results show that resting platelets primarily metabolize glucose to lactate via glycolysis, while acetate is oxidized to fuel the tricarboxylic acid cycle. Upon activation with thrombin, a potent platelet agonist, platelets increase their uptake of glucose 3-fold. This results in an absolute increase in flux throughout central metabolism, but when compared to resting platelets they redistribute carbon dramatically. Activated platelets decrease relative flux to the oxidative pentose phosphate pathway and TCA cycle from glucose and increase relative flux to lactate. These results provide the first report of reaction-level carbon fluxes in platelets and allow us to distinguish metabolic fluxes with much higher resolution than previous studies.

Sample Preparation as a Critical Aspect of Blood Platelet Mitochondrial Respiration Measurements-The Impact of Platelet Activation on Mitochondrial Respiration

Blood platelets are considered as promising candidates as easily-accessible biomarkers of mitochondrial functioning. However, their high sensitivity to various stimulus types may potentially affect mitochondrial respiration and lead to artefactual outcomes. Therefore, it is crucial to identify the factors associated with platelet preparation that may lead to changes in mitochondrial respiration. A combination of flow cytometry and advanced respirometry was used to examine the effect of blood anticoagulants, the media used to suspend isolated platelets, respiration buffers, storage time and ADP stimulation on platelet activation and platelet mitochondria respiration. Our results clearly show that all the mentioned factors can affect platelet mitochondrial respiration. Briefly, (i) the use of EDTA as anticoagulant led to a significant increase in the dissipative component of respiration (LEAK), (ii) the use of plasma for the suspension of isolated platelets with MiR05 as a respiration buffer allows high electron transfer capacity and low platelet activation, and (iii) ADP stimulation increases physiological coupling respiration (ROUTINE). Significant associations were observed between platelet activation markers and mitochondrial respiration at different preparation steps; however, the fact that these relationships were not always apparent suggests that the method of platelet preparation may have a greater impact on mitochondrial respiration than the platelet activation itself.

ROS in Platelet Biology: Functional Aspects and Methodological Insights

Reactive oxygen species (ROS) and mitochondria play a pivotal role in regulating platelet functions. Platelet activation determines a drastic change in redox balance and in platelet metabolism. Indeed, several signaling pathways have been demonstrated to induce ROS production by NAPDH oxidase (NOX) and mitochondria, upon platelet activation. Platelet-derived ROS, in turn, boost further ROS production and consequent platelet activation, adhesion and recruitment in an auto-amplifying loop. This vicious circle results in a platelet procoagulant phenotype and apoptosis, both accounting for the high thrombotic risk in oxidative stress-related diseases. This review sought to elucidate molecular mechanisms underlying ROS production upon platelet activation and the effects of an altered redox balance on platelet function, focusing on the main advances that have been made in platelet redox biology. Furthermore, given the increasing interest in this field, we also describe the up-to-date methods for detecting platelets, ROS and the platelet bioenergetic profile, which have been proposed as potential disease biomarkers.

Regulation of mitochondrial function as a promising target in platelet activation-related diseases

Platelets are anucleated cell elements produced by fragmentation of the cytoplasm of megakaryocytes and have a unique metabolic phenotype compared with circulating leukocytes, exhibiting a high coupling efficiency to mitochondrial adenosine triphosphate production with reduced respiratory reserve capacity. Platelet mitochondria are well suited for ex vivo analysis of different diseases. Even some diseases induce mitochondrial changes in platelets without reflecting them in other organs. During platelet activation, an integrated participation of glycolysis and oxidative phosphorylation is mediated by oxidative stress production-dependent signaling. The platelet activation-dependent procoagulant activity mediated by collagen, thrombin and hyperglycemia induce mitochondrial dysfunction to promote thrombosis in oxidative stress-associated pathological conditions. Interestingly, some compounds exhibit a protective action on platelet mitochondrial dysfunction through control of mitochondrial oxidative stress production or inhibition of respiratory complexes. They can be grouped in a) Natural source-derived compounds (e.g. Xanthohumol, Salvianoloc acid A and Sila-amide derivatives of NAC), b) TPP⁺-linked small molecules (e.g. mitoTEMPO and mitoQuinone) and c) FDA-approved drugs (e.g. metformin and statins), illustrating the wide range of molecular structures capable of effectively interacting with platelet mitochondria. The present review article aims to discuss the mechanisms of mitochondrial dysfunction and their association with platelet activation-related diseases.