Potential Role of Mitochondria as Modulators of Blood Platelet Activation and Reactivity in Diabetes and Effect of Metformin on Blood Platelet Bioenergetics and Platelet Activation

Increased platelet mitochondrial function correlates with clot strength in a rodent fracture model

BACKGROUND

Thromboelastographic measures of clot strength increase early after injury, portending higher risks for thromboembolic complications during recovery. Understanding the specific role of platelets is challenging because of a lack of clinically relevant measures of platelet function. Platelet mitochondrial respirometry may provide insight to global platelet function but has not yet been correlated with functional coagulation studies.

METHODS

Wistar rats underwent anesthesia and either immediate sacrifice for baseline values (n = 6) or (1) bilateral hindlimb orthopedic injury (n = 12), versus (2) sham anesthesia (n = 12) with terminal phlebotomy/hepatectomy after 24 hours. High-resolution respirometry was used to measure basal respiration, mitochondrial leak, maximal oxidative phosphorylation, and Complex IV activity in intact platelets; Complex I- and Complex II-driven respiration was measured in isolated liver mitochondria. Results were normalized to platelet number and protein mass, respectively. Citrated native thromboelastography (TEG) was performed in triplicate.

RESULTS

Citrated native TEG maximal amplitude was significantly higher (81.0 ± 3.0 vs. 73.3 ± 3.5 mm, p < 0.001) in trauma compared with sham rats 24 hours after injury. Intact platelets from injured rats had higher basal oxygen consumption (17.7 ± 2.5 vs. 15.1 ± 3.2 pmol O 2 /[s × 10 8 cells], p = 0.045), with similar trends in mitochondrial leak rate ( p = 0.19) when compared with sham animals. Overall, platelet basal respiration significantly correlated with TEG maximal amplitude ( r = 0.44, p = 0.034). As a control for sex-dependent systemic mitochondrial differences, females displayed higher liver mitochondria Complex I-driven respiration (895.6 ± 123.7 vs. 622.1 ± 48.7 mmol e - /min/mg protein, p = 0.02); as a control for systemic mitochondrial effects of injury, no liver mitochondrial respiration differences were seen.

CONCLUSION

Platelet mitochondrial basal respiration is increased after injury and correlates with clot strength in this rodent hindlimb fracture model. Several mitochondrial-targeted therapeutics exist in common use that are underexplored but hold promise as potential antithrombotic adjuncts that can be sensitively evaluated in this preclinical model.

Metformin-suppressed platelet's function in vitro: Possible relation to delayed or failure of platelet-rich fibrin preparation

Platelet-rich fibrin (PRF) is a popular autologous blood-derived biomaterial that is used in regenerative therapy. Owing to its simple preparation without additional factors, the PRF quality directly reflects the characteristics of individual blood samples. Antiplatelet or anticoagulant drugs can hamper the successful preparation of PRF. We recently observed similar phenomena in metformin-taking type-2 diabetics (T2DM). Thus, we hypothesized that metformin interferes with platelet function, thereby suppressing coagulation. For practical reasons, leukocyte- and platelet-rich plasma was prepared from healthy male donors (n = 9-15, age: 26-80 years) and treated with metformin (1-10 mM) for 24-72 h. Intrinsic and extrinsic coagulation activities were evaluated using prothrombin time (PT) and activated partial thromboplastin time (ATPP). Platelet adhesion and aggregation assays were performed using ADP stimulation. Among the parameters tested, APTT was the most sensitive and was significantly prolonged in the concentration range of 1-10 mM in a time- and concentration-dependent manner. Although obtained from healthy platelets and relatively higher concentrations of metformin, these findings suggest that metformin may induce further dysfunction of platelets to suppress intrinsic coagulation activity in T2DM patients, leading to failure of PRF preparation. This phenomenon may not have a severe impact on clinical diabetology or hematology. However, clinicians using PRF are recommended to be more sensitive to such information to avoid unexpected events in clinical settings.

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.

Platelet mitochondria: the mighty few

PURPOSE OF REVIEW

Platelet mitochondrial dysfunction is both caused by, as well as a source of oxidative stress. Oxidative stress is a key hallmark of metabolic disorders such as dyslipidemia and diabetes, which are known to have higher risks for thrombotic complications.

RECENT FINDINGS

Increasing evidence supports a critical role for platelet mitochondria beyond energy production and apoptosis. Mitochondria are key regulators of reactive oxygen species and procoagulant platelets, which both contribute to pathological thrombosis. Studies targeting platelet mitochondrial pathways have reported promising results suggesting antithrombotic effects with limited impact on hemostasis in animal models.

SUMMARY

Targeting platelet mitochondria holds promise for the reduction of thrombotic complications in patients with metabolic disorders. Future studies should aim at validating these preclinical findings and translate them to the clinic.

High Dose of Metformin Decreases Susceptibility to Occlusive Arterial Thrombosis in Diabetic Mice

Introduction

Metformin is the most prescribed medication in Diabetes Mellitus(DM). Metformin has shown to decrease mean platelet volume, with promising antiplatelet effects. High doses of Metformin have also been associated with hypercoagulation. We hypothesize that Metformin will protect DM mice from occlusive arterial thrombus formation by altering platelet activation and mitochondrial bioenergetics.

Methods

DM was developed by low dose of Streptozotocin, healthy (non-DM) mice are controls. Either vehicle or Metformin was administered twice daily via oral gavage for 7-days. Ferric chloride (FeCl3) arterial thrombosis and tail bleeding time were performed. Whole blood aggregometry, platelet activation/adhesion and mitochondrial bioenergetics were evaluated.

Results

Metformin decreased susceptibility of DM mice to arterial thrombosis. Platelet bioenergetics show DM mice have increased platelet mitochondrial respiration, but no differences were observed with Metformin treatment. In healthy mice, Metformin modulated ADP-dependent increase in platelet adhesion. In healthy mice, Metformin shortens bleeding time with faster thrombotic occlusion. Metformin also increased platelet mitochondrial maximal respiration and spare respiratory capacity uniquely in healthy mice.

Conclusion

Metformin regulates platelet bioenergetics and ADP-mediated platelet function in DM mice which attenuates susceptibility to arterial thrombosis. Future studies will evaluate clinically relevant doses of Metformin that regulates thrombotic function in diabetic platelets.

Platelet-Derived S1P and Its Relevance for the Communication with Immune Cells in Multiple Human Diseases

Sphingosine-1-phosphate (S1P) is a versatile signaling lipid involved in the regulation of numerous cellular processes. S1P regulates cellular proliferation, migration, and apoptosis as well as the function of immune cells. S1P is generated from sphingosine (Sph), which derives from the ceramide metabolism. In particular, high concentrations of S1P are present in the blood. This originates mainly from erythrocytes, endothelial cells (ECs), and platelets. While erythrocytes function as a storage pool for circulating S1P, platelets can rapidly generate S1P de novo, store it in large quantities, and release it when the platelet is activated. Platelets can thus provide S1P in a short time when needed or in the case of an injury with subsequent platelet activation and thereby regulate local cellular responses. In addition, platelet-dependently generated and released S1P may also influence long-term immune cell functions in various disease processes, such as inflammation-driven vascular diseases. In this review, the metabolism and release of platelet S1P are presented, and the autocrine versus paracrine functions of platelet-derived S1P and its relevance in various disease processes are discussed. New pharmacological approaches that target the auto- or paracrine effects of S1P may be therapeutically helpful in the future for pathological processes involving S1P.