Modulation of rabbit platelet aggregation and calcium mobilization by platelet cholesterol content

Monitoring platelet function in marine mammals: Intracellular Ca2+ mobilization as a biomarker of platelet activation

Platelet functionality plays a crucial role in marine mammals. Alterations in platelet function can result from stress, pathologies, or exposure to xenobiotics, among others. The early detection of platelet function abnormalities is essential in these species to prevent advanced pathology and mitigate potential risks. Our main objective was to establish a range of physiological values of platelet function in bottlenose dolphins (Tursiops truncatus), beluga whales (Delphinapterus leucas), sea lions (Otaria flavescens) and walruses (Odobenus rosmarus). Intraplatelet Ca2+ mobilization using adenosine diphosphate (ADP) as a platelet agonist was used as a platelet function biomarker, adapting the methodology previously described by us in dolphins (Felipo-Benavent et al., 2022) to the rest of the species. The assay was also adapted to a seal (Phoca vitulina). Numerical indicators of intraplatelet Ca2+ mobilization kinetics were established, and statistical analyses were performed to compare the effects of species, sex, age, aquarium and species. Significant differences were observed between species, being the platelets of the sea lions the more reactive to the agonist. This work demonstrates the usefulness of this assay in the diagnosis or monitoring of animals with hemostatic diseases, showing two clinical cases in which intraplatelet calcium mobilization values were altered in marine mammals suffering haemorrhages. This assay may also serve as a means to monitor environmental changes and their potential impact on the health of marine mammal populations.

TLR4 inhibitor alleviates sepsis-induced organ failure by inhibiting platelet mtROS production, autophagy, and GPIIb/IIIa expression

Thrombocytopenia and impaired platelet function are associated with sepsis-induced organ failure. Numerous studies have shown that mitochondrial ROS (mtROS) and autophagy are related to organ injury in sepsis. However, the relationships between platelet mtROS, autophagy and sepsis organ failure remain unclear. Herein, we explored whether toll like receptor 4 (TLR4) inhibitor alleviates sepsis organ failure by inhibiting platelet mtROS production, autophagy, and GPIIb/IIIa expression.Mice were administrated with LPS, LPS + TAK242 or vehicle. The lungs and kidneys were harvested and analyzed using hematoxylin and eosin staining assay. Platelet rich plasma (PRP) was isolated from blood and platelets aggregation and TLR4 expression were analyzed using flow cytometer and western blot. PRP from healthy volunteers was treated with saline, LPS, or LPS + TAK242, and then mitoSOX and calcium were detected using flow cytometer, and NOX2 and LC3B were tested using western blot.Results showed that TAK242 effectively alleviated LPS-induced acute kidney and lung injury in mice, and decreased CD41 expression more significantly than CD62P. In vitro, by inhibiting TLR4, TAK242 suppressed Ca²⁺, mitoSOX fluorescence, NOX2 expression and LC3BII/LC3BI ratio in LPS treated platelets.TLR4 inhibitor TAK242 may effectively alleviate mouse lung and kidney injury by inhibition of mouse platelet GPIIb/IIIa, and reduce LPS-induced mtROS generation related to Ca²⁺ influx, thus reducing platelet activation.

Dynamic cycling of t-SNARE acylation regulates platelet exocytosis

Platelets regulate vascular integrity by secreting a host of molecules that promote hemostasis and its sequelae. Given the importance of platelet exocytosis, it is critical to understand how it is controlled. The t-SNAREs, SNAP-23 and syntaxin-11, lack classical transmembrane domains (TMDs), yet both are associated with platelet membranes and redistributed into cholesterol-dependent lipid rafts when platelets are activated. Using metabolic labeling and hydroxylamine (HA)/HCl treatment, we showed that both contain thioester-linked acyl groups. Mass spectrometry mapping further showed that syntaxin-11 was modified on cysteine 275, 279, 280, 282, 283, and 285, and SNAP-23 was modified on cysteine 79, 80, 83, 85, and 87. Interestingly, metabolic labeling studies showed incorporation of [3H]palmitate into the t-SNAREs increased although the protein levels were unchanged, suggesting that acylation turns over on the two t-SNAREs in resting platelets. Exogenously added fatty acids did compete with [3H]palmitate for t-SNARE labeling. To determine the effects of acylation, we measured aggregation, ADP/ATP release, as well as P-selectin exposure in platelets treated with the acyltransferase inhibitor cerulenin or the thioesterase inhibitor palmostatin B. We found that cerulenin pretreatment inhibited t-SNARE acylation and platelet function in a dose- and time-dependent manner whereas palmostatin B had no detectable effect. Interestingly, pretreatment with palmostatin B blocked the inhibitory effects of cerulenin, suggesting that maintaining the acylation state is important for platelet function. Thus, our work shows that t-SNARE acylation is actively cycling in platelets and suggests that the enzymes regulating protein acylation could be potential targets to control platelet exocytosis in vivo.

Cholesterol enrichment of rabbit platelets enhances the Ca(2+) entry pathway induced by platelet-derived secondary feedback agonists

AIMS

Hypersensitivity of platelets due to increased platelet cholesterol levels has been reported in hypercholesterolemia. However, the signaling pathways linking increased platelet reactivity and cholesterol contents are not fully understood. This study aims to determine the direct effect of cholesterol enrichment of platelets on the pathways including Ca(2+) mobilization and secondary feedback agonists such as adenosine diphosphate (ADP) and thromboxane A2 (TXA2).

MAIN METHODS

In vitro cholesterol enrichment of rabbit platelets was performed by incubation with cholesterol complexed with methyl-β-cyclodextrin. Ca(2+) mobilization was monitored using platelets loaded with fura-PE3/AM, a fluorescent calcium indicator. Released ATP and TXB2 from platelets were measured by a luciferin-luciferase ATP assay system and a TXB2 ELISA Kit, respectively.

KEY FINDINGS

Cholesterol enrichment of rabbit platelets significantly enhanced Ca(2+) mobilization induced by thrombin, accompanying an augmented Ca(2+) entry. The augmentation of Ca(2+) entry by cholesterol enrichment was significantly suppressed by treatment with inhibitors for secondary feedback agonists. In cholesterol-enriched platelets, the amount of released ATP or TXB2 induced by thrombin was not significantly altered in comparison with control platelets, whereas an increase in [Ca(2+)]i induced by ADP or U46619, a TXA2 mimetic, was significantly enhanced.

SIGNIFICANCE

These results suggest that cholesterol enrichment of rabbit platelets results in enhanced Ca(2+) mobilization via ADP/TXA2-dependent augmentation of the Ca(2+) entry pathway. The results reveal a novel mechanism by which platelet hypersensitivity is regulated by cholesterol contents.

Platelet dysfunction in vascular pathologies and how can it be treated

Cardiovascular diseases are one of the leading causes of morbidity and mortality in industrialized countries, and although many processes play a role in the development of vascular disease, thrombosis is the primary event that precipitates stroke and acute coronary syndromes. The blood platelets are of significant importance in medicine. These cells are involved in many physiological processes, particularly haemostasis through their ability to aggregate and form clots in response to activation. In addition, these dynamic cells display activities that extend beyond thrombosis, including an important role in initiating and sustaining vascular inflammation. The expansion of knowledge from basic and clinical research has highlighted the critical position of platelets in several inflammatory diseases such as arthritis and atherosclerosis. Platelets are emerging as important mediators of inflammation and provide important signals to mediate phenotype of other blood and vascular cells. The important role of platelets in arterial thrombosis and the onset of acute myocardial infarction after atherosclerotic plaque rupture make inhibition of platelet aggregation a critical step in preventing thrombotic events associated with stroke, heart attack, and peripheral arterial thrombosis. However, the use of platelet inhibitors for thrombosis prevention must seek a delicate balance between inhibiting platelet activation and an associated increased bleeding risk. The aim of this review is to up-date the knowledge on platelets physiology and dysfunction in pathologies, such as diabetes mellitus, hypercholesterolemia, and hypertension, emphasizing the link between platelets and the inflammation-related atherosclerosis. The review evaluates the opportunities offered by the novel platelet inhibitors to efficiently alleviate the thrombotic events.