Platelet Serotonin Aggravates Myocardial Ischemia/Reperfusion Injury via Neutrophil Degranulation.

BACKGROUND: Platelets store large amounts of serotonin that they release during thrombus formation or acute inflammation. This facilitates hemostasis and modulates the inflammatory response. METHODS: Infarct size, heart function, and inflammatory cell composition were analyzed in mouse models of myocardial reperfusion injury with genetic and pharmacological depletion of platelet serotonin. These studies were complemented by in vitro serotonin stimulation assays of platelets and leukocytes in mice and men, and by measuring plasma serotonin levels and leukocyte activation in patients with acute coronary syndrome. RESULTS: Platelet-derived serotonin induced neutrophil degranulation with release of myeloperoxidase and hydrogen peroxide (H2O2) and increased expression of membrane-bound leukocyte adhesion molecule CD11b, leading to enhanced inflammation in the infarct area and reduced myocardial salvage. In patients hospitalized with acute coronary syndrome, plasmatic serotonin levels correlated with CD11b expression on neutrophils and myeloperoxidase plasma levels. Long-term serotonin reuptake inhibition-reported to protect patients with depression from cardiovascular events-resulted in the depletion of platelet serotonin stores in mice. These mice displayed a reduction in neutrophil degranulation and preserved cardiac function. In line, patients with depression using serotonin reuptake inhibition, presented with suppressed levels of CD11b surface expression on neutrophils and lower myeloperoxidase levels in blood. CONCLUSIONS: Taken together, we identify serotonin as a potent therapeutic target in neutrophil-dependent thromboinflammation during myocardial reperfusion injury.

Peripheral serotonin lacks effects on endothelial adhesion molecule expression in acute inflammation.

BACKGROUND: Peripheral, non-neuronal serotonin promotes the recruitment of neutrophils to sites of acute inflammation and tissue damage. Direct effects of serotonin on neutrophil function were shown to be involved. However, the influence of serotonin on the endothelial counterpart is unknown. OBJECTIVES: To investigate whether serotonin alters the function of endothelial cells in leukocyte recruitment during acute inflammation. METHODS: We used two murine models of acute inflammation: intraperitoneal lipopolysaccharide (LPS) injection and mesenteric ischemia/reperfusion injury (I/R). To study effects of peripheral serotonin, leukocyte recruitment and endothelial adhesion molecule expression were compared in wild type (WT) and tryptophan hydroxylase 1 deficient (Tph1-/- ) mice, which are unable to synthesize peripheral serotonin. RESULTS: As expected, neutrophil transmigration into the peritoneal cavity following LPS injection was impaired in Tph1-/- mice. Abdominal blood vessels, however, showed no difference in adhesion molecule expression. In the early reperfusion phase after mesenteric I/R, the number of rolling leukocytes was significantly lower in Tph1-/- compared to WT. In line with the LPS model, endothelial adhesion molecule expression was independent of serotonin. In vitro experiments using human umbilical vein endothelial cells (HUVECs) confirmed that serotonin does not affect endothelial adhesion molecules. CONCLUSIONS: The inflammatory release of peripheral serotonin is dispensable for the regulation of endothelial adhesion molecules.

Role of platelet serotonin in innate immune cell recruitment.

Serotonin (5-Hydroxytryptamine, 5-HT) was discovered as a vasoconstrictor in 1937. Since its discovery, the involvement of serotonin in numerous physiological processes was described. It acts as an important neurotransmitter, regulates bowel movement, can be released as a tissue hormone and acts as a growth factor. Among the years, a link between serotonin and inflammation has been identified and further evidence suggests an important role of serotonergic components in immune responses. Peripheral serotonin is synthesized by the enzyme tryptophan hydroxylase (Tph), which exists in two different isoforms: Tph2 being responsible for serotonin synthesis in neurons and Tph1 for generation of serotonin in peripheral organs. After synthesis in intestinal enterochromaffin cells, serotonin is stored in platelets and released upon stimulation. Several immune cells express the serotonin transporter SERT and enzymes for serotonin metabolism (monoamine oxygenase, MAO). To be susceptible to changes in serotonin levels, serotonin receptors are required and almost all of the 15 receptor subtypes are represented on immune cells. In this review, we describe the distribution of serotonergic components in cells of the immune system and the impact of platelet-derived serotonin on these cells. In particular, we aim to understand the effect of serotonin on immune cell recruitment to sites of inflammation.

Platelet-neutrophil complex formation-a detailed in vitro analysis of murine and human blood samples.

Platelets form complexes with neutrophils during inflammatory processes. These aggregates migrate into affected tissues and also circulate within the organism. Several studies have evaluated platelet-neutrophil complexes as a marker of cardiovascular diseases in human and mouse. Although multiple publications have reported platelet-neutrophil complex counts, we noticed that different methods were used to analyze platelet-neutrophil complex formation, resulting in significant differences, even in baseline values. We established a protocol for platelet-neutrophil complex measurement with flow cytometry in murine and human whole blood samples. In vitro platelet-neutrophil complex formation was stimulated with ADP or PMA. We tested the effect of different sample preparation steps and cytometer settings on platelet-neutrophil complex detection and noticed false-positive counts with increasing acquisition speed. Platelet-neutrophil complex formation depends on platelet P-selectin expression, and antibody blocking of P-selectin consequently prevented ADP-induced platelet-neutrophil complex formation. These findings may help generating more comparable data among different research groups that examine platelet-neutrophil complexes as a marker for cardiovascular disease and novel therapeutic interventions.