Normal platelet function is important for the body to maintain haemostasis. Under pathological conditions, platelet activity may be altered by elevated circulating priming factors. Primers can enhance platelet responses to physiological agonists such as thrombin and ADP, which leads to an increased risk of thrombus formation and resistance to anti-thrombotic drugs. Previous studies have identified several platelet primers including thrombopoietin, insulin growth factor 1 and sCD40 ligands. There is evidence to suggest that more platelet primers may exist, however, the identity and underlying mechanism of novel platelet primers is still largely unknown. This project therefore aims to identify novel primers and the underlying mechanism by which they affect platelet function. Through literature research and platelet transcriptome expression studies, a list of potential platelet primers was generated. Using platelet aggregation studies, primers that increased protease-activated receptor 1 (PAR1)-mediated platelet responses were taken for further studies. Of these primers, sphingosine-1-phosphate (S1P) was identified as the novel platelet primer to be studied in more detail. This study demonstrated that S1P exhibited a concentration dependent biphasic effect on platelet function, with low concentrations of S1P enhancing PAR1-mediated platelet responses, while inhibitory effects were observed at high concentrations of S1P. To demonstrate the mechanism of S1P biphasic effects and probe the S1P receptor subtypes expression on platelets, S1P receptors agonists and antagonists were tested in aggregation, flow cytometry, immunoblotting, TxB2 ELISA, Ca2+ mobilization assay and platelet spreading experiments. Current results suggest that the S1P’s priming effect was mediated via activation of phosphoinositide 3-kinase (PI3K) signalling pathway, resulting in increased integrin αIIbβ3 activation and P-selectin exposure. No significant effect of S1P on platelet calcium signalling or thromboxane A2 synthesis was observed. This study also indirectly suggests that the S1PR1, S1PR4 and S1PR5 receptors are expressed on human platelets.
Identification and Elucidation of Novel Platelet Primers
Liu, H. (Author). 23 Jan 2019
Student thesis: Master's Thesis › Master of Science by Research (MScR)