The platelet proteome

PURPOSE OF REVIEW

The proteome is the pool of proteins expressed at a given time and circumstance. The word 'proteomics' summarizes several technologies for visualization, quantitation and identification of these proteins. Recent advances in these techniques are helping to elucidate platelet processes which are relevant to bleeding and clotting disorders, transfusion medicine and regulation of angiogenesis.

RECENT FINDINGS

Over 1100 platelet proteins have been identified using proteomic techniques. Various subproteomes have been characterized, including platelet releasates (the 'secretome'), alpha and dense granules, membrane and cytoskeletal proteins, platelet-derived microparticles, and the platelet 'phosphoproteome'. Proteomic data about platelets have become increasingly available in integrated databases.

SUMMARY

Proteomic experiments in resting and activated platelets have identified novel signaling pathways and secreted proteins which may represent therapeutic targets, as well as potential cancer biomarkers.

The dopamine agonism on ADP-stimulated platelets is mediated through D2-like but not D1-like dopamine receptors

Monoamines such as serotonin and epinephrine are known to be involved in platelet activation and aggregation. Dopamine is another monoamine identified in platelets, but published data about its effect on platelets and the receptors involved are controversial. In the present study, we investigated the dopamine agonism in platelets and the receptors involved in these pathways. Platelet-rich plasma (PRP) of healthy individuals was treated with agonists (ADP, epinephrine, dopamine) and various dopamine receptor and transporter antagonists such as SCH-23390, raclopride, clozapine, methylphenidate, and cocaine. Platelet activation was investigated by flow cytometry (CD62P and CD63 surface expression), optical aggregometry, and microaggregate adhesion to collagen IV in a flow chamber system. In our study, dopamine on its own had no effect on platelet activation in the different assays. However, when used in combination with ADP (10 muM), dopamine in a range of 1 to 100 muM significantly potentiated platelet microaggregate formation and adhesion to collagen under low shear flow conditions. Specific antagonists for D2-like receptors (L-741,626, raclopride, and clozapine) completely diminished the dopamine effect at selective concentrations, but not the effect of epinephrine. Neither the D1-like receptor antagonist SCH-23390 nor dopamine transporter antagonists (methylphenidate, cocaine) showed inhibitory effects on the dopamine agonism. Thus, dopamine is an ADP-dependent platelet agonist which acts via D2-like but not D1-like receptors or adrenergic receptors. Because many psychopharmacological drugs are directed to D2-like receptors, platelet dysfunction in patients being treated with such drugs may be linked to these mechanisms.

Platelet transcriptome and cardiovascular disease

Platelet hyper-reactivity is likely to play a role in cardiovascular disease, but there are no standardized tests to evaluate platelet responsiveness. A 'platelet chip' (a synthetic oligonucleotide microarray representing all platelet-restricted genes) is under development as a tool for high-throughput characterization of platelet-based bleeding and clotting disorders. In future, platelet gene profiling may be used to improve thrombohemorrhagic risk assessment and to guide antiplatelet therapy for patients at risk of cardiovascular disease.