Structural insights in platelet receptor synergism-antiplatelet therapy in post-ischemic cerebrovascular events

P2Y12 receptor expression is a critical determinant of functional responsiveness to ATX's MORFO domain

In the central nervous system, the formation of the myelin sheath and the differentiation of the myelinating cells, namely oligodendrocytes, are regulated by complex signaling networks that involve purinergic receptors and the extracellular matrix. However, the exact nature of the molecular interactions underlying these networks still needs to be defined. In this respect, the data presented here reveal a signaling mechanism that is characterized by an interaction between the purinergic P2Y(12) receptor and the matricellular extracellular matrix protein autotaxin (ATX), also known as ENPP2, phosphodiesterase-Iα/ATX, or lysoPLD. ATX has been previously described by us to mediate intermediate states of oligodendrocyte adhesion and to enable changes in oligodendrocyte morphology that are thought to be crucial for the formation of a fully functional myelin sheath. This functional property of ATX is mediated by ATX's modulator of oligodendrocyte remodeling and focal adhesion organization (MORFO) domain. Here, we show that the expression of the P2Y(12) receptor is necessary for ATX's MORFO domain to exert its effects on differentiating oligodendrocytes. In addition, our data demonstrate that exogenous expression of the P2Y(12) receptor can render cells responsive to the known effects of ATX's MORFO domain, and they identify Rac1 as an intracellular factor mediating the effect of ATX-MORFO-P2Y(12) signaling on the assembly of focal adhesions. Our data further support the idea that a physical interaction between ATX and the P2Y(12) receptor provides the basis for an ATX-MORFO-P2Y(12) signaling axis that is crucial for mediating cellular states of intermediate adhesion and morphological/structural plasticity.

Surface tunability of nanoparticles in modulating platelet functions

Metallic nanoparticles are attractive candidates as MRI contrast agents and mediators for drug delivery, diagnostics, and therapy. Direct contact and exposure to blood circulation is common in many such applications. The consequent thrombotic response may therefore be important to study. The main objective of the present work was to study how platelet functions were changed in the presence of different nano-surface or surface capping, which may provide a measure for the safety of a nanoparticle, and also assess the use of such nanoparticles in platelet modulation. Aggregometry, ATP release reaction, flow cytometry and immune-blotting studies were performed to study platelet response to different nano-particles (iron oxide, gold and silver). For each nanoparticle surface conjugation (capping) was varied. It was found that citric acid functionalized iron oxide nanoparticles have anti-platelet activity, with a decrease in aggregation, tyrosine phosphorylation level, and granule release. On the other hand in other cases (e.g. gold nanoparticles) pro-aggregatory response was observed in the presence of nanoparticles and, in some cases, the nanoparticles behaved neutrally (e.g. for starch-coated iron oxide nanoparticles). Therefore, nanoparticles can induce antiplatelet or a pro-aggregatory response, or remain neutral depending on surface capping. A related observation is that antiplatelet drugs can be made more potent by nanoparticle conjugation.

Platelet responsiveness to yohimbine hydrochloride and MRS2179 in the context of the interaction between collagen and epinephrine in acute coronary syndrome

Acute coronary syndrome (ACS) covers a spectrum of clinical conditions ranging from unstable angina, Non-ST segment elevation myocardial infarction (NSTEMI), or ST segment elevation myocardial infarction (STEMI). This study encompasses patients with acute coronary syndrome, who were receiving the dual antiplatelet therapy of aspirin and clopidogrel. The focus of the study was to gain insight into the role of selective P2Y1 antagonism using MRS2179 in such cases as well as its effects, if any, on collagen-epinephrine interaction. All the cases showed greater potency of inhibition of the interaction when yohimbine hydrochloride (YH), a blocker of alpha2A-adrenoreceptor, was used compared to MRS2179, a P2Y1 antagonist, although there was variability in responsiveness to the antiplatelet drugs. These findings indicate that alpha2A-adrenoreceptors of platelets in this group play a major role in precipitating the interactive effect of collagen and epinephrine. The dose-response effect as studied by platelet aggregometry showed that the required molar concentration to block the interactive effect in the case of YH was less than that of MRS2179. Hence, it is postulated that although there may be an impairment of collagen-induced aggregation by MRS2179, the interactive effect of collagen-epinephrine may not be impaired by MRS2179 as efficaciously as YH.

The role of ATP and adenosine in the brain under normoxic and ischemic conditions

By taking advantage of some recently synthesized compounds that are able to block ecto-ATPase activity, we demonstrated that adenosine triphosphate (ATP) in the hippocampus exerts an inhibitory action independent of its degradation to adenosine. In addition, tonic activation of P2 receptors contributes to the normally recorded excitatory neurotransmission. The role of P2 receptors becomes critical during ischemia when extracellular ATP concentrations increase. Under such conditions, P2 antagonism is protective. Although ATP exerts a detrimental role under ischemia, it also exerts a trophic role in terms of cell division and differentiation. We recently reported that ATP is spontaneously released from human mesenchymal stem cells (hMSCs) in culture. Moreover, it decreases hMSC proliferation rate at early stages of culture. Increased hMSC differentiation could account for an ATP-induced decrease in cell proliferation. ATP as a homeostatic regulator might exert a different effect on cell trophism according to the rate of its efflux and receptor expression during the cell life cycle. During ischemia, adenosine formed by intracellular ATP escapes from cells through the equilibrative transporter. The protective role of adenosine A(1) receptors during ischemia is well accepted. However, the use of selective A(1) agonists is hampered by unwanted peripheral effects, thus attention has been focused on A(2A) and A(3) receptors. The protective effects of A(2A) antagonists in brain ischemia may be largely due to reduced glutamate outflow from neurones and glial cells. Reduced activation of p38 mitogen-activated protein kinases that are involved in neuronal death through transcriptional mechanisms may also contribute to protection by A(2A) antagonism. Evidence that A(3) receptor antagonism may be protective after ischemia is also reported.