CLEC‐2‐dependent activation of mouse platelets is weakly inhibited by cAMP but not by cGMP

BACKGROUND: The activation of platelet CLEC‐2 by podoplanin on lymphatic endothelial cells (LECs) has a critical role in prevention of mixing of lymphatic and blood vasculatures during embryonic development. Paradoxically, LECs release cAMP and cGMP‐elevating agents, prostacyclin (PGI(2)) and nitric oxide (NO), respectively, which are powerful inhibitors of platelet activation. This raises the question of how podoplanin is able to activate CLEC‐2 in the presence of the inhibitory cyclic nucleotides. OBJECTIVES: We investigated the influence of cyclic nucleotides on CLEC‐2 signaling in platelets. METHODS: We used rhodocytin, CLEC‐2 monoclonal antibody, LECs and recombinant podoplanin as CLEC‐2 agonists on mouse platelets. The effects of the cyclic nucleotide‐elevating agents PGI(2), forskolin and the NO‐donor GSNO were assessed with light transmission aggregometry, flow cytometry, protein phosphorylation and fluorescent imaging of platelets on LECs. RESULTS: We show that platelet aggregation induced by CLEC‐2 agonists is resistant to GSNO but inhibited by PGI(2). The effect of PGI(2) is mediated through decreased phosphorylation of CLEC‐2, Syk and PLCγ2. In contrast, adhesion and spreading of platelets on recombinant podoplanin, CLEC‐2 antibody and LECs is not affected by PGI(2) and GSNO. Consistent with this, CLEC‐2 activation of Rac, which is required for platelet spreading, is not altered in the presence of PGI(2). CONCLUSIONS: The present results demonstrate that platelet adhesion and activation on CLEC‐2 ligands or LECs is maintained in the presence of PGI(2) and NO.