The regulation of Sema4D exodomain shedding by protein kinase A in platelets.

We have previously shown that Sema4D expressed on the platelet plasma membrane can be cleaved by the metalloprotease ADAM17, producing a 120-kDa exodomain fragment that retains biological activity and remnant fragments of 24-28 kDa that remain associated with the platelet membrane. This process is modulated by calmodulin. Here we investigated the potential role of protein kinase A (PKA) in these events. Using a pharmacological approach, we now show that inhibition of PKA by H89 is sufficient to induce Sema4D exodomain shedding, while activation of PKA inhibits agonist-initiated shedding. Studies on the regulatory mechanism show that the shedding induced by PKA inhibition is mediated by ADAM17, but, unlike agonist-induced shedding, does not involve the dissociation of calmodulin from the Sema4D cytoplasmic domain. In attempt to identify the cleavage sites for shedding, we found that ADAM17 mediates variable cleavages in the juxtamembrane region. Therefore, our data reveal a potential regulatory mechanism for the shedding of Sema4D in platelets.